bei48482_FM

Answers to Odd-Numbered Exercises 523

CHAPTER 12

1. 1 4.


2. 3.10 10 ^4.


3. 34.8 h.


4. 1.6 103 y.


5. 1.23 104 Bq.


6. 2.22 10 ^9 kg.


7. 52 min.


8. 1.64 109 y.


9. 1.4 104 y.


10. 5.9 109 y.
    21.^20682 Pb; 48.64 MeV.


11. An electron leaving a nucleus is attracted by the positive nuclear charge, which
    reduces its energy. A positron leaving a nucleus, on the other hand, is repelled
    and is accordingly accelerated outward.


12. The energy available is less than 2mec^2.


13. 2.01 MeV; 0.85 MeV; 1.87 MeV.


14. 1.80 MeV.


15. The thirty-ninth proton in^89 Y is normally in a p 1  2 state and the next higher
    state available to this proton is a g 9  2 state, hence a radiative transition between
    them has a low probability.


16. The neutron cross section decreases with increasing E because the likelihood that
    a neutron will be captured depends on how much time it spends near a partic-
    ular nucleus, which is inversely proportional to its speed. The proton cross section
    is smaller at low energies because of the repulsive force exerted by the positive
    nuclear charge.


17. (a) 71 percent. (b) 3.0 mm.


18. 0.087 mm.


19. 0.766 Ci.
    43.^21 H;^11 H;^11 n;^7936 Kr.


20. 3.33 MeV.


21. 3.1 106 m/s; 4 MeV.


22. 
    
    
    
    4. 
       
    
    
    
    


23. E*Q KEA(1mAmC); 4.34 MeV.


24. The neutron/proton ratio required for stability decreases with decreasing A, hence
    there is an excess of neutrons when fission occurs. Some of the excess neutrons
    are released directly, and the others change to protons by beta decay in the fission
    fragments.


25. 253 MeV.


26. The^11 H nuclei in ordinary water are protons, which readily capture neutrons to
    form^21 H (deuterium) nuclei. These neutrons cannot contribute to the chain
    reaction in a reactor, so a reactor using ordinary water as moderator needs
    enriched uranium with a greater content of the fissionable^235 U isotope to func-
    tion. Deuterium nuclei are less likely to capture neutrons than are protons; hence
    a reactor moderated with heavy water can operate with ordinary uranium as fuel.


27. (b)100 percent; 89 percent; 29 percent; 1.7 percent.


28. 2.37 MeV.


29. (a) 2.2 109 K. (b) This temperature corresponds to the average deuteron
    energy, but many deuterons have considerably higher energies than the average.
    Also, quantum-mechanical tunneling through the potential barrier can occur,

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