ism_chapter_40 - Chapter 40 Nuclear Physics Conceptual...

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1355 Chapter 40 Nuclear Physics Conceptual Problems 1 Determine the Concept Two or more nuclides with the same atomic number Z but different N and A numbers are called isotopes. ( a ) Two other isotopes of 14 N are: 15 N, 16 N ( b ) Two other isotopes of 56 Fe are: 54 Fe, 55 Fe ( c ) Two other isotopes of 118 Sn are: 54 Fe, 55 Fe 2 Determine the Concept The parent of that series, 237 Np, has a half-life of 2 × 10 6 y that is much shorter than the age of the earth. There is no naturally occurring Np remaining on earth. 3 Determine the Concept Generally, β -decay leaves the daughter nucleus neutron rich, i.e., above the line of stability. The daughter nucleus therefore tends to decay via emission which converts a nuclear neutron to a proton. *4 Determine the Concept 14 C is found on earth because it is constantly being formed by cosmic rays in the upper atmosphere in the reaction 14 N + n 14 C + 1 H. 5 Determine the Concept It would make the dating unreliable because the current concentration of 14 C is not equal to that at some earlier time. 6 Determine the Concept An element with such a high Z value would either fission spontaneously or decay almost immediately by α emission (see Figure 40-3). 7 Determine the Concept The probability for neutron capture by the fissionable nucleus is large only for slow (thermal) neutrons. The neutrons emitted in the fission process are fast (high energy) neutrons and must be slowed to thermal neutrons before they are likely to be captured by another fissionable nucleus.
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Chapter 40 1356 8 Determine the Concept The process of slowing down involves the sharing of energy of a fast neutron and another nucleus in an elastic collision. The fast particle will lose maximum energy in such a collision if the target particle is of the same mass as the incident particle. Hence, neutron-proton collisions are most effective in slowing down neutrons. However, ordinary water cannot be used as a moderator because protons will capture the slow neutrons and form deuterons. 9 Determine the Concept Beta decay occurs in nuclei that have too many or too few neutrons for stability. In β decay, A remains the same while Z either increases by 1 ( decay) or decreases by 1 ( + decay). ( a ) The reaction is: 0 1 22 10 22 11 Ne Na + + + decay ( b ) The reaction is: 0 1 24 12 24 11 Mg Na + decay 10 Advantages Disadvantages The reactor uses 238 U, which, by neutron capture and subsequent decays, produces 239 Pu. Thus plutonium isotope fissions by fast neutron capture. Thus, the breeder reactor uses the plentiful uranium isotope and does not need a moderator to slow the neutrons needed for fission. The fraction of delayed neutrons emitted in the fission of 239 Pu is very small. Consequently, control of the fission reaction is very difficult, and the safety hazards are more severe than for the ordinary reactor that uses 235 U as fuel.
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This homework help was uploaded on 02/26/2008 for the course PHYSICS 11 taught by Professor Licini during the Spring '07 term at Lehigh University .

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ism_chapter_40 - Chapter 40 Nuclear Physics Conceptual...

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