DOCUMENT RESUME

ED 194 321 SE 033 142

AUTHOR Mayer, William V.TITLE Radiation and Its Use in Biology: A Laboratory

Block.INSTITUTION Biological Sciences Curriculum Study, Boulder,

Colo.SPONS AGENCY National Science Foundation, Washington, D.C.PUB DATE 70NOTE 66p.

EDPS PRICE MF01/PC03 Plus Postage.DESCRIPTORS Biology; *Instructin-1 Materials; *Radiation

Biology; *Science Activities; Science CourseImprovement Projects; Science Curriculum; ScienceEducation; Secondary Education: Secondary SchoolScience; *Supplementary Reading Materials; *TeachingGuides: Units of Study

IDENTIFIERS *Biological Sciences Curriculum Study

ABSTRACTThis booklet contains a six-week series of laboratory

investigations that may be used individually or in combination tocotplement other biology course materials or as an independentlaboratory course in radiation biology. Contents include twelveactivities dealing with radiation biology, five additional activitiessuitable for individual work, and four appendices which cover thehandling of microorganisms, a discussion on radiation, howradioactive materials are produced, and how a Geiger-Muller counteris operated. (CS)

************************************************************************ Reproductions supplied by EDRS are the best that can be made

from the original document.***********************************************************************

BIOLOGICAL SCIENCES CURRICULUM STUDYCOMMITTEE ON INNOVATION IN LABORATORY INSTRUCTION

U S DEPARTMENT OF HEALTH.EDUCATION L WELFARE.NATIONAL INSTITUTE OF

EDUCATION

THIS DOCUMENT HAS BEEN REPRO-DUCED EXACTLY AS RECEIVED FROMTHE PERSON OR ORGANIZATION ORIGINATING IT POINTS OF VIEW OR OPINIONSSTATED DO NOT NECESSARILY REPRE-SENT OFFICIAL NATIONAL INSTITUTE OFEDUCATION POSITION OR POLICY

A Laboratory Block

Radiation and Its Usein Biology

by

WILLIAM V.JVIAYER

Department of iologyUniversity of Colorado

"PERMISSION TO REPRODUCE THISMATERIAL HAS BEEN GRANTED BY

Nary L. rtriescrP NSF

TO THE EDUCATIONAL RESOURCESINFORMATION CENTER (ERIC)."

Published by

EDUCATIONAL PROGRAMSIMPROVEMENT CORPORATION

Copyright © 1970

by

Educational Programs Improvement Corporation,

Boulder, Colorado. All rights reserved.

Except for the rights reserved by others, the publisher and the cupyrightowner hereby grant permission to domestic persons of the United States andCanada for use of this work in whole or in part without charge in the Eng-lish language in the United States and Canada after January 1, 1977, pro-vided that written notice is made to the Director, Biological Sciences Cur-riculum Study and that the publication incorporating material covered bythis copyright contains the original copyright nalce and a statement thethe publication is not endorsed by the original copyright owner.For conditions of use and permission to use materials contained herein forforeign publication or publication in other than the English language, applyto the Director, Biological Sciences Curriculum Study, P.O. Box 930, Boul-der, Colorado, 80302.

3

LABORATORY BLOCKS FOR BIOLOGY

The Laboratory Blocks are a series of books,each of which provides for the investigation indepth of a specific topic in biology. The serieshas been developed by the Committee on Inno-vation in Laboratory Instruction of the Biolog-ical Sciences Curriculum Study and has beensupported by the National Science Foundation.

The Committee on Innovation in Laboratory InstructionADDISON E. LEE, Chairman, The University of Texas at Austin

HARPER FOLLANSBEE, Phillips AcademyBENTLEY GLASS, State University of New York

WILLIAM P. JACOBS, Princeton UniversityFLORENCE Moot, Washington University

EDWIN A. PHILLIPS, Pomona CollegeA. GLENN RICHARDS, University of MinnesotaALFRED S. SUSSMAN, University of Michigan

Biological Sciences Curriculum Study, University of Colorado

Addison E. Lee, ChairmanWilliam V. Mayer, Director

Manert H. Kennedy, Associate DirectorRobert F. Wilson, Art Director

iii

BSCS Publications

BSCS VERSIONS-:-cee introductory approaches to biological science as inquiry.

Biological Science. !..,1 ,nquiry Into Life, Second Edition, Student Laboratory Guide and Teachers Manual, HB.

High School Biology: 3SCS Green Version, Revised Edition, and Teacher's Guide, R.Biological Science: Molecules to Man, Revised Edition, Teacher's Edition, and Answer Key and Guide, HM.

TestsComprehensive Final Examination for use with the 1963 editions, PC.Version Quarterly Tests and Final Examinations, 1968 editions, HB, HM, R.

BSCS SECOND COURSE-an extension of the investigative aspects of biology for students who have completed an introductorycourse.

Biological Science: Interaction of Experiments and Ideas, Second Edition, PH.

TestsQuarterly Tests and Final Examination, first edition, PH; second edition, EPIC.

BSCS PATTERNS OF LIFE SERIES-enrichment materials for teachers and students, or interested laymen, R.

Animals of the Islands Bird MigrationAntibiotics Defensive Secretions of AnthropodsBiological Effects of Ionizing Radiation

Energy Transfer in Ecological SystemsPlant Morphogenes!Tortoise Behavior and Survival

BSCS SPECIAL MATERIALS-biology as a science for students previously unsuccessful in academic achievement.

Biological Science: Patterns and Processes, Second Edition, HR.

TestsUnit Tests and Final Examination, first edition, PC; second edition, Test Item Booklet, EPIC.

BSCS LABORATORY BLOCKS-six-week laboratory investigations that may be used individually or in combination to comple-ment other biology course materials or as an independent laboratory course in biology, H.

Animal Behavior Microbes: Their Growth, Nutrition Regulation in Plants by Hormones-

Animal Growth and Development and Interaction A Study in Experimental Design

Ele2iution Physiological Adaptation The Complementarity of Structure and

Field Ecology Plant Growth and Development Function

Genetic Continuity Radiation and Its Use in Biology The Molecular Basis of Metabolism

Life in the Soil (Available only from EPIC)Innovations in Equipment and Techniques for the Biology Teaching Laboratory, a resource book on laboratory apparatus, materials,

Testsand prccedures, H.

Tests and Teacher's Resource Book, H

BSCS PAMPHLET SERIES-enrichment materials on biological topics for teachers and students, or interested laymen. Allavailable from EPIC at 75 ea. plus postage.

Animal Language Cell Division Guideposts of Animal Population Genetics

Bioelectricity Cellulose in Animal Navigation Present Problems About

Biogeography Nutrition Hibernation the Past

Biological Clocks Courtship in Animals Homeostatic Regulation Slime Molds and Research

Biology of Coral Atolls Early Evolution of Life Metabolites of the SeaBiology of Termites Ecology of the African Photoperiodism in AnimalsBiomechanics of the Body Elephant Photosynthesis

Blood Cell Physiology Growth and Age Plant Systematics

BSCS RESEARCH PROBLEMS-challenging research written for students by practicing research scientists.

Research Problems in Biology: Investigations for Students, Series 1, 2, 3, and 4, D.

B SCS SELF-INSTRUCTIONAL PROGRAMS-programmed materials.

Population Genetics: A Self-Instructional Program (other titles in preparation), G.

B SCS BULLETIN SERIES-materials prepared for teachers, science supervisors, and school administrators.

1. Biological Education in American Secondary Schools 1890-1960, by Paul DeH. Hurd, BSCS, $3.502. Teaching High School Biology: A Guide to Working With Potential Biologists, by Paul F. Brandwein, Jerome Metzner, Evelyn Mor-

holt, Anne Roe, and Walter Rosen, BSCS, $2.003. BSCS Biology-Implementation in the Schools, by Arnold B. Grobman, Paul DeH. Hurd, Paul Klinge, Margaret McKibben Lawler,

and Elra Palmer, BSCS, clothbound, $5.00, paperbound, $3.504. The Changing Classroom: The Role of the Biological Sciences Curriculum Study, by Arnold B. Grobman, D. $6.95

5iv

BSCS SPECIAL PUBLICATIONSmaterials prepared primarily for teachers, science supervisors, institute directors, and col-lege science methods course instructors, BSCS, F. (1, 2, and 3 unavailable)

4. The Teacher and BSCS Special Materials 6. New Materials and Techniques in the Preparation of High School Biology Teachers5. Laboratory Blocks in Teaching Biology 7. Life Sciences in the Middle School

BSCS TEACHERS' HANDBOOK-a resource of background information on teaching modern biology and a reference on teach-Biology Teachers' Handbook, Second Edition, W. ing strategies for the biology teacher.

BSCS SINGLE TOPIC INQUIRY FILMS-a series of 40 filmed biological inquiries that are designed to be stopped frequentlyfor class discussion and student participation, HB, HM, and R.

An Example of the Biological Fossil Interpretation Mountain TreesAn Regeneration in AcetabulariaSignificance of Color Frog Development Ecological Study Social Behavior in Chickens

An InquiryThe Importanceof the Nucleus

Gene Flow in a CaliforniaSalamander

Nerves and Heartbeat RateOak ppulations

Structure, Function andFeeding Behavior in Herons

Australian Marsupials Genetics of Bacterial Photoreception and Temperature and ActivityBehavior of a Purple Nutrition Flowering in Reptiles

Bacterium GrouseA Species Problem Phototropism Temporal Patterns of AnimalChemical Communication Imprinting Planarian Behavior ActivityConvergence Life in the Intertidal Region Prairies and Deciduous The Intertidal RegionEngelmann's Inquiry into Locomotion in an Amoeba Forests The Kidney and

Photosynthesis Mating Behavior in the Predation and Protection HomeostasisFeeding Behavior of Hydra Cockroach in the Ocean The Peppered Moth: AFeeding Mechanisms of Mimicry Prey Detection in the Population Study

Oyster Drills Mitosis Rattlesnake Water and Desert AnimalsFlowering Water and Desert Plants

BSCS INQUIRY SLIDES-20 sequences of 6 to 15 full-color 35mm slides for daylight blackboard projection, HB.

Accuracy in Measurement Dietary Deficiency in Chickens Light Intensity and Quantitative RelationshipsCardiac Circulation Effects of Thyroid Action Photoc inthes is Sensing Mechanisms andControl of Blood Sugar: A Endocrine Pathways Metamorphosis in Cecropia Homeostasis

Homeostatic Mechmism Genetic Resistance to Moths Sources of Plant NutritionControl of Molting in Insects Pesticides Paper Chromatography Structure and FunctionControl of the Pancreas Light and Plant Growth Plant-Animal Physiology The Cell NucleusControl of Thyroid Secretion

BSCS TEACHER PREPARATION FILM -an orientation film for teacher grcups on the inquiry use of the BSCS Single TopicFilms.A BSCS Single Topic Inquiry Film Presentation, BSCS, FL, NA. Inquiry

BSCS INFORMATION FILM-a revised film on the history and philosophy of the BSCS.The Story of BSCS, BSCS, FL, P0.

BSCS NEWSLETTERS, BSCS, F.

BSCS INTERNATIONAL NEWS NOTES, escs, F.

BSCS SCIENCE AND SOCIETY SERIES-a projected series of 40 paperbacks by recognized authorities in the sciences ontopics of interest and concern to a large segment of our society.

The Personal Meaning of Birth Control, by Garrett Hardin, P. (Other titles in progress.)

BSCS TOPICS IN BIOLOGICAL SCIENCE SERIES -a series of paperbacks written by scientists on biological topics of con.

Symbiosis, by Thomas Cheng, P. (Other titles in progress.) cern to the informed general reader.

LEGEND

BSCSBiological Sciences Curriculum Study, P.O. Box 930, Boulder, Colorado 80302. DDoubleday & Co., 277 Park Ave., NewYork, N.Y. 10017. EPICEducational Programs Improvement Corporation, P.O. Box 3406 Boulder, Colorado 80302. FFree whenrequested on official letterhead; state proposed use. FLFree loan when requested on official letterhead; state viewing date(s)and audience. GGeneral Learning Corp., Silver Burdett Company Division, Morristown, N.J. 07960. HD.C. Heath & Co., 285Columbus Ave., Boston, Mass. 02116. HB Harcourt, Brace & World, Inc., 757 Third Ave., New York, N.Y. 10017. HMHoughtonMifflin Co., 110 Tremont St., Boston. Miss. 02107. HRHolt, Rinehart and Winston, Int.., 383 Madison Ave., New York, N.Y. 10017.NANot available for shipment outside U.S.A. PPegasus, A Division of Western Publishing Co., Inc., 850 Third Ave., New York,N.Y. 10022. PCThe Psychological Corporation, 304 E. 45th St., New York, N.Y. 10017. PHPrenticeHall, Inc., Englewood Cliffs,N.J. 07632. POPostage charge of $3.00 payable in advance for shipment outside U.S.A. RRand McNally & Co., P.O. Box 7600,Chicago, III. 60680. WJohn Wiley & Sons, Inc., 605 Third Ave., New York, N.Y. 10016.

V

6

FOREWORD

The Biological Sciences Curriculum Study,supported by the National Science Foundation,was initiated in 1958, with the charge to im-prove biological education at all levels. For thispurpose, instructional materials have been de-signed that reflect the modern content of biologywithout neglecting the wisdom of earlier schol-ars and authors. The basic approach of the Bio-logical Sciences Curriculum Study has been toemphasize independent laboratory investigationand inquiry as a means of acquiring significantknowledge in science. Materials produced by theBSCS utilize a variety of approaches to thestudy of biology, for there does not seem to bea single best way of presenting biology to thediversity of students studying the subject. TheBSCS has produced materials designed for allstudents regardleE2 of aptitudes or career goals.These materials have been successfully masteredat a level of student sophistication that washeretofore thought completely unrealistic.

Taken together, the BSCS materials comprisea series of balanced and enriched programs, notsimply a set of independent publications andaudio-visual materials. The BSCS has developeda wide variety of materials for use at both highschool and college levels, including three dif-ferent approaches to introductory biology forsecondary school students, a program for theacademically unsuccessful, a second course inbiology, Research Problems in Biology, a vari-ety of pamphlets on specific biological subjects,programmed materials, a Biology Teachers'Handbook, bulletins, special publications, News-letters, Single Topic Inquiry Films, InquirySlides, and other material.

Innovation has been a keynote of the BSCS,and among the innovations has been the investi-gation of ways to encourage use of the labora-tory as a means of promoting student inquiryand self-reliance. One of_the ways of emphasiz-ing the laboratory as a productive learning ex-perience has been the series of LaboratoryBlocks produced under the direction of theBSCS Committee on Innovation in LaboratoryInstruction. Each Block is a program requiringapproximately six weeks of time, during whichthe student investigates a specific biologicaltopic in depth. Ahy one of the LaboratoryBlocks may be used in conjunction with a wide

vi

variety of course materials to replace, to supple-ment, to complement, to enrich, or to augmenta specific component of the curriculum as neces-sary.

The Laboratory Block program has resultedin a group of books written either by a single.author or, at most, a small group of specialistsin a given field. The preliminary books weretested by BSCS Project Associates in a specialBSCS testing laboratory at the University ofTexas before undergoing trial use in a series ofclassrooms across the country. Based on thisextensive experience, the books were thoroughlyrevised before release for general use. The pres-ent volume is an example of one of these Labo-ratory Blocks, and the Preface, which is ad-dressed to the student, illustrates the rationalebehind the development of these Blocks.

The BSCS feels biological education is a con-tinuum, from kindergarten through universitygraduate school, and is attempting to devise anonrepetitive series of biological experiences tobe used at the most appropriate level in a givenprogram. The Laboratory Blocks constitute onekind of materials that can be used at a widevariety of instructional levels and in a largenumber of different situations. Adaptability andflexibility make this varied use not only possiblebut academically profitable within a particularcurricular sequence. More detailed informationon the philosophy and use of Laboratory Blocksis contained in BSCS Special Publication No. 5,Laboratory Blocks in Teaching Biology, avail-able free upon request to the Director of theBSCS.

With each new generation, our fund of scien-tific knowledge increases fivefold. This remark-able growth indicates in part that only selectedareas can be covered in depth at a given pointin time. Laboratory Blocks provide this in-depthcoverage of a specific topic; they have been usedas individual items in given parts of a curricu-lum, or several have been combined in variouspermutations to create entire laboratory-cen-tered, inquiry-oriented courses in biology.

The philosophy that has developed aroundthe BSCS Laboratory Blocks evolved fromstressing the importance of laboratory work inthe teaching of biology. Dr. Bentley Glass, for-mer Chairman of the BSCS, suggested that if

one were to design a continuous experience inthe laboratory, it would be possible to have thelaboratory work carry an entire topic within acourse and give the studeut significant and ex-tensive laboratory experience that could not begained by laboratory segments scattered peri-odically throughout a course. The LaboratoryBlocks are one mechanism for implementingthis approach to laboratory activities.

Hundreds of the nation's biological scientists,educators, and teachers have worked diligentlyon BSCS programs. Continued success andgrowth of the program is largely due to these

ADDISON E. LEEChairman of the Steering CommitteeBiological Sciences Curriculum StudyThe University of Texas at AustinAustin, Texas 78712

dedicated individuals and to those who havegiven unselfishly of their opinions and experi-ence in the classroom.

One of the strengths of the BSCS programhas been the incorporation of reactions gainedby the use of the materials in their betterment.This feedback has contributed constantly to theimprovement of existing materials and providedideas for still newer ones. Your comments andsuggestions are invited. Send them, togetherwith requests for information about other BSCSprojects intended to improve biological educa-tion, to the Director at the address below.

WILLIAM V. MAYER, DIRECTORBiological Sciences Curriculum StudyPost Office Box 930Boulder, Colorado 80302

vii

PREFACE

During the next six weeks we expect you towork and to think like a scientist or researchworker. Your actions in the laboratory or whiledoing homework should be directed by logicalthinking. Each step of your investigations shouldbe governed by the basic questions: How?What? Where? and Why? The answers you getwill be precisely as correct as your combinationof background knowledge, logical thinking, andaccurate experimentation let them be.

The scientist does not know all the answers;he is seeking to learn the answers just as youare. He has spent many years reading widely onthe subject of his interest so that his backgroundknowledge is extensive. By carefully studyingthe work of others and comparing it with hisown, he can make a critical evaluation of ourpresent state of knowledge. Then, carefully, heplans experiments which will provide observa-tions, or data, that can be interpreted and thatmay add new knowledge.

Since you do not have the background ofreading and experience in scientific research thatwould let you plan the material in this Labora-tory Block, you are asked to take our presenta-tion on faithup to a certain point. Thescientist would decide what to study and plan

how to study it. However, you will need tofollow the choices and procedural instructionsthat we will give you in this book. Both you andthe scientist obtain data, and from this point onyour work will be the sameanalyzing the dataand interpreting the results. At this point inresearch the scientist would make a decision onwhat to do nextrepeat the experiment, dis-card it, or plan the logical next step.

The first experiments could end in failure foreither you or the scientist. The scientist some-times learns more from his failures than fromhis successes. But he does make discoveries; thatis, he may find information to indicate that hisworking hypothesis is correct or that it is notcorrect, or he may find something he had notanticipated. So will you.

The exercises that follow have been designedto guide you in making some discoveriessomethat have al --ady been made by others and per-haps some that have not. The procedure given inthis Laboratory Block is one that a scientist,might follow if he were studying the same prob-lem. Therefore, during the next six weeks weexpect you to work like a scientist. And afterthat, who knows? You may want to become ascientist.

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9

The Committee on Innovationin Laboratory Instruction

ACKNOWLEDGMENTS

I A work such as this cannot be credited to oneman alone. While one man is ultimately respon-sible for the assemblage, order, style, andmistakes that may occur, this Block is a co-operative effort that has involved a large num-ber of individuals, from the members of theBSCS Committee on Innovation in LaboratoryInstruction to those who tested the experimentalBlock in actual classroom situations.

Three Project fr. ,sociates of the BSCS TestingLaboratory at the University of Texas testedand evaluated the various inquiries and recom-mended modifications prior to the appearance ofthe experimental edition used in the generalBSCS trial program. To them, the author is mostgrateful. They are: Dr. Marjorie Behringer,Department of Biology, University of NorthDakota, Grand Forks, North Dakota; Dr. DavidL. Lehman, 709 Leonard, Austin, Texas; andDr. H. Edwin Steiner, University of SouthFlorida, Tampa Campus, Tampa, Florida. Mrs.Mary Anne Hunter, Administrative Secretary atthe BSCS Austin Testing Laboratory, greatlyfacilitated work in conjunction with this Block.

Thirteen teachers tested the Radiation Blockin their classes and provided valuable feedbackthat contributed to the book in its present form.Without their aid. this volume would be far lessuseful in actual classroom situations. They are:Bob Anderson, Gunn Senior High School, PaloAlto, California; Andrew Browne, Los AltosHigh School, Los Altos, California; Rex Can,Shaw. ee Mission South High School, ShawneeMission, Kansas; Larry Hull, Cubberley SeniorHigh School, Palo Alto, California; C. T.Lange, Clayton High School, Clayton, Misso-iri;Dr. Leonard Molotsky, Shawnee Mission HighSchool District, Shawnee Mission, Kansas; RoyRosendahl, Homestead High School, Sunnyvale,California; Frank W. Smith, Jr., Los Altos HighSchool, Los Altos, California; Nancy E. Stees,Kennett Junior-Senior High School, KennettSquare, Pennsylvania; Larry Webster, ShawneeMission West High School, Shawnee Mission,

Kansas; Marlin Welch, Shawnee Mission EastHigh School, Shawnee Mission, Kansas; LillyWing, Ravenswood High School, East PaloAlto, California; and Harry Wong, Menlo-Atherton High School, Atherton, California.

Four individuals deserve special mention inconjunction with the preparation of this Labo-ratory Block. Dr. Walter Chavin of the Depart-ment of Biology of Wayne State University inDetroit, Michigan used the experimental editionin his jointly sponsored NSF-AEC SummerInstitute in Radiation Biology. The feedbackand contributions from the teacher-participantsin this institute were of great help,and only spacelimitations prohibit listing each by name. Theauthor is grateful to Dr. Chavin for the rigoroustesting his institute participants gave the experi-mental edition.

Mr. Patrick Balch of the Yuba City HighSOlool, Yuba City, California critically re-viewed the final draft of this manuscript andmade numerous helpful suggestions regardingorganiz ttion and emphasis on inquiry whichgreatly improved the teachability of the Block.

Dr. H. Edwin Steiner was the Project Asso-ciate most closely involved with the materials ofthis Laboratory Block. He was inordinatelyhelpful, patient, constructively critical and aidedimmeasurably in assessing the practical value ofthe inquiries herein.

Mr. Clarence T. Lange of Clayton HighSchool in Clayton, Missouri gave the authorexceptionally helpful feedback from the use ofthe experimental edition. He was extremelygenerous in supplying class results from hisusage of the experimental book and gave gen-erously of his own thoughts, materials, and ex-perience garnered from years of classroomteaching involving radiation biology.

To those many others whose suggestions,comments, and freely offered materials havebeen incorporated into this work, the authorextends his thanks and the gratitude of studentswho will profit by these contributions.

ix 1 0

IntroductionInquiry IInquiry H

Introduction

Inquiry IIIInquiry IVInquiry V

Inquiry VIInquiry VIIInquiry VIIIInquiry IXInquiry XInquiry XI

Inquiry XII

CONTENTS

What is radiation? 1

How are microorganisms affected by irradiation? 5Can the effects of irradiation be modified? 9

to radionuclide usecautions and precautions 11

How can you see radiation? 1 5How can we know when radioactive materials are present? 17How is the Geiger-Muller counter affected by radiation

present in the environment? 19What does a Geiger-Muller counter detect? 21How much radioactive material falls to earth? 23How do plants react to radionuclides? 23What happens to a radionuclide injected into animals? 27What effect does irradiation have on seeds? 29How can you detect the movemcat of materials

through an ecosystem? 31How can radionuclides be used to solve

biological problems? 33

fn your own1. How are radionuclides used to measure volumes in

living systems? 372. What effects does irradiation have on metabolism? 373. Can irradiation act as a preservative? 384. What types of genetic damage are caused by in adiation? 385. How does irradiation affect chemical reactions

in organisms? 39How are microorganisms handled? 41What is radiation? 45How are radioactive materials produced? 53How do you operate a Geiger-Muller counter? 57

Appendix IAppendix IIAppendix IIIAppendix IV

References

37

61

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11

INTRODUCTION WHAT IS RADIATION?

You turn over on a Florida beach so yourback will get a more uniform tan. At the end ofa hard day's skiing in Colorado you warm your-self in front of the blazing logs in the lodge fire-place. In New York City, you watch the RoseBowl game from California on television. InSeattle, your radio hammers out the "Top 40"for the week. Your doctor takes a chest x rayduring a routine physical. What do these eventsall have in common?

You are already familiar with most of thefollowing words:

radio radiation radiumradiant radioactivity radiatorradius radar irradiate

What do these words have in Common?The examples in the first paragraph all in-

volve radiation of some kind. The library willprovide a source of references so that you candetermine what kind of radiation is involved ineach case. In the second paragraph, all thewords contain the same three letters (rad) andmost the same four (radi). The base word forall of these is the Latin radius, which means ray.Thus, each of these words has something to dolyith a ray. By using the dictionary, you candetermine this relationship. Man in space isexposed to cosmic rays, and man on earth tosun's rays. With what other kinds of rays are youfamiliar?

The suit's rays are the ones most frequentlymentioned. The reason radiation from the sunis most familiar is that it can be seen daily asvisible light. Other types of radiation are ascommon but not so well known because theyare invisible to the naked eye. Our own experi-ence with sunlight forms a basis for understand-ing some of the effects of radiation. The sun'srays provide us with the visible light by whichwe see, the comforting warmth when we lie onthe beach, and our suntans. We can artificiallyduplicate some of these effects by the substitu-tion of electric lights for the sun's rays, infraredlamps for :he sun's heat, and ultraviolet lampsfor the sun's tanning effect. Why do we needthree different types of electric lamps to dupli-cate some of the effects of solar radiation? Asimple and obvious answer is that there is not

simply one kind of ray coming from the sun, butseveral different kinds. If, for example, we passthe apparently .colorless sunlight through aprism, it is broken up into a rainbow of colorsranging from violet through red (Figure 1).

Red

OrangeYellow

Green

Blue

Violet

Figure 1. When a beam of white light passes througha prism, the shortest light rays bend the most and arefound in the violet end of the spectrum. The longestrays are bent the least and are found in the red end ofthe spectrum.

The formation of this spectrum of colors isunderstandable if the assumption is made thatthe various parts that make up sunlight havedifferent wavelengths that are each bent at aslightly different angle. If you think of light astraveling in waves somewhat like the waves ofwater, wavelength is the distance from thecrest of one wavc to that of the next (Figure 2).

1

Wavelength

Distance

Figure 2. Electromagnetic radiation travels in waves.WaN elength is measured from the crest of one waveto the crest of the next. Wavelength may vary fromfractions of a millionth of an inch to several miles.

1

1 0

According to the wave theory of light, the higherthe wave, the greater the intensity of light. Theselight waves are similar to electrical waves pass-ihg along power lines and to heat wavesradiating from hot surfaces.

While the wave theory of light in useful inworking with certain physical phenomena, suchas refraction, there is another theory of light,called the quantum theory, that describes light asbeing composed of small quantities of energy(quanta or photons). According to this theory,it is the number of photons per unit of time thatinfluences the intensity of light. Thus, a verybright light shining for a short period of timecould transmit the same number of photons asa dimmer light shining over a longer period.

While the quantum theory of light is useful instudying the interaction of light and matter, thewave theory provides us a better introduction tothe energy carried by all electromagnetic radia-tions. Electromagnetic radiation is energy in theform of rays produced by the increase in speedof an electrical charge. We can identify variouskinds of radiant energy by wavelength. Thelength of a light wave is measured in angstromunits (A). Ten angstrom units are equal to 1millimicron or 0.00000001 centimeters, usuallyexpressed as 10-8 centimeters. The sunlight wesee ranges between 3800 and 7600 A in wave-length. Wavelengths of less than 3800 A in-clude ultraviolet light, x rays, gamma rays andcosmic rays, each smaller in wavelength thanthe other. Above 7600 A, we deal with wave-lengths for infrared, TV, radio, and electricwaves of various kinds. Radiant energy can thusbe described as extending from the visible spec-trum on both ends to form what is known asthe electromagnetic spectrum whose parts areidentified by their wavelengths (Figure 3).

Gamma rays

X raysVisiblelight

Another advantage of identifying radiantenergy by its wavelength is the possibility ofreproducing precisely the required part of theelectromagnetic spectrum. Using visible light asan example, a certain shade of green can beidentified as being of 5156 A, or a particularshade of red can be described as 6600 A. Theregion of the spectrum around 2850 A is thatwhich causes reddening of the skin and sunburn.What does this information suggest must beone of the properties of oils and other sub-stances used as sunburn preventatives?

Electric heaters or steam radiators radiateheat at wavelengths greater than 7600 A. Tele-vision and radio sets receive radiation in thatpart of the electromagnetic spectrum between 1and 500 meters. Radiant energy of differentwavelengths has different properties. One ofthese properties is penetration. It is this prop-erty that we need to understand to protect our-selves from various kinds of electromagneticradiation. We can be shielded very easily fromthe sun. A thin piece of black paper, for exam-ple, will cut off all visible light, but you can gointo a closet and close the door and still yourportable transistor radio will play, indicatingthat radio waves are not stopped by the thick-ness of the closet door. You can devise numer-ous experiments indicating what wavelengthsare transmitted through ordinary window glassor water or, by using your transistor radio, de-sign containers that will stop the passage ofradio waves.

We will, in using till; Laboratory Block, beconcerned with radiant energy beyond that ofthe visible spectrum. We will deal with radiantenergy primarily of small wavelengths fromultraviolet light to gamma rays and includingatomic particles.

Radio waves

Cosmic rays Ultraviolet Infrared

ffitO'io-2A

WAVELENGTH IN METERS

T.V. Electricwaves

Figure 3. The electromagnetic spectrum is a continuous band of radiation. The physical nature of the radiationis the same throughout the whole range except for frequency and wavelength. The names of the sections aresomewhat arbitrary, and they overlap. Radio waves result from electrons moving in conductors, infrared fromhot objects, visible light from very hot objects, ultraviolet from arcs and gas discharges, x rays from electronsstriking a target, and gamma rays from nuclei of radioactive atoms.

2

But first a word about safety. Everyday life isfull of hazards. One may be killed or injuredcrossing the street, driving an automobile, fall-ing down stairs, tripping over an untied shoe-string, slipping in the bathtub, cutting oneselfwith a knife, or in any number of ways. Life isthus a hazardous situation, but we reduce haz-ards with ordinary caution and precautions. Welook both ways before we cross the street. Wedo not step in front of a speeding car. As carefuldrivers, we keep our automobiles under control.We use handrails and do not run downstairs;we behave in ways that minimize the possibilityof falling. And we tend, for the most part, tonotice where accidents can occur and to act toprevent them.

In the laboratory, we are accustomed also tohazards. There are Bunsen burr ers beakersof boiling water, acids, knives, pins, needles,and many other potential sources of danger.However, it is a very rare day in a laboratorywhen someone is injured, and then usually onlywhen he or she fails to observe the normallaboratory precautions that would minimizesuch risks. Just as we can handle boiling waterand caustic chemicals in the laboratory withoutdanger, so can we handle radioactive materialssafely. With radioactive materials, however,more care is required because the amounts arevery small and their radiation can neither beseen, felt, smelled, tasted, nor heard. It is be-

3

cause this radiation is not evident to our sensesthat we think of it as mysterious and dangerous.Also, the fact that the effects of radiation arenot immediately visible may give one a falsesense of security. Various periods of timeranging from hours to generations may be re-quired before the effects of radiation can benoted. However, just as we handy: microbes inthe biology laboratory when they cannot beseen, so by a series of somewhat similar tech-niques, radionuclides (radioactive chemicals)can be handled, transferred, and their effectsobserved as safely as any other laboratory mate-rial if the proper precautions are used. Thespecific techniques that are to be followed indealing with radionuclides and radiation in gen-eral will be described where and when they areuseful in the following inquiries. Strict attentionmust be paid to the directions in the LaboratoryBlock and the instructions given by the teachermust be followed exactly! Play safe with sourcesof radiation and you will have a tool that will re-veal much about biological systems that couldnot be understood by any other method. Care-less use, however, makes radiation not only un-usable as a biological tool but exposes theinvestigator and th., rest of the class to unneces-sary hazards. Reasonable care and precautionsare all that is necessary to use radiation toinvestigate biological phenomena with completesafety.

INQUIRY I HOW ARE MICROORGANISMSAFFECTED BY IRRADIATION?

In this inquiry we will use radiant energy veryclose to the violet end of the visible light spec-trum. We will use ultraviolet light which, whilenot seen by human eyes, can be perceived byinsects such as the fruit fly, Drosophila. Thewavelength perceived by Drosophila is roughly3660 A. Ants and many other insects,as well aspersons with aphakic (lensless) eyes, can alsosee with this wavelength of light. If you couldsee ultraviolet light, in what ways would such anaddition to your visual spectrum affect yourinterpretation of your environment? While youare not to look directly at the ultraviolet lamp,you will know when it is off and when it is onbecause you will notice a certain amount ofvisible radiation. However, this is not ultravioletlight but is visible light produced because theultraviolet lamp is not 100 percent efficient inthe production of pure UV light.

This first inquiry is designed to supply datafrom which you can determine some effects ofirradiation on a particular cell population.Because of the easy availability of ultravioletlight, it will be used as a radiation source in thisinquiry. The observations you make with theultraviolet light will be useful when you useother types of radiation, even though the actionof ultraviolet radiation is technically differentfrom that of x rays or gamma rays.

CAUTION

The ultraviolet light source should be wellshielded and turned on only for the actual pur-pose of irradiating the specimens. Materialsshould be moved in and out of the radiationfield with tongs. Do not expose hands to theultraviolet light. Do not look directly into thelamp as its radiation is particularly harmful tothe retinal tissue of the eye.

MATERIAL (per group)Agar, potato dextrose, 150 mlAutoclave or pressure cookerCotton, sterileFlask, Erlenmeyer, 250 ml (2)Flask, Erlenmeyer, 1000 mlHydrochloric acid, 0.1N, 2 mlLamp, ultraviolet (2537 A)Marking pen, felt

5

Paper, black (marked off in 1 cm squares foruse in counting colonies)

Paper, pHPetri dishes, (8)Pipette (with bulb), 1 ml in 0.1 ml unitsRod, glass (bent like a hockey stick to use as

spreader)TongsWater, distilled, 200 mlYeast, dry, 1 g

PROCEDURE

This inquiry assumes you are familiar withthe handling of microorganisms. If your labora-tory work so far has included microbiologicaltechniques, you may wish only casually to referto Appendix I ("How Are MicroorganismsHandled?") to refresh yourself as to the proce-dures to be used. If you have not yet masteredmicrobiological techniques, this appendix shouldbe read with care as microorganisms are excel-lent for radiation studies and will be used againin this Block.

1. Follow the directions on the package forthe preparation of potato dextrose agar. Themedium works best if adjusted to a pH of 3.5by the addition of 0.1N HC1 one drop at atime. Check the pH by means of pH paper.Sterilize the medium in the autoclave or pres-sure cooker at 15 pounds pressure for 15 min-utes. A 150 ml flask of agar is sufficient for the8 petri dishes. Sterilize the petri dishes in a hotair oven at 175°C for an hour. Pour the meltedpotato dextrose agar into the 8 sterile dishesusing careful aseptic techniques. Label the petridishes with a felt marking pen indicating yourname, the date, and later add the length of timeeach dish is to be irradiated.2. Prepare a yeast solution with a concentra-

tion of 10' grams per milliliter as follows:a. Weigh out 1.0 gram of dry yeast.b. Pour the dry yeast into 1000 ml of

sterile, distilled water. Plug the containerwith cotton and swirl the contents to mixthoroughly.

c. Transfer 1.0 ml of this first solution to99 1111 of sterile, distilled water. Again,mix well.

d. From this second yeast solution, transfer1.0 ml to 99 ml of sterile, distilled waterand mix well. This last solution is a 10-7g /ml concentration.

3. Pipette by bulb 0.1 ml of the 104 g /mlyeast solution onto the agar in each of the auto-claved petri dishes. One person can inoculateseveral agar plates (up to 10) at one time withone pipette (use a 1 ml pipette marked in 0.1ml divisions) by filling the pipette then releas-ing 0.1 ml of the yeast solution into each petridish. The yeast solution can be spread over thesurface by the use of a sterile glass rod bent toresemble a hockey stick. Be sure to distributethe yeast evenly over the surface of agar but trynot to let the solution reach the edge of the petridish.4. Fasten the ultraviolet lamp 10 cm above

the table top.5. By means of tongs, place one of the petri

dishes, with cover removed, containing the 0.1ml of the yeast suspension under the lamp forexactly five seconds. Do not keep your handsunder the lamp. Do not look at the light source.

6. After the five-second exposure, remove thepetri dish with tongs and replace the cover.

7. Repeat step 5 and irradiate other samples(one at a time) for periods of 15 seconds, 30seconds, 60 seconds, 2 minutes, 5 minutes, and10 minutes.8. One of the 8 prepared petri dishes of 0.1

ml of yeast suspension has not been exposed toultraviolet radiation. What is the purpose of thisdish?

9. Incubate all inoculated plates in the dark atabout 25°C for 48 hours. The cultures growbetter at 37°C in an incubator if available.10. Count the number of colonies formed fromthe untreated cells. Since the colonies are lightin color, place a black piece of paper markedoff in centimeter squares with a white pencilunder the petri dish. The paper squares allowfor more accurate counting. Count the numberof colonies present in the untreated culture. Usethis number to represent 100 percent survival.11. Count the number of colonies in eachtreated cultui-e. Use this figure to represent thenumber of survivors in each petri dish. Comparethis figure with that for the untreated culture.Calculate the percentage of survivors for eachof the treated cultures. One hundred percentminus this percentage indicates the percent af-fected by the radiation treatment.

6

12. Prepare a graph with the length of time ofirradiation on the horizontal axis and the per-cent of the cells inactivated on the vertical axis.Of course, at 0 time none was affected. Thegreatest exposure-10 minutesmay affect allthe cells. In plotting the time on th, horizontalaxis, remember that each square on the graphpaper represents a given unit of measurement inseconds or minutes. Thus, the 5-minute and 10-minute points will be much further from eachother than the 15-second and 30-second points.After marking appropriate points on the graphpaper representing the percent of the cells in-activated per unit of time, draw a smooth lineconnecting the points plotted on the graph. Iftime allows, plot the survival curve using as thevertical axis the percent of growing cells. Howdo these data compare with those for inactivatedcells?

ANALYSIS OF DATAIn general what effect did irradiation have on

individual yeast cells? Now, referring to yourdata, at what point, if any, were all the cellsinactivated by irradiation? Between what twotimes was the effect of the irradiation greatest?What is the significance of the shape of the lineon the graph paper? What does it indicate con-ci=qming the pov, er of radiation? At approxi-mately what time of exposure would one-halfof the cells havel-en inactivated? What was thefate of those cells which did not grow on theirradiated plates? Were they necessarily dead?What other explanation besides death wouldexplain the data? Given the data obtained, couldthe effect of radiation on other species be pre-dicted? Would the amount of irradiation re-quired to inactivate half the cells of a newspecies be about the same as that in this inquiry?Would the general shape of the graph obtainedfrom this inquiry be about the same if a differentspecies were used? What possible effects wouldthe irradiation have had on the still-livingorganisms in this inquiry? Having observed theeffects of ultraviolet radiation on a micro-organism, suggest what practical applicationsmight be made of such irradiation.

PROBLEMS

As time allows, certain groups may usemicroorganisms other than yeast and preparedata on dosages required to inactivate 50 per-cent of the individuals of different species of

16

microorganisms, such as Serratia marcescens,Sarcina lutea, or Penicillium.

The effect of ultraviolet radiation on streak-plate and pour-plate cultures may be contrasted.Further data may be obtained by the introduc-ticn of glass, a film of water, or other filtersbetween the ultraviolet source and the micro-organisms. Such experiments will give someindication of the penetrating power of the ultra-violet rays through the various media by theireffect on microorganisms.

p

Similar investigations may be performedusing either different types of radiation ororganisms. For example, earthworms exposed tosunlight readily show the effects of solar radia-tion. If the data obtained from experiments ofthis nature are to be attributed solely to theeffect of sunlight. what are the factors that needto be controlled?

INQUIRY II CAN THE EFFECTS OF IRRADIATIONBE MODIFIED?

In the firsi Inquiry, you determined an effectof ultraviolet radiation on microorganisms. Forthese organisms, it is possible to think of theultraviolet light as a "death ray." It is also pos-sible for gamma rays, x rays, and other parts ofthe electromagnetic spectrum to act, in effect, asdeath rays. Except that they cannot be shotdramatically from a pistol and that their effectsare not as instantaneous as in science fictionstories, these rays do affect living organisms.

Not only is science fiction full of reference todeath rays, but even to antideath rays that canturn away or otherwise render harmless theeffects of radiation. Death rays and antideathrays were products of the imagination of sciencefiction writers just as were voyages to the moon.While the United States has now placed men onthe moon, we have not yet perfected a death raysimilar to that used by Flash Gordon. We have,however, come to understand the lethal proper-ties of radiation. Similarly, though we have notperfected an antideath ray, scientists haveinvestigated radiation itself as a means of com-bating radiation effects. In this Inquiry, we willinvestigate a process known as photoreactiva-tion. We will determine what effect visible lighthas on cells previously exposed to ultravioletlight.

As you remember from Inquiry I, ultravioletradiation can damage and kill living cells. Isthere some way that this damage can be lessenedor prevented? For the purposes of this Inquiry,we will use yeast and ultraviolet radiation as inInquiry I. We will add the additional step ofexposing the yeast cells, after they have beensubjected to ultraviolet irradiation, to variouswavelengths of visible light and compare theireffects.

MATERIAL (per group)

Agar, potato dextrose, 75 mlAutoclave or pressure cookerCotton, sterileFlask, Erlenmeyer, 250 ml (2)Flask, Erlenmeyer, 1000 mlForcepsHydrochloric acid, 0.1N, 2 mlLamp, ultraviolet (2537 A)

Light sources (2one for white light, one forblue light)

Marking pen, feltPaper, black (marked in 1 cm squares for use in

colony counting)Paper, pHPetri dishes, (4)Pipette (with bulb), 1 ml in 0.1 ml unitsRod, glass (bent like a hockey stick to use as

spreader)Water, distilled, 200 mlYeast, dry, 1 g

PROCEDURE1. This Inquiry begins with the same tech-

niques as in Inquiry I. Follow steps 1, 2, and 3of the procedure for Inquiry I. The 75 ml ofagar will be sufficient for the four petri dishesper group used in this Inquiry.2. Label with the felt marking pen the petri

dishes before adding the yeast solution. Labeldish 1 "no irradiation," dish 2 "UV light," dish3 "UV light -I- white light," dish 4 "UV light+ blue light."3. Before exposing dishes 2, 3, and 4 to the

UV light, determine the length of time thedishes are to be exposed to irradiation. Refer tothe results of Inquiry I for data by which tomake this determination.

4. Remove the cover from petri dish 2 and, bymeans of forceps, place it under the ultravioletlamp for the period of time determined in 3above. Do not keep your ;tends under the lamp.Do not look at the light source.5. After the exposure, remove the petri dish

with the forceps and replace the cover over theyeast suspension.

6. Repeat step 4 with petri dish 3. After theirradiation, replace the cover and immediatelyplace this dish 10 to 20 cm from a strong, whitelight for a period of 30 minutes. Be careful thatthe culture does not get overheated.7. Irradiate dish 4 as for dish 2 and, after

irradiation, replace the cover and immediatelyplace this dish 10 to 20 cm under a source ofblue light for a period of 30 minutes.

8. Place all four plates together in the dark atabout 25°C for 48 hours. The cultures growbetter at 37°C. in an incubator if available.

9 18

9. After 48 hours, count the number of colo-nies as indicated in step 10 of Inquiry I.10. Record the di 'a and examine them. If de-sired, these data cult be graphed.

ANALYSIS OF DATAWhat is the significance of these data? In your

data analysis, compare the difference in thenumber of colonies between dishes 1 and 2 todetermine the effect of irradiation. How doesdish 2 compare with dishes 3 and 4 regardingthe number of colonies present? How do youaccount for these differences?

The exposure to visible light after irradiationresults in photoreactivation. How is photoreac-tivation a kind of antideath ray? Support your

10

statement with reference to the data of thisInquiry.

PROBLEMS

If the plates had been exposed to the white orblue light for a longer period of time, would youexpect more or fewer colonies to be present?What would have been the effect of exposureto red light or green light or to some color otherthan blue or white? Would a longer exposure toa less intense light source have been more orless effective than a short exposure to a highintensity light source? Investigations designedby you can provide data to answer these ques-tions.

9

INTRODUCTION TO RADIONUCLIDE USE--CAUTIONSAND PRECAUTIONS

In the remainder of this Block we will beusing radionuclides as radiation sources. Usefulbackground material on radiation is to be foundin Appendix II, which may be reviewed at thistime. As the energy available in various formsof radiation is inversely proportional to theirwavelengths, gamma rays have more energythan the ultraviolet light used in Inquiries I andII. Similarly, the tissue-penetrating power ofradiation is directly proportional to the energyof its wavelength. This means that we will bedealing with radiation of higher energy anddeeper potential penetration than the ultravioletlight of Inquiries I and II. Therefore, radio-nuclides should be handled with care, and thedirections in the Laboratory Block and thosegiven by your teacher should be followedexactly.

The Inquiries that use radionuclides demandthoughtful preparation and careful techniques.When we handle strong acids or heated mate-rials, we are careful because we. know thatcareless handling of eiliter will produce animmediate and dramatic reaction. However,radionuclides carelessly handled do not produceeither an immediate or a dramatic reaction. Onecould be exposed for a considerable period oftime before there would be any evidences of theeffects of radionuclides on one's health. Use ofradionuclides requires constant care on yourpart in order that unseen radiation does notcontaminate either the laboratory or individuals.Properly handled, radionuclicws are no moredangerous than properly handled boiling wateror lye solution.

The following general precautions should beobserved when dealing with all radionuclides.

1. All radionuclides must be handled care-fully, avoiding unnecessary exposure toionizing radiation. Good habits of safehandling of materials must be formedearly. Radioactive materials are neverhandled directly. They are handled withforceps or tongs. Solutions are neverpipetted by applying suction with themouth. Mechanical or bulb type- pipettesare always used instead. Wherever vapor-ization of radioactive materials is re-quired, this operation should be per-

formed in a fume hood. The room shouldalways be well ventilated.

2. All inquiries must be carefully plannedin advance and you should understandcompletely what is to be done at alltimes. Experiments should be runthrough without use of radioactive chem-icals before using radioactive nuclides inorder to perfect techniques and to reducepossibilities of error and loss of radio-active materials.

3. Avoid any direct contact with radioactivematerials. They should not touch theskin or be taken into the body eitherthrough the mouth or by breathing.Nothing must be placed in the mouth oron the lips in the laboratory. This in-cludes food, beverages, ciorettes, cos-metics, gum, or any other materials.

4. The laboratory and equipment must bothbe monitored. That is, possible radio-activity should be checked with a Geiger-Muller (G-M) counter, which you willlearn use later, to make sure that boththe laboratory and equipment are free ofcontamination. This must be done duringand after each Inquiry and at regularintervals in addition. At the conclusionof the Inquiry, hands are to be washedcarefully and placed in front of theGeiger-Muller tube window (not againstit or over the tube itself) to determine ifradioactive contamination is being car-ried away on the hands.

5. Protective clothing prevents contamina-tion of the skin or personal clothing.Rubber gloves or plastic disposable onesare always used to cover the hands whenworking with radioactive material or po-tentially contaminated equipment. Pro-tective clothing, such as a laboratorycoat or apron, should be used to coverpersonal clothing when handling radio-active materials.

6. The most frequent cause of contamina-tion is spillage. It is easier to preventspillage than to decontaminate after-wards. Spills can be avoided by placingsmaller open containers into larger ones,

0

such as beakers, or into a hole drilled ina flat block of wood that is not easilyupset. Possible spills should be confinedby working on a tray or a large shallowpan lined with an absorbent materialwith a water-repellent backing. Suchabsorbent material is usually available inlarge rolls and can be obtained fromlocal paper companies. Disposable dia-pers or newspapers backed with a layerof oiled or waxed paper may be used in-stead to prevent the spill from contami-inating the working surface. Cover theworking area where radionuclides arehandled with several layers of newspaperbacked with plastic wrap or wax paper.At the end of the Inquiry, these are to bediscarded in the radioactive waste con-tainer (see 10 below). The laboratorytable, sink, and equipment are to be freeof contamination at the end of the In-quiry. Proper procedure not only pre-vents exposure of other people using thelaboratory, but also prevents contamina-tion that vill interfere with the accurateusage of the Geiger-Muller counter infuture Inquiries.

7. All radioactive materials must be labeledso that they may be recognized. Glass-ware and equipment that have beenused and decontaminated, if found stillto have traces of radioactivity, should belabeled radioactive and separated fromgeneral use. As a common practice, it isbest to segregate all apparatus used withradioactive materials, whether it has beendecontaminated or not. The area inwhich work with radiation is done mustbe marked with the magenta three-bladedsymbol on the yellow background asprescribed by the Atomic Energy Com-mission (Figure 4). All containers ofradioactive materials must likewise bemarked.

8. All radioactive materials must be keptunder lock and key when the responsibleindividual is not in the laboratory.

9. The working area should never be clut-tered. Unnecessary equipment should beremoved from the working area in whichit might be contaminated.

10. Radioactive waste should be collectedand plainly labeled radioactive. Quanti-

12

CAUTION

oliRADIOACTIVE

MATERIALSFigure 4. This symbol indicates the presence of radio-active materials. The three-bladed symbol is magentaon a yellow background and is prescribed by theAtomic Energy Commission.

ties of soluble material obtained undergeneral license can generally be dis-charged into the sanitary sink if dilutedwith large volumes of water. All othermaterials should be placed in a specialcovered container clearly marked "Ra-dioactive Waste" for later disposal. Intothis container place all exposed soil,paper, sticks, bottles, and other suppliesthat are used in these Inquiries. Neverdiscard radioactive wastes in any con-tainer but that specially marked for thepurpose.

11. Despite all precautions, accidents some-times happen and decontamination pro-cedures are ne 'essary. Knowing this, youwill find it easier to decontaminate anarea if the working surfaces are imper-meable to aqueous solutions. This can beaccomplished by coating the workingsurface with wax. If, despite all precau-tions, an area or part thereof becomescontaminated with the radioactive mate-rial, the following procedure is to beused:a. For spills of dry material, apply

sticky paper, such as masking tape orcellophane tape to pick up the mate-rial. Do not try to swef.» it up. Sweep-ing usually spreads the contaminationwithout removing it.

b. With liquids, absorbent papers can beused to blot up as much contamina-tion as possible. They should be dis-carded in the radioactive waste can.

c. The contaminated surfaces can thenbe further cleaned with liquid clean-ers. Your teacher has a list of generalreagents for decontamination and pro-cedures for handling specific types ofcontamination.

21

BASIC MATERIALS

Jr order that the above general precautionscan be observed, the following materials must beavailable and used in every Inquiry in whichradionuclides are handled:Clothing, protective, such as laboratory coat or

apronDecontamination equipmentForceps or tongs for handling radionuclidesGloves, rubber or plastic, disposableMonitoring equipment (Geiger-Muller counter)Radiation labels, including a large warning sign

for the work area, and small warning stickersand tape to label appropriate containers andmaterials

Radioactive waste container

Spillage control materials such as larger con-tainers for smaller open containers, largetrays or shallow pans, absorbent material withwater-repellent backing, such as newspaperbacked with plastic wrap or wax paper

This is a master list for each laboratory inwhich radioactive materials are to be used. Theintroductory Inquiries using radionuclides listsome of this material, but later ones do not inorder to clarify the essential materials requiredfor the individual experimental design. How-ever, it must be kept in mind that the abovematerials must be available for each of theInquiries using radionuclides whether listed ornot.

INQUIRY III HOW CAN YOU SEE RADIATION?

In the Introduction it was pointed out thatvisible light is but a small parr of the electro-magnetic spectrum. In Inquiries I and II youwere using light that was specifically stated notto be visible to the unaided eye but whose ef-fects could be noted. The individual yeast cellsused in Inquiries I and II were also invisible tothe unaided eye, yet they could be handled andlater their presence was made known by theircolonial growth. In these examples the invisiblecould be observed by its effects, and initiallyinvisible objects handled and later seen

When you drive your car across a shiny rail-road track, you look both ways becat. le thetrack alone indicates the potential presence oftrains. Even though none is visible at themoment, one can deduce that a train may havejust passed or may arrive shortly at the crossingwithout having seen it. On a dirt road, tiretracks indicate the presence of automobiles, andto a hunter, game is indicated by tracks in earthor snow. Thus, you are familiar with the factthat if we do not actually see an object, its

presence in the vicinity can be deduced bytracks it leaves behind. Cosmic rays, gammarays, and certain radioactive particles also leavetracks in a specially prepared environment.

The environment to be used for this Inquiryis created in a cloud chamber (Figure 5). Acloud chamber is an enclosed space filled with asupersaturated gas that condenses and forms avapor trail whenever an electrically chargedparticle (an ion) passes through it. The ions

Figure 5. A commercially available cloud chamberused for the detection of radioactive particles. Photo-graph courtesy of Sargent-Welch Scientific Company.

become the centers for the condensation of thesupersaturated gas which then appears likestreaks of fog in the cloud chamber. What willbe observed in the cloud chamber is not actuallya particle or ray itself but the track it has takenthrough the cloud chamber.

In this Inquiry you are to observe carefullyradiation tracks from various sources and try toclassify or identify some of their characteristics.This will not be an easy task, but careful ob-servation of each event while looking for unify-ing or reoccurring patterns should allow you toclassify several different kinds of tracks andrecord their characteristics.

MATERIAL (per group)

Aluminum foil (less than a square centimeter)Cloud chamber and its associated materials,

such as methyl alcohol, black dye, dry ice(8" x 8" x 1" slab)

Paper (less than one centimeter square)Radioactive emitters marked as unknowns 1 and

2Ruler (millimeter, transparent)

PROCEDURE

Radiation Tracks1. Review the construction and operation of

the cloud chamber as directed by your teacher.2. Set up the cloud chamber and place one of

the unknown radiation sources in it as directed.3. After conditions inside the chamber have

stabilized, carefully observe the tracks of ioniz-ing radiation.

4. Describe the types of tracks that tend to re-occur frequently.5. After you have identified several reoccur-

ring patterns, count and record in columns thefreauency of eazh pattern observed over a givenperiod of time.6. Use a transparent ruler to estimate the

length in centimeters of each type of path. Howdo they compare?

7. Summarize your observations indicating thenumber of different types of paths observed,their characteristics, and their frequency.

8. How do you account for the different pat-terns noted?

159

Particle EnergyEach of the tracks was produced by an ion

that possessed a certain amount of energy.Energy in these t it,t-s is measured in terms ofelectron volts. Some indication of particleenergy in million electron volts (Mev) canbe gained by the length of the track. Comparethe length of the tracks you measured with thefollowing data and record the approximateenergy in Mev for the tracks noted.

Track Length Mev1 cm 2

1.62.53.54.6

6

CM

CM

CM

cmcm

345

67

PenetrationEarlier in this Block it was noted that various

wavelengths of the electromagnetic spectrumhave different penetrating powers. Does thisdifference in penetration also apply to alpha orbeta particles or gamma rays?

For an explanation of these charged particles,refer to Appendix H ("What is Radiation?").

If the construction of the cloud chamber soallows, penetrating properties can be determinedby pacing a piece of paper or aluminum foilover the radiation source and observing if tracksare seen in the area beyond the shielded source.

With the radiation source shielded, repeatthe observations in procedures 4 and 5 above.

16

What types of tracks, if any, seem to be stoppedby paper? What types seem to be unaffected bycovering the radiation source? Repeat step 6.Does the shielding have an effect on tracklength? Prepare a statement regarding the pene-trating ability of the particles creating the typesof tracks noted above.

Analysis of DataHow many different types of tracks were

noted in the cloud chamber? What is the sig-nificance of the length of the tracks? Which typeis most numerous? Compare the data of studentsusing radioactive emitter No. 1 with the dataof those who used No., 2 when class data arepooled. What does this tell you about the radio-active sources used? Were certain types oftracks present in both samples? What is the sig-nificance of this observation?

Which type of path seems to have the greatestpenetrating ability? What does this tell youabout protection from radiation?

ProblemsRemove all radioactive sources from the

cloud chamber. Obser le the chamber and re-cord your observations. What explanations canyou give for these observations? Design aninvestigation to test your hypothesis as to thesource of this radiation.

Do particles passing through the cloud chamber have an electrical charge? How might thisbe checked simply? How could you determinewhether the charge was positive or negative?

94

INQUIRY IV HA40AWT ECRAI AN SWAE RKENFORWE WENT RADIOACTIVE

In the previous Inquiry, the ionization trailsin the cloud chamber indicated the presence ofradioactive particles and rays. However, thecloud chamber is not a convenient device withwhich to detect the presence of radioactivity.For one thing, it would be difficult, using acloud chamber, to localize sites of radioactivity.In this Inquiry, you will learn how film can beused to detect the presence of particles and howtc identify the sites from which they came. Dur-ing the course of this Inquiry be alert for appli-cations of this technique, known as autoradio-graphy, to the study of biological systems.

MATERIAL (per group)

Cartridge pen with empty cartridge (2 of dif-ferent colors or distinctive markings)

Cardboard [2 pieces, 13 x 18 cm (5" x 7")]Film [No Screen Medical x-ray, 13 x 18 cm (5"

x 7") or Kodak Royal Pan 10 x 15.5 cm(4" x 6")]

Film developer, 1 literFilm stop bath, 1 literFilm fixative. 1 literFilm holder, or black envelope, or black paper

(30 x 40 cm)ForcepsGloves, plastic, disposableLaboratory coatNeedles, hypodermic, No. 26 (2)Nonradioactive sodium phosphate solution, 2 mlNotebook and paperPaper, 7 x 9 cm (23/4" x 31/2")32-Phosphorus solution (radioactively labeled

sodium phosphate), 1 microcurie per 2 ml,2 ml

Radiation warning signRadiation warning stickersRadiation warning tapeRubber bandsScissorsStop bath, 1 literSyringe, hypodermic, 1 ml (2)Trays, developing (3)

PROCEDURE

1. With the hypodermic syringe and needle, fillone clean and empty fountain pen cartridge with

the 32P solution. Place the filled cartridge in thepen. Mark carefully the pen containing the 32P.

2. With the second syringe and needle fill thesecond cartridge with the nonradioactive so-dium phosphate solution and place the cartridgein the second pen.

3. On a piece of 7 x 9 cm notebook paper thathas been divided in half by a penciled line, writethe symbol 32P or your initials, as you prefer, onthe right side of the paper, using the 32P solu-tion. Each member of your team can write onthe same side of the piece of paper if desired.

4. Repeat the procedure in 3 above using thesecond pen filled with the nonradioactive so-dium phosphate, writing this time on the lefthalf of the paper.

5. Thoroughly dry the paper.6. In a darkroom, using no more than a photo-

graphic safe light, unwrap a piece of x-ray film.Hold it emulsion side up. (You can cut off theupper right-hand corner to identify the emul-sion side.) Do not touch the film surface withyour fingers. The film must not be exposed toany light until after it is developed. Place thethy notebook paper against the film. Eitherplace both in a film holder or between 2 piecesof cardboard. Hold this cardboard sandwichtogether with rubber bands and then place iteither in a black envelope or wrap it in blackpaper and seal it with rubber bands. Identify thispackage with your name or group number. Donot stack the films of other students on top of orbeneath yours. The radioactivity from theirpaper may also affect your film.

After 24 hours have elapsed, open the filmpackage or holder in a darkroom and separatethe film from the paper. Put the paper and card-board in the container marked "RadioactiveWaste."

7. Develop the film as follows:a. Place the three developing trays in a row

on a surface in the darkroom. Into thetray on the left, pour sufficient developerto completely cover the film. Pour thesame amount of stop bath into the trayin the middle and a similar amount offixative into the tray on the right. Do notcontaminate one solution with the other.Do not spill or splash solutions. Cau-

17

lion: Photographic chemicals stain andare harmful to clothing. Make sure thatno fixative gets into the developer.

b. Place the film in the developer, coveringit completely, for about 4 minutes at68°F. Occasionally wiggle the film in thedeveloper with forceps.

c. After 4 minutes, remove the film fromthe developer tray with forceps. Let theexcess developer drip back into the trayand then transfer the film to the stopbath for about 15 seconds. Remove withrinsed forceps and allow to drain. Trans-fer to the fixative for 5 minutes or untilclear.

d. Rinse the film in cool, running water for30 minutes. Hang the film up to dry.

ANALYSIS OF DATA

What could you see on the dried notebookpaper on which you had written? Compare theright and left halves of the developed photo-graphic film. How does the nonradioactivephosphorus solution affect the film? How has the

"2P solution affected the film? How can you ac-count for these differences? To what uses mightphotographic film be put to detect radiation?What other instances do you know in whichfilm is affected by radiation we cannot see? Flowcould the film be used to detect the amount ofradiation? Would the film be a good or a poordetector of amount of radiation? Why or whynot?

18

PROBLEMS

Design experiments to determine the effect ofradiation on film by varying the times of expo-sure or distances from the sozIrce to the film.

In many areas Where radionuclides are used,workers wear film badges. These are simply littlecarriers into which a piece of unexposed film isinserted. The film is removed and devebped atstated intervals. What would the state of the filmindicate about the environment of the worker?Design and, if time allows, perform investiga-tions to determine the types of radiation towhich the film is sensitive.

INQUIRY V HOW IS THE GEIGER-MULLER COUNTER AFFECTED BYRADIATION PRESENT IN THE ENVIRONMEN I?

A review of Appendix IV on how to operatea G-M counter shows that it is sensitive to radia-tion but not equally sensitive to all types ofradiation. You also know from earlier Inquiriesthat radiation is naturally present in the atmos-phere. What is the relationship between thisnaturally occurring radiation and the operationof the G-M counter?

You perhaps noticed in the work with theGeiger-Muller counter that even with no sourcenearby, the counter was registering radiationevents. This activity recorded without any spe-cific radioactive source in the vicinity is referredto as the background count. We know that therecorded counts are in response to radiation.But where does the ionizing radiation comefrom with no specific radioactive source in theimmediate vicinity? This inquiry will concernitself with the observation of such backgroundcounts and hypotheses as to their source.

MATERIAL (per group)Foil, aluminum, 12" x 12" pieceForcepsGeiger-Muller counter with holder or ring stand

and clampLead and/or iron sheets, approximately 5" x 5"

(6)

PROCEDURE

1. Hold the Geiger-Muller tube vertically withthe window end down and the cover closed.Mount in either a holder or by means of a clampto the ring stand.2. Check the high voltage of the counter to in-

sure that it is set to the proper, previously deter-mined operating voltage. (Check Appendix IVfor detailed instructions.)3. Be sure that no radioactive materials are

near the tube.4. Take counts during periods of from 5 to 10

minutes. Record the results.5. Record the count with the tube window un-

covered.6. With the shield again covering the window,

wrap the tube in several layers of aluminum foil.Again record the count.

7. What does the data obtained with the shieldcovering the window and the aluminum foil

around the tube tell you about the type of ra-diation producing the background count?8. Or the basis of your current knowledge of

radiation and with reference to the introductorymaterials in this Block, list the possible sourcesthat could account for this background radia-tion. Your list will form a basis for furtherdiscussionof this Inquiry.

FURTHER PROCEDUREPlace a piece of iron or lead flat on the labo-

ratory table. Place similar pieces in such a wayas to form a four-sided box open at the top.Place the Geiger-Muller tube carefully insidethe box and cover the box, making sure that thelead cable is not crushed by providing a smallopening for it. Record the effect of this thickmetal box on the background count. What isthe thickness of the iron or lead being used?Based on the reduction in counts per minuteand the thickness of the metal, by what percent-age is background radiation reduced with agiven thickness of metal shielding?

ANALYSIS OF DATA

What seems to be the average backgroundradiation level in your classroom? By whatpercentage is this reduced by having the tubewindow covered? By having the tube wrappedin aluminum foil? By placing the tube in a metalbox? What do you think would be required toshield the background radiation to zero?

Because background radiation is always pres-ent, it influences all counting done with theGeiger-Muller counter. Therefore, for an accu-rate count, background radiation must be sub-tracted from the total count to give you the netactivity of the sample being tested. A simpleformula for this determination is Rt (totalcount) RI, (background count) = net activityof the sample.

Would you assume that background radiationis constant? Why or why not? If it is not con-stant, what factors must be taken into considera-tion -luring any experiment that will extend overa prolonged period of time? What is the bio-logical significance of background radiation'?Prepare a summary statement that answers thequestion in the title of this Inquiry.

19 ?7

PROBLEMS

Experiment with different materials to deter-mine their effect on reducing background radia-tion. Does the time of day have anything to dowith the intensity of background radiation?

Figure 6a. A portable battery-powered counter. Theend window of the Geiger-Muller tube is protected bya removable cap-like shield. Counts per minute arerecorded on a survey meter and also can be heard asa series of clicks.

Would you expect it to be the same at night asin the daytime? Test various areas in your im-mediate locality for background radiation. Howdoes it vary, if at all? How would you accountfor this?

20

Figure 6b. This counter uses 115-volt alternating cur-rent and is equipped with a side-window Geiger-Mullertube. The calibrated board on which the tube is tem-porarily mounted can be used in both a vertical and ahorizontal position.

.28

VI WHAT DOES A CET? HULLERI Naum ECTyCOUNTER DET?

The first response to the question in the title isprobably, "Counts per minute." But counts ofwhat? Remember the work with the cloud cham-ber? To what kinds of radiation was it particu-larly sensitive? Was the photographic film usedfor the autoradiograms equally sensitive toradiation of various types? On the basis of 'hesedata from previous Inquiries, what hypothesescan you state regarding the anticipated sensitiv-ity of the Geiger-Muller tube to various kinds ofradiation? Data re'ative to your hypothesis canbe obtained by a series of experiments designedin a similar fashion to those you performed withthe cloud chamber.

You will be provided with three unknownradiation sources. They will be numbered 1, 2,and 3. You will be allowed to use paper, alum-inum, and lead as filters. From the reaction ofthe tube in response to your experiments, youare to deduce the major type of radiation fromeach of the three unknowns.

MATERIAL (per group)Filters, paper, aluminum, leadForcepsGeiger-Muller counter with clamp ring and

stand (holder optional)Radioactive samples, 3 unknownsRuler, 10 cm

PROCEDURE

1. Be sure you are familiar with the operationof the Geiger-Muller counter. Review, if neces-sary, Appendix IV ("How Do You Operate aGeiger-Muller Counter ? ")

2. Mount the Geiger-Muller tube in a verticalposition, window down (shield closed) in eithera holder or a clamp mounted on a ring stand.Be sure the scaler voltage is properly set as ex-plained in Appendix IV.

3. Place one of the numbered samples either atthe bottom of the card mount holder or on apiece of glass mounted on a ring fastened to aring stand no more than 10 cm below theGeiger-Muller tube.

4. Record the counts per minute registered onthe scaler or rate meter. If no counts abovebackground are recorded, place the sample onthe next highest shelf in the holder or raise the

21

ring stand, recording in each case the distancefrom the sample to the shield of the tube.

5. Continue raising the sample and recordingcounts at each distance.

6. Again lower the sample to the bottom of thering stand or holder as in 3 above. Remove theshield from the window and repeat procedures 4and 5, recording counts per minute at eachlevel.

7. Place the sample at a point where the countno longer increases rapidly when the sample isbrought closer to the tube, but in no case closerthan 1 cm to the tube.

8. Now, carefully place a paper filter over theradioactive sample. Then replace it with analuminum foil filter, and finally replace this witha lead filter. Record the count for each filter.

9. Repeat steps 3-8 with the other two radio-active samples.

ANALYSIS OF DATAWhat was the effect of distance on the counts

per minute? Compare the three samples. Whichgave the greatest number of counts per minute?Which the least? Did any sample not register onthe counter? What was the effect of use of thepaper filter? The aluminum filter? The lead fil-ter? From the data of your earlier Inquiries,which type of radiation could be shielded bypaper? By aluminum? By lead? On this basis,what type of emitter do you deduce sample 1to be? Sample 2? Sample 3?

With the identical counting geometry (seeAppendix IV), to which sample was the Geiger-Muller counter most sensitive? The least sensi-tive? How do you account for this? What as-sumptions have to be made in order to make thestatement that the counter is more sensitive toone kind of radiation than to another? For thedetection of what type of radiation would theGeiger-Muller counter be most effective?

After all of your data have been recorded,have they supported or refuted your originalhypothesis regarding the sensitivity of theGeiger-Muller tube to various kinds of radia-tion?

Your teacher will tell you the nature of thesource material in samples 1, 2, and 3. If yourdata do not agree with the sources as revealed,

re-examine your techniques for possible expla-natious. How do these data relate to data of theprevious Inquiry? Write a summary statementof the significance of these data for biology.

PROBLEMS

Using the above techniques you can deter-mine the major types of radiation emitted fromother unknown samples that may be available.Try different materials as filters to measure theireffects.

You noticed that the counts per minute var-

ied with the distance of the sample from the endof the Geiger-Muller tube. Design investigationsto show the influence of distance on radiation.

How effective are biological materials asradiation filters? Make filters of scales, feathers,fur, frog skin, or other biological materials andtest them. Compare the results. How effective isa leaf as a radiation filter? Would an aphidunder a leaf receive as much radiation as oneliving on top of a leaf? Design investigations toanswer these and similar questions that mayarise in your discussions.

22

30

INQUIRY VII HOWFALLS

UCH RADIOACTIVE MATERIALMTO EARTH?

The term "fallout" is commonly used in thisnuclear age. Fallout is made up of airborneparticles containing radioactive materials thatsettle to earth (Figure 7). There has alwaysbeen some fallout from natural causes. Withinthe last decades, as man has set off nucleardevices, the amount of radioactive material inthe atmosphere has increased. So has theamount of fallout. In the case of short-livedradionuclides, this is not a serious problem. But,with ones such as 90-strontium, with a half-lifeof 28 years, and 14-carbon, with a half-life of5730 years, there are possible hazards. Not onlyare these radionuclides long-lasting, but bothare taken up by plants and animals. 90-stron-tium, for example, replaces calcium in bone,and the potential danger from its radioactivityincreases markedly when it becomes an emitterfrom within the body.

You have established a background count foryour immediate area. Is this background countevidence of fallout? In a way it is. But if weconcentrate some kind of material that has hadthe opportunity to pick up radioactive particlesfrom the atmosphere, we should be able tomeasure a count higher than that of background.This difference we consider a measure of fallout,even though the total background also must beconsidered of this nature.

What materials are available that could pickup radioactive debris from the atmosphere? Youmight think of rain, snow, and dust, amongothers. What other kinds of substances couldyou suggest that might be sources of fallout?

MATERIAL (per group)Aluminum foil, 4" x 2"Geiger-Muller counter with holder or ring stand

and clampPen, ball-pointPlastic wrap, 3" x 3"Rubber band, largeSack, cheesecloth or other fabric, 4" x 4"ScissorsTape, cellophaneVacuum cleaner

23

FALLOUT FROM ATMOSPHERE

Figure 7. Atmospheric fallout can be taken up by bothplants and animals and passed from one to another inthe food chain.

PROCEDURE

1. Fasten the cheesecloth sack or one of someother fabric over the intake hose of a vacuumcleaner by means of a large rubber band.2. Mark a circle on the sack around the edge

of the opening in the hose with a ball-point pen.3. Stick the hose out of the window and secure

it firmly. Turn on the vacuum cleaner and allowit to run for 15 minutes.4. Fold the piece of aluminum foil in half to

form a 2-inch square.5. Turn off the vacuum cleaner and carefully

remove the sack. Cut out the circle previouslymarked. Center it on the piece of aluminumfoil.6. Place the aluminum foil with the circle of

cloth beneath the end-window of a mountedGeiger-Muller tube. Open the shield and countfor at least 5 minutes. Record your results.

7. Place the piece of plastic wrap over the topof the circle of cloth and fold it back under thealuminum foil. Fasten it in place with cello-phane tape on the back of the aluminum. Repeatthe counting technique in 6 above.

8. After from 30 minutes to an hour, take an-other count of the plastic-coated sample. Recordyour data.

ANALYSIS OF DATACompare the counts per minute between the

plastic-wrapped sample and the uncovered one.Subtract the background count as determined inInquiry V from your figures. If we consider thedifference to be because of fallout, what state-ment can you make regarding the status of fall-out at the time this sample was taken? Do youregard this amount of fallout as significant? Whyor why not? How do the counts taken a halfhour later compare with the initial ones? Howdo you account for this difference, if any? Whatis the biological implication of these data?

24

PROJECTSWhat type of radiation is this fallout? Use

filters as a possible way of determining this.Your teacher will suggest other possible sourcesto sfa,-ined for fallout. Periodic falloutchecks a 'pan of time can give comparinformation about variation in amount of fall-out. Investigate the phenomena of half-life anddecay of radionuclides in relation to fallout.Check Appendix II for a discussion of theseterms. Determine which radionuclides are takenup by organisms. What hazards are involvedfrom this uptake?

3?

INQUIRY VIII HRAODWIODNOUPCLEIDNETSS?REACT TO

In Inquiry I we calculated the LD-50 (lethaldose-50 percent) for ultraviolet light on a yeastpopulation. From this we learned of the lethalnature of radiation. But is all radiation lethal?If radioactive materials are in an organism, dothey produce the same effects as if the organismwere subjected to an equal amount of externalradiation? What would be the effect of uptakeof a radionuclide by a plant? Would you expectthe plant to be killed or damaged? Inquiry Iwas concerned with a microorganism, but howdoes a multicellular organism react? Are eachof its cells as sensitive as individual micro-organisms? What happens to a radionuclideincorporated into a multicellular plant, and howdoes the plant react? Answers to only some ofthese questions can be provided by a simpleinquiry. The others may serve as a basis forfurther future study.

Some idea of the metabolic requirements of amulticellular organism and the path taken by aspecific element can be obtained by the use ofradionuclides. For this purpose we will use anelement which is normally taken up by plantsfor their metabolism. A common element re-quired by plants is phosphorus and the usualform of administering it is in phosphate.

To determine the effect of a radionuclide on amulticellular plant we will use the phosphorusin a radioactive sodium phosphate solution anda tomato plant. With your knowledge of plantstructure and function, where would be the mostreasonable site to apply the phosphate solution?Would other places on the plant serve as well?

MATERIAL (per group)Cardboard, 13 x 18 cm (5" x 7"), (2, or 4 if

plants are large)Envelope, black, for 13 x 18 cm cardboards (or

black paper or film holder)Film developer, 1 literFilm stop bath, 1 literFilm fixative, 1 literFilm, no screen x-ray, 13 x 18 cm (5" x 7")ForcepsNeedle, hypodermic, No. 2632-Phosphorus (labeled sodium phosphate

NaH232PO4), 0.3 microcuriePipette, medicine dropper, with drawn-out tip

Plastic wrap, 1 footRubber bandsSticks, ice cream or equivalent (4" to 8" in

length for propping plant leaf and supportingplant)

Stop bath, 1 literSyringe, hypodermic, 1 mlTape, cellophaneTest tubes, 30 ml (2)Test tube holderTomato or bean seedlings (2), 4" to 8" in

heightTrays, developing (3)Water, 15 ml

25

PROCEDURE

1. Search for a horizontal leaf on one of theplanted seedlings. If none is found, prop a leafin a horizontal position by means of sticks.2. Using the medicine dropper pipette, place a

small drop of 32P solution in the center of theleaf. Be careful that none of the solution runsoff. Allow the liquid to dry before moving theplant.3. Write a statement in your notebook as to

what you believe will be the result of havingapplied 32P to the leaf.4. Carefully remove a second plant from the

vermiculite or soil in which it is growing.Loosen the vermiculite or soil with a stick orpencil so that the roots can be pulled out withoutdamaging them. Carefully shake off as much ofthe vermiculite from the roots as you can.

5. Place the root system of this second plant ina 30 ml test tube. Add 15 ml of water. Supportthe plant on a stick taped to the side of the testtube and place the tube in a holder. Carefullyadd 0.3 microcurie of 32P in 1 ml of sodiumphospliate solution to the test tube using thehypodermic syringe. Shake the test tube care-fully to mix the 32P with water. Be sure the rootsare covered by the solution.6. Place this plant beside the other one and

allow both to stand for 24 hours_ . Why shouldthe plants be kept together?7. At this point analyze the experimental de-

sign for defects. What has not been done so farthat should be done for every experiment?Decide as a class how to rectify this omission

before proceeding further.8. After 24 hours remove the one plant from

the soil or vermiculite and the other from thetest tube. In the sink rinse the "P solution fromthe roots. Be careful not to splatter. Remember,do not touch the plants with your handsuseforceps or glove:., Ask your teacher whether tosave or to dispose of the "P solution remainingin the test tube.

9. Using your forceps, place the plants on the13 x 18 cm piece of cardboard. Spread out theleaves so that they are flat but not lying on topof one another. Spread out the roots as much aspossible. Bend the stems if necessary but keepall parts of the plant on the cardboard. If bothplants are too large to fit together on one card-board they may be placed on the separate piecesof cardboard and steps 10 through 13 repeatedfor each plant. Identify each plant (whetherfrom test tube or soil) by labeling it on the card-board or by drawing in your notebook.10. Cover the cardboard and plants completelywith a piece of plastic wrap. Make sure that theplants cannot move on the cardboard. Use thecellophane tape to secure the plastic wrap on ti,e,side of the cardboard opposite the plants.11. In a photographic darkroom place a pieceof no screen x-ray film, emulsion side down,over the plants on the cardboard and cover itwith another piece of cardboard. Fasten thissandwich together with two rubber bands andplace it inside a black envelope.12. Up until this point all the procedures forthe class have been the same. At this time athird of the group shoula store the film in thedark for one day, another third for two days,and the remaining third for three days.13. At the proper time open the film packagein the darkroom. Discard the cardboard, plasticwrap, and plants in the radioactive waste con-tainer. Develop the film as outlined in InquiryIV.ANALYSIS OF DATA

On the basis of the autoradiogram what state-

ment can be made about the relative absorptionof 32P in each of the two experimental plants?Note the distribution of "2P in the plant. Whatdoes this distribution tell you about the localiza-tion of phosphorus in the growing plant? Thinkof some of the common metabolic processes inwhich phosphorus may take part. Do thesedata indicate to you some reason for the distri-bution of phosphorus? What radiation effects,if any, were noticeable in the plants after 24hours? What do you suppose would be the effectof "2P on the plant after a longer period of time?How can you check your answer to this ques-tion?

How does the phosphorus move through theplant when deposited on a leaf? How does thispath differ from that of the phosphorus ab-sorbed by the root? If phosphorus can enterthe plant through the leaf would spraying foliageseem an effective way of fertilizing a plant?

Do you have any indication that phosphorusis used more in one part of the plant than inanother? What indications., if any, do you havethat phosphorus is localized or stored in anyparticular part of the plant? In what tissues, ifany, does it seem to be localized? Answer asmany of the questions posed in the introductoryparagraph of this Inquiry as you can on thebasis of your experimental data.

26

PROBLEMS

If a tomato plant was used in this Inquiry,would you expect similar responses to phos-phorus if other types of plants had been used?Design an investigation to test your hypothesis.

Design an investigation to determine the ef-fects of "2P on the plant over a long period oftime. Can phosphorus be absorbed throughplant parts other than leaves and roots?

What relation does the movement of phos-phorus bear to the process of photosynthesis?By repeating this experiment with plants in thelight and in the dark, data on your hypothesismay be obtained.

34

INQUIRY IX WHAT HAPPENS TO A RADIONUCLIDEINJECTED INTO ANIMALS?

In the previous Inquiry you saw how phos-phorus was distributed in a plant. If 32-phos-phorus is injected into an animal, what arethe results? On the basis of your knowledge ofanimal physiology formulate hypotheses as towhat you believe will happen. In this Inquiry wewill use radioactive phosphorus injected into afrog. The data from this .1.aquiry should supportor refute your hypotheses.

MATERIAL (per group)Cardboard, 13 x 18 cm (5" x 7") (2)Chloroform, 5 mlContainer for frogCotton, absorbent, small padDissecting kitDissecting pan with wax bottomEnvelope, black, for 13 x 18 cm cardboards (or

black paper or film 'oolder)Film developer, 1 literFilm stop bath, 1 literFilm fixative, 1 literFilm, no screen x-ray, 13 x 18 cm (5" x 7")FrogJar with lid, 8 oz. or larger for anesthetizing frogNeedle, hypodermic, No. 26Pins (8)Plastic wrap (8" x 10"), (2)32-Phosphorus solution (labeled sodium phos-

phate Nal-1.2"2PO4), 1.0 microcurieRubber bandsScissorsSodium phosphate solution, nonradioactiveSyringe, hypodermic, 1 mlTape, cellophaneTrays, developing (3)

PROCEDURE

1. Inject a frog with approximately 1 micro-curie of "2P in 1 ml of sodium phosphate solu-tion. This is done as follows:

a. Hold the frog firmly. Insert the hypoder-mic needle through the thigh muscle.Direct the needle forward to the dorsallymph sac. (The space under the looseskin of the lower middle back of thefrog.) Inject the sodium phosphate solu-tion into the space under the skin. Becareful not to allow the needle to extend

through the skin or into your fingers.When the needle is withdrawn the mus-cle seals the injection site and preventsthe loss of the "2P solution.

b. Place the frog in a marked containerwhere it will have both water and air,but will not be able to splash water out-side the container. Leave the frog in thecontainer for from 24 to 48 hours be-fore proceeding with step 2.

2. Kill the frog by overanesthetizing withchloroform. Pour about 5 ml of chloroform ontoa cotton pad and place in a large jar. Put thefrog inside and cover the jar. Wait about 10minutes until you are sure the frog is dead.3. Place the frog, ventral side up, in a wax-

uottom dissecting pan and pin the legs in anextended position.

4. Carefully open the body cavity as directed.Remove the liver and spleen and place themside by side on a piece of plastic wrap. Alsoremove the heart and one lung and place themon the plastic wrap. Carefully slit the skin of theleft hind leg from the ankle to the pelvic region.Cut the muscle away from the bone and place apiece of muscle on the plastic wrap.

At the pelvic joint of the left hind leg separatethe upper end of the thigh bone from the pelvicgirdle. Be careful not to break the bones. Twobones are attached to the thigh bone. Separatetheir lower ends from the bones at the end ofthe leg. You now have a pair of bones attachedto each other and to the thigh bone by liga-ments. Most of the muscle has been removedearlier. If any remains remove it now. Placethese bones on the plastic wrap with the otherparts.

5. It is considerably more difficult to dissectbut, if time allows, remove the brain and placeit with the other parts.6. Arrange the parts on the plastic wrap so

that they will fit within a 13 x 18 cm rectangle.Cover the parts with a second sheet of plasticwrap and press it down gently so the frog partsand the two pieces of plastic wrap stick togetheras a unit.7. On one of the 13 x 18 cm pieces of card-

board trace the outlines of the parts placed onthe plastic wrap and label them. You will need

27.1S

this diagram later to identify the position of theparts on the film.

8. Place the plastic-wrapped parts againstthe cardboard so that each matches the labeleddiagram. Fold the excess plastic wrap aroundthe edge of the cardboard and firmly secure it tothe back of the cardboard with cellophane tape.9. In a darkroom place a sheet of x-ray film

over the parts and cover it with the second13 x 18 cm piece of cardboard. Fasten the card-boards, the film, and the plastic-wrapped partstogether with rubber bands and place the packetin a film holder or black envelope.10. Allow the parts and the x-ray film to re-main in contact in the dark for three days.11. After three days remove the film in a dark-room, discard the plastic, the frog parts, card-boards, and the rubber bands in the radioactivewaste can. Develop the film as outlined inInquiry IV.

ANALYSIS OF DATA

Examine the autoradiogram of each of thefrog parts. How do the various parts selectedcompare as to radioactivity? On the basis ofyour answer to this question list the parts inorder of the amount of "213 they apparently con-tain from most to least. On the basis of this datawhat kind of hypothesis can you formulate re-garding phosphorus metabolism in the variousparts concerned?

If the interval between the injection andthe examination of the parts were two weeks,what would you expect the results to be? Howcan you account for this answer? With whatyou have just observed how can you be sure thatthe results you have seen are due to the radio-active phosphorus you injected? Why couldthey not be due to natural radioactivity withinfrogs? How could you rule out this possibility?What would you expect the effect of the radio-active phosphorus to be on the frog? How canyou gather data to test this hypothesis?

Compare the frog tissues with 'those of theplants in the previous Inquiry. Contrast the dis-tribution of radioactive phosphorus in bothorganisms.

PROBLEMS

Frogs may be injected with other radio-

28

nuclides such as 131-iodine or 45-calcium.Autoradiograms could show specificity of theseelements for various organs.

Various time intervals can be used to tracethe movements of radionuclides in the organsconcerned and their rate of elimination, withallowances made for decay.

Animals other than frogs can be usedmiceand fish, for example. Radionuclides need not beinjected. They can be administered in drinkingwater or added to the water of a small aquariumto determine absorption by an organism such asa fish.

It is interesting to follow the path of an ele-ment through stages of a life cycle of an organ-ism. This is a long-term project and a radio-nuclide with a long half-life is necessary. Itmight first be introduced into a plant knnw--to be the food of a caterpillar. A number ofcaterpillars might then feed upon the plant. Anexamination of the tissues of sacrificed cater-pillars, pupae, and adult insects would provideinteresting data on localization and utilization ofelements during the life cycle of an insect.

24NaCI solutioninfected here

Constriction

WO

High reading(good circulation)

Low reading(poor circulation)

Figure 8. Radionuclides are used as tracers in a widevariety of ways. Not only can they be localized inspecific organs as in this Inquiry, but they can be usedto detect rates of flow. In this case, labeled sodium isused to detect the site of restriction of blood flow tothe lower leg.

_

INQUIRY X HAVEWHAT

ON SEEDS?EFFECT DOES IRRADIATION

In the first Inquiry we observed that radiationcan inactivate and kill organisms. SubsequentInquiries have been concerned with detectionand uses of radiation. Let us now return toradiation effects. From what you now know, itshould be fairly easy to devise investigations todetermine the effects of radiation on a widevariety of organisms. It should only be necessaryto irradiate the organism concerned with givendosages of particular radiation and observe theresults. This type of experimental design is rela-tively simple; it is in the observations andtheir interpretations that difficulties arise. First,one needs to know the normal sequence ofevents with which the data obtained from anirradiated specimen can be compared. Whatdoes this then mean in terms of experimentaldesign?

As an example of such an irradiation study,we will use seedling growth. Seeds can be pur-chased already irradiated with various dosagesof radiation from 10,000 to 40,000 roentgens.They can be irradiated locally if a high-energysource is available. Even with such a simpleexperimental design, requiring only irradiatedseeds and a control, there is a wide range ofpossible variables. For example, this design canbe used to determine the effects of various irra-diation dosages on seeds of similar types. It canbe used to compare irradiation effects on seedsfrom various plants. It can be used to check theeffect of irradiation on germination time, on thefirst appearance of leaves or roots, or on growthplotted against time. What other variables canbe tested by this design?

PROCEDURE1. You are to select a variable you wish to

study. Write your hypothesis concerning whatyou believe to be the effect of irradiation uponthe variable you have selected.2. Write out the experimental design that will

provide the data necessary to evaluate yourhypothesis. Included in the design should be thetype of seed you are using, the number of seeds,

the amount of irradiation, the type of controls,the time you expect the experiment to take, anda complete list of materials.3. After you have received your teacher's ap-

proval, assemble the required materials andbegin the investigation. During the course ofyour observations you will want to note care-fully whether the expected changes do indeedoccur, as well as to observe those changes, ifany, that occur that you had not anticipated.

4. As your experiments undoubtedly involvesome measurements, how are you going to pre-pare the data for analysis? Will you place themin tabular form? Will you make a bar graph ora line graph? Be prepared to present your datain a form that can be understood by other mem-bert, of the class.

After the 0 is tove been assembled, ana-lyze the data Ind :-pare a c- mar turresults. What has your experiment sho-, aboutthe effects of radiation on seeds? What kinds ofquestions can you answer on the basis ofyour experiment?6. Be prepared to present the results of your

experiment to the class and to discuss them.

PROBLEMSThis basic experimental design leads to a

variety of long-term projects. For example, theplants from the irradiated seeds can be grown tomaturity -,nd the effects of irradiation noted onflowers, fruits, seeds, ,z-e. Experiments can beconducted to show relative effects of radiationvarious stages in the plant's growth and develop-ment. Seeds from the plants grown from theirradiated seeds can be collected and, in turn,planted to observe any hereditary effects ofirradiation. In this latter case, individual flowersmust be covered by transparent bags to insureself-pollination.

Seeds can be soaked in solutions of radio-nuclides previously used in class, such as 32P,and the effects of this type of radiation con-trasted with that of others.

3725i7

INQUIRY XI HOW CAN YOU DETECT THE MOVEMENT OFMATERIALS THROUCH AN ECOSYSTEM?

One of the uses to which radionuclides areput is to act as tracers (see Appendix III). InInquiries VIII and IX you were able to trace thedistribution of phosphorus in a plant and in ananimal. Similarly, tracers can be used to followan element through the principal components ofan ecosystem. The basic experimental design issimple. It involves introducing a radionuclide ata given time in an ecosystem and, by detectioninstrumentation, measuring its uptake in variousorganisms. Different quantitative techniques canbe used to measure uptake and concentration ofthe element concerned. For one illustration ofsuch a technique, a simple. aquarium will serve.To clarify the gathering of data and analysis ofresults, a single type of plant, such as elodea,and a single type of animal, such as the snail,are all that is required. The radionuclide will beintroduced into the water and its uptake andaccumulation in the plant and in the snail willbe observed. A week or two prior to the time ofinitiating this Inquiry a balanced aquariumshould be set up containing elodea and pondsnails.

MATERIAL (per group)Absorbent paper or paper towels, 3-cm-square

pieces (20)Aquarium, balanced (with elodea and snails)Elodea plantsForceps, extra longForceps, regularGeiger-Muller counter with holder or ring stand

and clampGraph paper, semilog, 5-cycle, 1 sheet per

studentLight source32-Phosphorus solution (0.5 to 1.0 microcurie

per liter of aquarium water)Pipette (2 ml or larger)Planchets or bottle caps (30)Rod, glass, stirringSnails, pond (a minimum of a dozen per aquar-

ium)Wax paper or plastic wrap, 3-cm-square pieces

(20)

PROCEDURE

1. Pipette a 2 ml sample of water from thebalanced aquarium into a planchet or a bottle

31

cap. Evaporate and measure its activity with anend-window Geiger-Muller tube. (Rememberthat all measurements should be taken the samedistance from the tube.)

2. Remove a leaf of elodea from a plant grow-ing in the balanced aquarium. Remove the ex-cess water by placing it for about a minute ona small (3-cm-square) piece of absorbent paperor paper towel backed with a similar piece ofwax paper. Place it in a bottle cap or planchetand measure its activity as in 1 above.3. Repeat step 2 with a single snail. As the

snail is not damaged, it can be returned to theaquarium after being checked for activity.4. Add 0.5 to 1.0 microcurie of 32-phos-

phorus per liter of water to the balancedaquarium. Stir the water with a glass rod gentlyso as not to disturb the organisms.5. Again determine the activity of the water,

the snail, and the leaf as in steps 1-3. Recordyour data.

6. Discard the pieces of paper and bottle capsin the radioactive waste container.7. Leave the aquarium in an area lit by mod-

erate natural light (not direct sunlight) withnecessary supplement by artificial light to pro-vide a relatively constant level of illuminationover a given time period daily (10 to 12 hours).8. The following day and on alternate days

thereafter for a period of 2 weeks repeat steps1-3, recording the data and discarding the paperand caps as in 6 above.9. Plot this data on a graph showing counts

per minute on the vertical axis and time in dayson the horizontal axis for the water, the snail,and the elodea leaf.

ANALYSIS OF DATA

What happens to the counts per minute in thewater? Between what two time intervals is thechange the greatest? What happens to thecounts per minute in the elodea leaf? For howmany days is the count the highest? Betweenwhat two days is there the greatest increase incount?

How does the graph for the counts per minutein the snail differ from that for water and theelodea leaf? By what day have the counts in thesnail reached their maximum? How do you ac-

count for these data? What do they tell youabout the movement of an element through anecosystem? After 14.3 days, (two weeks) only50 percent of the radioactive phosphorus is stillpresent. But how is it distributed at that time?Is the radioactivity of the water now 50 percentof what it was initially? For the elodea? For thesnail? On the graph that shows the counts perminute for elodea, snails, and water in relationto time, plot the decay rate of 32P. Where is the32P being concentrated? Is all of the 32P foundin snails due to the fact that snails ate theelodea? What would happen if a second orderconsumer (a fish that ate snails) were added tothis ecosystem? After two weeks would you re-ceive a greater dosage of 32P per unit of mass ifyou drank the water from the aquarium or if you

ate the elodea? What does this tell you about therelative hazards of consuming contaminatedmaterials?

PROBLEMS

More complicated food webs can be set up.The introduction of several kinds of plants, fish,Daphnia, or crustaceans can add to the com-plexity of the ecosystem and provide alternatepathways for the 32P.

Other radionuclides, such as 14-carbon, canbe used rather than 32-phosphorus. They willgive somewhat different results.

In areas near the ocean, marine aquaria canbe set up using shellfish, echinoderms, anem-ones, etc.

0000.0>

Birds

raft-at--

Crops

0

00o.0

Figure 9. Diagramatic representation of the flow of radionuclides through a natural ecosystem. Contrast thecomplexities shown in this figure with the simple food chain used in this Inquiry.

32 39

INQUIRY XII TO SOLVEBRAcelf00 GN AC LLI pD RE mUSsE

In Inquiries VIII and IX ,32P was used toshow uptake of phosphorus in a plant and inan animal. These data told us somettifi aboutthe distribution of phosphorus in these organ-isms. From these data, certain hypotheses couldbe made about the role of phosphorus in plantand animal metabolism. These hypotheses thencould be tested by further experimentation.

Are there more specific ways to use radio-nuclides in solving biological problems? Forexample, how might radionuclides be used toexplain the events that occur during such aprocess as photosynthesis? We are all familiarwith the simplified formula for photosynthesiswhereby carbon dioxide and water, using lightenergy and chlorophyll, form carbohydrates andliberate oxygen:

sunlight6 CO2

4-6 H2O

chlorophyllC6111206 4- 6 02

If you accept the above simple formula, youmust assume that carbon dioxide and water areused in the presence of light to form the carbo-hydrate. If there is no light, there is no photo-synthesis and no uptake of carbon dioxide or useof water for this purpose.

You are familiar with the fact that this sum-mary omits many of the separate intermediatesteps that occur. Many questions can be asked.For example, does the oxygen in the carbohy-drate come from the water or the carbondioxide? Or both? And what about the carbonitself? At what stage of the process is the carbonof carbon dioxide involved and what is its rela-tin to light enftrgy ftnd chlorophyll? Couldthere even be other sources of carbon for thecarbohydrates? Perhaps the carbon comes fromelsewhere in the plant for the purposes of photo-synthesis, and the carbon in the carbon dioxideis a replacement for that carbon so used.

On the basis of what you now know aboutradionuclides, you can probably think of waysto devise experiments that will show the role ofthe carbon in the carbon dioxide or the role ofthe oxygen in either water or carbon dioxide.Such experimental designs quickly become com-plex and require skills perhaps not yet mastered.However, you can use 14- carbon in 1 #CO2 and,

by varying the experimental techniques, demon-strate several features relative to photosynthesis.Using 14C in the form of sodium carbonate(Na214CO3), you will compare the uptake ofcarbon dioxide (released from the Na214CO3 as14CO2) in the light and in the dark and attemptto localize it within the plant. Other questionsregarding the role of CO2 in photosynthesis mayoccur to you. An alteration of this basic ex-perimental design can provide some data foranswers.

MATERIAL (per group)Acetic acid, 0.1M (CH3COOH) 2 mlBarium hydroxide or calcium hydroxide, 0.2M,

4 ml50 ml beaker or small glass container (2)Black paper or black paint to darken one of the

two-gallon jars14-Carbon in sodium carbonate (Na214CO3)

aqueous solution, 20 microcuriesColeus plant, potted, small (2)Film developer, 1 literFilm stop bath, 1 literFilm fixative, 1 literFilm holder or black envelopeFilm, no screen x-ray, 13 x 18 cm (5" x 7")Fumehood or properly ventilated areaGallon glass jars, screwtop with lids (2)Lamp as light source, 100 WNeedle, hypodermic, No. 26 (2)Plastic wrapRubber bandsSyringe, hypodermic, 1 ml (2)Tape, cellophaneTrays, developing (3)Wire for beaker support in jar, l -foot piece (2)

33

PROCEDUREThe procedure using each of the two,one-gal-

Jon jars is identical except for the fact that onejar must be light-tight. It can be covered withblack paper or painted with flat, black paint.The following description for steps 1-4 will begiven in terms of a single jar and plant, but thesetups for both jars should be performed at thesame time.

1. Place a small potted coleus plant in thebottom of the gallon glass jar.

2. Support a small beaker or similar glass con-tainer with wire within 4 cm of the top of theglass jar (see Figure10). By means of the hypo-dermic syringe and needle, add 10 microcuriesof Na2"CO3 (sodium carbonate) in aqueoussolution to the beaker. Cover the jar with plasticwrap, fastening it tightly around the jar mouthwith rubber bands to form as tight a seal as pos-sible.

Plastic wrap

_Rubber band

50m1 Beaker

Coleus plant

Gallon jar

Figure 10. For Inquiry XII, two jars are prepared asshown. One, however, is light-tight. The beaker issuspended from the jar lip by means of wire and theplastic wrap forms a tight seal over both the jar andits contents.

3. Or the plastic-wrap cover at a point di-rectly over the sodium carbonate container,make a cross of cellophane tape about 3 incheslong to reinforce the area.4. Carefully draw 2 ml of 0.1M acetic acid

into the second hypodermic syringe. Stick thehypodermic needle through the center of thecrossed cellophane tape. Discharge the 2 ml ofacetic acid into the sodium carbonate container.Seal the puncture hole with tape. The gas that isbeing generated is carbon dioxide, part of whichis in the form of "CO2.

S. Place both jars carefully in a fumehood orin a well ventilated area. Illuminate the clear jarwith a strong light. What is the purpose of thelight?

34

6. Review the experimental design to thispoint. What is it designed to test? On the basisof your knowledge of photosynthesis from pre-vious experiments in biology, hypothesize thecondition of both plants when the experimentis done.7. Depending on the time available, you may

proceed with the experiment after 10 minutes,or you may wait until the next class period 24hours later. Before opening the containers, the"CO2 must be removed. This may be done byintroducing by means of the hypodermic syringeand needle either barium hydroxide or calciumhydroxide solution into the container of sodiumcarbonate at least an hour before the containersare opened.

If adequate ventilation is provided, the plasticseal can be cut with a knife and the containerleft in the closed chemical hood for at least anhour before examination.8. Carefully remove the wire support and

beaker and take the coleus plant from the jar.Place the plastic wrap, the beaker and its con-tents, the wire supports, and the rubber bandsin the jar and seal with the lid until step 11.9. Remove 1 or 2 leaves from the plant kept in

the light and prepare an autoradiogram follow-ing the procedures outlined in Inquiry IV. Re-peat this process with leaves from the plant keptin the dark, being careful not to expose theleaves to light. If it can be arranged, this entiresecond transfer should be done in a darkenedroom.10. Open the jars, place the coleus plants in thejars with the rest of the material used in theexperiment and discard in the radioactive wastecan.11. After 24 hours, develop the autoradiogramsand examine them.

ANALYSIS OF DATAWhat does the autoradiogram indicate about

the uptake of carbon dioxide by the plant? Howdoes the autoradiogram of the leaf of the plantkept in the light, compare with the one of theleaf of the plant kept in the dark? How do youaccount for this? Is photosynthesis apparentlytaking place both in the light and in the dark?What does the autoradiogram indicate about thesites of photosynthesis? What does this Inquirytell you about the source of the carbon used inphotosynthesis?

PROBLEMS

This experimental design can be altered touse different plants, different times of exposure,and different plant parts. What would you ex-pect to be the condition of the stern or roots ofthese plants? If roots absorb CO2, what must bedone to the above experimental design to insurethat only the leaves and stems are exposed tocarbon dioxide?

Test the leaves removed from the coleusplant with the end-window of the G-M tube.

In this case, count both upper and lower sur-faces of the leaves. Are the experimental resultsconsistent for both the upper and lower sur-faces?

A quantitative measure of the count can beobtained by taking a 1-cm square of leaf surfaceand exposing it to the end-window of the G-Mtube.

Can you think of other ways in which radio-active compounds could be used to help explainthe processes of photosynthesis?

35

9.4.

ON YOUR OWN

The initial 12 Inquiries have given you someunderstanding of radiation and its use in biol-ogy. You have mastered some techniques forhandling sources of radiation and are now ableto identify and measure certain types of ionizingradiation. You have become familiar with someof the basic techniques and experimental designsfor the use of radionuclides. You are now fami-liar with the use of radionuclides as tracers andsome effects of radiation on certain organismsand their processes. You have seen how radio-nuclides can be used for a spectrum of problemsranging from those pertaining to microbiologyto those that are ecological in nature.

The following suggestions offer opportunitiesfor individual work in addition to the problemsin the individual inquiries. They are completelyunstructured and are open to a wide variety ofvariations on a single experimental design andto the testing of a wide variety of hypotheses.For each, you will have to develop a hypothesis,an experimental design, and materials lists thatshould be checked with your instructor beforeyou proceed. As time and facilities are always afactor, this checking will insure that your designis feasible within the limits of your situation.

1. How Are Radionuclides Used To MeasureVolumes In Living Systems?Radionuclides have a considerable use as

tools in determining volumes in living systemsthat are otherwise difficult or impossible to de-termine by direct measurement. For example,frequently biologists are asked how much bloodthere is in a given animal. One way to determinethis, of course, is to draw the blood from theanimal concerned and measure its volume.However, it is extremely difficult to draw bloodfrom all the tiny capillaries in the body andimpossible to do so without the destruction ofthe animal. A simple, quicker, more accuratetechnique to answer questions concerning blob('volume is through the use of radionuclides.

Blood volume studies are based on the equa-tion

V/S = CU/CDV is the animal's total blood volume that we aretrying to determine. S is the volume of radio-nuclide mixture to be injected in the animal (use

0.5 ml for this example). CU (for Counts Un-diluted) is the counts per minute of 0.5 ml of S.CD (for Counts Diluted) is the counts perminute of 0.5 ml of the animal's blood with-drawn 5 minutes after the animal is injectedwith 0.5 ml of the radionuclide mixture. Five-tenths ml of the radionuclide mixture injectedinto the tail vein of a rat or other mammal willbe diluted in proportion to the total bloodvolume of the animal after about five minutes.When making the counts per minute with theGeiger-Muller counter, the samples are firstplaced in a planchet and evaporatA

What modifications would have to be madefor the radionuclid4 injected into the bloodvolume to be recovered in the urine? Whatradionuclide would you use for this purpose?One could be used that would simply be dilutedin the blood plasma. Would this provide anaccurate measure of blood volume? Othernuclides can be incorporated into the red bloodcells. Would this then give a measure of bloodvolume? In your experimental design, select theliquid volume to be determined and insure thatthe above formula will be applicable. Select aproper radionuclide to make the measurementsyou are seeking. Be sure the experimental designwill provide you with data capable of analysis.

* * *

The following three inquiries require a high-energy radiation source. For this purpose, youmust be able to enlist the aid of some localphysician, dentist, hospital, university, or re-search center to attain access to x-ray or otherirradiation equipment. Arrangements shouldbe made well in advance. Make sure yourteacher is involved in such arrangements inorder to avoid difficulties with those whooperate the radiation sources.

2. What Effects Does Irradiation Have OnMetabolism?Studies of living organisms show that the

metabolic rate of the entire organism varies atdifferent times and in different places. Differenttissues metabolize at different rates. It is notdifficult for you to answer the question as towhether or not an active muscle or piece of bonehas a higher metabolic rate. What relationship,

37

if any, exists between radiation and the metabo-lic ate of different tissues?

For an experimental design to provide ananswer to this question, one must first have ac-cess to a radiation source of sufficient intensityto produce radiation capable of tissue dam-age. Secondly, there needs to be a living organismthat is easily obtainable and one whose tissuemetabolism is fairly well known. An ideal ani-mal for this source is the common planarianDugesia. It is small, easily obtainable, and theeffects of radiation damage can be observedreadily. Expose the planarians to various dos-ages of irradiation.. After irradiation, makecareful daily observations of its effects on theplanarians.

3. Can Irradiation Act As A Preservative?Food preservation has been a problem since

man first produced more food than he could eatat one time. He was then in a position of tryingto preserve this surplus for the next meal or totide him over the hard winter. Drying and salt-ing were undoubtedly two of the first ways ofpreserving food. They were followed by pick-ling, canning, and nowadays, freezing. Today,many of the fresh foods eaten are really freshfrozen foods. Freezing, however, is an expensivetechnique requiring a constant supply of elec-trical energy, not only for the initial freezing,but for maintenance of food in the frozen state.In the event of a major power failure over anextended period of time, frozen foods wouldhave to be eaten immediately or spoilage wouldresult. Thus, we have not yet, after many centu-ries, found the ideal way to preserve food withfresh taste and nutrition intact.

As you know, food spoilage is the result ofthe multiplication of microorganisms, primarilybacteria or fungi. Theoretically, this multiplica-tion can be stopped by killing all the micro-organisms in the food and preventing furthercontamination. What problems can you suggestthat would make such an apparently simpletechnique so difficult to achieve? Here again, thepermanent preservation of food, often calledsterilization, requires large amounts of energy.More frequently, food is preserved simply byextending spoiling time, which gives the food alonger shelf life.

You may have heard of the use of high-inten-sity irradiation as a food preservation technique.For this experimental design, ground meat is a

38

good food to use. By its color and odor, it givesan easy indication of spoilage and, at room tem-perature, tends to spoil relatively rapidly. Usingground meat, develop an experimental designthat will expose the food to high intensity radia-tion and allow you to measure the degree ofpreservation of the food so exposed. Won ld youuse thick or thin preparations of food to bepreserved by irradiation? How do you preventsubsequent contamination of the irradiatedfood? Will the food itself be made radioactiveby exposure to high intensity radiation? Yourexperimental design should be so structured toprovide answers to these and other similarquestions.

4. What Types Of Genetic Damage AreCaused By Irradiation?

Frequently you hear of mutations that areoccasioned by expbsure to radiation. From yourstudy of evolution, you recognize that mutationsare one of the devices whereby change is intro-duced into the hereditary material. The changemay be good, bad, or indifferent. However, asmost organisms are fairly well adapted to theirenvironment, a random change is more likely torender them less fit rather than more fit.

If a mutation is a change in a gene, it wouldbe difficult to make a direct observation of agene change in a chromosome. Such changecould only be shown by observing the offspringof parents so irradiated. In Inquiry X, you hadan opportunity to see some of the gross effects ofirradiation on seeds. There are also observablechromosomal changes that become apparentafter irradiation. These include translocations,nondisjunctions, chromosomal duplications, andthe appearance of acentric fragments.

However, because chromosome morphologyis relatively complex, for the purposes of thisinvestigation it is easiest to compare chromo-somes that have been irradiated with normal,nonirradiated chromosomes from the sameorganism and notice the differences, withoutspecifically trying to name each change. Theimportant observations will be whether chromo-somal effects have been produced or not as theresult of irradiation, not so much which typeshave occurred.

A good source of materials for this Inquirywould be the cells of onion root tips undergoingmitosis. The onion tips will need to be exposedto high - intensity radiation, then fixed and

stained to observe the chromosomes. The ex-perimental design should be such that you canmake a statement regarding the effect of irradia-tion on hereditary materials.

5. How Does Irradiation Affect ChemicalReactions In Organisms?From previous work in biology, you have

come to understand that enzymes acting asorganic catalysts are necessary for certain chem-ical reactions to occur in organisms. Enzymesare also protein, and their biological activity inpart depends on their particular structure andspecific chemical groups. You may also remem-ber that enzymes are specific; that is, theycatalyze a particular type of reaction only. Theenzyme pepsin, for example, is specific forsplitting proteins into smaller units. By irradiat-ing dry, powdered pepsin with a high-energyradiation source such as 60-cobalt, normally

39

available only in hospitals and research institu-tions, the effect of such irradiation on enzymeactivity can be noted.

Develop an experimental design that will testthe effects of irradiation on the activity of pep-sin. The design should include the amount ofpepsin to be irradiated and irradiation dosesto be used. Plans for controls should alsobe included. Egg albumin is a convenient pro-tein. When placed in capillary glass tubes andheated, it turns to a white solid. Using this tech-nique or cutting egg white into small cubes,determine how the effect of the action of pepsinwill be measured. Remember the conditions foroptimum enzyme actions, such as pH andtemperature. Are these accounted for in yourexperimental design? Include in your designplans for the presentation of data. Will graphsor tables best suit your design?

J

APPENDIX I HANDLED?MICROORGANISMS

If your laboratory work so far has includedmicrobiological techniques, this Appendix canbe skimmed rapidly simply to refresh in yourmind the procedures to be used. If you have notyet mastered microbiological techniques, thissection should be read with care, for micro-organisms are excellent organisms for radiationstudies.

What is a microorganism?A microorganism is not a specific kind of

animal or plant, but simply one too small to beseen effectively with the unaided eye. Consid-ered as microorganisms are bacteria, yeast,molds, protozoa, and some algae.

MethodsMost of the methods used to study micro-

organisms in general were first developed tostudy a particular groupthe free-living bac-teria, which are the smallest of the micro-organisms with which you will normally deal.Because you will be dealing with organisms youcannot see, the techniques of handling andgrowing microorganisms must be mastered be-fore any constructive laboratory work can beundertaken. The same care shown in handlinginvisible microorganisms will provide valuableclues for proper handling of invisible radiation.It is essential that you learn the methods bywhich microorganisms are handled and grown.While these methods may seem complicated atfirst, actually they are simple to learn and essen-tial for a study of microorganisms. The tech-niques can be mastered with a little practice.

Aseptic TechniquesIn dealing with microorganisms, the glass-

ware, needles, loops, and all materials used inhandling and growing these organisms must beabsolutely clean. This does not mean simplywashing in soap and water to free these objectsof dirt visible to the naked eye; it also meansthey must be sterilized to remove or kill allmicroorganisms present. Use of materials thathave no living microorganisms on them is anaseptic technique.

Microorganisms can be removed or killed byusing heat, as in an autoclave, by flaming, or bychemical means, such as the use of acids, alka-

lies, salts, and various other compounds, includ-ing alcohol and phenol. The chemicals used insterilization of glassware and equipment arecalled disinfectants. Those used on living tis-sues, such as human skin, are called antisepticsand are not as powerful as the disinfectants.Antiseptics and disinfectants will either killmicroorganisms or inhibit their growth.

When you work with microorganisms, allequipment and culture media must be sterilized.However, when microorganisms are transferredfrom one piece of equipment to another, theyare at least briefly exposed to an environmentin which there are other unwanted micro-organisms. If these other microorganisms areintroduced into the colony with which you areworking, they are called contaminants. We can-not see them with our eyes so we must see themwith our minds. In handling microorganismsyou must visualize every way in which con-taminants can get into cultures. Then, the cul-tures must be handled in such a way as toprevent the entrance of contaminants.

EquipmentSome of the equipment for working with

microorganisms is illustrated in Figure 11. Theautoclave is used to sterilize all equipment andmedia not damaged by heat. A pressure cookercan serve the same purpose. Pipettes are of twotypes: calibrated glass tubes into which liquidscan be drawn and medicine dropper pipettesprovided with a rubber bulb at one end to givenecessary suction. Do not suck liquids by mouthinto pipettes. Always use a bulb so that noliquid enters the mouth. The petri dish comesin two parts. Each part is a flat-bottomed glassdish with a small rim. A culture medium ispoured into the smaller half of the dish; theother half serves as a lid to prevent micro-organisms in the air from getting onto the cul-ture medium. Glassware used in microbiologicalinvestigations can be washed and autoclaved.

Microorganisms are so small that it is almostimpossible to handle them individually. Theinoculating loop and inoculating needle aresimple tools designed to handle clusters ofmicroorganisms. Before using either the loop orneedle, sterilize them by passing their tips

41 4

through a flame until they are red hot, then al-low them to cool in the air before use. Thesterile loop tip must not be allowed to touchanything except the material to be handled.

Glass rod for streaking

Inoculating loop and needle

Pipettes

Autoclave

Petri dish Bunsen burner

Figure 11. Apparatus necessary for handling micro-organisms. A pressure cooker can be substituted forthe autoclave.

Culture MediaIn nature many kinds of microorganisms are

found growing together. But, when they aregrown or cultured in the laboratory, we usuallywant to separate the different kinds and groweach separately as a pure culture. One of the bigproblems in culturing microorganisms is findinga substance on which they can grow.

42

The technique of separating one species ofmicroorganism from others so that it can begrown in a pure culture was discovered in thelaboratory of the German microbiologist, Ro-bert Koch (1843-1910). Koch, as well asseveral other biologists, discovered that a solidrather than a liquid culture medium could beused to isolate the cultures of microorganisms.As is often the case in science, the studies ofKoch and his group were based on the observa-tions of another investigator.

In 1872, Johann Samuel Schroeter noticedthat bacteria would grow on the surface ofseveral materials, including that of a cut potato.Here the bacteria formed a large mass whichwas later called a colony. When the individualcolonies were examined under a microscope,each was found to consist of a great number ofbacteria. The most important thing was that allthe organisms in a single colony were of thesame species. Apparently individual cells hadlanded on the surface of the potato and had thenmultiplied into millions. These colonies of mil-lions of cells grew large enough to becomevisible to the naked eye. All the bacteria in asingle colony were derived from one cell andhence each colony formed a pure culture.

The potato was not a satisfactory culturemedium for a number of reasons. First, it wasnot an adequate food for all species of micro-organisms because only a few kinds would growon it. Secondly, it was not sterile and contami-nants from the knife or from the air would alsobecome attached to its surface and form colo-nies. Thus, it was not always possible to be surewhether the colonies were from contaminants orwere from the microorganisms purposely placedon the potato by the investigator. What wasneeded was a culture medium that would con-tain all the substances necessary for growth ofbacteria and also be sterile. Koch first usednutrient beef broth soup, which he mixed withgelatin to make it solid. The sterilized mixturewas poured into petri dishes to harden. On thissurface bacteria were spread, a process calledstreaking. (Figure 12 shows several differentpatterns to be used in the streak-plate method.)However, gelatin also has its disadvantages. Itmelts at 28°C, which is below the best tempera-ture for growth of many bacteria. In addition,many microorganisms make an enzymegela-tinasewhich digests gelatin and causes it toliquefy. Whenever the medium liquefies, the

A

CD

Figure 12. A pattern should be used in streakingmicroorganisms on agar in petri dishes. The above arefour examples of streaking patterns.

separate pure colonies run together and mix.Agar, an extract of seaweed, proved to be the

ideal medium for the solid culture technique. Itis attacked by only a few microbial species andit does not soften until a temperature of 100°Cis reached. The liquid agar does not solidifyuntil it is cooled to 42°C. The fact that it re-mains liquid until cooled to this temperaturemakes it possible to refine methods of obtainingcultures. Thus, instead of using the streakingtechnique to isolate colonies of bacteria, theculti're can be mixed with the cooling agar whilethe agar is still liquid. The agar-microorganismmixture can then be poured into sterile petridishes. When the agar solidifies, the individualbacteria are found to be trapped at different lo-cations in the agar, where they develop into

Figure 13. Note the difference in patterns of isolationof bacteria by streaking (left) and use of the pour-plate technique (right). Photo courtesy of Mrs.Lucille Seguin.

43

separate pure colonies. This technique of ob-taining pure bacterial cultures is referred to asthe pour-plate method. Figure 13 shows the iso-lation of bacteria both by the streak-platemethod and by the pour-plate method.

We still use the pure culture techniques ofKoch in microbiological studies and then growthe microorganisms in an incubator at theproper temperature.

Contamination PreventionIn any microbial culture the medium must

first be freed of foreign organisms by steriliza-tion, and then barriers must be provided to pre-vent the introduction of contaminants. Cottonhas been used to filter out airborne microorgan-isms since 1855, when a pharmacist used a cot-ton plug to keep molds out of his drug prepara-tions. Making sterile media, protecting theculture containers with cotton plugs, and usingsterilized glassware are necessary for growingmicroorganisms.

TechniquesIn handling microorganisms you must be

careful not to distribute them carelessly aboutthe working area. This precaution would beessential if microorganisms caused dis-ease were being used. Even though you will notbe working with disease-producing microor-ganisms you should learn to treat all cultures asif they were. Develop a steady hand for thecareful transfer of microbial growth. Do notleave a trail of microorganisms behind you.Sterilize the inoculating loop and needle afteryou have used them and before putting themdown. Put all cultures and other materials con-taining living microorganisms in containers tobe sterilized. Use disinfectant solutions to cleanwork areas. These practices are designed foreveryone's welfare. The same types of carefultechniques must be used in handling radioactivematerials.

It is essential that you practice the techniquesto become skilled at working with microorgan-isms before you actually begin using them. Com-petence in handling microorganisms will makeyour later work in this Block with radionuclidesmuch easier and greatly reduce possibilities ofaccidents due to careless handling.

Your teacher will demonstrate how to handlemicroorganisms. Watch the techniques carefullyand repeat them until you make no mistakes.

0

APPENDIX II WHAT IS RADIATION?

In order to understand the phenomenon ofradiation and some of its properties, it is neces-sary to know something about matter, energy,and radionuclides. This Appendix providessome of the background you will find valuablein answering certain of your questions aboutradiation. All of it is not essential for perform-ance of the work of this Block. Some of theterms that are discussed here are used in theBlock, and knowledge of them will be helpfulto you.

Basic Units of MatterAll matter is made up of three basic units

protons, neutrons, and electrons. Because neu-trons and protons are ordinarily in the nucleusof an atom they are referred to as nucleons(Figure 14). There are many other kinds ofatomic particles that have been demonstratedexperimentally. You may have read of posi-trons, mesons, and hyperons, among others. Butthese are beyond the scope of this Block andthey will be neither used nor defined in con-nection with it. It is necessary only for us todeal with the three fundamental units. Knowl-edge of their behavior makes the phenomenonof radioactivity understandable.

0 Proton

Neutron

9 Electron

Z=2 A=4Figure 14. A helium atom made up of protons, neu-trons, and electrons. The protons and neutrons togetherin the nucleus are referred to as nucleons.

Nucleons

Mass and EnergyThe mass of protons and neutrons is approxi-

mately 1800 times that of an electron. (Actuallyprotons are 1836.13 times the mass of an elec-tron, and neutrons have a mass of 1838.13times that of an electron.) To many people,mass is weight. Before Einstein's statement ofthe theory of relativity it was assumed that themass of a body was independent of its motion.However, the variation of mass with speed(velocity) was determined experimentally asearly as 1906, using high-speed electrons emit-ted by radioactive substances. Without going

into detail, the general feature of this relationship is that the mass tends to increase as maxi-mum velocity is reached (Table 1).

TABLE 1INCREASE OF ELECTRON MASSWITH INCREASING VELOCITY

Velocity of ElectronExpressed as a Fraction

of Velocity of LightRelative Mass

of Electron

At rest 1.000

0.3742 1.078

0.5025 1.157

0.6954 1.392

0.8629 1.979

0.9988 20.58

1.0000(Velocity of light) Infinite

Table 1. Mass tends to increase with velocity. Prior tothe theory of relativity, it was assumed that the massof a body remained constant regardless of its motion.

It was to account for this that Einstein intro-duced the idea that change of mass withchanged velocity is equivalent to a change inthe energy of the system. This, of course, led tothe conclusion that mass is a form of energy.Thus, mass and energy can be expressed in thesame unit. This idea is stated in the familiarequation E = me". In this equation, energy(E) is equal to the mass (m) times a conver-sion factor, in this case the square of the speedof light (c').

If a process could be devised whereby onegram of mass could be converted into energy,such as electrical energy, we would dealingwith a very large amount of energy indeed. Onegram of mass would convert into 25,000,000kilowatt hours of energy. The average homeuses about 4,000 kilowatt hours of electricalenergy per year. One gram of mass,then,wouidbe enough to supply 6,250 homes with all theneeded electrical energy for a period of oneyear. It is this factthat a given unit of masscontains huge amounts of energythat accountsfor the force of an atom bomb or makes possiblethe generation of power from nuclear fission.

45

Electrical and Nuclear ForcesNot only are the masses of the units that make

up an atom different, but their electrical chargesare different as well. Neutrons have no charge.Protons are positively charged and electronsnegatively charged (Figure 14). Protons andelectrons have equal but opposite charges. It isthese charges that give rise to an electrostaticforce. Unlike charges attract each other, and likecharges repel. Thus, protons and electrons at-tract each other, but one proton repels another,and one electron repels another electron. Thisconcept of attraction and repulsion can bedemonstrated by using bar magnets as ananalogous model. --nd a bar magnet by astring so that it is free to rotate in any direction.Bring a second bar magnet close to it to demon-strate the effect of attraction and repulsion oflike and unlike charges when different poles areused.

It is not possible to demonstrate easily inclass but, despite the fact that neutrons possessno electrical charge, when two neutrons arebrought very close together they do attract oneanother. This is not an electrical force. It is

known as a nuclear force. Nuclear force is re-duced with increasing distance and is not notedat distances slightly greater than 10-13 centi-meters. This nuclear force also exists when neu-trons and protons or two protons are broughtvery close together. Normally when two protonsare separated by relatively large distances, theirelectrical charges tend to repel one another.However, when the distances are very short, thenuclear force overcomes the electrical repulsionand they are attracted to each other. Electricalforces vary inversely as the square of the dis-tance between the charged particles. Nuclearforces vary much more rapidly but not accord-ing to an accurately known law of variation.

Atomic Structure, Nuclides, and IsotopesWe can use the above introduction to the

three fundamental units of matter to consideratoms. Atoms are the basic building blocks ofchemical elements. They are indivisible bychemical means. They are made up of protons,neutrons, and electrons. Their nuclear and elec-trical forces are important when we considerthe energy they contain. The simplest atom thatwe can imagine is that of common hydrogen(Figure 15). It consists of a single protonaround which one electron revolves. All other

atoms consist of a compact nucleus containing aspecific number of protons and neutrons, andsurrounding it, a specific number of electronsrevolving in orbits (Figure 16). Figure 16 is amore accurate representation of the atom but itis difficult to use as a model. As an introductorymodel, Figure 15, known as the Bohr model,allows us to make a number of generalizations.

46

0 Proton

0 Electron

Z=1 A=1

Figure 15. A hydrogen atom. It consists of a nucleusof one proton and one orbital electron. This Bohrmodel makes it look as if the electron spins around thenucleus in a single plane.

Nucleus

OrbitingElectrons

Figure 16. The electrons orbiting an atomic nucleusspin around in a wide variety of planes that seem toform a cloud around the nucleus. Compare this moreaccurate representation of a helium atom with that ofFigure 14, which is a Bohr model of the same atom.

It is nuclear force that binds protons andneutrons in the nucleus and maintains the elec-trons in closed orbits around the nucleus. Sincethe atom, as a whole, is electrically neutral, thenumber of positively charged protons equals thenumber of negatively charged electrons. Thenumber of protons in an atom is known as theatomic number (Z) (Figures 14 and 15). Asthe atom is normally electrically neutral, if theZ of an atom is 3, what would you expect itsnumber of electrons to be? Another useful num-ber is the mass number (A), which is the sumof the protons and neutroos in an atom (Figures14 and 15). It is possible to have atoms withthe same atomic number but different massnumbers. These atoms are called nuclides.Nuclide is a term generally applied to all atomicforms of the elements. They are distinguished

by their atomic number, mass number, andenergy state. The name "nuclide" is oftenwrongly used in place of the term isotope, whichapplies to two or more nuclides having the sameatomic number. Isotope has a more limitedmeaning and is a term that we do not usegenerally in this Block. Isotopes are the variousforms of a single element and thus are a par-ticular grouping of nuclides. Nuclides, on theother hand, include all the isotopic forms of allthe elements. All isotopes of the same elementhave the same atomic number but differ in massnumber, that is, they have the same number ofprotons but different numbers of neutrons. Anunstable isotope of an element that emits radia-tion is termed a radioisotope, of which morethan 1300 natural and artificial types have beenidentified. The term radionuclide indicates aradioactive nuclide.

Atomic SizeAll atoms are built on a plan similar to that

of the hydrogen atom. The Z number of pro-tons and the neutrons (A minus Z) form thecompact nucleus. The electrons circle the nu-cleus in orbits of varying radii. As Z increases,the size of the nucleus increases. One might thenexpect the atom to become larger and larger.However, such is not the case. As Z increases,the electrons are attracted more strongly to thenucleus and their orbits decrease in size. Thus,the overall effect of increasing nuclear size anddecreasing orbital size is to maintain an almostconstant size of atoms, from hydrogen to uran-ium. The diameter of the largest atom is onlyabout 4 times that of the smallest (Figure 17).

Atomic EnergyAn idea of how much energy is trapped in an

atom can be gained if we first deal with distance.Atoms are mainly empty space. If, for example,the nucleus of a hydrogen atom were a ball 4inches in diameter, the electron would be a thirdof a mile away. (Actually, the electron orbit ofa hydrogen atom has a radius of 0.53 x 10's cmand does not approach the nucleus closer thanthis distance.) Because of the electrostatic forcebetween the electron and the proton, it takesenergy to remove the electron. The very smallestamount of energy required to remove the elec-tron from a hydrogen atom is 13.54 electronvolts (ev). (An electron volt is the amount ofkinetic energy gained by an electron acceleratedthrough an electrical potential difference of one

Hydrogennucleus

Hydrogenatom

Figure 17. The number of protons in an atomicnucleus (Z) varies from one in hydrogen to 92 inuranium. Despite this fact, the uranium atom is not92 times as large as the hydrogen atom because, whilethe nuclear size increases, the diameters of the electronorbits decrease so that the difference between thelargest atom and the smallest is only about 4 times.

Common hydrogen

Z=1A=1

Tritium

Deuterium

C) Proton

e Electron

Neutron

Z=1 A=3

Z=1A=2

Figure i8. The atomic number (Z) of these threeforms of hydrogen is the same. The mass number (A),which includes both protons and neutrons, varies fromone to three. These three forms of the element hydro-gen are referred to as isotopes. They have almost thesame chemical properties but somewhat differentphysical properties. Isotopes are the various forms ofa single element. The term nuclide is used to refer toall the isotopic forms of all the elements. Thus, iso-topes of a single element can be considered to be afamily of nuclides.

volt. It is a unit of energy for work, not of volt-age.)

Atomic energy relationships may be demon-strated in a deuterium atom, which is a nuclideof hydrogen known as heavy hydrogen (Figure18). It differs from ordinary hydrogen only in

47 51

that a neutron is attached to the proton in thenucleus. The atomic number is 1 but the massnumber is 2. The electron is relatively unaffectedbecause there is no appreciable force betweena neutron and an electron. Because the electronorbits are almost the same in atoms which arenuclides of one another, this applies generallyto all nuclides. In the deuterium atom only13.54 electron volts of energy are required toovercome the electrostatic force between theelectron and the proton. However, the nuclearforce is so great that about 2.2 million electronvolts (2.2 Mev) are needed to break apart theneutron and the proton in the deuterium nu-cleus. The energy needed to separate the neu-tron from the proton is called binding energy.

Just as the energy required to separate neu-trons from protons is called binding energy,so the energy required to break molecules intocompletely separate atoms is called bindingenergy. The amount of energy needed to sep-arate carbon from oxygen in a carbon monoxidemolecule is about 10 electron volts. How doesthis compare to the binding energy required toseparate a neutron from a proton in a deuteriumnucleus?

Naturally occurring atoms have atomic num-bers (Z) ranging from 1, as in hydrogen, to92, as in uranium. In an earlier stage in theevolution of the universe, atoms of higher Zthan 92 probably existed and may still exist insuch environments as the interior of the sun. Formost of these Z values, limited numbers of stablenuclides exist.

Attractive ForcesWhat has been said so far indicates that an

atom possesses no net electrical charge. How-ever, there are attractive forces between two ofthese "neutral" atoms at any particular instantdue to an unequal distribution of electricalcharges in the electrons circling the atom whichcreate electrostatic fields. Thus, when the posi-tive charges are in one position in one atom andthe negative charges in an opposite position inanother, a temporary attractive force .can existbetween atoms (Figure 19). There are atomicforces that exist between atoms which areweakly attractive in many cases. These areknown as van der Waals forces. At muchsmaller distances, frequently the forces become,more strongly attractive. This larger attractiveforce is a chemical force or chemical bond. It is

48

Atom "A" Atom"B"

Figure 19. While atoms are normally considered topossess no net electrical charge, unequal distributionof electrical charges due to position of the electrons,as shown in atoms A and B above, may result in atemporary attractive force.

Hydrogen electronsin outer orbit

G Oxygen electronsin outer orbit

Figure 20. Two representations of the water molecule.In the Stuart model, above, hydrogen atoms -rebonded to a single oxygen atom. In the Bohr diagram,below, each hydrogen atom is seen to share its elec-tron with the oxygen and also to share one of theoxygen's electrons. This sharing of electrons bonds theatoms together. The bond is covalent.

this attractive chemical force between atoms thatcauses them' to combine to form molecules,which are clusters of two or more atoms (Figure20).

Apart from their greater strength, chemicalforces differ from van der Waals forces inexhibiting a saturation effect that allows a par-ticular atom to be able to combine with only alimited number of other atoms at any one time.A saturated molecule is one in which all thechemical attracting powers of the atoms ofwhich it is composed are used so that the mole-cule has no chemical attraction for another of

Electrons in outer 0 Electrons in outerorbit of carbon atom orbit of chlorine atom

Figure 21. In the carbon tetrachloride molecule all thechemical attracting powers of the carbon atom areused so that there are no more electrons to share. Themolecule is said to be saturated.

the same kind. In carbon tetrachloride (Figure21), four chlorine atoms combine with oneatom of carbon. In this molecule, usually noneof the atoms has any further chemical attractionfor any others.

Nuclear ReactionsJust as we have chemical reactions between

atoms and molecules, we may have rearrange-ments of nucleons between atomic nuclei. Thesereactions are spoken of as nuclear reactions.When we compare a nuclear reaction to achemical reaction, the energy released in a nu-clear reaction is much higher than that releasedin a chemical reaction. The reason for this isthat the binding energy of nucleons in a nucleusis about a million times greater than that ofatoms in molecules. If a uranium nucleus breaksup into two nearly equal parts, about 150 mil-lion electron volts (150 Mev) of energy isreleased. Contrast this with the 5 electron voltsof energy released when two hydrogen atomscombine with one of oxygen to form a moleculeof water in a standard chemical reaction.

FissionRemember that mass is also a form of energy.

Since the sum of the masses of the two frag-ments of the uranium nucleus is less than themass of the original nucleus, the difference isreleased as 150 Mev of energy. This breakup ofnuclei with the release of energy occurs in theprocess of nuclear fission. Nuclear fission usuallytakes place in nuclei of mass numbers greaterthan 56.

FusionIn those elements with nuclei of mass num-

bers of less than 56, energy is released whencertain nuclei are formed from lighter ones. If,for example, helium nuclei could be formedfrom protons and neutrons, the sum of themasses of these nucleons is greater than themass of the newly formed nucleus. The differ-ence in mass is released in the form of 28 Mevof energy per helium nucleus formed. Thisprocess resulting in nuclear energy release isknown as fusion. It is the process that is respon-sible for the heat of the sun and of the stars.The use of the E --= mc2 formula shows that themass of a proton is equivalent to 940 Mev ofenergy. The mass of a uranium nucleus is 238times greater.

Radioactivity and RadionuclidesCertain atomic nuclei are unstable and break

up spontaneously. These are said to be radio-active and include such naturally occurringelements as potassium, rubidium, samarium,lutetium, polonium, radium, radon, thorium,and uranium. When radioactive nuclei releasetheir energy, they do so in the form of alpha orbeta particles, or gamma radiations, or a com-bination of any of these three. For example,131-iodine emits both gamma rays and betaparticles, 32-phosphorus only beta particles, and210-polonium alpha particles. These radio-nuclides are written as 131I, 32P, and 21°Po. Theletters are the chemical symbols for the elementsconcerned. The numbers are the mass numbers(A), the sums of the protons and neutrons inthe nucleus. A species of atom that is identifiedby the number of neutrons and protons in itsnucleus, as well as the energy state of thenucleus, is a nuclide. There are over 320 natur-ally occurring nuclides and almost 900 that havebeen artificially produced. Nuclides are writtenwith a subscript atomic number (Z) precedingthe chemical symbol and a superscript mass

49

numbe. (A) preceding it. For example, calciumhas an atomic number of 20 and a mass numberof 40. This nuclide would be written gCa.

ions and IonizationMolecules can be broken up into electrically

charged fragments. These fragments of mole-cules or more complex units are called ions. Anion can be an atom or a group of atoms thatcarries either a positive or a negative electricalcharge. Positive ions are formed when electronsare lost. Negative ions are formed when elec-trons are gained.

When a substance contains ions it is ionized.Radiation from radionuclides can break upmolecules into ions. This radiation is calledionizing radiation. The cloud chamber operateson this principle. The charged particles passingthrough the gas of the cloud chamber leave atrail of positive and negative ions around whichvapor condenses. Both ionized gases and liquidsare conductors.

Half-LifeRadionuclides emit ionizing radiation. Any

emission causes the nucleus of that particularnuclide to change to that of another. Thus, eachatom can emit its characteristic radiation onlyonce. The entire sample of radioactive materialwill ultimately cease its radioactivity in time. Itis possible to predict mathematically the de-crease in the radioactivity of any given radio-active sample. Scientists commonly use the timefor the rate to fall to one-half the initial rate.This value is the half-life of the radionuclide.The decrease in radioactivity over a period oftime is spoken of as the decay rate. The half-lifeof radionuclides varies greatly. For 45-calcium,it is 165 days; for 14-carbon, 5730 years; for210-polonium, 138 days; and for 211-polon-ium, it is 0.52 seconds. Table 2 shows half-livesfor some typical radionuclides. Figure 22 showsa generalized decay graph plotting decreasedradioactivity against time expressed as half-lives.After five half-lives, how much radioactivityremains ;11 the individual sample? What wouldbe some of the advantages of a radionuclidewith a long half-life? With a short half-life?Which type would you use for a long-termexperiment?

Particles and RaysAlpha and beta emissions are not a part of

the electromagnetic spectrum, but are particles.

50

TABLE 2HALF-LIVES OF REPRESENTATIVE RADIO-NUCLIDES AND THEIR MAJOR RADIATIONS

Radionuclide Half-Life Radiation

131-Barium 12 days gamma

199-Bismuth 24 minutes alpha

14-Carbon 5730 years beta

137-Cesium 30 years beta, gamma

140-Cesium 66 seconds beta

60-Cobalt 5.25 years beta, gamma

60-Copper 23 minutes beta,gamma

3-Hydrogen 12 years beta

131 - Iodine 8.04 days beta, gamma

210-Lead 21 years beta, gamma

27-Magnesium 9.6 minutes beta, gamma

16-Nitrogen 7.3 seconds beta, gamma

32-Phosphorus 14.3 days beta

206-Polonium 9 days alpha. gamma

209-Polonium 103 years alpha

38-Potassium 7.5 minutes beta, gamma

222-Rodium 38 seconds alpha

225-Radium 14.8 days beta

226-Radium 1600 years alpha, gamma

22-Sodium 2.6 years beta, gamma

90-Strontium 28 years beta

35-Sulphur 88 days beta

230-Uranium 20.8 days alpha

234-Uranium 2.48x105 years alpha, gamma

238-Uranium 4.498x109 years alpha, gamma

65-Zinc 244 days beta, gamma

Table 2. Can you determine a relationship betweenelements and half-life or between type of radiation andeither half-life or element?

Alpha particles are helium nuclei. They areemitted from radioactive material usually atenergies of between 3 to 8 million electronvolts. They are positively charged and relativelylarge. They travel at a speed less than 1/10 thevelocity of light.

Beta particles are electrons 1/1837 the massof a hydrogen atom with energies ranging froma few thousand to a few million electron volts

traveling at speeds of from 25 to 99 percent ofthe velocity of light.

Gamma rays are short wavelengths of elec-1.000.800.60

0.40

0.20

0.10.08.06

.04

.02

.01

.008

.006

.004

.002

.0010 2 4 6 8

NUMBER OF HALF-LIVES

10

Figure 22. Decrease in radioactivity plotted againsthalf-life. The time for half-life varies with differentelements from fractions of seconds to thousands ofyears. This generalized graph applies to all radioactivematerials. After ten half-lives, the radioactivity isreduced to a thousandth of its original intensity. Willit ever reach zero?

tromagnetic radiation emitted as small packets(photons). They travel at th.t speed of light.Some characteristics of these radiations aresummarized in Table 3. The symbols for alphaand beta particles and gamma radiation are theGreek symbols: Alpha a, Beta /3, and Gamma y.You will see the biological significance of thesethree types of radiation as you perform theInquiries in this Block.

Radiation UnitsRadiation is measured in curies (c). A

curie is a relatively large unit of activity of aradionuclide which is equal to 3.7 x 1010 radio-active disintegrations per second. A microcurieis a more convenient unit. It is one millionth ofa curie and is equal to 3.7 x 104 radioactivedisintegrations per second.

In biological work, there are three radiationunits that are commonly used. They are asfollows:

1. The roentgen (r) is a unit of exposure tox or gamma radiation based on the ioniza-tions that these radiations produce in air. Anexposure of 1 roentgen results in 2.584 x 10-4coulomb (a unit of electric charge) perkilogram of air. This unit is not applied toparticulate radiation, such as that of alpha,beta, or neutron particles. Exposure rate isusually expressed as roentgens per unit oftime, roentgens per hour, per minute, persecond, etc.2. The rad is a way of expressing the energyabsorbed per gram of materials from any ion-

OF RADIATIONTABLE 3PROPERTIES

RADIATION CHARGE APPROXIMATE

ENERGY RANGE

APPROXIMATE RANGE PRIMARY SOURCE

OF RADIATIONAIR WATER

11)LU

U1.-

<

Alpha

Beta

Neutrons

+2

-1

0

3-9 Mev

0-3 Mev

0-10 Mev

2-8 cm

0-10 m

0-100 m

20 -40µ

0-1 mm

0-1 m

Heavy nuclei

Nuclei withhigh n/p ratio

Nuclearbombardment

U1--

li"0123u,...1ILA

X-rays

Gamma ray's

None

None

ev-100 Key

10 Key-10 Mev

mm-10 m

cm-100 m

is -cm

mm-10 cm

Orbital electrontransitions

Nucleartransitions

Table 3. X rays and gamma rays are part of the electromagnetic spectrum. Alpha and beta particles are producedby radioactive elements. Neutrons can be released by use of such devices as high-energy accelerators.

51

izing radiation. One rad is 100 ergs (a unit ofwork) absorbed per gram of any substance.The rad is a relatively recent term. A roent-gen is a much more common term, particu-larly in the older literature. In most cases, thenumber of rads is approximately equal to thenumber of roentgens. The rad is used forparticulate as well as electromagnetic radia-tion.3. The rem is a unit of dose equivalent. Thisis numerically equal to the dose in rads multi-plied by the proper modifying factors (rbe,q.f., or d.f.see below).There are three factors to be considered in

radiation measurement. They are as follows:1. Relative Biological Effectiveness (rbe).This term is limited to use in radiobiology toexpress relative effectiveness of various types

52

of radiations. The rbe values are experi-mentally determined and pertain only to thesystem under study.2. The quality factor (q.f.) relates the effectof radiations to that of gamma rays from60-cobalt. It makes allowances for the factthat the same dose in rads from differenttypes of radiation does not necessarily pro-duce the same biological effect. It is used pri-marily for the purposes of radiation protec-tion. If gamma rays have a quality factor of1, alpha particles have a quality factor of 10.3. The dose distribution factor (d.f.) is usedto calculate the rem when considerina intern-ally deposited radionuclides. It takes into ac-count the nonuniform distribution of a givenradionuclide in various organs in the body.

APPENDIX III HPROOWDUARCEEDRADIOACTIVE MATERIALS

When particles are emitted from an atomicnucleus, the atom is changed. Through thesechanges it becomes an atom of a different ele-ment. The alchemists of ancient times soughtvainly to change (transmute) lead into gold.Now we can perform such transmutations, butthe cost of so doing is far greater than the valueof the elements produced. As an example oftransmutation let's first consider an alpha parti-cle. It consists of two protons and two neutrons.It has an atomic number of 2 and a mass num-ber of 4. Any nucleus which emits an alphaparticle changes into a nucleus with a mass num-ber less by four and an atomic number less bytwo than that of the original element. Thus,polonium with a mass number of 210 and anatomic number of 84 consists of 126 neutronsand 34 protons. The element resulting after theemission of an alpha particle from polonium islead with a mass number of 206 and an atomicnumber of 82 (Figure 23).

Another example is provided in a study byRutherford in 1919 concerning alpha particlesand nitrogen. This work showed that if an alphaparticle hit a nitrogen nucleus, its large kineticenergy (K.E.) of 7.7 Mev (million electronvolts) overcame the repulsion of the nuclearcharges. A highly unstable compound nucleusof a rare nuclide of flourine was then formed.This nuclide disintegrates immediately to releasea high-K.E. proton and a stable nucleus of arare nuclide of oxygen (Figure 24). This wasthe first example of an artificially producedtransmutation.

Alphaparticle

(Pie) Nitrogennucleus

(IN)

Protuns=84Isktutrons=126

ow*

Alphaparticle

Protons=2Neutrons=2

Protons=82Neutrons=124

Protons=84Neutrons=126

Figure 23. Polonium can be changed to lead byemission of an alpha particle, which changes both themass number (A) and the atomic number (Z) .

Nuclear reactions can be written as formulaethe same way as can nonnuclear chemicalreactions. The summary of the helium-nitrogenreaction above is as follows:

:He + N 1H + 1Z0

This equation balances. The sum of theatomic numbers (nuclear charges or position inthe periodic table) is the same on both sides ofthe reaction ( +9). The sums of the mass num-bers (18) are also the same. Since the interme-diate flourine nucleus disappears immediately, itis usually omitted from the written reaction.

Similarly, if we split a single electron fromthe nucleus of a lead nuclide with an atomicnumber of 82 and a mass number of 210, a Us-

High-energyproton

Unstablefluorinenuclide

(I8F)9

OH)

Stable oxygennucleus

(70)

Figure 24. The first laboratory-produced transmutation occurred in 1919, when a nitrogen nucleus was used as atarget for an alpha particle, The nitrogen nucleus changes into a rare nuclide of flourine, which rapidly breaksdown to form oxygen with the release of a high-energy proton.

53

muth nuclide is formed with a mass number of210 and an atomic number of 83 (Figure 25).The emission of a beta particlean electronby a nucleus increases the atomic number byone. It does not change the mass number sincethe mass of an electron is small. The reason theatomic number increases by one is that it is

assumed a neutron in the nucleus splits into aproton and an electron. The proton remains inthe nucleus and the electron is ejected from it.Thus, the mass remains unchanged but the num-ber of neutrons is decreased by one and thenumber of protons is increased by one.

RadionuclidesRadionuclides can be manufactured when a

neutron, proton, alpha particle, deuteron, orgamma ray from outside an atom reaches itsnucleus and interacts with its nucleons. When31-phosphorus captures a neutron in its nu-cleus it becomes the radionuclide 32-phosphorus

Betaparticle

Protons=82 Protons=83Neutrons=128 Neutrons= 127

Figure 25. Transmutations can occur not only byemission of alpha particles, but also by the emission ofa beta particlean electron. The loss of the electronincreases the atomic number by one and results in theproduction of bismuth from lead in this example.

Beta particle

Neutron

32-Phosphorus

Figure 26. Radionuclides can be manufactured whenan atom is bombarded by neutrons. In this case, stablephosphorus absorbing a neutron becomes radioactive32-phosphorus. This type of radioactivity can be pro-duced in a nuclear reactor.

Heliumnucleus

(zH.)

Boronnucleus

(1;8)

Boron nucleus (5 protons, 6 neutrons) about tocapture helium nucleus (2 proton's, 2 neutrons).

Carbonnucleus

(1:c)

+Hydrogennucleus

OH)

Boron nucleus bombarded by helium nucleusbecomes radioactive carbon (6 protons,8 neutrons) plus one hydrogen nucleus.

Figure 27. Bombarding nuclei with alpha particles(helium nuclei) also causes transmutation. In this case,boron is changed to radioactive carbon with an emis-sion of a proton (hydrogen nucleus).

54

and emits beta particles. The original stabletarget nucleus has been transformed into aradioactive one (Figure 26). This type of nu-clear reaction also can be shown when the nu-cleus of the element boron is bombarded by analpha particle (helium nucleus with two pro-tons and two neutrons). It is transformed intoa 14-carbon nucleus and releases a proton(hydrogen nucleus) (Figure 27).

AcceleratorsCharged particles such as alpha particles or

protons must have a very high energy of motionin order to reach the interior of a target nucleus.They must overcome being repulsed by theelectrical charge of the nucleus. Neutrons arenormally used because, being uncharged, theydo not show this repulsion effect. The lack ofcharge is of value in penetrating the nucleus, butneutrons cannot be accelerated by electricalfields because of this very lack of charge. Theyare, however, released with considerable energyin certain nuclear reactions. In a mixture of

plutonium and beryllium, the plutonium emitsalpha particles. These react with the beryllium,which then releases neutrons. These can be usedas target material to hit other nuclei. The beryl-lium itself is transformed into 12-carbon (Fig-ure 28).

Alpha particle

let

12-Carbon

Plutonium Beryllium Targetmaterial

Figure 28. Neutrons are generated from a mixture ofplutonium and beryllium. The alpha particle from theplutonium reacts with the beryllium nucleus, whichchanges into 12-carbon and emits a neutron th,,._ canthen be used to hit the nucleus of a given targetelement.

It is possible to use alpha particles fromnatural sources to bombard nuclei. A far greatervariety of nuclear reactions, however, can bebrought about with beams of charged particlesfrom such accelerators as cyclotrons, betatrons,synchrotrons, linear accelerators, and similardevices. In these machines, ions are speeded upand directed by a combination of electric andmagnetic fields under vacuum conditions. Asmany as 31 billion electron volts (Bev) havebeen given to protons in these devices. Nuclearreactions in accelerators are accompanied byrelease of energy. But this is not the way to

mlawa'Neutron

generate energy commercially, because, like thesituation of turning lead to gold, more energyhas to be used up to produce the reaction thancan be released. Therefore, these instrumentsare basically for msearch.

Chain ReactionsFission with uranium is a chain reaction.

Naturally occurring uranium consists of twonuclides. One is of the mass 238 (238U) and isthe far more abundant. The other (2"U) ispresent in only 1/140 part of 238U. A slowneutron is readily captured on impact with the2"51.J. This capture creates a new nucleus of thesame atomic number but with its mass numberincreased by one. Thus, '"U is formed. "9..1is an unstable element and undergoes fissionwhich releases a few more neutrons, usually twoor three, These neutrons then are captured byother nuclei of 23JU, which split and releasemore neutrons. Thus a chain reaction is set up(Figure 29). A large mass of uranium can befissioned with great generation of energy, about20,000 kilowatt hours per gram of 2"51.J. Onecondition necessary for the chain reaction :s asufficiently large mass of uranium. If only asmall volume of uranium is available, most ofthe neutrons escape from the small mass with-out producing fission. In a larger mass, however,most of the neutrons,are captured by the nucleiand relatively few escape. This is a critical massone of such size that fission occurs within it.With natural uranium alone, it would not bepossible to set up a chain reaction because the2"81_3, which is most abundant, absorbs neutronswithout undergoing fission. By artificial nuclide

235U capturesneutron, becomesunstable 236Uand splits

235U Nucleus aboutto capture neutron

Two or threeneutrons arereleased andcontinue fissionprocess

Figure 29. Fission occurs when a heavy nucleus is into two approximately equal parts tolighter elements. This splitting is accompanied by the rele.p,r of a large amount of energy andneat -ons that continue the chain reaction.

55 59

lei ofe re

separation in our laboratories, relatively pure235U can be produced and chain reactions maybe initiated within a mass of critical size orlarger. This can be used either in an atomicbomb or in reactors to produce energy thatcould be used for constructive purposes.

Nuclear ReactorsA pile of uranium of critical mass or larger

that includes devices whereby the energy ofmotion of neutrons is reduced is called a nuclearreactor. The energy absorber in this case is

spoken of as a moderator. The moderator isusually graphite or heavy water (water in whichhydrogen is replaced by the nuclide deuterium).If the uranium is sufficiently rich in 235U,ordinary water can be used as a moderator. Asthe neutrons become slowed by the moderators,they are not absorbed appreciably by 238U.Thus, the moderator slows down neutrons. Butenough still escapes absorption by 238U tomaintain the chain reaction. In a nuclear re-actor, the rate of generation of energy is con-trolled by insertion of adjustable rods of mate-rials such as boron or cadmium, which arestrong neutron absorbers.

The nuclear reactor generates power throughits heat emission. It also creates radionuclidesfor use in various ways, because nuclear frag-ments resulting from fission are unstable. It ispossible to create radioactive nuclides of dif-ferent elements by inserting the natural elementinto the reactor so that it captures neutrons andbecomes radioactive. It is these radionuclidesthat we will be using in this Laboratory Block.

Use of RadionuclidesTracersRadionuclides have a wide range of applica-

tion in industry, agriculture, medicine, andresearch. It is important to remember that theradionuclide of an element behaves chemicallythe same way as does the normal element. How-ever, because it is radioactive, it can be followedthrough biological processes even when presentin very small quantities. Because it can be fol-lowed in this fashion, a radionuclide can bereferred to as a tracer. In addition to use ofradionuclides as tracers, they can be used toinduce mutations, diagnose disease, treat certainkinds of diseases such as cancer, preserve food,and eliminate insect pests. What other uses ofradionuclides occur to you?

56

APPENDIX IV HG:GwEPRIYUOLLUERCWC

EORLIANT TE EAR?

Inquiries III and IV provide data on tworadiation detection devicesthe cloud chamberand photographic film. Various other devices,including proportional and scintillation count-ers, are used for the immediate detection ofradiation and its measurement. One of the mostcommon devices is the Geiger-Muller (G-M)tube and its accompanying measuring instru-ment (scaler or rate meter). Together, the tubeand the measuring instrument are called acounter.

The structure and operation of the Geiger-Muller tube are essentially simple. An under-standing of its operation is necessary in order touse it to make meaningful determinations ofradioactivity. The tube itself is basically a pairof electrodes separated by an easily ionizablegas. The tube case can act as the negative elec-trode; the center wire is the positive one. A highelectrical potential is maintained between thetwo electrodes. When radiation ionizes the gas,the ions thus produced travel to the electrodes.The motion of these ions produces an electricalcurrent detected in the tube and recorded byeither a scaler or a rate meter.

A simplified diagram of a Geiger-Mullertube is shown in Figure 30. This is a typicalend-window type of tube closed by a thin win-dow at one end. There are also side-windowtubes not illustrated. Both are filled with eitherhelium or argon.gas. These are called countinggases. In operation, electrons travel to the posi-tive anode. The positively charged ions traveltoward the negative cathode. Both acquireenergy. These particles produce still other ionswhen they collide with the gas molecules. This

Pulsecounter

0 Positively charged ione Negatively charged electron

Cathode ) Gas

Highvoltage Anode+) Window

Figure 30. A diagram of an end-window Geiger-Muller tube. The gas consists of a counting gas, suchas helium or argon, and a quenching gas, such as bu-tane or chlorine. The window is very thin and shouldnever be touched. To prevent breakage, it is protectedby a metal shield.

is almost like an ion chain reaction. If therewere no way of controlling it, one ionizationwould cause the tube to d;scharge continuously.

One way to prevent this from happening is byplacing a quenching gas in the tube. Thequenching gas absorbs some of the energy ofthe electrons and positive ions. If the quenchinggas is alcohol or butane, the tube is said tobe organically quenched. These molecules ofquenching gas are decomposed so that the tubehas a useful life of about 108 counts (about 20months of counting at the rate of 1000 countsper minute). Halogen-quenched tubes utilizebromine, chlorine, or some other similar mole-cules as quenching agents. They have a muchlonger life because atoms of these quenchinggases recombine.

Tubes that use these gases are said to beinternally quenched. Another way tc )reventcontinuous discharge is to remove the high volt-age from the tube momentarily. This can bedone electronically. The tube is then said to beexternally quenched.

Most Geiger-Muller tubes have certain char-acteristics in common that affect their countingability. There are four major factors 'that influ-ence the efficiency of the tube. They are:

1. Resolving time. After ionization has be-gun, there are short periods of time when thetube will not register additional ions. Theseperiods constitute the resolving time. Duringresolving time, two or more ionizing particleswill be counted as one. Because of this, thenumber of counts recorded will always be lessthan the actual number of particles or rayspassing into the tube. The maximum numberof radiation events that can be registered by asingle tube amounts to about 50,000 countsper minute.2. Absorption factor. All particles or rays

emitted from a radioactive source are notdetected. Some of the radiation is absorbedby the sample itself, some by the air betweenthe sample and the tube, and some by thewindow of the Geiger-Muller tube. For thework in this Laboratory Block, the absorptionfactor is considered insignificant.3. Counting geometry. This term refers tothe physical relationship (position, distance,

57 61

holder, etc.) between the radiation sourceand the detecting equipment (Geiger-Mullertube). It commonly expresses the percent ofradiation reaching the detection equipmentfrom the radioactive source. The closer theradioactive source to the tube the greater thepercent of emitted radiation detected. Forcomparable results the same counting geom-etry should be maintained when making anumber of readings.4. Scattering. This is the process by which aparticle's path is changed. Scattering may bedue to the mass of the sample itself, the airbetween the radiation source and the tube, orby the wall of the tube shield. When particlesare directed back toward the counter, wespeak of backscattering. The degree of back-scattering varies with the energy of the radia-tion, atomic number, and the thickness of thesupport materials. It may increase the countby as much as 50 percent.

In addition to this background information,your teacher will demonstrate the use of theGeiger-Muller counter to you. Follow the writ-ten directions as the counter is being demon-strated. You may be asked to plot a curve of theoperating characteristics of the counter. If so,prepare a graph with counts per minute on thevertical axis from 0 to 6,000 in 1000's. Thehorizontal axis should be for voltage. Begin with800 volts and increase by 100's to 1300 volts.You may have to change these figures dependingen the operating characteristics of your counter.

CautionsBefore beginning to use a Geiger-Muller

counter, the following cautirns should be ob-served:

1. The high voltage carried by the Geiger-Muller tube is dangerous. The tube and itsconnections should be inspected prior to usefor possible defects before high voltage is ap-plied. Instruments should z,..!ways be handledcarefully.2. The tube window is extremely thin andvery easily broken. Under no conditionsshould it be touched. Damage to the windowrenders the tube useless. To prevent suchdamage, three operational procedures shouldbe followed. Keep the shield over the windowwhen the tube is handled. Remove it onlywhen directed to do so. Mount the tube in a

-'ical position and bring the specimens to

it rather than using the tube as a movingprobe.3. Operating at too high a voltage is thecause of most failures of detection equipment.Dependable operation requires a proper volt-age setting. The tube can be permanentlydamaged by an increase in voltage. Neverexceed the voltage setting recommended foryour instrument.

Just as there are various kinds of microscopesin different biology classrooms, so there may bedifferent scalers or rate meters to which aGeiger-Muller tube can be attached. However,the principles are similar for all and the fol-lowing generalized description can be modifiedas necessary:

1. Always read the operating manual forthe particular instrument you are to use.Learn the location and use of each of thecontrols and understand the scale divisions ofthe meter. Should any questions arise at anypoint in the operation of the Geiger-Mullercounter, always check with your instructorbefore proceeding.2. Make sure the chassis of the scaler or

rate meter is grounded if the line cord doesnot have a grounding connection.3. Before plugging the meter cord into a

110-volt, 60-cycle outlet, check to see that themaster power switch is off. Check to be surethat the high-voltage switch is off. Turn it asfar counterclockwise as it will go, which setsthe voltage as low as possible.4. Make sure that the Geiger-Muller tube

is connected to the proper jack on the scaler.The anode connects to the positive terminalof the high-voltage supply. The cathode con-nects to the negative terminal, which is nor-mally the chassis ground.

5. Turn the master switch on and allow theinstrument to warm up. In vacuum tubeinstruments, this may take from 20 to 30minutes. For normal classroom use, the mas-ter switch is not turned off unless the. instru-ment is to be used less than twice a week.Most instruments have a pilot light to indicatewhen the power is on.

6. After the instrument has warmed up,turn the high-voltage switch on. Do not in-crease the voltage beyond the minimal settingfor 20 to 30 minutes. This not only protectsthe electronics, but stabilizes the high voltage

58

Geiger tube

Card mountsupports

Geiger tube

Figure 31. To preserve counting geometry, a special holder for the Geiger-Muller tube and mounted radio-active samples may be used.

power supply so that future readings will beconsistent.7. Place the Geiger-Muller tube with the

end window down in a special holder (Figure31) or secure it by means of a clamp to a ringstand (Figure 32). Place a card mount cc-t-taining the sample on a counting shelf or ona piece of glass beneath the Geiger-Mullertube. Uncover the window, turn the countswitch to the count postion, and slowly in-crease the high voltage until the scaler beginsto indicate a count.

Plot the point on your graph where thescaler begins to indicate a count. This is thestarting potential for the G-M tube.

8. Increase the voltage in 50-volt steps andtake a count at each step. As radiation is asequence of random events, the longer thecounting period, the better. Short periodstend to be unreliable. Count for at least 5-minute periods at each voltage setting. Goodtechnique calls for taking the average fromthree or more such counting periods. Ploteach of these points on,your graph.9. Note the point where an increase in

voltage incre%ses the count very little. This isthe beginning of the plateau. The point iscalled the threshold potential of the tube.What is it for the tube you are using? Indetermining the extent of the plateau, notewhen a further small increase in voltage be-gins to cause a rapid increase in count rate.At this point turn down the voltage at once!Do not increase it when the graph slopebegins to rise from the plateau!

10. In organically quenched tubes, the pla-teau region usually has a slope of less than3 percent. For halogen-quenched tubes, theslope is higher. Determining the plateau isimportant, for it is in this region that theproper operating voltage is selected. The tubeshould be operated with voltage relativelyclose to the threshold. Good practice calls foroperating voltages within the lower 25 per-

-To meter

G-M tube

Radiooctive sampleon glass plate

Iron ring

Ring stand

Figure 32. If special holders are not available, theGeiger-Muller tube can be held in a clamp mountedon a ring stand. The sample can be placed on a glassplate and moved toward the tube by adjusting theheight of the iron ring. Possible tube damage is mini-mized by having the tube held steady and by movingthe sample rather than by moving the tube toward thesample.

593

cent of the plateau to conserve the life thetube. Color this section of your graph. Whatvoltages does it span?11. After the high voltage is properly ad-justed, the count switch should be turned tostop and the reset switch depressed. Thescaler is now ready for operation. All that isnecessary for operation now is to turn thehigh voltage switc'i to on and the countswitch to count.12. Your teacher may wish to demonstratethe consequences of raised voltage. If thevoltage is increased above that for the plateauregion, the tube begins to discharge continu-ously and may be seriously damaged. When-ever count-rate increases rapidly above thatof the plateau region, the voltage should beturned down immediately to prevent tubedamage. It is necessary to estimate count-rateat high voltages rather than to damage thetube by attempting to obtain measurementsat such levels. If by accident the voltage isset too high, the scaler jams. The only actionto take is to turn the voltage down instantly!Switching the count switch to stop does notcut off the high voltage to the tube!

13. Having plotted the counts per minute ongraph paper, compare your graph with onethat the teacher will place on the board. Youhave now determined the threshold and eoperating plateau for the Geiger-Mullercounter you will use. Remember that voltagefor subsequent experiments thust be set inthe lower 25 percent of plateau operatingrange. Tube characteristics change with ageand use. With long usage, it is necessaryperiodically to redetermine the characteristiccurve for the tube being used. However, forthe short time the Geiger-Muller counter willbe used in this Block, this step will not benecessary.

Analysis of DataWhat are the first steps in preparing to use a

Geiger-Muller counter? What precautions mustbe observed in its operation? At what voltageshould it be operated? What is the greatest singlesource of danger to the counter?

To what uses might the counter be put? Con-trast the counter with the cloud chamber andthe film badge as a detection and measurementinstrument. Of what value is a counter to abiologist?

60 64

I

p

SELECTED REFERENCES ON RADIATION

The Atomic Energy Commission (AEC) hasproduced a large number of printed materials,some of which are available free upon requestin small numbers. The "Understanding theAtom" series contains over 40 titles. A singlecopy of any one booklet in this series, or of nomore than three different booklets, will be sentfree upon request. Teachers and librarians canobtain complete sets if requests are on schoolletterhead and their proposed use indicated.

Requests relative to nuclear science can also

be directed to the AEC if the exact topic ofinterest is stated and the reason for the requestis presented.

The address for both requests and publica-tions is:

United States Atomic Energy CommissionDivision of Technical Information

ExtensionEducational Materials SectionP. 0. Box 62Oak Ridge, Tennessee 37830

BiologicalArnold, J. R. and E. A. Marte11.1959 (Sept.).

The circulation of radioactive isotopes. Scien-tific American 201(3) :84.

Asimov, I. and T. Dobzhansky. 1966 (Sept.).Genetic effects of radiation. 2 ± 49 pp.USAEC, Oak Ridge, Tennessee.

Atomic Energy Commission. 1967. Nuclearterms: a brief glossary. 80 pp. USAEC, OakRidge, Tennessee.

Beadle, G. W. 1959 (Sept.). Ionizing radiationand the citizen. Scientific American 201(3):219.

Casarett, A. P. .1968. Radiation biology. xiv368. Prentice-Hall, Englewood Cliffs, N.J.

Crow, J. F. 1959 (Sept.). Ionizing radiationand evolution. Scientific American 201 (3 ) :138.

Davis, G. E. 1967. Radiation and life. IowaState University Press, Ames. (Ninety percentof this volume deals with physical factors.One chapter concerns biological effects).

Ebert, M. and A. Howard. Current topics inradiation research. Vol. 1, 272 pp. 1965, Vol.2, 398 pp. 1966, North Holland Pub. Co.,Amsterdam. Vol. 3, 226 pp. 1967. JohnWiley and Sons, New York.

Henry, H. F. 1969. Fundamentals of radiationprotection. xviii + 485 pp. Wiley-Intersci-ence, New York.

Hollaender, A. and G. E. Stapleton. 1959(Sept.). Ionizing radiation and the living cell.Scientific American 201(3) :94.

Hurlburt, E. M. 1966(Sept.). Radioisotope ex-periments in high school biology, an anno-tated selected bibliography. iii + 20 pp.USAEC, Oak Ridge, Tennessee.

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Kimeldorf, D. J. and E. L. Hunt. 1965. Ioniz-ing radiation; neural function and behavior.331 pp. Academic Press, New York.

Kisieleski, W. E. and R. Baserga.1966(April).Radioisotopes and life processes. 2 + 50 pp.USAEC, Oak Ridge, Tennessee.

Leblond, C. and K. B. Warren. 1965. The useof radioautography in investigating proteinsynthesis. 348 pp. Academic Press, NewYork.

Loutit, J. F. 1959 (Sept.). Ionizing radiationand the whole animal. Scientific American201(3):117.

Miner, H. A., R. W. Shaaleton, and F. G.Watson. 1959 (Jan.). Teaching with radio-isotopes. iv + 60 pp. U.S. GovernmentPrinting Office, Washington, D.C.

Morgan, K. Z. and J. E. Turner (Eds.). 1967.Principles of radiation protection. 622 pp.John Wiley and Sons, New York.

National Association of Biology Teachers.1965. Radioisotopes in biological researchand teaching. American Biology Teacher 27(6) :402 -503.

Phelan, E. W. 1966 (Sept.).Radioisotopes inmedicine. 2 + 50 pp. USAEC, Oak Ridge,Tennessee.

Pizzarello, D. J. and R. L. Witcofski. 1967.Basic radiation biology. x + 301 pp. Lea andFebiger, Philadelphia.

Pollard, E. C. 1969. The biological action ofionizing radiation. American Scientist 57(2):206-236.

Radiation botany. An international researchjournal published by Pergamon Press, NewYork.

Shaw, E.I.1965 (Apr.) .Laboratory experimentsin radiation biology. vi 81 pp. USAEC,Oak Ridge, Tennessee.

Spiers, F. W. 1968. Radioisotopes in the humanbody. xiv 346 pp. Academic Press, NewYork.

Wallace, A. M. 1969. Determining rates ofphotosynthesis in elodea by C1 method.Turtox News 47 (5) :158.

Wang, C. H. and D. L. Willis. 1965. Radio-tracer methodology in' biological science.Prentice-Hall, Englewood Cliffs, N.J.

Woodwell, G. M. 1969. Radioactivity and fall-out: the model pollution. BioScience 19(10):884-887.

PhysicalAlfven, Hannes. 1967 (April). Antimatter and

cosmology. A new look at the particulatenature of matter. Scientific American 216(4):106.

Atomic Energy Commission. AEC rules andregulations. U.S. Government Printing Office,Washington, D.C.

Badash, L. 1966 (Aug.). How the "newer al-chemy" was received. (A historical look attransmutation by radioactive decay.) Scien-tific American 215 (2) :89.

Bullard, E. C. 1966 (July). The detection ofunderground explosions. Scientific Ameri-can 215(1):19.

Burbidge, G. 1966 (Aug.). The origin of cos-mic rays. Scientific American 215 (2) :32.

Chase, G. D. and J. L. Rabinowitz. 1967. 3rded. Principles of radioisotope methodology.Burgess, Minneapolis.

Cranberg, L. 1964 (March). Fast-neutron spec-troscopy. Scientific American 210 (3) :79.

Ford, K. W. 1963. The world of elementaryparticles. xi 246 pp. Blaisdell, New York.

Fowler, T. K. and R. F. Post. 1966 (Dec.).Progress toward fusion power. ScientificAmerican 215(6) :21.

Glasstone, S. 1967. Sourcebook on atomicenergy. vii 883 pp. illus. Van Nostrand,Princeton, New Jersey.

Heckman, H. H. and P. W. Starring. 1963.Nuclear physics and the fundamental parti-cles. vi + 410 pp. Holt, Rinehart and Wins-ton, New York.

Hermias, M. and M. Joecile. 1963. Radioactiv-ity: fundamentals and experimenti. xii209 pp. Holt, Rinehart and Winston, NewYork.

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Korn, J. 1961. Atoms; the core of all matter.54 pp. Golden Press, New York.

Laurence, W. 1959. Men and atoms: the dis-covery, the uses, and the future of atomicenergy. 302 pp. Simon and Schuster, NewYork.

Leachman, R. B. 1965 (Aug.). Nuclear fission.Scientific American 213 (2) :49.

Lederer, C. M., J. M. Hollander, and I. Perl-man. 1967. Table of isotopes. xii + 594 pp.John Wiley and Sons, New York.

Leeds, R. D. 1967. Introducing the atom. x143 pp. illus. Harper and Row, New York.

Levinger, J. S. 1967. Secrets of the nucleus.Scholastic Book Services, New York.

Mawson, C. A. 1969. The story of radioactivity.64 pp. illus. Prentice-Hall, Englewood Cliffs,N.J.

Platzman, R. L. 1959 (Sept.). What is ionizingradiation? Scientific American 201(3) :74.

Reines, F. and J. P. F. Sellschop. 1966 (Feb.).Neutrinos from the atmosphere and beyond.Scientific American 214 (2) :40.

Romer, A. 1960. The restless atom. 198 pp.Doubleday and Company, New York.

Wahl, W. and H. Kramer. 1967 (April).Neutron-activation analysis. (An analyticaltool to measure trace elements to less than abillionth of a gram.) Scientific American 216(4):68.

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