Chapter 30 Nuclear Physics

30 Nuclear Physics Slide 30-2

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Nuclear Structure Different isotopes of the same element have the same atomic number but different mass numbers. Slide 30-12

Checking Understanding How many neutrons are in the following isotope? (The isotope may be uncommon or unstable.) 8 7 6 5 4 Answer: A Slide 30-13

Answer How many neutrons are in the following isotope? (The isotope may be uncommon or unstable.) 8 7 6 5 4 Answer: A Slide 30-14

Checking Understanding How many neutrons are in the following isotope? (The isotope may be uncommon or unstable.) 8 7 6 5 4 Answer: B Slide 30-15

Answer How many neutrons are in the following isotope? (The isotope may be uncommon or unstable.) 8 7 6 5 4 Answer: B Slide 30-16

Checking Understanding How many neutrons are in the following isotope? (The isotope may be uncommon or unstable.) 8 7 6 5 4 Answer: C Slide 30-17

Answer How many neutrons are in the following isotope? (The isotope may be uncommon or unstable.) 8 7 6 5 4 Answer: C Slide 30-18

Checking Understanding How many neutrons are in the following isotope? (The isotope may be uncommon or unstable.) 8 7 6 5 4 Answer: D Slide 30-19

Answer How many neutrons are in the following isotope? (The isotope may be uncommon or unstable.) 8 7 6 5 4 Answer: D Slide 30-20

Checking Understanding 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 Answer: B Slide 30-21

How do I find out about Elemental abundances Web Elements is a good site at URL: http://www.webelements.com/

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 Answer: B Slide 30-22

Example Problem Magnesium has three stable isotopes, with the following natural abundances: 79% of naturally occurring magnesium is 24Mg, with u=23.99 10% of naturally occurring magnesium is 25Mg, with u=24.99 11% of naturally occurring magnesium is 26Mg, with u=25.98 What is the chemical atomic mass of magnesium? Slide 30-23

Stability Slide 30-24

Example Problems 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? Slide 30-25

Binding Energy Slide 30-26

Binding Energy of a Helium Nucleus Slide 30-27

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. Slide 30-28

Nuclear Forces Two protons also experience a smaller electrostatic repulsive force, but it is smaller than the strong nuclear force! Slide 30-29

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 Slide 30-30

Example Problem The beryllium isotope 11Be decays to the boron isotope 11B. Show the nucleons of both nuclei on the shell-model energy-level diagrams below. Explain why this decay is energetically favorable. Slide 30-31

Nuclear Radiation Slide 30-32

Alpha Decay Nothing missing here. Slide 30-33

Beta Decay They did not show the neutrino, neutron goes to proton + electron + electron neutrino or proton goes to neutron + positron (positive electron) + electron neutrino Slide 30-34

Checking Understanding What is the daughter nucleus for this decay: 90Sr → ?X+e- 90Y 89Y 90Rb 89Rb Answer: A Slide 30-35

Answer What is the daughter nucleus for this decay: 90Sr → ?X+e- 90Y 90Rb 89Rb Answer: A Slide 30-36

Checking Understanding What is the daughter nucleus for this decay: 222Rn → ?X+α 220Po 218Po 220Ra 218Ra Answer: B Slide 30-37

Answer What is the daughter nucleus for this decay: 222Rn → ?X+α 220Po 220Ra 218Ra Answer: B Slide 30-38

Checking Understanding What is the daughter nucleus for this decay: 99Tc → ?X+γ 99Tc 99Mo 99Nb 99Ru Answer: A Slide 30-39

Answer What is the daughter nucleus for this decay: 99Tc → ?X+γ 99Tc 99Mo 99Nb 99Ru Answer: A Slide 30-40

Example Problem 11Li 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. Slide 30-41

Operation of a Geiger Counter Slide 30-42

Example Problem: Activity 0.693N t1/2 ______ R = rN = 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? Slide 30-43

Half Life Slide 30-44

Nuclear Decay Slide 30-45

Example Problems: Decay Times The 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? A 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. Slide 30-46

Dose and Dose Equivalent 1 Gy = 1.00 J/kg of absorbed energy Dose equivalent in Sv = (dose in Gy) x RBE Slide 30-47

Example Problems: Determining Dose 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? A passenger on an airplane flying across the Atlantic will receive an extra radiation dose of about 5 microsieverts per hour from cosmic rays. How many hours of flying would it take in one year for a person to double his or her yearly radiation dose? Assume there are no other significant radiation sources besides natural background. Slide 30-48

Conceptual Example Problem: Radioactive Cookies Suppose you have three cookies, each of which is radioactive. They have the same activity, but one is an alpha source, one a beta source, and one a gamma source. You must put one cookie in your pocket, eat one, and place one in a lead box. Which one do you put in the lead box, which one do you eat, and which one do you put in your pocket? Slide 30-49

Summary Slide 30-50

Summary Slide 30-51

Additional Questions What is the decay mode of the following decay? 137Cs → 137Ba + ? Alpha decay Beta-minus decay Beta-plus decay Gamma decay Answer: B Slide 30-52

Answer What is the decay mode of the following decay? 137Cs → 137Ba + ? Alpha decay Beta-minus decay Beta-plus decay Gamma decay Answer: B Slide 30-53

Additional Questions What is the decay mode of the following decay? 222Rn → 218Po + ? Alpha decay Beta-minus decay Beta-plus decay Gamma decay Answer: A Slide 30-54

Answer What is the decay mode of the following decay? 222Rn → 218Po + ? Alpha decay Beta-minus decay Beta-plus decay Gamma decay Answer: A Slide 30-55

Additional Questions What is the decay mode of the following decay? 60Ni* → 60Ni + ? Alpha decay Beta-minus decay Beta-plus decay Gamma decay Answer: D Slide 30-56

Answer What is the decay mode of the following decay? 60Ni* → 60Ni + ? Alpha decay Beta-minus decay Beta-plus decay Gamma decay Answer: D Slide 30-57

Additional Example Problem A 60 kg laboratory worker receives a whole-body x-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. Slide 30-58