Chapter 30

© 2010 Pearson Education, Inc.

PowerPoint® Lectures forCollege Physics: A Strategic Approach, Second Edition

Chapter 30

Nuclear Physics

© 2010 Pearson Education, Inc. Slide 30-2

30 Nuclear Physics





Reading Quiz 1. The mass number A of a nucleus is the number of

___________ in the nucleus.

A. protons

B. protons plus neutrons

C. neutrons

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Answer 1. The mass number A of a nucleus is the number of

___________ in the nucleus.

A. protons

B. protons plus neutrons

C. neutrons


Reading Quiz 2. When a nucleus decays by giving off an electron, we call this

________ decay.

A. alpha

B. beta

C. gamma

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Answer 2. When a nucleus decays by giving off an electron, we call this

________ decay.

A. alpha

B. beta

C. gamma


Reading Quiz 3. ________ decay results in a daughter nucleus that is the same

as the parent nucleus.

A. Alpha

B. Beta

C. Gamma

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Answer 3. ________ decay results in a daughter nucleus that is the same

as the parent nucleus.

A. Alpha

B. Beta

C. Gamma


Nuclear Structure

Different isotopes of the same element have the same atomic number but different mass numbers.

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Checking UnderstandingHow many neutrons are in the following isotope? (The isotope may be uncommon or unstable.)

A. 8B. 7C. 6D. 5E. 4

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Answer How many neutrons are in the following isotope? (The isotope may be uncommon or unstable.)

A. 8B. 7C. 6D. 5E. 4



A. 8B. 7C. 6D. 5E. 4

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A. 8B. 7C. 6D. 5E. 4



A. 8B. 7C. 6D. 5E. 4

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A. 8B. 7C. 6D. 5E. 4



A. 8B. 7C. 6D. 5E. 4

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A. 8B. 7C. 6D. 5E. 4


Boron, with atomic number Z=5, has two stable isotopes, with atomic mass numbers A=10 and A=11. Boron’s chemical atomic mass is 10.81. What are the approximate fractions of the two stable boron isotopes found in nature?

A. 92% 11B, 8% 10B

B. 80% 11B, 20% 10B

C. 50% 11B, 50% 10B

D. 20% 11B, 80% 10B

E. 8% 11B, 92% 10B

Checking Understanding

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Answer Boron, with atomic number Z=5, has two stable isotopes, with atomic mass numbers A=10 and A=11. Boron’s chemical atomic mass is 10.81. What are the approximate fractions of the two stable boron isotopes found in nature?

A. 92% 11B, 8% 10B

B. 80% 11B, 20% 10B

C. 50% 11B, 50% 10B

D. 20% 11B, 80% 10B

E. 8% 11B, 92% 10B


Magnesium has three stable isotopes, with the following natural abundances:

• 79% of naturally occurring magnesium is 24Mg, with u=23.99



What is the chemical atomic mass of magnesium?

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Example Problem


Stability


Binding Energy

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Binding Energy of a Helium Nucleus

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There are several elements for which there is only one stable isotope, or for which one stable isotope dominates the natural abundance. Three examples are:

• All but 0.00013% of naturally occurring helium is the stable isotope 4He.

• 100% of naturally occurring niobium is the stable isotope 93Nb.

• 100% of naturally occurring bismuth is the stable isotope 209Bi.

What is the ratio of neutrons to protons for these three isotopes?

16O, with u=15.994915, is stable; 19O, with u=19.003577, is not. What is the binding energy per nucleon for each of these nuclei?

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Example Problems


Curve of Binding Energy

• Light nuclei can become more stable through fusion.• Heavy nuclei can become more stable through fission.• All nuclei larger than a certain size spontaneously fission.

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Nuclear Forces

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Nuclear Energy Levels and Decay• Different levels for neutrons

and protons• Energy difference between

levels is very large• Nuclei can become more

stable through certain decay modes

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The beryllium isotope 11Be decays to the boron isotope 11B.

a. Show the nucleons of both nuclei on the shell-model energy-level diagrams below.

b. Explain why this decay is energetically favorable.

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Example Problem


Nuclear Radiation

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Alpha Decay

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Beta Decay

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What is the daughter nucleus for this decay:

90Sr → ?X+e-

A. 90YB. 89YC. 90RbD. 89Rb

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Answer What is the daughter nucleus for this decay:

90Sr → ?X+e-

• 90Y• 89Y• 90Rb• 89Rb



222Rn → ?X+α

A. 220PoB. 218PoC. 220RaD. 218Ra

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222Rn → ?X+α

• 220Po• 218Po• 220Ra• 218Ra



99Tc → ?X+γ

A. 99TcB. 99MoC. 99NbD. 99Ru

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99Tc → ?X+γ

• 99Tc• 99Mo• 99Nb• 99Ru


Example Problem11Li is an unstable isotope of lithium. Sketch the energy level structure for the neutrons and the protons in this nucleus. What decay mode would you expect for this nucleus? Write the full equation for the decay you expect, including the daughter nucleus.


Decay Series

• 235U is unstable

• Through a series of alpha beta decay, settles on 207Pb

• In a star, everything ends up as 56Fe


Fission

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Operation of a Geiger Counter


Example Problem: Activity

Most of the internal radiation of the human body is due to a single isotope, the beta emitter 40K, with half life of 1.28×109 years. The body contains about 0.35% potassium by mass; of this potassium, about 0.012% is 40K. What is the total activity, in Bq, of a 70 kg human?

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R = rN = ______0.693Nt1/2


Half Life

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Nuclear Decay

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Example Problems: Decay TimesThe Chernobyl nuclear reactor accident in the Soviet Union in 1986 released a large plume of radioactive isotopes into the atmosphere. Of particular health concern was the short-lived (half life: 8.0 days) isotope 131I, which, when ingested, is concentrated in and damages the thyroid gland. This isotope was deposited on plants that were eaten by cows, which then gave milk with dangerous levels of 131I. This milk couldn’t be used for drinking, but it could be used to make cheese, which can be stored until radiation levels have decreased. How long would a sample of cheese need to be stored until the number of radioactive atoms decreased to 3% of the initial value?

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Example Problems: Decay TimesA scrap of parchment from the Dead Sea Scrolls was found to have a 14C/12C ratio that is 79.5% of the modern value. Determine the age of this parchment.

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Dose and Dose Equivalent

1 Gy = 1.00 J/kg of absorbed energy

Dose equivalent in Sv = (dose in Gy) x RBE

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In a previous example, we computed the activity of the 40K in a typical person. Each 40K decay produces a 1.3 MeV beta particle. If 40% of the energy of these decays is absorbed by the body, what dose, and what dose equivalent, will a typical person receive in one year from the decay of these nuclei in the body?

Example Problems: Determining Dose

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Summary

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Summary

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What is the decay mode of the following decay?

137Cs → 137Ba + ?

A. Alpha decayB. Beta-minus decayC. Beta-plus decayD. Gamma decay

Additional Questions

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Answer What is the decay mode of the following decay?

137Cs → 137Ba + ?




222Rn → 218Po + ?



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222Rn → 218Po + ?




60Ni* → 60Ni + ?



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60Ni* → 60Ni + ?



Additional Example ProblemA 60 kg laboratory worker receives a whole-bodyx-ray exposure of 0.50 mSv. The x-ray wavelength is 0.15 nm. How many x-ray photons are absorbed in the worker’s body? X rays have an RBE of 1.

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