Conceptual Physics Fundamentalssrjcstaff.santarosa.edu/~alee3/Physics 11/Powerpoint...

Copyright © 2008 Pearson Education, Inc., publishing as Pearson Addison-Wesley

Conceptual Physics

Fundamentals

Chapter 16:

THE ATOMIC NUCLEUS AND

RADIOACTIVITY


This lecture will help you

understand:

• Radioactivity

• Alpha, Beta, and Gamma Rays

• The Atomic Nucleus and the Strong Force

• Radioactive Half-Life

• Transmutation of the Elements

• Radiometric Dating

• Nuclear Fission

• Mass-Energy Equivalence—E = mc2


The Atomic Nucleus and

Radioactivity “The release of atomic energy has not

created a new problem. It has merely made

more urgent the necessity of solving an

existing one.”

—Albert Einstein


Radioactivity

Radioactivity

• Radioactivity is the process of nuclear decay

(radioactive decay).

• Nothing new in the environment; it’s been going

on since time zero.

• It warms Earth’s interior, is in the air we breathe,

and is present in all rocks (some in trace

amounts).

• It is natural.


The radioactive decay of nature’s elements occurs in the

A. soil we walk on.

B. air we breathe.

C. interior of Earth.

D. all of the above

Radioactivity

CHECK YOUR NEIGHBOR


The radioactive decay of nature’s elements occurs in the

A. soil we walk on.

B. air we breathe.

C. interior of Earth.

D. all of the above.

Radioactivity

CHECK YOUR ANSWER


Alpha, Beta, and Gamma Rays

Radioactive elements emit three distinct types of radiation: • —alpha: positively charged (helium nuclei)

• — beta: negatively charged (electrons)

• —gamma (electromagnetic radiation)



Relative penetrations


Alpha Radiation

• Alpha radiation is a heavy, very short-range

particle and is actually an ejected helium nucleus.

Some characteristics of alpha radiation are:

– Most alpha radiation is not able to penetrate human

skin or clothing.

– Alpha-emitting materials can be harmful to humans

if the materials are inhaled, swallowed, or absorbed

through open wounds.

– A thin-window Geiger-Mueller (GM) probe can

detect the presence of alpha radiation.

• Examples of some alpha emitters: radium, radon,

uranium, thorium.

//upload.wikimedia.org/wikipedia/commons/7/79/Alpha_Decay.svg


Beta Radiation • Beta radiation is a light, short-range particle,and

is actually an ejected electron. Some

characteristics of beta radiation are:

– Beta radiation may travel several feet in air and is

moderately penetrating.

– Beta radiation can penetrate human skin to the

"germinal layer," where new skin cells are

produced. If high levels of beta-emitting

contaminants are allowed to remain on the skin for

a prolonged period of time, they may cause skin

injury.

– Beta-emitting contaminants may be harmful if

deposited internally.

– Clothing provides some protection against beta

radiation.

• Examples of some pure beta emitters: strontium-

90, carbon-14, tritium, and sulfur-35.

//upload.wikimedia.org/wikipedia/commons/a/aa/Beta-minus_Decay.svg


Gamma Radiation • Gamma radiation and x rays are highly penetrating

electromagnetic radiation. Some characteristics of

these radiations are:

– Gamma radiation or x rays are able to travel many feet in

air and many inches in human tissue.

– Gamma radiation and x rays are electromagnetic radiation

like visible light, radiowaves, and ultraviolet light.

– Dense materials are needed for shielding from gamma

radiation. Clothing provides little shielding from penetrating

radiation, but will prevent contamination of the skin by

gamma-emitting materials.

– Gamma radiation and/or characteristic x rays frequently

accompany the emission of alpha and beta radiation

during radioactive decay.

• Examples of some gamma emitters: iodine-131,

cesium-137, cobalt-60, radium-226, and technetium-

99m.

//upload.wikimedia.org/wikipedia/commons/7/79/Alpha_Decay.svg


Gamma Radiation What happens if you overdose on Gamma Radiation?


The origins of radioactivity go back to

A. military activities in the mid-20th century.

B. the industrial revolution two centuries ago.

C. the beginning of human error.

D. before humans emerged on Earth.


CHECK YOUR NEIGHBOR


The origins of radioactivity go back to

A. military activists in the mid-20th century.

B. the industrial revolution two centuries ago.

C. the beginning of human error.

D. before humans emerged on Earth.


CHECK YOUR ANSWER


Any atom that emits an alpha particle or beta particle

A. becomes an atom of a different element, always.

B. may become an atom of a different element.

C. becomes a different isotope of the same element.

D. increases its mass.


CHECK YOUR NEIGHBOR


Any atom that emits an alpha particle or beta particle

A. becomes an atom of a different element, always.

B. may become an atom of a different element.

C. becomes a different isotope of the same element.

D. increases its mass.

Explanation:

Contrary to the failures of alchemists of old to change elements

from one to another, this was going on all around them—

unnoticed.


CHECK YOUR ANSWER



Food irradiation kills microbes

• doesn’t make the food radioactive

• there is no diarrhea with astronauts in space (their food is first irradiated).


Which of these is the nucleus of the helium atom?

A. alpha

B. beta

C. gamma

D. all are different forms of helium


CHECK YOUR NEIGHBOR


Which of these is the nucleus of the helium atom?

A. alpha

B. beta

C. gamma

D. all are different forms of helium


CHECK YOUR ANSWER


Which of these is actually a high-speed electron?

A. alpha

B. beta

C. gamma

D. all are high speed


CHECK YOUR NEIGHBOR


Which of these is actually a high-speed electron?

A. alpha

B. beta

C. gamma

D. all are high speed

Explanation:

Choice D may be true, but doesn’t directly answer the question.


CHECK YOUR ANSWER


Environmental Radiation

Radon, a common environmental hazard

• Most radiation from natural background

• About 1/5 from non-natural sources.



Units of radiation Particle Radiation Dosage Factor Health effect

alpha 1 rad 10 = 10 rems

beta 10 rad 1 = 10 rems

• Doses of radiation – Lethal doses of radiation begin at 500 rems.



Source received annually Typical dose (mrem)

Natural origin

Cosmic radiation 26

Ground 33

Air (Radon-222) 198

Human tissues (K-40; Ra-226) 35



Doses of radiation Typical dose (mrem)

Human origin

Medical procedures

Diagnostic X-rays 40

Nuclear diagnostics 15

TV tubes, other consumer products 11

Weapons-test fallout 1

Commercial fossil-fuel power plants <1

Commercial nuclear power plants <<1


Radiation



Radioactive tracers

• Radioactive isotopes used to trace such

pathways are called tracers.


The Atomic Nucleus and the

Strong Force The strong force holds nucleons together.



Strong Force The strong force is effective over a small

distance.



Strong Force The strong force is more effective with

smaller nuclei.


Strong Force vs number of nucleons



Strong Force A lone neutron is radioactive, and

spontaneously transforms to a proton and

an electron.

• A neutron needs protons around to keep this from

happening.



Strong Force Alpha emission


The strong force is a force in the

A. atom that holds electrons in orbit.

B. nucleus that holds nucleons together.

C. both A and B

D. neither A nor B

The Atomic Nucleus and the Strong Force

CHECK YOUR NEIGHBOR


The strong force is a force in the

A. atom that holds electrons in orbit.

B. nucleus that holds nucleons together.

C. both A and B

D. neither A nor B


CHECK YOUR ANSWER


In the nucleus of an atom, the strong force is a relatively

A. short-range force.

B. long-range force.

C. unstable force.

D. neutralizing force.


CHECK YOUR NEIGHBOR


In the nucleus of an atom, the strong force is a relatively

A. short-range force.

B. long-range force.

C. unstable force.

D. neutralizing force.


CHECK YOUR ANSWER


Strong Force

Strong force

Binding energy and fission

http://www.youtube.com/watch?v=UkLkiXiOCWU

http://www.youtube.com/watch?v=9gERUtbtkRc&feature=endscreen&NR=1


Radioactive Decay


Radioactive Half-Life

The rate of decay for a radioactive isotope is

measured in terms of a characteristic time, the

half-life. The time for half of an original quantity of

an element to decay.



Uranium-238 to lead-206 through a series of alpha

and beta decays. In 4.5 billion years, half the

uranium presently in Earth will be lead.



Some radiation detectors

(a) a Geiger counter

(b) a scintillation counter


A certain isotope has a half-life of 10 years. This means the

amount of that isotope remaining at the end of 10 years will

be

A. zero.

B. one quarter.

C. half.

D. the same.


CHECK YOUR NEIGHBOR


A certain isotope has a half-life of 10 years. This means the

amount of that isotope remaining at the end of 10 years will

be

A. zero.

B. one quarter.

C. half.

D. the same.


CHECK YOUR ANSWER


Suppose the number of neutrons in a reactor that is starting

up doubles each minute, reaching 1 billion neutrons in 10

minutes. When did the number of neutrons reach half a

billion?

A. 1 minute

B. 2 minutes

C. 5 minutes

D. 9 minutes


A challenge…


Suppose the number of neutrons in a reactor that is starting

up doubles each minute, reaching 1 billion neutrons in 10

minutes. When did the number of neutrons reach half a

billion?

A. 1 minute

B. 2 minutes

C. 5 minutes

D. 9 minutes

Explanation:

This question would be appropriate with Appendix D, Exponential Growth and

Doubling Time. Can you see that working backward, each minute has half the

number of neutrons?

Radioactive Half-life

CHECK YOUR ANSWER


Transmutation of the Elements

• Nuclear transmutation is the

conversion of one chemical

element or isotope into another.

• This occurs either through

nuclear reactions (in which an

outside particle reacts with a

nucleus), or through radioactive

decay (where no outside particle

is needed).



With alpha or beta particle, a different element is

formed. This is transmutation, which occurs in

natural events, and also initiated artificially in the

laboratory.

Uranium naturally transmutes to thorium when an

alpha particle is emitted.



Natural transmutation

• Thorium naturally transmutes to protactinium when a beta particle is emitted.

• An electron is e. – Superscript 0 indicates electron’s mass is insignificant compared with

nucleons.

– Superscript -1 is the electric charge of the electron.

0-1


When an element ejects an alpha particle and a beta

particle, the atomic number of the resulting element

A. reduces by 2.

B. reduces by 4.

C. increases by 2.

D. increases by 4.


CHECK YOUR NEIGHBOR


When an element ejects an alpha particle, the atomic

number of the resulting element

A. reduces by 2.

B. reduces by 4.

C. increases by 2.

D. increases by 4.

Explanation:

An alpha particle (a helium nucleus) has atomic number 2. So ejection of an alpha particle means a loss of 2 protons. So the atomic number of the element is lowered by 2.


CHECK YOUR ANSWER



particle, the atomic number of that element

A. reduces by 1.

B. increases by 1.

C. reduces by 2.

D. increases by 2.


CHECK YOUR NEIGHBOR



particle, the atomic number of that element

A. reduces by 1.

B. increases by 1.

C. reduces by 2.

D. increases by 2.

Explanation:

Alpha emission reduces atomic number by 2, and beta emission increases atomic number by 1, so net result is 1.


CHECK YOUR ANSWER


Transmutation of the Element

Artificial transmutation

• an alpha particle fired at and impacting on a

nitrogen atom, which transmutes to oxygen and

hydrogen


Atoms can transmute into completely different atoms in

A. nature.

B. laboratories.

C. both A and B

D. neither A nor B


CHECK YOUR NEIGHBOR


Atoms can transmute into completely different atoms in

A. nature.

B. laboratories.

C. both A and B

D. neither A nor B

Explanation:

Atomic transmutation occurs in nature, in laboratories, and as far

as we know, throughout the cosmos.


CHECK YOUR ANSWER


An element emits 1 beta particle, and its product then emits

1 alpha particle. The atomic number of the resulting

element is changed by

A. zero.

B. −1.

C. −2.

D. none of the above


CHECK YOUR NEIGHBOR


An element emits 1 beta particle, and its product then emits

1 alpha particle. The atomic number of the resulting

element is changed by

A. zero.

B. −1.

C. −2.


Explanation:

Beta emission increases atomic number by 1, then alpha

emission decreases atomic number by 2, so the net change is –1.


CHECK YOUR ANSWER


Radiometric Dating

• Earth’s atmosphere is continuously bombarded by cosmic rays, which causes many atoms in the upper atmosphere to transmute. These transmutations result in many protons.

• a nitrogen that captures a neutron and becomes an isotope of carbon by emitting a proton:


Radiometric Dating

• Carbon-14 is a beta emitter and decays back to

nitrogen.

• Because living plants take in carbon dioxide, any C-14

lost by decay is immediately replenished with fresh C-14

from the atmosphere.

• Dead plants continue emitting C-14 without

replenishment.


Radiometric Dating

Relative amounts of C-12 to C-14 enable dating of

organic materials.


The half-life of carbon-14 is about 5730 years, which

means that the present amount in your bones will reduce to

zero

A. when you die.

B. in about 5730 years.

C. in about twice 5730 years.


Radiometric Dating

CHECK YOUR NEIGHBOR


The half-life of carbon-14 is about 5730 years, which

means that the present amount in your bones will reduce to

zero

A. when you die.

B. in about 5730 years.

C. in about twice 5730 years.


Explanation:

In theory, the amount never reaches zero. In eons to come, trace

amounts of the carbon-14 in your bones, even if completely

dissolved, will still exist.

Radiometric Dating

CHECK YOUR ANSWER


Nuclear Fission

German scientists Otto Hahn and Fritz

Strassmann in 1938 accidentally discovered

nuclear fission.


Nuclear Fission

A typical uranium fission reaction


Nuclear Fission

Chain reaction—a self-sustaining reaction in which

the products of one reaction event stimulate further

reaction events.


The greater the surface area of a piece of fission material,

the

A. less likely an explosion.

B. more likely an explosion.

C. neither A nor B; mass, rather than surface area is significant


Nuclear Fission

CHECK YOUR NEIGHBOR


The greater the surface area of a piece of fission material,

the

A. less likely an explosion.

B. more likely an explosion.

C. neither, A nor B; mass, rather than surface area is significant


Explanation:

When a chain reaction occurs, it fizzles out when neutrons escape

a surface. Therefore, the greater the surface area, the less likely

an explosion will occur.

Nuclear Fission

CHECK YOUR ANSWER


Mass per Nucleon M

ass p

er

Nucle

on


Which of these nuclei has the greatest mass?

A. hydrogen

B. iron

C. lead

D. uranium

Nuclear Fission

CHECK YOUR NEIGHBOR


Which of these nuclei has the greatest mass?

A. hydrogen

B. iron

C. lead

D. uranium

Nuclear Fission

CHECK YOUR ANSWER


In which of these nuclei does the nucleon have the greatest

mass?

A. hydrogen

B. iron

C. lead

D. uranium

Nuclear Fission

CHECK YOUR NEIGHBOR


In which of these nuclei does the nucleon have the greatest

mass?

A. hydrogen

B. iron

C. lead

D. uranium

Nuclear Fission

CHECK YOUR ANSWER


In which of these nuclei does the nucleon have the least

mass?

A. hydrogen

B. iron

C. lead

D. uranium

Nuclear Fission

CHECK YOUR NEIGHBOR


In which of these nuclei does the nucleon have the least

mass?

A. hydrogen

B. iron

C. lead

D. uranium

Explanation:

Iron has the least mass per nucleon, but the strongest binding

energy.

Nuclear Fission

CHECK YOUR ANSWER


When a uranium nucleus undergoes fission, the energy

released is primarily in the form of kinetic energy of

fission fragments

Kinetic energy of fragments is what becomes heat energy.

Interestingly, gamma-ray energy is tiny in comparison. Neutrons,

although important for the chain reaction, contribute a small part

of the energy release.

Nuclear Fission


Nuclear Fission

Fission bomb

• A bomb in which pieces of uranium are driven together is a so-called “gun-type” weapon, as opposed to the now more common “implosion weapon.”

• Constructing a fission bomb is a formidable task. The difficulty is separating enough U-235 fuel.


Nuclear Fission

Nuclear fission reactors • About 20% of electric energy in the

United States is generated by nuclear

fission reactors.

• more in some other countries—about

75% in France

• Reactors are simply nuclear furnaces.


Nuclear Fission

Diagram of a typical power plant.


Nuclear Fission

• The benefits are plentiful electricity, conservation of

billions of tons of fossil fuels every year that are

converted to heat and smoke (which in the long run may

be far more precious as sources of organic molecules

than as sources of heat), and the elimination of

megatons of carbon dioxide, sulfur oxides, and other

deleterious substances put into the air each year by the

burning of fossil fuels.

• Drawbacks include risks of release of radioactive

isotopes into the atmosphere, by accident or by terrorist

activities. Radioactive waste disposal is a problem

(although not for some countries that monitor it for

potential use later).


Nuclear Fission

Plutonium-239, like uranium-235, undergoes fission when it

captures a neutron.


Nuclear Fission

The breeder reactor • A breeder reactor breeds Pu-239 from U-238 while

“burning” U-235.

– occurs in all reactors to some extent.

– in few years can produce twice as much fissionable

fuel as it begins with.

– a more attractive alternative when U-235 reserves are

limited.

– fuel for a breeder may be today’s radioactive wastes


Nuclear Fusion

• nuclear fusion is the opposite of nuclear fission

• fission, nuclei “fizz” apart

• fusion, nuclei fuse together

• Each releases energy in accord with the graph

in Figure 16.33.


Nuclear Fusion


Nuclear Fusion

Fission and fusion compared

• Less mass per nucleon occurs in both processes.


Nuclear Fusion

Typical fusion reactions


When a fusion reaction converts a pair of hydrogen

isotopes to an alpha particle and a neutron, most of the

energy released is in the form of

A. gamma radiation.

B. kinetic energy of the alpha particle.

C. kinetic energy of the neutron.

D. all of the above about equally

Nuclear Fusion

CHECK YOUR NEIGHBOR


When a fusion reaction converts a pair of hydrogen

isotopes to an alpha particle and a neutron, most of the

energy released is in the form of

A. gamma radiation.

B. kinetic energy of the alpha particle.

C. kinetic energy of the neutron.

D. all of the above about equally

Explanation:

By momentum conservation, the ejected neutrons have a high

speed compared with the alpha particle, and therefore much

kinetic energy. It is the kinetic energy of the neutrons that

becomes the heat needed for power. Gamma rays play a small

energy role, as they do in fission.

Nuclear Fusion

CHECK YOUR ANSWER


Nuclear Fusion

Controlling fusion

• Carrying out fusion is more difficult than thought when fission succeeded. – plasma reactors have not been successful

– other schemes, including lasers, are being considered

– deuterium pellets rhythmically dropped into synchronized laser crossfire; heat used to produce steam


In either a fission event or a fusion event, the quantity that

remains unchanged is

A. energy.

B. the mass of nucleons.

C. the number of nucleons.


Nuclear Fission

CHECK YOUR NEIGHBOR


In either a fission event or a fusion event, the quantity that

remains unchanged is

A. energy.

B. the mass of nucleons.

C. the number of nucleons.


Explanation:

This is a premise of reaction equations, whether nuclear or

chemical. Although energy and mass undergo changes, the

number of particles and amount of charge remains unchanged.

Kinetic Energy

CHECK YOUR ANSWER


Backup


Mass-Energy Equivalence—

E = mc2 • Early in the early 1900s, Albert Einstein

discovered that mass is actually “congealed” energy.

• Enormous work is required to pull nucleons from a nucleus. This work is energy added to the nucleon that is pulled out.



E = mc2

Measurements of atomic mass are made with this

device.



E = mc2 • more explanation of the mass

spectrometer

• Electrically charged isotopes

directed into the semicircular

“drum” are forced into curved

paths by a strong magnetic

field. Lighter isotopes with less

inertia (mass) easily change

direction and follow curves of

smaller radii. Heavier isotopes

with greater inertia (mass)

follow larger curves. Mass of

an isotope ~ distance from

entrance slit.



E = mc2 The plot shows how nuclear mass increases with increasing atomic number.



E = mc2 A very important graph results from the plot of

nuclear mass per nucleon from hydrogen through

uranium.



E = mc2 The same graph, with emphasis on nuclear fission.

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