STUDY OF FISSION FRAGMENT TRACKS IN SOLIDS AND ITS APPLICATIONS IN

URANIUM DETERMINATION

DISSERTATION SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS

FOR THE AWARD OF THE DEGREE OF

M^^ttv of ^{)ilos(op{)p IN

APPLIED PHYSICS

BY

PADAM SINGH

Under the Supervision of

Prof. D. S. SRIVASTAVA

DEPARTMENT OF APPLIED PHYSICS Z H. COLLEGE OF ENGINEERING & TECNOLOGY

ALIGARH M U S L I M UNIVERSITY ALIGARH ( INDIA)

1993

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Univ. Telex: 564 230 AMU IN

DEPARTMENT OF APPLIED PHYSICS Z. H COLLEGE OF ENGG. & TECH. ALIGARH MUSLIM UNIVERSITY ALIGARH, U P. 202002, INDIA

Dated 2 7 . 1 1 . 9 3

5:ERTIFICATE

Certified that the results reported in this

dissertation are from the original work carried out by

Mr. Padam Singh under ^y supervision

Prof. D.S. SRIVASTAVA

Supervi sor

ACKNOWLEDGEMENTS

I would like to express my sincere gratitude to my

supervisor Dr. D.S. Srivastava, Professor and Chairman,

Deptt. of Applied Physics, Z.H. College of Engg & Technology,

A.M.U., Aligarh for his continued guidance and valuable

discussions throughout the tennure of my research work.

I am also thankful to my friends Mr. N.P. Singh Rana

and Mr. Ameer Azam for their co-operations received from

time to time during this work.

Finally, I want to express my thanks to the Department

of Atomic Energy (DAE), Govt, of India for assisting me

financially by granting the Junior Research Fellowship under

the running DAE Project No. 27/1/91-G sanctioned to Prof.

D.S. Srivastava.

(PADAM SINGH)

PREFACE

This dissertation is being submitted in partial

fulfilment of the degree of Master of Philosophy (M. Phil)

which is an essential pre-Ph.D., requirement in the Aligarh

Muslim University.

The preliminary experimental work in this dissertation

has been carried out under the research scheme No.27/1/91-G

entitled, 'Application of ^ U (n,f) nuclear reaction for the

analysis of trace quantities of Uranium present in various

solid and liquid materials' granted to Dr. D.S. Srivastava

by the Department of Atomic Energy, Govt, of India.

Uranium contents in drinking and river water samples

collected from Jhansi and Allahabad cities have been

estimated by using Melinex-0 plastic track detector. Fission

track etch characteristics of some plastics have also been

studied as a part of the M.Phil topic entitled, 'Study of

fission ,f-ragment tracks sol ids and its application in uranium

determinations'. The knowledge of these characteristics is

essential for the estimation of uranium in liquids using the

plastic track detectors.

This dissertation is divided into four chapters as

follows:

The first chapter describes historical development of

Solid State Nuclear Track Detection Techniques. Realistic

track formation mechanism, relevant models, for track

registration, the revelation of fission tracks by selective

chemical etching, visualization and evaluation of tracks have

also been discussed in this chapter. At the end of this

chapter the aims of present work and the extent of

achievement have been mentioned. Finally the chapter ends

with the list of references consulted by author.

The second chapter contains brief description about

relative study of the known techniques for Uranium estimation

and applications of SSNTD technique for the determination of

uranium in -different solid and liquid materials. The SSNTD

methods for uranium determination have also been discussed

and special emphasis is made on 'dry' method used by author

for the determination of uranium in water samples.

The third chapter has the details about the actual

experimental work carried out by the author himself on

study of fission tracks in three plastic track detectors

(Viz. Lexan-8030, Melinex-0 and Makrofol - DE).

Trackological characteristics (Viz. bulk etch rates, track

etch rates, critical angles, and etching efficiencies) for

fission tracks in the three plastics etched in 6N NaOH at

60°C have been determined. Finally, it was decided that

Melinex-0 plastic used as a detector for fission tracks had

better observational properties in comparison to Lexan-

8030 and Makrofol-DE for the trace analysis of Uranium

(described in Chapter 4).

The fourth chapter is based on actual determination of

uranium in drinking and river water samples made by the

author using the 'Dry' method. All experimental details, the

obtained results and discussion on the trace analysis of

uranium in four drinking water samples from Jhansi and seven

water samples (i.e. three drinking water and four river

water) from Allahabad have been presented in this chapter.

It has been found that on the average the drinking water

samples from Jhansi have higher Uranium content than those

from Allahabad.

C O N T E N T S

1. Fission Tracks in Solids 1-21

1.1 Introduction 1

1.2 Track Formation Mechanism and Models 3

1.3 Track Revelation by Selective Chemical Etching 9

1.4 Track Visualization and Evaluation 14

1.5 The Aims of Present Work and the Extent of 15 Achievement References 17

2. SSNTD Techniques For Trace Analysis of Uranium 22-35

2.1 Introduction 22

2.2 Relative Study of the Prevalent Techniques for Uranium Estimation 23

2.3 The SSNTD Techniques for Uranium Determination 25

2.4 Uranium Analysis in Different Solid Materials by SSNTD Technique 26

(a) Determination of Uranium in Homogeneous Solid Materials 27

(b) Determination of Uranium in Heterogeneous Solid Samples 32

(c) Another Geometry for Using SSNTD Technique for the Trace Analysis of Average Uranium in Heterogeneous Solids. 35

2.5 Determination of Uranium in Liquid Materials by SSNTD Technique

(a) Wet Method

38

39

(b) Dry Method ,„

References _

3. Trackological Characteristics of Solid State Nuclear Track Detectors

3. 1 Introduction

56-84

56

3.2 Experimental Details 59

(a) Detectors and Fission Fragment Source Used 59

(b) Irradiation, Etching and Measurements 61

3.3 Results and Discussion 63

(a) Fission Track Registration Characteristics of 63 Lexan-8030 Plastic Track Detectors

(b) Fission Track Registration Characteristics of Melinex-0 Plastic Track Detectors 68

(c) Fission Track Registration Characteristics of Makrofol-DE Plastic Track Detectors. 73

3.4 Conclusions 77

References S3

4. Determination of Uranium Content in Some Water Samples 85-105

4.1 Introduction 85

4.2 Experimental Details 90

(a) Sample Collection and Preparation 90

(b) Irradiation and Etching 91

(c) Track Observation and Analysis of Uranium Content 92

4.3 Results and Discussion 94

References .^QQ

« »>|c«:tc«:|c : 34C

CHAPTER - 1

FISSION TRACKS IN SOLIDS

CHAPTER 1

FISSION TRACKS IN SOLIDS

1.1 Introduct ion

The f i r s t o b s e r v a t i o n o f f i s s i o n f r a g m e n t t r a c k s i n

s o l i d was made by D.A. young [ 4 ] i n 1958 a t Ha rwe l l i n

Eng land, when he i r r a d i a t e d t h e L i F c r y s t a l w i t h f i s s i o n

f ragments; e tched and viewed i t under an o p t i c a l microscope.

Just a year l a t e r ( i n 1959) E.C.H. S i l k and R.S. Barnes [5 ]

r e p o r t e d the o b s e r v a t i o n o f t h e o r i g i n a l damage t r a i l s o f

f i s s i o n f ragmen ts i n mica u s i n g a t r a n s m i s s i o n e l e c t r o n

microscope (TEM). They found t h a t t h e f i s s i o n f ragmen ts

produce m a t e r i a l damage along t h e i r t r a j e c t o r y in mica and

t h a t the damaged core region has a d iameter o f about 50-100

A°.

A sys temat ic work on obse rva t i on o f charged p a r t i c l e

t r acks in so l i ds ,was however s t a r t e d on ly i n ea r l y s i x tees

by a team of t h ree American s c i e n t i s t s namely R.L. F le i scher ,

P.B. Pr ice and R.M. Walker, working a t G.E.C. Schenectady,

New York . They found t h a t the h e a v i l y i o n i s i n g charged

p a r t i c l e s produce e tchab le r a d i a t i o n damage i n not only mica

but i n many o the r i n s u l a t i n g s o l i d s v i z ; i no rgan ic minerals

( c r y s t a l s ) and glasses as we l l as o rgan ic polymers or the

p l a s t i c s [ 2 1 ] . They showed t h a t the damaged reg ion could be

e tched by s e l e c t i v e chemical e t c h i n g - t h u s e n l a r g i n g t h e

"latent tracks" to size of a few microns and making them

visible under an optical microscope.

Using different heavy ions of various energies from

linear accelerators, they observed different thresholds of

detection for different solids and also developed a semi

quantitative theory [26] for the registration mechanism of

charged particle tracks in solids. The meteoretic minerals,

20 mica and glasses record heavy ions beyond Ne, while the

polymeric solids or plastics (viz. Lexan, Makrofol, Cellulose

acetate, Cellulose nitrate etc.) also register the low

energy alpha particles besides recording the fission

fragments and heavier ions. More sensitive Cellulose

nitrate plastics (Nixon Baldwin, Kodak Pathe's CN-80, LR-115

etc.) register the tracks of all heavy ions down to 0.5 Mev

protons. The discovery of a most sensitive plastic detector

which is diglycol carbonate (trade name CR-39) by Cartwright

et al [2] revolutionized the applications of plastic

detectors in cosmic ray studies and radon dosimetry [20],

Fossil tracks of fission fragments were observed in

geologically old inorganic mi nerals (crystal s such as mica,

apatite^ biotite, hornblende, zircon, quartz, glasses etc.

from terrestrial and extraterrestrial samples [22] and led to

the development of fission track dating (FTD) method for age

determination [2^]. Soon there was an exponential increase in

the applications of charged particle tracks in solids in the

fields of nuclear physics, geophysics, cosmic rays, space

physics, radiation dosimetry, trace element analysis,

biophysics, metal 1urgy, heavy ion physics etc. [25, 27, 31].

Most of the useful information about the development

and applications of Solid State Nuclear Track Detectors

(SSNTDS) can be found in two standard books: One written by

Fleisher et al [27] in 1975 in USA and the other authored

by Durrani and Bull [31] in 1987 in England. Also very

useful information are collected into the proceedings of the

international conferences on SSNTDs [ 7, 8,12, 13, 15l>,20].

1.2 Track Formation Mechanisms and Models

Today more than 150 dielectric solids are known which

store etchable tracks of charged particles passing through

them. Fleishcher [28] has showed that barring a few

exceptions, almost all the solids that register the etchable

tracks of charged particles, have electrical resistivity 2

2000 ohm cm and thermal diffusivity of <_ 0.06 cm^/sec.

Several realistic models [27,31] of track formation in

solids have been put forward viz. (a) critical total rate of

energy loss (dE/dx)^,^^^ model, (b) critical primary

ionization (dJ/dx)^^^^ model, (c) critical restricted energy

loss (REL)^^^^ model (d) critical secondary electron energy

loss model (e) critical radius restricted energy loss

(RREL)(,j,^^ model and (f) critical lineal evet density

(LED)^^^^ model. Although none of the models fully explains

all the observed facts about track formation, it is found

that for the inorganic solids the (dJ/dx)^^.^^ criterion of

Fleisher et al [26] fits the observed data most

satisfactorily while for the polymers the (REL)j,^^^_ of

Benton [6] turns out to be most useful for all practical

purposes.

Tracks in dielectric solids are generally formed by the

positive ions at the energies for which the electronic

interactions are the dominant mode of energy loss [31] of

the particle. The formation of etchable tracks in fact takes

place in two steps (i) the creation of defect and (ii) the

relaxation of defect. The former takes place in a time span

of about 10~ to 10~ seconds while the latter may extend

upto one second.

The heavy ions entering a solid lose energy primarily

by Coulomb interaction with the orbital electrons of the

atoms of the target material lying along their trajectory.

The time span of this interaction is about 10~^^ sec. Then

the cascade process of electronic collision starts and the

colliding electrons move outward around the particles

trajectory producing chemically more reactive molecules (in

the form of free radicals in polymers) outside the core zone

and leaving positive unstable ions along the trajectory. This

process is over by about 10~ seconds. The unstable

positive ions so produced repel each other with coulomb force

and move into the interstitial spaces thus creating vacancies

into the crystal lattice. This process is known as ion

— 1 2 explosion [23] and lasts for about 10 " sec.

The atomic defect produced within the core zone is an

extended defect and aggregates within a time of about 10~

sec. Finally the relaxation of molecular defects takes place

by secondary reactions of chemically activated species in the

volume around the core zone known as 'track halo'. The

relaxation process takes place on a time scale of one second.

The diameter of the track core zone produced by

interstitial vacancies caused by positive ions is of the

order of 100 A° or about 10 nm while that of the track halo

generated by electronic collision cascade lies between 100-

1000 nm. (0.1 to 1 um) . The formation of charged

particle track in an inorganic solid and an organic polymer

are schematically shown in fig.1 (a,b) while the track zone

and surrounding track halo are depicted in fig.1c.

o o o o o o o o o o o o o o © "Sr - S L ^ ® O O Ionization

O O © © ®" "S; - ^ ^

O O O O O O O v£

O O O O O O O

O O O O O O O

^ 55 o cr o o o ^ - 1 . ^ / ^ ^ Q J^ Electrostatic

(^ "^ ^ Y displacement

O Oy © " " - ^ ^

0 0 0 0 / 0 o \ o ""

O O O O O O O

f ® y ® y p n - ^ ^ Q O ® P Relaxation

O O " - ^ O „ °"d O ^ - ^ ^ ^ elastic strain

° © O o C C df c i ° ®

Fig.la. The ion explosion spike mechanism for track formation in inorganic solids. The original ionization left by the passage of a charged particle (top) is unstable, and ejects ions into the solid, creating vacancies and interstitials (middle). Later, the stressed region relaxes elastically (bottom), straining the undamaged matrix, thus forming 'latent tracks' Fleischer et al, [25].

Fig.lb. Track formation in organic polymers. The charged particle ionises and excites the molecules, breaking the polymer chains. The chain ends rarely reunite in the same place, but usually react with oxygen or other dissolved gases in the polymer forming new species along the particle's trajectory (shown by filled circles) that are highly chemically reactive.

8

100-1000 nm

Fig.lc. Radial section of the 'latent track' produced by a charged particle in an insulating solid.

The heat treatment of the inorganic solid having

latent damage trails may erase the tracks due to filling up

of the vacancies partially or fully (track annealing) [3].

Similarly the irradiations of the plastic detectors having

'latent tracks' with U.V. light or their exposure to oxygen

etc. may also affect the track stability and detection

sensitivity due to their interaction with molecular defects

in the track halo (environmental effect) [29,33].

1.3 Track Revelation by Selective Chemical Etching

The original damage trails produced by the incident

heavy ion is known as 'latent' track as it is not seen even

by the optical microscopes. It is visible only in electron

microscopes. Important applications of nuclear tracks in

solids started only after the selective chemical etching

process of track revelation that was established by Price

and Walker [17]. For etching, the solid containing latent

tracks is dipped in a chemical solution of fixed

concentration maintained at a particular temperature for a

specific length of time which is generally determined by

trial and error. Although every 'trackologist' has to find

the etching conditions for his own detector system under his

own laboratory conditions, a general guideline can be

obtained from the etching conditions given in table 2 in the

book of Fleisher et al [25]. Generally plastics are etched in

10

aqueous solution of NaOH having concentration between IN to

10N and suitable temperatures between 40°C and 70°C. The

glasses and micas are etched in HF acid (48% or less) for a

few seconds at room temperature for revealing the fission

tracks. It has been found that using lower concentration of

HF for longer time at room temperature gives better result in

case of glasses [14]. Useful etching conditions for revealing

fission tracks in some commonly used track detectors are

given in table 1 .

Development of etched track [11] is the result of

competition between two etching processes: (i) the bulk

etching (ii) the track etching. In the first process, the

surface of the detector is dissolved at a constant rate known

as the bulk etch rate (VQ) , while in second process the

detector material along the particle trajectory is dissolved

at a faster rate termed as the track etch rate (Vj), For the

formation of an etched track pit, it is necessary that Vj >

VQ. Thus V = VJ/VQ, known as the track etch ratio, must be

greater than unity.

Henke Benton [30] in 1971, Somogyi and Szalay [10] in

1973, Ali and Durrani [1] in 1977 and Somogyi [11] in 1980

have discussed the geometry of etched tracks in great details

for the isotropic as well as anisotropic solids.

11

TABLE 1

a. 3o«e Useful Etching Conditions For Reveling Fission Tracks in Soae Solid State Detectors

Name of the Detector Material Etching conditions for fission track

Lithium Fluoride (LiF)

Apatite [Ca^ (F,C1) (PO^)^]

MiCa

HjO + 0.13 g/L LiF + 0.5 ppa Fe,23°C 1 min

0.25X HNOj, 23°C , 1 min

(i) [Biotite, K (Mg, Fe)^ AlSigO^Q (OH)^] 20% HF, 23 , 1-2 min.

(ii) [Muscovite, KAI^ Si^O^g (OH)^] 48X HF, 23 C, 10-40 Bin.

(iii) [Phlogopita, KHg^Al^SigO^Q(OH)^] *6X HF, 23 C, 1-5 Bin.

Glass

(i) Soda lime glass slide 48X HF, 23 C, 5 sec.

(bettor 5X HF, 23°C, 2 ain)

(ii) Phosphate glass 48X HF, 23 C, 5-20 Bin.

Cellulose Acetate Plastics

(Cell it, Kodacol, Triaphol -T)

25 g NaOH + 20 KOH + 4.5 g

KMnO^ + 90 gm H O at 50°C, 2-30 Bin.

1 Bl 15X NaClO -f 2B1 6.25 NaOH, 40 C, 1 hour

Cellulose acetate Butyrate 6.25 NaOH, 7 0 0 , 12 Min.

Cellulose nitrate (Diacell,

Nixon-Baldwin)

6.25 N NaOH, 23 C, 2-4 hour

Polycarbonate Plastic

(Lexan, Makrofol, Merlon,

Kimfol)

6.25 N NaOH, 50 C, 20-60 Bin

12

Polyethylene 10 g K Cr 0 : 35 •! 30X H SO ,85 C, 30 min

Polyethylene terephthalate

(My1a.r, Chronar, Melinex,

Terphane)

6.25 N NaOH; 70 C, 10 min or

6 N NaOH,80°C, 1 hour

D|'g1yco1 carbonate

Plastic (CR-39)

6N NaOH^ 70 C, 2 hour

13

For the isotropic solids the bulk etch rate (VQ) is

found to be an exponential function of concentration and its

temperature T[' 5]. For a given concentration of the etchant,

VQ obeys a Arhinus relation of the form.

VQ = A exp (-EQ/KT)

where A is a fitting constant, K is Boltzmann constant, T is

temp, in Kelvin and EQ is known as activation energy of

bulk etching.

The track etch rate (Vj) is a function of the particle

energy. It is also a material parameter i-e, for a particle of

given charge and energy, Vj depends on the detector

material, the etchant temperature and concentration. For a

particular particle track in a particular detector etched in

a given concentration of etchant, Vj also generally, obeys

Arhinus relation of the form.

Vy = B exp, (-Ey/KT)

Here Ey is called the activation energy of track etching.

The order of activation energy for bulk etching is ' 1 ev

while for track etching is still smaller.

The track shapes, their etched length, cone-angle,

diameter, etc. can be derived using the concept of bulk and

14

t r ack e tch ing f o r the case of homogeneous so l i d s ( p l a s t i c and

g l a s s ) as w e l l as heterogeneous c r y s t a l s mak ing c e r t a i n

assumptions 1 J,1 . The f a c t t h a t V j i s a l so a parameter of

p a r t i c l e energy and charge Z, has been u t i l i z e d f o r p a r t i c l e

i d e n t i f i c a t i o n u s i n g SSNTDs, i n t h e a s t r o p h y s i c a l and

p lane ta ry s tud ies [ 1 8 , 1 9 ]

A n o t h e r k i n d o f c h e m i c a l e t c h i n g known as

e lec t rochemica l e t c h i n g (ECE) was i n t roduced by Tommasino

[15a] in 1970 to revea l and enlarge the t r a c k s q u i c k l y . In

t h i s type of e t c h i n g the de tec tor c o n t a i n i n g l a t e n t damage

t r a i l s separates the e tchant i n t o two compartments and each

compartment has an e l e c t r o d e s . Due t o e l e c t r i c break down

and " t r e e i n g " phenomenon, enlarged "bushy t r a c k s " appear.

The advantage of ECE i s t h a t i t en larges the t r a c k s to very

l a r g e s i z e i n much s m a l l e r t i m e and may be used w i t h

advantage in low l e v e l count ing exper iments .

1.4 Track V i s u a l i z a t i o n and Eva luat ion

The most p o p u l a r method o f t r a c k v i s u a l i z a t i o n and

eva lua t ion i s based on us ing a b inocu la r research microscope

a t m a g n i f i c a t i o n o f 100 x to 1000 x . I t f a c i l i t a t e s t he

c o u n t i n g o f number o f t r a c k s and m e a s u r e m e n t s o f t h e

parameters o f e t ched t r a c k s v i z . t h e i r d i a m e t e r ; l e n g t h ,

cone ang le , d i p . ang le e t c . V i s u a l t r a c k o b s e r v a t i o n s i n

o p t i c a l microscope i s very good method, but i t i s ted icus and

15

time consuming. Some advanced laboratories have started using

semi automatic or completely automatic method [16] for

etched track evaluation such as jumping spark counter [32],

quantimat or image analyser system and microprocessor based

systems. Except the jumping spark counter which has limited

applications, others are very expensive costing several

lakhs of rupees.

In certain applications direct projection of etched

tracks using a slide projector or a projection microscope

may be employed. For very high track density evaluation,

reflectance and transmittance of light (optical density)

measurements have also been found useful [9].

1.5 The Aims of Present work and the extent of achievement

It was intended by me to first aquaint myself with the

development of fission tracks in solids by selective chemical

etching and then to use the U(n,f) reaction for the analysis

of trace quantities of uranium especially in drinking water

samples collected from various cities of U.P.

I have tried myself the revelation of fission tracks

in various solids (plastics) by selective chemical etching

and acquired first hand experience. I have also evaluated

the etching efficiency from the measurement of bulk and track

etch rates in certain cases. I found appropriate etching

16

conditions for revealing the fission tracks in Melinex-0

plastics by etching them till the end of the trajectory,

producingnOf track ends. These are discussed in chapter 3. I

further used this condition for revealing the fission tracks

produced by U(n,f) reaction undergo!ng^^the U-content of

dried water drop sandwiched between two foils of Milinex-0

plastic. A count of total No. of tracks produced was used to

determine the Uranium content in 4 water samples collected

from Jhansi and 7 water samples from Allahabad. The

details are given in chapter 4.

17

REFERENCES

1. A. Ali and S.A. Durrani, "Etched Track Kinetics in

Isotropic Detectors", Nuci. Track Det. 1 (1977), 107-

121 .

2. B.G. Cartwright, E.K. Shirk and P.B. Price, "CR-39: A

Nuclear Track Recording Solid of Unique Sensitivity and

Resolution", Nucl. Instr. Meth. 153 (1978), 457-460.

3. C.W. Naeser and H. Paul, "Fission Track Annealing in

Apatite and Sphene", J. Geophysics Res. 74 (1969),

705-710.

4. D.A. Young "Etching of Radiation Damage in Lithium

Floride", Nature 182 (1958), 375-377.

5. E.C.H. Silk and R.S. Barnes, "Examination of Fission

Fragment Tracks with an Electron Microscope", Phil.

Mag. 4 (1959), 970-972.

6. E.V. Benton, "On the Latent Track Formation in Organic

Nuclear Charged Particles Detectors", Radiat. Effects 2

(1970), 273-280.

7. E.V. Benton and A.F. Starage, "An International Conf.

On Track Physics, Lincoln, Nebraska, 1988, Nucl. Tracks

and Radiat. Meas. Vol. 16 No. 2/3, 1989.

8. E.V. Benton, A.M. Marenny, Istvan Csige, 2nd. All union

conf. of Solid State Nuclear Track Detectors and

Autoradiography, Odessa, U.S.S.R. 1989, Nucl. Tracks

and Radiat. Meas. Vol.20 No.2, 1992.

18

9. G. Somogyi and D.S. Srivastava, "Alpha Radiography with

Plastic Track Detectors, "Int. J. Appl. Rad. Isotope

22 (1971), 289-299.

10. G. Somogyi and S.A. Szalay, "Track Diameter Kinetics in

Dielectric Track Detectors", Nuc1. Instr. and Meth. 109

(1973), 211-223.

11. G. Somogyi, "Development of Etched Nuclear Tracks",

Nucl. Instr. and Meth. 173 (1980), 21-42.

12. G. Espinosa, P.V. Grifith, L. Tommasino, S.A. Durrani

and E.V. Benton, Eds., Proc. 12th Int. Conf. on Solid

State Nuclear Track Detectors, Accapulco 1983, Nucl.

Tracks and Radiat. Meas. 8 (1984).

13. H.A. Khan, I.E. Qureshi and I. Ahmad, Eds. Proc, 14th

Int. Conf. on Solid State Nuclear Track Detectors,

Lahore (Pakistan), 1988, Nucl. Tracks and Radiat.

Meas. 15 (1988).

14. H.H. Mansoor, "Study of Fission Tracks Characteristics

for Measuring Neutron Flux, A M.SC. Thesis, Univ. of

Salahaddin, April, 1988.

15a. L. Tommasino, "Electrochemical Etching of Damaged Track

Detectors by High Voltage Pulse and Sinusoidal Wave

forms", CNEN Report 1973, RT/PROT, 1.

15b. L. Tommasino, G. Barni and G. Campos - Venuti , Eds.,

Proc. 13th Int. Conf. on Solid State Nuclear Track

19

D e t e c t o r s , Rome, 1985, N u c l . T r a c k s and R a d i a t . Meas.

12 ( 1 9 8 6 ) .

16. M.M. Mon in , "Methods o f A u t o m a t i c Scann ing o f SSNTDs",

N u c l . I n t r . M e t h . 173 ( 1 9 8 0 ) , 6 3 - 7 2 .

17. P .B . P r i c e and R.M. W a l k e r , " C h e m i c a l E t c h i n g o f

Charged P a r t i c l e T r a c k s , " J . A p p l . R h y s . 33 ( 1 9 6 2 )

3407-3412 .

18. P.B. P r i c e and R.L . F l e i s c h e r , " P a r t i c l e I d e n t i f i c a t i o n

by D i e l e c t r i c T rack D e t e c t o r s , " Rad. E f f e c t s 2 (1970)

291 -298 .

19. P .B . P r i c e and R . L . F l e i s c h e r , " I d e n t i f i c a t i o n o f

E n e r g e t i c Heavy N u c l e i w i t h S o l i d D i e l e c t r i c T r a c k

D e t e c t o r s : A p p l i c a t i o n s t o A s t r o p h y s i c a l and P l a n e t a r y

S t u d i e s " , Ann . Rev. N u c l . Sc i . 21 ( 1 9 7 1 ) , 295 -334 .

20 . P .H. F o w l e r and V . M . C lapham, E d . , P r o c . 1 1 t h I n t .

Conf . on S o l i d S t a t e Nuc lea r T rack D e t e c t o r s , B r i s t o l

1 9 8 1 , Pergamon P r e s s , O x f o r d , N u c l . T r a c k s S u p p l . 3

( 1 9 8 2 ) .

2 1 . R . L . F l e i s c h e r , P . B , P r i c e , R . M . W a l k e r and E . L .

H u b b a r d , " T r a c k R e g i s t r a t i o n i n V a r i o u s S o l i d S t a t e

Nuc lea r T rack D e t e c t o r s " , Phys. Rev. 133A ( 1 9 6 4 ) , 1443-

1449.

22 . R .L . F l e i s c h e r , P .B . P r i c e and R.M. W a l k e r , " F o s s i l

Records o f N u c l e a r F i s s i o n " , New S c i e n t i s t s 21 (1964)

406-408 .

20

23. R.L. F l e i s c h e r , P.B. P r i c e and R.M. W a l k e r , "The Ion

E x p l o s i o n S p i k e Mechanism f o r f o r m a t i o n o f C h a r g e d

P a r t i c l e T r a c k s i n S o l i d s " , J . Appl . Phys . 36 ( 1 9 6 5 ) ,

3645-3652 .

24 . R . L . F l e i s c h e r and P . B . P r i c e , " T e c h n i q u e s f o r

G e o l o g i c a l D a t i n g o f M i n e r a l s by Chemica l E t c h i n g o f

F i s s i o n Fragment T r a c k s " , Geochim. Cosmochim. Ac ta 28

( 1 9 6 4 ) , 1705 -1714 .

25 . R .L , F l e i s c h e r , P .B , P r i c e and R .M. W a l k e r , " S o l i d

S t a t e N u c l e a r T rack D e t e c t o r s : A p p l i c a t i o n t o Nuc lear

S c i e n c e a n d G e o p h y s i c s , A n n . R e v . N u c l . S c i . 15

( 1 9 6 5 a ) , 1-28.

26 . R . L . F l e i s c h e r , P . B . P r i c e , R . M . W a l k e r and E . L .

H u b b a r d , " C r i t e r i a f o r R e g i s t r a t i o n i n D i e l e c t r i c

Track D e t e c t o r s " , P h y s . . R e v . 156 ( 1 9 6 7 ) , 353 -355 .

27 . R.L. F l e i s c h e r , P.B. P r i c e and R.M. W a l k e r , "Nuc lea r

T racks i n S o l i d s : P r i c i p l e s and A p p l i c a t i o n s " . U n i v .

C a l f . P r e s s , B e r k e l e y ( 1 9 7 5 ) .

28 . R .L . F l e i s c h e r , " N u c l . T r a c k P r o d u c t i o n i n S o l i d s "

P r o g r e s s i n M a t e r i a l S c i e n c e s ; C h a l m e r s A n n i v e r s a r y

Volume, Pergamon P r e s s , O x f o r d ( 1 9 8 1 ) , 9 8 - 1 2 8 .

29 . R.P. Henke, E.V. Benton and H.H. Heckman, " S e n s i t i v i t y

Enhancement o f P l a s t i c N u c l e a r T r a c k t h r o u g h P h o t o

o x i d a t i o n " , Rad. E f f e c t s 3 (197cJ, 4 3 - 4 9 .

21

30. R.P. Hfc_nke and E.V. Benton, "On Geometry of Tracks in

Dielectric Nuclear Track Detectors", Nucl. Instr. Meth.

97 (1971), 483-489.

31. S.A. Durrani and R.K. Bull, "Solid State Nuclear Track

Detection: Principles, Methods and Applications",

Pergamon Press, Oxford (1987).

32. W.G. Cross and L. Tommasino, "A rapid Reading Technique

for nuclear particle Damage Track in thin Foils",

Radiat. Effect 5 (1970), 83-85.

33. W.T. Crawfad, W. De Sorbo and J.S. Humphrey,

"Enhancement of Track Etching Rates in charged particle

irradiated Plastics by a Photo Oxidation Effects",

Nature 220 (1968), 1313-1314.

CHAPTER - 2

SSNTD TECHNIQUES FOR TRACE ANALYSIS OF URANIUM

22

CHAPTER 2

SSNTD TECHNIQUES FOR TRACE ANALYSIS OF URANIUM

2.1 Introduction:

Uranium is a radioactive element having the highest

atomic number of all the naturally occurring nuclides. It is

found in all the rocks, ores, soils, earth crusts and

different types of water etc. in some proportion or the

other, Mostly uranium is found in hexavalent and tetravalent

states instead of the +2, +3, +4, +5 or +6 valence states.

The hexavalent state of uranium in water is most important

because all the tetravalent compounds of uranium are

practically insoluble [2].

p O Q

Natural uranium has three isotopes i-e .> U

(99.27%), - U (0.72%) and ^ " U (0.006%). Since uranium is

ubiquitous in nature, it is transferred to the inner part of

human body through foods and drinking water etc. and there

its alpha radioactivity causes greater health hazard.

Therefore.the analysis of uranium in various solid materials

(i.e. rocks, vegetables, cereals, wheat, tea, tobaccos, coal,

soils etc.) is most important from the point of view of

health services and environmental radiation protection. The

advices on health effects from these materials are given by

several agencies in the world like the U.S. Environmental

23

Protection Agency and Division of Radiological Protection,

BARC (India) etc.

Various reliable methods [6] are available for

Uranium determinations for example

(a) Isotopic dilution mass spectrometry,

(b) Activation analysis,

(c) Fluorescence,

(d) Delayed neutron counting,

(e) Radiometric method,

(f) Alternating current polarography.

In the following lines we first briefly review these

methods and then describe the SSNTD techniques which have

many distinct advantages.

2.2 Relative Study of the Prevalent Techniques for Uranium

Estimation

(a) Mass Spectrometry: This method can be used for Isotopic

analysis. Although this is very accurate and sensitive,

nevertheless it has some shortcomings as follows:

(i) This technique is very expensive,

(ii) A high level of training is necessary before taking

observations.

(iii) Each observation is so difficult that its use is not

advisable in those cases where several samples are to

be analysed.

24

(iv) The problem of contamination level causes special

difficulties especially at the ppb levels.

(b) Activation Analysis: This method has relatively low cost

and has no contamination problems. The short comings in this

technique are as follows:

(i) Uranium analysis is most difficult with this method

because it is generally done by separating one selected

fission product from all the host produced.

(ii) Nuclear reactor is necessary in this technique.

(iii) The experimenter is required to be within the

experimental distance of a reactor.

(iv) This gives results with an accuracy of + 10%

(c) Fluorescence Technique: This techniques is very cheap

and rapid but the shortcomings in this method are :

(i) This can be used only for ppm limit of Uranium

determi nation.

(ii) Contamination always creates problems.

(d) Delayed Neutron counting:

This technique is very rapid but not much sensitive.

The short comings in this method are the following;

(i) Reactor for this technique is also necessary.

(ii) It also requires the experimenter's presence near the

reactor.

25

iii) It gives + 10% accuracy and the problem of

contamination is same as with the other methods.

(e) Radiometric method: This method is also cheap and rapid

but the difficulty arises due to the requirement of large

size of samples. Uranium can be determined up to ppm limit.

(f) Alternating Current Polarography

This technique is cheap and fairly rapid. But the

disadvantage of this method is the determination of uranium

up to ^— ppm limit.

2.3 The SSNTD Technique for Uranium Determination

The Solid State Nuclear Track Detector technique first

reported by Price and Walker [17,18] has unique capabilities

for measuring the concentration of uranium via the detection

of fission fragments in samples irradiated with thermal

neutrons and counting the resulting density of tracks. This

technique is very cheap and rapid. The contamination

problems is recognizable if not avoidable. It requires

small size of samples and can be used to lowest levels (Sub-

ppb mapping) .

Although the reactor is also necessary for this

technique but it does not require the experimentalist's

presence near the reactor. In this method the prepared

samples are sent to nuclear reactor for neutron irradiation

26

in the "service facility" of the reactor. After irradiation,

the samples may be allowed to "cool" as long as necessary

before commercial shipment to the experimenter. The

experimenter after suitable etching can collect data at his

convenience.

Since SSNTD technique of uranium estimation is based on

^^^U (n,f) reaction, the isotopic abundance is required to

be known accurately. Where isotopic abundance ' ^ u/U

alters, and is unknown, this method is not applicable.

2.4 Uranium Analysis in different Solid Materials by the

SSNTD Technique

The SSNTD technique has become very useful for trace

element analysis. Besides it has been put to many other

applications in Nuclear Physics, Geophysics, Medical Sciences

and Environmental Studies and Radiation Protection etc.

[26,29].

Several workers [1,3-5,8,11,14,17.18,22-24] have used

it for the analysis of uranium concentration in various solid

materials ie. tobaccos, plants, soils, coal, vegetables, milk

powder etc.,

Here we describe some useful situations of its

applications for uranium analysis.

27

(a) Determination of Uranium in Homogenous Solid Materials

The method is applicable to the homogenous solids

having Uranium concentration > 50 ng/g (0.050 ppm). In this

method a uranium-poor track detector, for example Plastic

track detector, is kept in perfect contact with the sample

whose uranium concentration is to be determined. A standard

glass or some other material of known uranium concentration

is placed on the opposite side of the same detector. This

type of combination of known, and standard samples is shown

in Fig. (1). This combination is irradiated with thermal

neutrons in a nuclear reactor.

p O C

The interaction of neutrons produces fission of " U

nuclides and the tracks of fission fragments are registered

as latent tracks in the plastic detector. Even if Th is

also present in the sample, its contribution will be

negligible to the total tracks in the plastic from the (n,f)

reaction because thermal neutrons have a cross section of 580

barn for ^^^U (n,f) reaction while for the - Th (n,f)

reaction the cross section is 40 microbarn.

After irradiation the detectors are washed and etched

in a suitable chemical etchant. Proper etching reveals the

latent tracks of fission fragments and makes them visible

under optical microscopes.

28

CL Q

29

The u r a n i u m c o n c e n t r a t i o n o f t h e unknown m a t e r i a l

h a v i n g t h e same e l e m e n t a l c o m p o s i t i o n as t h e s t a n d a r d

m a t e r i a l i s g i v e n by t h e f o l l o w i n g r e l a t i o n [ 2 6 ]

Ig / x C^ (U) = ( ) — p — C^ (U) ( 1 )

Where s u b s c r i p t s x and s refer t o t h e unknown and

s t a n d a r d r e s p e c t i v e l y , I i s t h e r a t i o o f ^^U t o U, a n d /

t h e t r a c k d e n s i t y induced by f i s s i o n t r a c k s i n t h e d e t e c t o r .

When t h e e l e m e n t a l c o m p o s i t i o n o f t h e u n k n o w n i s

d i f f e r e n t f r o m t h a t o f t h e s t a n d a r d t h e n t h e U - c o n t e n t i n

s o l i d s has t o be e s t i m a t e d by a s l i g h t l y m o d i f i e d r e l a t i o n

I s

C, (U) = C^(U) ( — ^ — ) ( - - - — ) ( ) (2) X

?s X — s , • - p - • ^ X }S

Where R represents the effective range (in mg/cm^) of

fission fragments in the sample material- This quantity (R)

increases with increasing Z because of the tighter bonding

of atomic electrons. If this term is ignored it will cause

gross errors in the uranium determination. In the simple

cases where we assume (i) that the isotopic abundance

ratio ^ " U/ '^ U is the same in the unknown and standard

samples, so that I^I,. "is equal to unity, and (ii) that the

ranges of fission fragment in the unknown and the standard

2\- • ABDULLAEV et al(1967) *

o MORY et Ql (1970)

0

o

o

o

oS

o s

o

30

t t I ! 1

0 10 20 30 40 50 60 70 80 90 100

ATOMIC NUMBER

2 Fig.2. Relative ranges (gm/cm ) for average fission

fragments obsorbers of different atomic number [26].

31

materials are also equal (which is not always true), then

the above formula for the estimation of uranium in solid

materials is reduced to the simplest form

X

Cx (U) = — 7 — Cgi u; (3)

Where the terms shown in the above formula have their

usual meanings.

In those cases where the second assumption is not fully

satisfied, the ranges of fission fragments in various Z

materials have to be estimated [26]. This can be done from

the plot [26] shown in fig.(2). This figure shows that in

the elements having atomic number below 50, there is a

monotonic increase in the ranges with atomic number (with

2$ the exception of one point Cu) and the two seteof

observations agree with each other to about 5%. Above Z ~ 50

the absolute determination of U is probably no better than

20%. Standards of high Z composition would improve the

ability to measure absolute values.

An additional complication arises if the unknown sample

is crystalline and not amorphous. In this case it is possible

to get anomalously high transmission along certain crystal

axis (Mory 1969) [12]. Although these effects have not been

32

studied in most materials, they are expected to contribute <

10% error to the absolute uranium determination [26]. The

uranium in homogenous solid samples determined by this

method is well within 10% accuracy,

(b) Determination of Uranium in Heterogeneous Solid

Samples

Many geological materials show considerable

discrepancies in the distribution of uranium. D.E. Fisher

[3] in 1970 modifi,ed the fission track analysis technique

to make it suitable for whole-rock U-determinations/or the

solid materials containing heterogeneous U-distribution. The

degree of heterogeneity of the U-distributions within the

total rock/solid can be removed by crushing the sample to a

scale that is much finer than the scale of the

heterogeneities. Thoroughly crushed solids are passed through

a 100 - mesh sieve. A homogenous mixture of accurately

weighed sample powder and methyl cellulose are used for

making a pellet of target samples. Such pellets of samples

are sandwitched between two detector discs of same diameter

(see Fig.3) and irradiated with thermal neutrons in a

reactor.

The etching, counting and calculations are performed in

the similar manner as in (2.4a). Fisher [3] in 1970 has

found that the track density of both powdered and unaltered

33

e 0) m CO (0

0)

a

u •H

C (0

m

•H

34

glass standards remains same. The uranium concentrations

within the particles which are smaller than " 100 jum will

pass undisturbed into the powdered rock surface and will show

countable clusters of tracks in the plastic detectors. In

some cases clustering effects is found to be large and the

tracks are not countable. In these cases it is difficult to

decide the average uranium concentrations by this method.

Some results of variation in the U-concentration due

to clusters reported by Fisher [3] in 1970 are reproduced

here in table 1. In case of G2-granite and GSP-1 grandiorite

he stated that due to large clustering effect average uranium

could not be estimated.

Such clusters can be removed by the thermal and

chemical treatment of the unknown sample taking into

consideration the following important conditions: (i) the

uranium content of the given amount of sample material under

both chemical and thermal treatment should remain constant,

even if the U-concentration varies as a consequence of the

mass variation of this amount (ii) the final elemental

composition of the changed samples material does not

essentially differ from initial elemental composition.

These treatments are as follows: a given quantity of

the sample is treated with 3N HNO3 and heated until all the

35

TABLE 1

U-Contents of USGS Standard Rocks fyom l?ef-3

U (1n ppm) U (in ppm) Sample Source Without With

clusters clusters

1. G-2 granite FLa^TdSd^ - 1.0 3.5

2. DST-1 dunite " 0.0028 + 0.0005 0.0024 + 0.0005

3. BCR-1 basalt Hamaguchi 0.0044 + 0.0008 1.75 + 0.2

Flanagan 1.87 + 0.2 2.00 + 0.2

4. PCC-1 periodtite Tatsumoto 1.70 + 0.2 1.62 + 0.2 Flanagan 0.0046 + 0.0005 0.0041 + 0.0006

5. AGV-1 andesite Hamaguchi 0.0058 + 0.0006 0.0042 ± 0.0005 Flanagan 1.90 + 0.2 1.92 + 0.2 Tatsumoto 1.77 + 0.2 2.0 + 0.2

6. GSP-1 grandiorite Flanagan Overwhelming Clusters

36

uranium present in it is completely dissolved. Thus attained

uranyl nitrate is converted into UO3 under a controlled

infra-red evaporation and heating. Thereafter the sample is

heated upto < 400* 0 in order to change UO3 into suitable

stable chemical form, UgOg. This treatment gives a

redistribution of the UOOQ in solid state form.

Thus uniform track distribution patterns are obtained.

Therefore the heterogeneous track distribution can be

converted into homogenous distribution by this process. The

same treatment can be used for any powdered samples that are

to be analysed by one of the track method (2.4a or b).

(c) Another Geometry for Using SSNTD Technique for the

Trace Analysis of Average Uranium in Heterogeneous Solicis:

Geisler et al [8] in 1974 had suggested another method

for such solids. In this method the detector is placed at a

fixed distance from the samples in powdered form and the

irradiations are performed in vacuum. The schematic diagram

of this arrangement is shown in fig.4.

A uniform pattern of tracks is observed instead of the

fission "stars" due to the separation between the sample and

the detector.

37

3.5 jLi

KIMFOL

i

sxwwwww^

N /

'''10.5mm

•6m

MICA DETECTOR ON QUARTZ PLATE

QUARTZ STRUCTURE

POWDERED SAMPLE

STANDARD

MICA DETECTOR ON QUARTZ PLATE

Fig.4. Schematic diagram of cell used for determination of average uranium concentration in samples with highly heterogeneous distribution of uranium. Typically a number of such cells are mounted in an evacuated quartz cylinder for neutron irradiation. (After Geisler et al, 1947a.)

38

This geometry gives 5% accuracy in highly

heterogeneous lunar samples as small as 15 mg in mass [8].

However, a large area is required to get a representative

number of exposed high-uranium regions.

2.5 Determination of Uranium in Liquid Materials by SSNTD Technique

Basically? there are two methods which are used for the

trace analysis of uranium in liquid materials. One is known

as "Dry" method, in which a very thin deposit of dried drop

on a planchet is kept in perfect contd..ct with the detector.

This method requires counting of total number of tracks

instead of track density because the track distribution

pattern produced by U(n,f) reaction in the U-content of

dried drop is not uniform throughout.

The other is the "wet" method where strips of plastic

detector is immersed in the fissile material solution and is

together irradiated with reactor neutrons. This method gives

a uniform track distribution pattern. Hence only average

track density is required for the determination of uranium in

liquid instead counting the total no. of tracks.

Several workers have used the above techniques for

uranium estimation in various liquids i.e. waters, blood,

fruit, juice, milk etc. [2, 10, 13, 15, 16, 19-21, 25, 27,

28, 30, 31].

39

Principle of both "Wet" &''Dry" methods is based on the

fissioning of ^^^U by thermal neutrons. In both cases the

samples containing known and unknown amount of uranium is

irradiated with the same fluence of thermal neutrons.

(a) Wet Method: Iyer et al [20,21] in 1973 and 1974 made a

detailed discussion on the experimental implications of the

method.

In this method sample solutions prepared in 3M HNO3

(nitric acid) are taken in a polypropylene tubes of internal

diameter 2-3 mm and a strip of the detector is immersed in

them and then sealed. Here natural uranium solution in 3M

HNO3 is taken as standard solution and is sealed along with a

plastic detector strip in the same way as for the unknown

sample.

The tubes containing standard and unknown samples are

then doubly sealed in the PVC bags. These sample tubes along

with the standard samples are irradiated with thermal

neutrons in a nuclear reactor. After irradiation the

detectors are washed with nitric acid, water and then etched.

This method is applicable for solutions containing

fissile materials. In this case the resulting track density

on either side of the detector is given by the relation

described by Iyer et al [20] in 1973 as ,

40

P- ^e t C - — (4) A

Where C i s the concen t ra t i on (we ight /vo lume) of uranium

in the s o l u t i o n , K^^^ i s the cons tan t o f p r o p o r t i o n a l i t y ( i n

cm) w h i c h i s d e f i n e d as a b s o l u t e t r a c k r e g i s t r a t i o n

e f f i c i e n c y in s o l u t i o n . The value o f K^Q^ depends upon the

d e t e c t o r used, e t c h i n g c o n d i t i o n s and average range o f

f i s s i o n f r a g m e n t s i n t h e s o l u t i o n , I i s t h e i s o t o p i c

abundance o f the f i s s i l e isotope ^"^^U i n the na tu ra l Uranium

(7.2 X 10 ), Cf i s the f i s s i o n c r o s s - s e c t i o n (580 barn) f o r

t h e f i s s i l e i s o t o p e , 0^ i s t h e i n t e g r a t e d f l u x i n

neutrons/cm , N i s the Avogadro's number (6 .02 x 10 per gm

mole) and A i s the atomic number o f the U expressed in gm

(235 gm).

If Cg and C^ are the track densities of standard

and unknown samples, then the eq (4.) can be written as

S = ^et ^s ^/A; I CTf 0t (5)

•> = ' wet ^^ ^N/A) I CTf 0^ (6)

By compa r i son of eq (6) and (6) we get

P.

41'

Thus the uranium in the unknown solution can be

calculated by knowing the values of Pg, P and Cg. Where

the track densities for both standard and unknown samples

(Og & 9 ) can be obtained by optical scanning in the middle

portion of both the detectors. Cg is the known concentration

of uranium for the standard solution. Practically it is found

that the track densities are quite uniform on the detector

strips and no clusters are obtained.

This method is very useful for routine analysis of

fissile material because it requires, the number of tracks

per unit area (i.e. track density) instead of the counting of

total number of tracks. Since the track density is directly

proportional to the amount of fissile isotope per unit

volume, there is no self absorption and the linearity is

maintained in the concentration range of 8x10 gm/mL to

— 9 . . .

4x10 gm/mL uramum. The time of scanning in this method

can be reduced from a few hours to a few minutes due to

counting of only few strips of the detector for track

density. It is very simple and even better than the case

where a U-planchet is made by vacuum evaporation method. It

is more accurate, precise and very rapid than the dry method

when the fissile material present in solution is in milligram

amount. The overall estimated accuracy of this is about 2.55>$.

42

Though this method has several merits which are

discussed above, it has some demerits viz. it is applicable

where the estimation of uranium in the concentration range of

8x10 gm/ml to 4x10 gm/mL is needed. When the

determination of uranium in the amount less than nanogram is

required, this method gives no more reliable results. While

by using "Dry" method, one can estimate uranium in the

nanogram or still lesser amount. Hence this method is less

sensitive than the dry method. Therefore we have used the dry

method for the determination of uranium in water samples in

chapter 4 instead of the "wet" method.

(b) Dry method

This method was first used by Fleischer et al [25] in

1968 for the determination of uranium in water. In this

method a liquid droplet of known volume (V) is allowed to

evaporate on a plastic detector. This leaves a thin residue

of nonvolatile constituents, including fissile material.

Another similar piece of the plastic detector is kept

in intimate contcict (SIT -geometry) with the dry planchetted

fissile material. This type of sandwiches along with a piece

of standard glass sealed in PVC bags are irradiated with

thermal neutrons in a reactor.

The thermal neutrons i nducect ( n , f ) reaction with U-

target nuclei present in the thin deposit and p-oduce

43

f i s s i o n f r a g m e n t s w h i c h a r e r e g i s t e r e d i n t h e p l a s t i c

d e t e c t o r s . When these d e t e c t o r s a re e t c h e d i n a p roper

e tchant under s u i t a b l e e t ch ing c o n d i t i o n s , t he t racks are

made v i s i b l e under o p t i c a l microscope. In t h i s case t o t a l no.

of t r acks must be counted t o e l i m i n a t e the e f f e c t s of non-

u n i f o r m i t y in the depos i t f i l m .

The U-concent ra t ion (C) i n u n i t s of weight /vo lume of

l i q u i d i s c a l c u l a t e d by the f o l l o w i n g r e l a t i o n g i v e n by

Fleischer e t a l [ 2 5 ] .

TM C = (8)

VG N^cr E 0

Where N^ is the Avagadro's number, M (238.03) andcT (4.2 x

10 cm ) are the atomic weight and reaction cross section

of the natural uranium respectively, G is the geometry factor

for the detection of tracks in the plastics. In this geometry

G is equal to unity because two fission fragments are

produced per reaction and half of those reach each detector

plate if the uranium deposit is thin. E is the etching

efficiency required to correct shallow tracks that are not

revealed by etching [23]. For the sandwich geometry E is

given by

^G 2 E = 1 - ( )2 .... (9)

^T

44

Where VQ/VJ is the ratio of bulk etch rate to track

etch rate. The value of E will be appreciably less than

unity for the detection of particles whose ionization rate is

close to threshold of detector or when ordinary glass is used

as a fission fragment detector.

The integrated neutron flux denoted by 0 (neutron/cm )

can be found with the help of standard glass dosimeter using

the relation given by Fleischer et al [25] viz.

0 = K P ... (10)

Where / is the track density in the freshly opened

surface of the standard glass dosimeter which has been

irradiated with thermal neutrons alongwith the sandwitches of

unknown samples and K is the conversion constant for the

glass used.

Later a detailed description on the experimental

implications of 'dry' method (in which detector is kept in

perfect contact with the dry planchetted target ) was given

by Iyer et al [20,21] using the following relation viz.

T = tVdry X W CN/M; (jf 0^ (11)

where T is the number of fission tracks registered on the

plastic detector, W the amount of uranium in gm, Kdry the

45

proportionality constant which gives a measure of track

registration efficiency of the detector and is dimension

less, N the Avogadro's number (6.02 x 10^^ molecules per gm

mole), X is the isotopic abundance of " U in the natural

uranium (7.2 x 10""^), cTf the fission cross-section for

the fissile isotope ^^^U (580x10~^^ cm^), M the gram atomic

weight of uranium ^^^U (235 gm), the thermal neutron flux in

neutrons/cm^ sec and t the time of irradiation in sec.

If S and X are the subscripts for standard and the

unknown samples respectively, then the above equation for

standard and unknown samples which are irradiated with the

same neutron fluence, may be written as

^s = ^dry WslN/M) XcTf 0t (12)

Tx = ^dry W^(N/MjXcrf 0t (13)

By comparing eq (12) and (13), we have

"x W^ r W_ (14)

^s

Thus the amount of uranium on the unknown planchet in gm is

obtained by counting the total number of tracks produced by

unknown and standard planchet and by the knowledge of the

mass of uranium on the standard planchet. Since the volume of

the drop dried on the standard planchet is known, the uranium

concentration (gm/cc) of the unknown drop may be determined.

46

The terms E and G in eq (8) given by Fleischer et al

are the etching efficiency of the detector and geometry

factor (viz. G=1 for this geometry) respectively. The

constant of proportionality K^^y in eq (11) given by Iyer et

al gives the measure of etching efficiency of the detector.

Hence for this geometry K^^y = E G. The basic

difference between the formula of Iyer et al and Fleischer et

al is that Iyer et al used the terms M, (jf for fissile

o o c

isotope ( U) while Fleischer et al used for natural p o c

uranium. Fleischer have used natural uranium instead of '^ U

used by Iyer et al, hence the term X has no place in eq (8).

Thus from the above discussion it is clear that the

formula given by Iyer et al is basically the same as the

formula given by r/eischev &i cu -

This method is very sensitive and can be used for the

determination of uranium in amounts less than or equal to

nanogram. The effects of non-uniformity in the thin deposited

film can be eliminated by counting the total number of

tracks. The accuracy achieved by this method is about 3%.

Since 100 tracks per/\centimeter can be counted easily,

concentration of uranium down to 0.002 /jg/1 can be

determined. The background of tracks from uranium in the

detector itself is negligible since it contains a

47

— 1 ? concentration of roughly 10 "^ uranium atoms per detector

atom [25].

Inspite of all such merits, this method has some

specific demerits as discussed below

Since a non-uniform distribution of tracks (i.e. high

track density on the periphery in comparison to the middle

portion of the drop) is obtained as shown in fig (5),

counting of total no. of tracks in the entire area of the

dried drop is required which is very time-consuming and

tiring. Very often clusters of fission tracks (sub burst type)

occur in this method that make the scanning difficult and

inaccurate.

Fleischer and Deleny [27] in 1976 exp 1 ai n<J that the

clusters and non-uniformity of tracks in water samples Mtre

due to the presence of a variable mass of suspended uranium-

bearing particles. There are many factors which affect the

abundance of such particles i.e. flow velocities, degree of

turbulence, local geology and vegetation.

Although the equation (8) given by Fleischer et al is

best for counting the entire tracks associated with a drop,

often high track abunaances make it cumbersome* to do so.

Therefore J Fleischer and Deleny [27] gave a new method for

48

evaluating total number of tracks in the cases where such

high track abundances occur by using the formula

N = 2 TT RNL + IT (R- cf)^ Hf^ ... (11)

Where N is the total number of tracks, N|_ the

constant number of tracks per unit length along the rim of

the distribution, Nj is constant number of tracks per unit

area in the interior distribution of tracks in the droplet, R

the radius of the circular track distribution of the droplet

and -j the width of radial non-uniformity near the rim and

is much less than R.

When the drop is not exactly circular but slightly

elliptical, straight forward variations on eq (11) can be

used. Experimental range of the ratio N^ (R-( ) /2jjRN|_ is

found to vary between 0.15 to 1,6 with a median of 0.3.

Here overcrowding of tracks and non-uniformity in their

spdfal distribution causes some other problems, for example>

i-F the track density N, is very much high in the rim, a

separate irradiation with a lower neutron flux is needed.

Fig.5 shows that the track density N-, is resolvable in part

but not all of the rim. This means that N|_ varies

circumferentially, where such cases exist either a full count

of the periphery or a series of equal spaced measurements

around the circumference is required. Fleischer and Deleney

49

i ^ ^ , , r - . f 0'> .-4 • 'I.

Fig.5. Etched fission track distribution pattern of deposited residue of uranium on the detector surface from a evaporated droplet.

50

also found that drop to drop variations exceeds somewhat

those expected from the bare statistics for individual

determinations, which are typically + 10%. More extensive

sampling of the tracks is necessary for greater

reproducibi1ity.

51

REFERENCES

1. Anal Danis , D. Dorcioman and Aurora Ponta , "Certified

Reference Materials with Uranium, Homogeniety on the

Microscale", Nucl. Tracks 12 (1986), 785-788.

2. C.R. Cothern and W.L. Lappenbusch, "Occurrence of

Uranium in Drinking water in U.S.", Health Phys.45

(1983), 89-99.

3. D.E. Fisher, "Homogenized Fission Track Determination

of Uranium in Whole Rock Geological Samples", Anal.

Chem. 42 (1970b), 414-416.

4. D.E. Fisher, "Homogenized Fission Track Analysis of

Uranium in some Ultramafic Rock of known Potassium

Content", Geochim. Cosmochim. Acta.34 (1970a), 630-634.

5. D.E. Fisher, "Uranium Content and Radiogenic ages of

Hyphersthene, Bronzite, Amphoterite and Carbonaceous

Chondrites", Geochim. Cosmochim. Acta, 36 (1972a), 15-

33.

6. D.E. Fisher, "Geoanalytic Applications of Particle

Tracks", Earth Sci. Rev. 11 (1975), 291-335.

7. E. Mark, M. Pahl, T.D. Mark and P. Riechs,

"Vergleichende Bestimmung Von Urangehalten nach der

Fission Track Method and Mittels Verzogerter Neutronen",

Preprinp, 1974.

52

8. F.H. G e i s l e r , J . S h i r c k and R. Walker "A New Method o f

Ave rage U - d e t e r m i n a t i o n i n H e t e r o g e n o u s s a m p l e s " ,

u n p u b l i s h e d r e p o r t ( 1 9 7 4 a ) .

9. H.A. Khan and S.A. D u r r a n i , " E f f i c i e n c y C a l i b r a t i o n o f

S o l i d S t a t e N u c l e a r T rack D e t e c t o r s " , N u c l . I n s t r . and

Meth . 98 ( 1 9 7 2 ) , 229-236.

10. I . Gamboa, I . J a c o b s o n , J . I . G o l z a r r i and G. Esp inosa ,

"Uranium C o n t e n t s D e t e r m i n a t i o n i n Commcerc ia l D r i n k i n g

M i l k " , N u c l . T r a c k s and R a d i a t . Meas. 8 ( 1 9 8 4 ) , 4 6 1 -

463.

1 1 . J . D . Kleeman and J . F . L e v e r i n g , "Uran ium D i s t r i b u t i o n

i n R o c k s by F i s s i o n T r a c k R e g i s t r a t i o n i n L e x a n

P l a s t i c " , S c i e n c e 156 ( 1 9 6 7 ) , 512 -513 .

12. J . M o r y , " C h a r g e d p a r t i c l e T r a c k s i n I n s u l a t i n g

s o l i d s " . S o l i d S t a t e D o s i m e t r y , S. A m e l i n c p x e t a l

(eds) ( 1 9 6 9 ) , 393 -424 New Y o r k : Gorden and B reach .

13. K.C. Das, A H a n i f a and T . D . Goswami , "The Uranium

C o n t e n t i n t h e B l o o d o f some v e r t e b r a t e s " , N u c l .

T racks 12 ( 1 9 8 6 ) , 7 8 9 - 7 9 2 .

14. M.K. Nagpaul , K .K . NagpacG and P . P . M e h t a , " U r a n i u m

Con ten ts i n M i n e r a l s by F i s s i o n T rack M e t h o d " , Pure

and A p p l i e d G e o p h y s i c s , 102 ( 1 9 7 3 a ) , 153 -160 .

15. N. L a i , K.K. Nagpau] and M.M. Dhai-Jian, " D e t e r m i n a t i o n o f

U ran ium C o n t e n t i n Wate r f r o m some H i m a l a y a n Ho t

S p r i n g s " , H ima layan Geology 5 ( 1 9 7 5 ) , 185 -192 .

53

16. N.P. S i n g h , Manwinder S i n g h , S u r i n d e r S ingh and H.S.

V i r k , "Uran ium and Radon E s t i m a t i o n i n Water and P l a n t s

u s i n g SSNTD", N u c l . T r a c k s and R a d i a t . M e a s . 8 ( 1 9 8 4 ) ,

4 8 3 - 4 8 6 .

17. P.B. P r i c e and P.M. W a l k e r , " F o s s i l T r a c k s o f charged

p a r t i c l e s i n M i c a a n d t h e Age o f M i n e r a l s " , J .

Geophys. Res. 68 ( 1 9 6 3 b ) , 4847 -4862 .

18. P . B . P r i c e a n d R . M . W a l k e r , "A S i m p l e m e t h o d o f

M e a s u r i n g Low U r a n i u m C o n e n t r a t i o n i n N a t u r a l

C r y s t a l s " , A p p l . Phys. L e t t e r s 2 ( 1 9 6 3 b ) , 2 3 - 2 5 .

19. R.C. Ramola, S u r i n d e r S ingh and H.S. V i r k , "Uran ium and

Radon E s t i m a t i o n i n some Water Samples f r o m H i m a l a y a s " ,

N u c l . T racks R a d i a t . Meas. 15 ( 1 9 8 8 ) , 7 9 1 - 7 9 3 .

20 . R . H . I y e r , M . L . S a g u , R. S a m p a t h Kumar and N . K .

C h a u d h u r i , " A p p l i c a t i o n o f t h e F i s s i o n T r a c k

R e g i s t r a t i o n T e c h n i q u e i n t h e E s t i m a t i o n o f F i s s i l e

2 " 5 M a t e r i a l s : "^^^U c o n t e n t i n N a t u r a l a n d D e p l e t e d

Uranium Samples and T o t a l Uran ium i n S o l u t i o n s " , N u c l .

I n s t , and Me th . 109 ( 1 9 7 3 ) , 4 5 3 - 4 5 9 .

2 1 . R . H . I y e r , R. S a m p a t h Kumar a n d N . K . C h a u d h u r i ,

" F i s s i o n T r a c k R e g i s t r a t i o n i n S o l i d S t a t e T r a c k

D e t e c t o r s Immersed i n F i s s i l e M a t e r i a l S o l u t i o n s " ,

N u c l . I n s t r . and Meth . 115 ( 1 9 7 4 ) , 2 3 - 2 7 .

54

22. R.L. Fleischer and P.B. Price, "Glass Dating by Fission

Fragment Tracks", J. Geophysical Res. 16 (1964), 331-

339.

23. R.L. Fleischer, P.B. Price and R.M. Walker, "Tracks of

Carged Particlesin Slids" Sci.149 (1965), 383-393.

24. R.L. Fleischer, P.B. Price and R.M. Walker, "Neutron

Flux Measurement by Fission Tracks in Solids", Nucl.

Sci and Engg. 22 (1965), 153-156.

25. R.L. Fleishcer and D.B. Lovett, "Uranium and Boron

Content of Water by Particle Track Ething", Geochimica

et Csmochimica Acta 32 (1968), 1126-1128.

26. R.L. Fleischer, P.B. Price and R.M. Walker, "Nuclear

Tracks in Solids: Principles and Applicataions", Univ.

of California Press, Berkeley, (1975).

27. R.L. Fleischer and A.C. Delney, "Determination of

Suspended and Dissolved Uranium in Water", Anal. Chem.

48 (1976), 642-645.

28. Rajinder Parshad, Nand Lai, K.K. Nagpaul,

"Determination Trace Content of Uraniumm in Normal

Blood by Particle Track Etch Rate Technique", Health

Phys. 38 (1988), 409-410.

29. S.A. Durrani and R.K. Bull, "Solid State Nuclear Track

Detection", 1987 Pergamon Press.

55

30. V. Bansal , A. Azam, R.K. Tyag i and R a j e n d r a Prasad ,

" M i c r o a n a l y s i s o f U r a n i u m i n Wate r s a m p l e s Us ing

F iss ion Track R e g i s t r a t i o n Technique" , I nd ian J . Pure

App l . Rhys. 28 (1990) , 194-196.

3 1 . Zhai Pengj i and Kang Tiesheng, "Trace Uranium Analys is

o f Va r i ous Water by F i s s i o n T rack T e c h n i q u e " , N u c l .

Tracks. Rad ia t . Meas. 15 (1988) , 759-762.

CHAPTER - 3

TRACKOLOGICAL CHARACTERISTICS OF SOLID STATE NUCLEAR TRACK DETECTORS

56

CHAPTER 3

TRACKOLOGICAL CHARACTERISTICS OF SOME SOLID STATE NUCLEAR TRACK DETECTORS

3.1 Introduction:

The solids which are capable of recording the tracks of

heavy charged particles are known as solid state nuclear

track detectors for example, the minerals (viz., Epidote,

Olivine, Mica, Zircon, Sphene, etc.), natural and man-made

glasses (viz, Sodalime glass, Tektite glass. Volcanic glass,

phosphate glass etc.), and organic polymers or plastics

(viz, Cellulose nitrate. Cellulose acetate, polycarbonate.

Polyethylene terephthalate, Di glycol carbonate (trac e name

CR-39), Kodak Pathe's LR-115 etc). A list of some well known

SSNTDs (i.e. Inorganic and Organic solids) in approximate

order of their sensitivity alongwith their atomic composition

and least ionizing ion that has been detected by them is

given in table 1. The track registration behaviour of a

detector material depends on the particular etching

conditions, particular formulation of a plastic and exposure

to various experimental conditions [10].

It is seen that the plastics in general are the most

sensitive class of solicir. Among Plastics the diglycol

carbonate (trade name CR-39) is the most sensitive and can

even record the tracks of IMev protons while polycarbonates

57

TABLE 1

List of Some Track Recording Solids in approximate order of increasing sensitivity

Name of Solid Atomic Least ionizing

composition ion detected

Olivine Mg FeSiO^ 100 Mev ^^Fe

Zircon ZrSi04 10 Mev "* Ar

Quartz Si02 100 Mev " Ar on

Muscovite Mica KAlgSigO^Q (0H)2 2 Mev " " Ne

Silica Glass " Si02 16 Mev " Ar

Tektite Glass 22Si02:2AI2O3:FeO 2-4 Mev^°Ne 9 0

Soda Lime Glass 23 Si02: 5Na20: 20 Mev Ne 5 CaO: AI2 OQ

Polyethylene CH2 Fission fragments

Polyethylene C5H4O2 28 Mev "" N Terephthalate (Cronar, Melinex)

Bisphenol A-poly- ^16^14°3 °-^ ^ " ^^^ carbonate (Lexan, Makrofol, Kimfol, Merlon)

Cellulose Triacetate C3H4O2 3 Mev ^He (Cellit-T, Triafol-T, Kodacel TA 401 , 1 ,unpl astici zed)

Cellulose Acetate ^12^18^7 ^ ^^"^ Butyrate

Cellulose Nitrate 16' 8°9' 2 '^^ ' ^ ^ (Daicel1, Nixon Baldwin)

CR-39 ^12'^8°7 •'- ^^^ Proton

58

can not register them. Polycarbonates register the etchable

tracks of very low energy c^-particles and all other heavier

ions including fission fragments while the polyethylene

terephthalate can only record the tracks of heavier ions

above B.

The Threshold characteristics of different solids can

be underst^.d in terms of critical value of primary

ionization (dJ/dx)^^^^ [8] or Critical value of restricted

energy loss rate (REL)Q^^-^_ or (dE/dx)^^ < wcrit ^^^- ^'^^^y

SSNTD has a critical value of material damage above which

latent tracks become etchable. These threshold values for

various solids are shown in fig (1) by horizontal lines [8].

Most of the workers have used Lexan and Makrofol

plastics while very few have used the Melinex-0 plastic which

is better than Lexan and Makrofol in that it does not

register the tracks of low energy ©(.-particles. Moreover it

shows much less background that can be ignored in comparison

to Lexan and Makrofol.

ENERGY/NUCLEON (MeV)

50 100 200 300 500 000 2000 T 1 1 r

METEORITIC MINERALS!

O.A Q5 0.6 0.7 VELOClTY,jB = v/c

F i g . l . Threshold c h a r a c t e r i s t i c s of various SSNTDs according to the c r i t i c a l (dJ/dx) c r i t e r i on of Fleischer et a l . [8] in (1967).

59

In this chapter we present the results of our study on

etching characteristics of the fission tracks in some solid

state nuclear track detectors viz. Lexan, Melinex-0 and

Mackrofol—DE . The calculated values of etching efficiency

for fission track revelation in these detectors are also

given.

3.2 Experimental Details

(a) Detectors and fission Fragment source used

The solid state nuclear track detectors which we have

used are listed in table 2 along with the names of their

manufacturers. A spontaneously fissioning source of gg''

in the form of planchet, imported from Oayr Ridge National

Laboratory, Tennessee 37830, U.S.A. was used for irradiation

of detector with fission fragments. The original activity at

the time of shipment (in 1980) was about ^ JJCI •

The source had been prepared by the deposition of 27 x _ H p P R 9

10 gm of Cf in a circular area of diameter 1.00 Cm on a

nickel planchet of about 3.00 cm diameter and 0.254 mm

thickness. This source is capable of producing fission

fragments as well as 6 Mev p(_-particles. This source now has

become too weak and it takes about 24 hrs for irradiation to

get a track density of the order of lo" tracks/ cm^.

60

This source can not be used for exact calibration of

the detectors, because the fission fragments emitted by it

have a continuous spectrum of mass, charge and energy. This

source gives fission fragments, most probably, 108 and 142

amu corresponding to light and heavy fission fragment groups.

The conservation of momentum shows that the heavier fission

fragment carries smaller energy of ( - 65 Mev) and lighter

fragment carries larger ( - 100 Mev) out of total 200 Mev

energy released in fission.

TABLE 2

Details of Plastic Track Detectors used in

Present Investigations

Name of Detector Name of Manufacture Original thickness (;jm)

Lexan (Bisphenol General Electric 250 -A polycarbonate) Plastics Dept. ••^°3P Mt. Vernon, Indiana

U.S.A.

Melinex-0 Imperial Chemical 100 (Polyethylene Industries Ltd. terepthalate) England *

Makrofol-DE Farbenfabriken 175 (Bisphenol A. Bayer, A.G. Lever polycarbonate) Kusen, F.R. Germany

61

(b) Irradiation, Etching and Measurements

The passive detectors were cut into the size of 2x2.5

cm and exposed to gs "*" fission fragment source in 2 TT-

geometry with the help of a punched card sheet of thickness

equal to that of a post card. Thus, fission fragments of

almost full energy but entering the detector at all angles

were recorded. The fission fragments whose entrance angle

were greater than the critical angle for the detector would

give etchable tracks.

The minimum etching time for the revelation of fission

fragment tracks in the three plastics namely Lexan-8030,

Melinex-0 and Makrofol-DE etched in 6N NaOH solution at 60°C

is 10 minutes. Under these conditions the observed projected

etched track length is greater than 3 microns and is easily

measurable by using a calibrated micrometer eyepiece.

In our measurement the plastics were etched in 6N NaOH

etchant at 60°C. The etching solutions were prepared in

double distilled water. The plastic detectors were etched by

suspending them in a conical flask containing the etching

solution aidLplacing them in a high precision thermostatic

c

water bath maintained at a constant temperature with + 0.1c

accuracy. After that the detectors were washed with running

tap water and double distilled water up to 30 min. and then

dried by putting them separately at a distance of '^ 50 cm.

62

from an infra-red lamp. A fresh etching solution was used

after etching of 2 to 4 samples to minimize the effects of

etching rates due to change of concentration and accumulation

of etch products.

A Japan-make dial type thickness gauge having a least

count of 1 /jm was used for the measurement of thickness of

the detectors before and after each etching. Each data o\

thickness is the mean of at least 30 measurements for each

etching time. The bulk etch rate (VQ) of the plastic track

detectors can be calculated with the graph plotted with

these measurements.

The projected track length of the longest tracks

observed has been measured with the help of a Poland—make

screw micrometer eye-piece (OK 15 KM) having a least count of

0.1163 jjm at a magnification of 750 x. The projected track

length data are the average values of measurements made for

the longest observed tracks in a field over 50 different

fields of view.

The track density was evaluated with the help of the

Huygen^s eye-piece fitted with a square graticule and

calibrated with the help of a stage micrometer slide (Japan-

make ).

63

3.3. Results and Discussion

(a) Fission Track Registration Characteristics of Lexan-8030

Plastic Track Detectors

The observed average values of thickness of the p K p

detector, projected track length for Cf^ fission fragment

tracks registered in Lexan-8030 plastic track detector

etched in 6N NaOH solution at 60°C as a function of etching

time are shown in table 3. A plot of removed layer from the

single surface of the detector as a function of etching time

is shown in fig 2a. The slope of the straight line gives the

bulk etch rate (VQ) of the detector. The value of bulk etch

rate (VQ), calculated either by slope of the graph or by

the least square fit method which gives the best fitted value

on the curve, comes out to be 0.016 jUm/mi n.

The fission tracks in Lexan plastic track detector were

long, thin and resembling the shape of match-stick. Since the

measurement of projected track length after 10 minutes

etching was difficult at ordinary magnification (100 X), the

measurements of projected track length for the longest

fission tracks were taken at 750 x magnification. The average

of about 50 longest observed fission tracks has been

taken as the representative value of projected track length.

The variation of projected track length with etching time is

64

O

o w CO

_ o ^ o

2 o • a m

3 £jQ

o CO

o 03 I

c X

O

o

o o

o CD

O CD

O ^

00

0 Z5 c

^

c -—'

E 1—

05 Cvl

O) LL

o CM

O

65

shown in Fig 2b. The value of track etch rate (Vj) can be

calculated from the least square fit to the initial data of

Fig.2b which lie on straight line.

TABLE 3

Trackological Characteristics of Lexan-8030 Plastic

Track Detector Etched in 6N NaOH AT 60°C

Etching time (in m i n.)

0

10

20

30

45

60

75

90

Average thickness Removed Layer Av. Projected (in microns) from single track length

surface (in (in microns) microns)

242.61

242.29

241.93

241.65

241.15

240.69

240.21

239.85

0

0.16

0.34

0.48

0.73

0.96

1 .20

1 .38

0

4.0

9.32

1 .29

1 .98

1 .97

1 .72

1 .68

Experimentally it was found that the track length

increased with etching time up to 45 minutes. Up to this

time, very fine conical tracks were observed, thereafter the

conical ends of tracks started rounding off. This is also

clear from the fig 2b which shows after 45 minutes of etching

the projected track length remains approximately constant.

66

CO O to 05 o CO

CO

o 0 CO

o

o

w p - J — ' o 0

CD " O

O CO jr;

O "co CO r^

o CO

o CD 1

c CD X CD _J

1

CO

g "co CO

o

o C\J

o o

o CD

O (D

O

00 ®

- I — '

C :i c

_Q C\J CO

LL

o C\J

o

67

Hence most appropriate etching time for Lexan in 6N NaOH

solution at 60°C is to be taken as 45 minutes. The value of

track etch rate in this case comes out to be 0.3919/Jm/min.

For the case of external point source, the value of

etching efficiency of the detector can be calculated by

using the relation described by Srivastava [1] in 1971 viz.

where N2^ and Hj^ are the number of particles emerging

from an isotropic point source placed on the detector

surface in solid angles 2Tr and JX. determined by the

critical angle (8c) for the detector respectively.

lKu5, we have - = -^^- (1 )

where dn, = Sin 0 dG d0 _, r

= 1 - Sin (Sc (2)

Where critical angle ^c is defined as the minimum

angle measured with the surface of the detector below which

68

tracks of charged particles can not be observed by etching.

The relation used for calculation of critical angle for the

detectors is as follows:

(3) <5c = S i n ^ (

hence eq (2 ) becomes

^ = (1 - - - - - - ) ^ ^T

^G

^

(4)

The calculated values of critical angle for etching

and etching efficiency are Sc = 2.34° and 95.92^ respectively

which are in good agreement with those reported by earlier

workers [5,6].

(b) Fission Track Registration Characteristics of Melinex-0

Plastic Track Detectors

Melinex-0 plastic track detector manufactured by

Imperial chemical Industries Ltd. England is more beneficial

over the Lexan and Makrofol plastic track detectors due to

its negligible background and less scratchejimperfections. The

measured data of the average values of thickness of the

plastic track detector and the projected track length of

2 5 2

Cf f i s s i o n f ragment t r a c k s r e g i s t e i ' e d i n M e l i n e x - 0

p l a s t i c d e t e c t o r e tched i n 6N NaOH s o l u t i o n a t 60°C as a

f u n c t i o n of e t ch ing t ime are shown i n t a b l e 4 .

69

The variation of removed layer from the single surface

of the detector with etching time is shown in fig (3a). The

value of bulk etch rate either calculated by the slope of the

straight line or least square fit method, comes out to be

0.01 73 jum/min.

The variation of average projected track length of

fission tracks as a function of etching time is shown in

Fig. 3b, From this figure it is seen that the saturation

value of projected track length reaches at the etching time

after 60 min. It is practically seen that the shape of tracks

remains conical upto 60 minutes of etching in 6N NaOH

solution maintained at 60°C. Thereafter the tips of fission

tracks start becoming spherical. It is therefore decided that

most appropriate time of etching for Melinex-0 in 6N NaOH

solution maintained at 60°C is 60 minutes.

70

0) o CO _ o ^ o

CD 2 03

O

o X CD C

e

O CD i

CO

_® CD C

f ^

^

0) ^ ^ • ^ • i

Z3

(0 03 Q.

CO

E o

cri

CD >

CD cr L

C\J

1

o CM

o

o

o o

o no

O (0

o

(7) CD

C

^

C

CD

E

G j (y)

o

o

71

TABLE 4

T r a c k o l o g i c a l C h a r a c t e r i s t i c s o f Me1 inex -0 P l a s t i c Track

Detector etched i n 6N NaOH a t 60°C

Etch ing t ime Average th i ckness Removed Layer Av. Pro jected ( i n min) ( i n microns) from s i n g l e t r a c k length

sur face ( i n ( i n microns)

microns)

0 104.21 0 0

10 103.85 0.18 3.08

20 103.49 0.35 6.43

30 103.19 0.51 9.62

45 102.67 0.77 11.29

60 102.19 1.01 12.21

75 101.65 1.28 12.23

90 101.23 1.49 12.22

105 100.95 1.63 12.15

120 100.23 1.99 12.08

The slope of straight line gives the track etch rate of

the detector. The value of track etch rate calculated by the

slope of fig. 3b, comes out to be 0.3022 jum/min.

Thus the values of critical angle of etching and

etching efficiency can be calculated from values obtained for

bulk etch rate and track etch rate. The calculated values of

critical angle d^c) of etching and track etching efficiency

72

cn o

• • • •

0) o 2 cc

o Q)

"2 ^

o ^ - ^ o ^

o <0

• ^

c o "w CO

L L

w b •4-^ o

^a^

o CO

^

o CO CO CL

"o

o 1 X

c CD

2

--1

o CM

O O

00 CD

c

o "^ CD (D

o

o OJ

CO

LL

o

73

C*^) for fission tracks in Me1inex-0 Plastic detector are

3.28° and 94.27% respectively. These values for critical

angle and etching efficiency for this detector are in good

agreement with the values as reported by Somogyi et al [4],

in 1969 and Iyer et al [6] in 1974 respectively.

It is concluded from our observations that the fission

tracks in Melinex-0 plastic track detector are of better

conicals shape than in Lexan which gives almost cylindrical

tracks. Also the number of background tracks and etched

scratches in Melinex-0 are very much smaller than in the

Lexan and Makrofol detectors. Therefore the Melinex-0

detector was used by us for the determination of Uranium in

water samples as described in chapter 4.

(c) Fission Track Registration Characteristics of Makrofol-

DE Plastic Track Detectors

Makrofol detectors manufactured by Bayer A.G. Germany

are being used widely for the registration of fission tracks.

Various type of Makrofol (K, KG, E,N, ..) produced by

different processing of manufacturing are expected to behave

in different way^CS, 11, 12]. Trackological characteristics

of Makrofol DE plastic detector (_(l ~nauj Makrofol have been

discussed here.

74

Makrofol-DE detector was supplied by Bayer (India) Ltd.

New Delhi: a local supplier of Bayer A.G. Germany. This

detector is well protected by thin plastic sheet laminated on

both sides of the detector. The process of irradiation and

etching used was the same as for other detectors excepts the

removing of laminated plastic layers from both sides before

irradiation. This detector is also transparent like the

Makrofol-E and Lexan. The thickness of Makrofol-DE detector

in our case was 175 microns. The fission tracks in this

plastic after 10 minute etching in 6N NaOH solution

maintained at SO°C was found to have length of 4.6 microns,

which is greater than track length measured in Lexan and

Melinex-0 as seen in table 3,4^5.

75

TABLE 5

Trackological Characteristics of Makrofol-DE Plastic Track

Detector Etched in 6N NaOH at 60°C

Etching time Average thickness Removed Layer Av. Projected (in min) (in microns) from single track length

surface (in (in microns) microns)

0 175.02 0 0

10 174.72 0.17 4.6

20 174.46 0.30 8.81

30 174.16 0.45 11.53

45 173.62 0.72 12.97

60 M3.20 0.93 13.22

75 172.72 1.17 12.99

90 172.48 1.29 12.84

Table 5 shows the measured data of average value of

thickness removed from the detector and projected track

length of 93 ''' fission fragment tracks recorded in

Makrofol-DE plastic detector. The measured values of removed

layer from the single surface of the detector with etching

time is shown in fig (4a). The value of bulk etch rate can

be calculated either from the slope of the straight line

obtained or with the help of least square fit method. It has

been found to be 0.0151 m/min.

76

(0 o

+ = CO CO 1 -

•- o

t5 ^ CO CD

c c ^ O o - . «3

_a) ^ "co , , v_ _0

HI3 i l CO ^ JO "m Q-

LJJ t 1

1

O O I

CO 3 ^

^ c

m

o

o

O O

CO

00 ^

c o "" CD ®

o

o <M

CO

* o 03

O

CO

O

^

o CM

O O

O

77

The observed fission tracks in this detector were thin,

long and resembling in the shapes to those observed in the

case of Lexan plastic detector. The variation of projected

track length as a function of etching time is shown in fig.

(4b). From the figure it is seen that the value of projected

track length increases upto 45 minutes of etching. After

that, the track ends start becoming spherical. Hence the most

appropriate time of etching for this detector also is 45

minutes under the etching conditions used. Thus the

calculated value of track etch rate, by applying least square

fit to the initial data on projected track length obtained

from f i g. C4b) which lie on straight line is found to be 0.3682

/jm/min. This value gives the critical angle of etching ^c =

2.35° and etching efficiency for fission tracks under these

conditions is 95.89%. These values are in the same range

(94.8 - 95.2;^ ^ s reported by several workers for other

Makrofol plastics (;: ,4-, 5 , 6 , , 11-13).

Separate experiments have shown that similar to Lexan

detector, the Makrofol-DE plastics are also capable of

recording the tracks of low energy alpha particles.

3.4 Conclusions:

Study of fission tracks in three different solids (i.e.

plastic track detectors) viz. Lexan, Melinex-0 anjl^Mak.rpfol-

DE leads us to the following conclusions.

C: ^ ^ 7 V >* ^^ i)C .. .,-

78

o i • • • •

03 "o CO

CO

0 "S sz o

• ^ - ^ o jji:: o CO

•t= c o

"CO CO

(0

b +^ O

"a

o CO

4r: o "co CO CI

"o

LU Q

o

CO

-=

^

o

o

O O

00 CD

- 9 3 00 c

c o (D CD

e

o

o CM

Ll_

o

79

Lexani: The most appropriate etching time for the revelation

of fission tracks in Lexan plastic tracks detector is 45

minutes when etched in 6N NaOH at 60°C. The track etching

efficiency of Lexan Plastic detector for fission tracks in 2-

TT-geometry under these conditions is 95.925g.

Melinex-0: The most appropriate etching time for getting

conical shaped tracks of fission fragments in Melinex-0

plastic track detector is 60 Min.when etched in 6N NaOH

solution maintained at 60°C. The track detection efficiency

of this detector for fission track revelation under these

conditions is 94.27%.

Makrofol-DE: The most suitable etching time of fission

tracks in Makrofol-DE is 45 minutes when etched in 6N NaOH

solution maintained at 60*^0. The track etching efficiency for

this detector comes out to be 95.895K under these conditions

of etching.

Since the behaviour of solid state nuclear track

detectors depends upon the structure of the polymer, the

etching behaviour of plastics manufactured by different

processes should be different even if the product may be

same as observed in the present studies. Although the tracks

of fission fragments in Lexan and Makrofol-DE plastics are

long and thin, the shapes of etched fission tracks in

Melinex-0 plastic is better and more conical.

80

Since Lexan and Makrofol-DE plastic are capable of

registering the tracks of alpha particles along with

the fission tracks, it is concluded that Melinex-0 Plastic

detector is more suitable for the analysis of nuclear fission

(n,f) reaction as compared to other plastics due to its

inability of recording the alpha tracks. Although the track

etching efficiency ^ of Melinex-0 is slightly less than

that of Lexan and Makrofol-'DE plastics, nevertheless it is

preferred over Leyan and Makrofol-DE for (J-determination

due to the presence of nice conical fission tracks, less

number of etched background tracks^scratches and other

imperfections.

Between the Makrofol - DE and Lexan - 8030 it was

found that the Makrofol-DE should be preferred although they

have nearly equal etching efficiencies under similar

conditions. The reasons for this preference are the

fol1owi ng:

(i) The obse-ved projected track length in Makrofol-DE is

greater than that in Lexan-8030.

(ii) The number of scratches in Makrofol-DE is also lesser

than that in the Lexan-8030.

81

For a handy comparison the observed values of bulk etch

rate (VQ), track etch rate (Vj) for fission tracks, the

calculated values of critical angle ^c for track registration

and the etching efficiency ( Tj ) for these detectors when

etched in 6N NaOH at 60° are given in table 6.

o 0 o (O

•p (0

I o ra z

z (O

c •^ •o (D J : 0 •p 0)

c 0) X 2

03 O

•r* •p

(O (0 CO

U2 r-j a « •< Q) 1 - 0)

Ni. X P

I f-0

(0 (J

•r-P w

'r~ L. Q) P O CO L. to sz o

r—

m o

•r— 0) 0

1 —

0 ^ o <a L. H-

1 >> 1 O 1 C 1 (D I -r— 1 0 1 ''~ 1 < + -1 4 -1 O

1 CD 1 C

1 x : 1 O 1 P

tu

Q) 1 —

1 CD 1 C

td

r—

1 (0 1 U I -f—

1 p •r— 1^

d? "v^

^

<u 0)

CD 0) •a

\ , o

O^-O

i : 1 O

o >

1 P \ o

- 1 U 1 (0 1 i-

h-

0) 1 -p

(0

x:" u p 0)

^ o

1 -> 0

- 1 —

p <c i_

c • t —

E

£ 3 .

CO \ ' s

1 -

0) p

x: o -p (D

j i : 1 —

3 CD

i_ O P o 0) +J 0)

Q

t ->

c •r-

e E 3^

\ O

>

1 OJ 1 O 1

in 01

CO 1 CO

• CM

OT 1 - ^

• 1 "'t

CM

05 T—

cn CO •

o

o <D ,— o

•

o

o CO

o 00 1 c <0 X 0)

_J

h-CM •

• ^

en

CO CJ •

CO

r~ -*

• r-T -

CM CM O CO •

o

CO

r~-• • -

o •

o

o 1 X Q) C

-T—

f ^

(D X

CD 1 CO 1

1 IT) 1 O) 1

ID 1 CO 1

1 CM 1

00 1 CO 1

1 • * 1

CM 1

CM 1 00 1 (O 1 CO 1

1 O 1

— 1 un 1 y- 1

O 1 1

O 1

LU 1 O 1 1 1

1— 1 O 1

>*- 1 O 1 ^ 1

-St 1 (0 1

2 : 1

82

83

REFERENCES

1- D.S. Srivastava, "Study of Plastic Track Detectors for

the Detection of Alpha Particles", Ph.D. thesis, 1971,

Aligarh Muslim University, Aligarh.

2. E.V. Benton, "On Latent Track Formation in Organic

Nuclear Charged Particle Track Detectors", Radiat.

Effects 2 (1970), 273-280.

3. E. Dietz and G. Rassl, "A Calculation of the

Efficiency of Makrofol Fission Track Detectors for

Fission Fragments From thin and thick Fission Foils",

Proc. 11th Int. Conf. on Solid State Nuclear Track

Detectors, Bristol 1982, 271-273.

4. G. Somogyi, M. Varnagy and L. Medveczky, "The Influence

of Etching Parameters on the Sensitivity of Plastics",

Radiat. Effects 5 (1969), 111-116.

5. H.A. Khan and S.A. Durrani, "Efficiency Calibration of

Solid State Nuclear Track Detectors", Nucl. Instr. and

Meth. 98 (1972), 229-236.

6. R.H. Iyer, R. Sampath Kumar and N.K. Chaudhuri,

"Fission Track Registration in Solid State Track

Detectors Immersed in Fissile Material Solutions",

Nucl. Instr. and Meth.115 (1974), 23-27.

7. R. L. Fleischer and P.B. Price, "Glass Dating by

Fission Fragment Tracks", J. Geophysical Research 69

(1964), 331-339.

84

8. R.L. Fleischer, P.B. Price, P.M. Walker and E.L.

Hubbard, "Criterion for Registration in Dielectric

Track Detectors", Phys. Rev. 156 (1967 ), 353-355.

9. R.L. Fleischer and D.B. Lovett, "Uranium and Boron

content of water by Particle Track etching", Geochim.

Cosmochim. Acta 32 (1968), 1126-1128.

10. R.L. Fleishcer, P.B. Price and P.M. Walker, "Nuclear

Tracks in Solids: Principles and Applications", Univ.

of California Press, Berkely: USA (1975), 19-20.

11. Shi - Lum Quo, Shu-Hua Zhou, Wu, Meng, Sheng-Fun Sun

and Ru-Fa Shi, "Detection Efficiencies of Solid State

Nuclear Track Detectors for Fission Fragments", Proc.

11th Int. Conf. on Solid State Nuclear Track Detectors,

Bristol 1982, 265-269.

12. V. Bansa] "Study of Fission Track

in Plastics and their Applications for the

determination of Uranium content in water samples", M.

Phil Dissertation, 1986: Aligarh Muslim University,

Al igarh.

13. W. Enge, K. Grabisch, E. Dallmeyer, K.P. Bartholoma and

R. Beaujean, "Etching Behaviour of Solid State Nuclear

Track Detectors", Nucl. Instr. and Meth. 127 (1975),

125-135.

CHAPTER - 4

DETERMINATION OF URANIUM CONTENT IN SOME WATER SAMPLES

85

CHAPTER 4

DETERMINATION OF URANIUM CONTENT IN SOME WATER SAMPLES

4.1 Introduction

Radioactive nuclides enter the human body mainly

through food and water. According to an estimate [6], food

contributes about 15% of the ingested uranium while the

drinking water contributes about 85% for U.S. citizens.

Uranium is found in the underground and surface waters due to

its natural occurrence and also due to man-made activities

like mining and milling of uranium and phosphate rocks. Other

sources of uranium in water are the rocks like granite,

lignite, monazite sands and minerals such as uraninite,

carnotite and Pitch blende etc., which come in contact with

flowing water during its long mountaneous course and seepage

inside the earth.

Uranium is present in almost everything in our natural

surroundings in varying proportions; for example the

average U-content in the earth's crust has been reported to

be 4 X 10""^% (or 4 ppm) by wt, (Hursh and Spoor 1973) [12]

while in the phosphate rocks its value may be as high as 1.2

X 10~^% (or 120 ppm) by wt. (Roessler et al 1979) [5].

Nearly one hundred mineral species possess almost 1% (or

86

10,000 ppm) of uranium by wt. while a few ores may contain

uranium to the extent of 40-60% (or 400,000 to 600,000 ppm).

The uranium from these rocks and minerals leaches out

and mixes with water. Consequently, uranium is found in small

quantities in dissolved and suspended particulate forms in

water-be it surface water (i.e.;, from rivers and lakes), sea

water or ground water (i«.;, drinking water from tube-wells

or taps). In the case of surface waters, the run-off /ifields

also causes changes of its uranium contents.

Determination of uranium in water samples has acquired

importance from the point of health services and

environmental studies [14], the ecological changes brought

about by uranium due to its absorption in plants and also for

assessment of its oral toxicity and effect on human kidney

[2,25].

Of several methods available for the trace quantity

determination of uranium, the fission track counting method

using (n,f) reaction is by far the most simple, less

expensive and equally accurate [7,22]. The accuracy claimed

is ^0% at the trace level [7,20].

Several workers have measured the U-content of water

samples collected from different sources like sea, riveij,

wells, hot springs, taps etc. In table 1, we present the

87

TABLE 1

Some Reported U-content of Water from Different Sources

Type of water U-Content Reference sample (;ig/L)

Sea Water 0.3 - 3 (15) Turekian and Chan (1971) (10) Rona et al (1956) (31) Ritter et al (1982)

U.S. Tap water 0.11-640 (9) Edgington (1965)

U.S. underground water 0.029-1948.9 (14) Drury et al (1981)

U.S. drinking water 0.0269-6.99 (6) Cothern & Lappen busch (0.009-2.08 pCv/L) (1983)

Gawhati water samples 0.08-5.32 (3) Talukdar et al (1983) (India)

Tube-well water from 38.46 -471.36 ^8) Bansal et al (1985) A.M.U. (India)

Domestic water samples 0.5-19.2 (29) Bansal et al (1988) (India)

Some water samples 1.02-35.83 (19) Ramola et al (1988) from Himalayas (India) Kumayu Hill (UP) and Siwalik (H.P.)

Ground water from Av. value = 58.3 (24) Betcher et al (1988) wells in southern Max. value= 2020 Monitobe (Canada)

Natural ground water 2-833 (1) Keer et al (1988) samples

Beijing tap water 1.3-8.8 (18) Zhai et al (1988) River water 0.03-1.3 lake-water 0.056-1.56

88

Water from Carboni- ^ 4.1 x 10^ (11) Pluta (1988) ferous formation

Water from lake < 177 (13) J.K. Otton etal (1989) Tahol - Carson Range area, Nevada and Cali­fornia (U.S.A.)

Ramgarh Lake 9.64 (30) Bansal et al (1990) Jaipur (India)

Some Indian rivers 1.93-4.49 of U.P.

Himalayan hot springs 1.6-7.3 (17) Lai et al (1975)

Hot springs 4000-8800 (26) Chakravarti et al (1979)

Hot springs, well 0.003-2.484 (4) Talukdar et al (1989) and River of North East (India)

Salt lake Iran 4.4 - 19 (31) Ritter et al (1982) Salt lake Israel 4.3

89

data c o l l e c t e d from pub l i shed l i t e r a t u r e , about the uranium

content in water samples o f some c o u n t r i e s . I n the case of

sea water U-content g e n e r a l l y l i e s between 0.3 and 3 >jg/L

(Turek ian and Chan [ 1 5 ] , Rona e t al [10] and R i t t e r [31]).

The r i v e r waters have s i m i l a r U-content [ 1 8 , 3 0 , 4 ] . But in

the case of d r i n k i n g water or tap water samples, very widely

d i f f e r e n t values (0.11 - 640 >jg/L) have been repor ted [ 9 ] .

G e n e r a l l y , the taps s u p p l i e d w i t h u n d e r g r o u n d w a t e r have

h igher U-content than those supp l ied w i t h r i v e r waters . The

w a t e r f r o m h o t s p r i n g s [ 4 ] and some w e l l s c o n t a i n i n g

u ran i f e rous c lays [ 1 ,24 ] show s t i l l l a rge r uranium conten ts .

Even in the case of concent ra ted underground sea water from

near the Lai2hou(Bay Zhou e t a l 1982) [32] have r e p o r t e d

average U-content of ^^^ 50 / jg /L w i t h a maximum of 100 mg/L

due to uranium depos i ted sediments.

We have used t h e f i s s i o n t r a c k r e g i s t r a t i o n

c h a r a c t e r i s t i c s o f t h e M e l i n e x - 0 p l a s t i c d e t e c t o r t o

determine the t r ace q u a n t i t i e s o f Uranium present in some

d r i n k i n g w a t e r s a m p l e s ( c o l l e c t e d f r o m t a p s f e d w i t h

underground water by j e t pumps or hand pumps) and a lso

some r i v e r water ( i^ .^surface water) samples. I n t h i s chapter

we present the exper imenta l d e t a i l s , the observed data and

the r e s u l t s of the U - d e t e r m i n a t i o n i n 4 d r i n k i n g wate r

samples c o l l e c t e d f rom Jhans i C i t y and 7 w a t e r samples

90

collected from Allahabad City, which also includes four

samples of river water viz. 2 from Yamuna, 1 from Ganga and 1

from Sangam.

4.2 Experimental Details

(a) Sample Collection and Preparation

Water samples were collected in small plastic bottles

which were already cleaned with 3M HN03 solution and double

distilled water and rinsed with the sample water. We used

the Melinex-0 Plastic (manufactured by Imperial Chemical

Industries Ltd., London, England) instead of the usual Lexan

because it gives very sharp conical tracks of fission

fragments which are very easily distinguishable from

background/woerf ect i ons and cracks etc. In comparison to

Lexan, Me1inex-0 plastic also gives negligible background of

surface scratches.

The Melinex-0 plastic was cut into circular pieces of

diameter 1.5 cm and coded with certain numbers. These pieces

were washed thoroughly with 3M HNO^ and dried in a clean

oven. Then a small drop of water having known volume (V)

0.0067 mL was dropped on the plastic detector surface with

the help of a calibrated syringe filled with the desired

water sample. The drop was dried by placing the plastic

inside a oven maintained at 80°C. The water got evaporated

91

leaving the non-volatile residue on the plastic detector

sheet. Another similar plastic detector sheet was placed in

perfect contact with the above plastic containing the

residue of the water drop-thus making a sandwich of the

dried drop. These two plastic detectors were sealed with the

help of a transparent tape.

Similar sandwi-ches of dried drops were prepared with

each sample. All the sandwiched samples were then packed

tightly by keeping them one over the other so as to fit in

an Aluminium cylinder of diameter 2 cm and height 5.3 cm. A

standard glass dosimeter containing 0.77 ppm uranium by

weight [27] (Kumar and Srivastava, 1984) was also doubly

sealed in a PVC plastic and put in the Aluminium cylinder

for evaluating integrated neutron flux at the plafe of

i rradiation.

(b) Irradiation and Etching

The cylinder containing all such sandwi ches and the

standard glass neutron dosimeter was sent to BRIT a unit of

Bhabha Atomic Research Centre Trombay, Bombay for

irradiation with thermal neutrons in the IA-3 core position

of the 'Apsara' reactor for three hours where the neutrons

flux was > 50 Pile factor (pf). After the irradiation the

cylinder was received back by Air when it was 'radioactive!y

cool'.

92

The sandwiches were opened. The residue was cleaned

again by dipping the detector in HNO3 solution and washing

by double distilled water. The 'Melinex-0' Plastic track

detectors were etched together in 6N NaOH solution at 60°C

for one hour using a temperature controlled water bath fitted

with a contact thermometer and a stirrer.

The standard glass dosimeter was broken to open a fresh

surface, and etched in 48% H.F. at 23°C for 5 sec.

(c) Track Observation and Analysis of Uranium Content

The etched fission tracks in the plastic detector were

observed by using the Hertel and Reuss binocular research

microscope at a magnification of 250X. Since the tracks were

found to be mere concentrated near the periphery of the

water drop than in the middle, total number of fission

tracks produced by the drop in the plastic detector were

counted by area scanning using a well calibrated graticule

covering an area of 2.25 x 10~ cm . The number of tracks

found on the two detector surfaces in contact with a dried

water drop generally showed a deviation of 0.5% to 10% from

the mean value. The mean value from the two internal surfaces

of a sandwitch was taken as the total number of tracks.

Such mean values were found for each water samples and used

for calculation of the uranium content.

93

The tracks on the freshly opened etched surface of the

standard glass detector were also counted. These tracks were

very uniformly distributed. Track density S^ of the tracks

in the glass detector was determined by using a magnification

of 250X in the same microscope and a well calibrated

graticule. The integrated neutron flux or fluence ( in' Vcm )

was calculated from the formula

0 = Kf

Where K - 1.028 + 0.008 x 10 neutrons/ track for this glass

[27] and P is the track density. This value of 0 was used

in the final formula for U-content as discussed below.

The U-content in the water in wt/volume units is given

by the following formula first described by Fleischer and

Lovett (1968) [23].

T M

V6>N^O- E 0 Cy = ...(1)

Where

T = total number of tracks

M = Atomic Weight of Uranium (238.03)

V = Volume of the water taken

23 N^ = Avagadro's number (6.02x10 molecules/mole)

0^= Reaction Cross-section of natural uranium

for thermal neutrons (4.20 x 10 cm' )

94

E - "'"(Xji/^T ' ^^^ etching efficiency factor

G = Geometry factor for detection of tracks in

solids. Here G = 1 because two fission fragments are produced

2 3 5 per U (n,f) reaction and half of these reach each

detector disc if the uranium deposit is thin. Using the

values of constants

cr- = 4.2 X 10"^"^ cm^

N^ = 6.02 X 10^"^ molecules/mole

E = 0.9902 for melinex-0

G = 1, for the present geometry

The formula becomes T

C,, = 95.074 (gU/mL) ... (2) \/<P

Since in our case V = 0.0067 mL

and 0 = 4.69 x 10^^ Neutron/cm^

Hence Cy = 3.025 x 10"" x T pg/L ... (3)

4.3 Results and Discussion

Table 2 and 3 show our results of U-determination in

the different water samples from Jhansi and Allahabad cities

respectively.

In the drinking water samples from Jhansi, it is seen

that the tap water supplied by Jal-Kal Vibhag has minimum U-

content (0.87 jjg/L) while the hand pump water from Jawahar

chawk has maximum U-content (6.45 ^g/L). The jet pump water

95

TABLE-2

URANIUM CONTENT IN WATER SAMPLES FROM JHANSI USING FISSION

TRACK ANALYSIS

S.No. Sample Details of water Total No. U-concen-Code No. saples of tracks tration

in ipg/L)

J Hand pump water from 21329 6.45 Jawahar Chawk

J2 Jet pump water from 15150 4.58 Pasarat

3. J3 Tap water from 3633 1.10 Pasarat

4. J4 Tap water from 2883 0.87 Tube-wel1 (Jal Kal Vibhag)

Average U contents for all samples = 3.25 + 2.73 ug/L

96

TABLE-3

URANIUM CONTENT IN WATER SAMPLES FKX!M ALLAHABAD USING

FISSION TRACK ANALYSIS

S.No. Sample Details of water Total No. U-concen-Code No. saples of tracks tration

in (>ig/L)

1. Ad-| Hand pump water 6679 2.02 from Ghanta Ghar

2. Ad ^ Tap water from 9311 2.82 Allahabad Univ. Campus

3. Ady Tap water from 12530 3.79 Rly. Station

4. Ad2 Yamuna River 3053 0.92 water from Balua-Ghat

5. Adg Yamuna River 5274 1.60 water from Gau Ghat

6. Adg Ganga River 4908 1.48 water from Sangam (from stream of Ganga)

7. Adg River water 7693 2.33 from Sangam

Average U content for all water sample

= 2.14+ 0.953 pg/L

Average U content of four drinking water samples

= 2.88 ± 0.886 /ig/L

Average U content of three River5

= 1.58 ± 0.580 pg/L

97

from the Pasarat area had a U-content of 4.58 g/L,whi1e the

Municipal tap water sample from the same locality has 1,1

ug/L. The average U-content for the drinking water sample^for

Jhansi comes out to be 3.25 + 2.73^g/L.

For the case of drinking water samples from Allahabad,

it is seen that the tap water collected from the Railway

Station shows maximum U-content (3.79 jug/L), while the |amuna

river water collected from Baluaghat has a minimum U-content

(0.92 ^g/L). The surface water (river water) samples are

generally found to contain lesser U-content than the

underground drinking water samples. The average value of U-

content for all the water samples collected from Allahabad

comes to be 2.14 + 0.953 /jg/L.

Thus the water samples from Jhansi show higher U-

content compared to those from Allahabad.

For a comparison one may refer to the reported values

of our results with the values collected in Table 1 which

shows that U.S. tap waters [9] contain (0.11 - 640 ^g/L);

U.S. underground water [14] contain (0.029 - 1948.9 jjg/l),

U.S. drinking water samples [6] contain (0.0269 - 6.9 ^g/L).

In India the Gt/wahati water [3] samples contain (0.08 -

5.32 jug/L), A.M.U. tube well [28] water contains (38.37 -

471.27 g/L), Domestic Indian water [29] samples contain (0.6

98

- 19.2 ^g/L), Himalayan water [l9 ] samples contain (1.02 -

35,82 jug/L), Beijing (China) tap water [18] samples contain

(1,3 - 8.8 jLjg/L) , whi le the Beijing river water samples

contains (0.03 - l,3jug/L) of uranium.

This comparison shows that U-content in water samples

from these two cities (Jhansi and Allahabad) is within the

range as reported by several workers for other places.

The Uranium concentration in drinking water samples

from Jhansi varies from 0.87 to 6.45 pg/L. The average value

for all water samples is 3.25 /jg/L. Since the activity of

1 jug uranium is 0.67 p Ci [6], the activity of drinking water

containing of 3.25 /jg/L uranium is 2.18 p Ci/L. The average

intake of uranium in human body from drinking water assuming

a consumption of 5 liter water per day comes to be 0.016

mg/day.

The range of U-concentration in water samples from

Allahabad is 0.92 - 3.79 jug/L. The average values for the

three drinking water samples and four river water samples are

2.88 and 1.58 ^g/L respectively. Thus the average intake of

uranium in human body through drinking water assuming a

consumption of 5 litre water per day comes out to be 0.014

mg/day. The activity by drinking water from Allahabad is

1. 93 p Ci/L.

99

Thus our results show that uranium content in drinking

water samples from both the cities is lower than the maximum

permissible intake of 40 mg/day reported by Morgan [16].

100

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