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Unformatted text preview: 301 Chapter 40 Nuclear Physics Conceptual Problems 1 Isotopes of nitrogen, iron and tin have stable isotopes 14 N, 56 Fe and 118 Sn. Give the symbols for two other isotopes of ( a ) nitrogen, ( b ) iron, and ( c ) tin. 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: 117 Sn, 119 Sn 2 Why is the decay chain A = 4 n + 1 not found in nature? Determine the Concept The parent of that series, 237 Np, the longest-lived member of this decay series, has a half-life of 2 10 6 y that is much shorter than the age of Earth. There is no naturally occurring Np remaining on Earth. 3 A decay by emission is often followed by decay. When this occurs, it is by and not + decay. Why? Determine the Concept Generally, -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 The half-life of 14 C is much less than the age of the universe, yet 14 C is found in nature. Why? 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 What effect would a long-term variation in cosmic-ray activity have on the accuracy of 14 C dating? 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. Chapter 40 302 6 Why does an element that has Z = 130 not exist? Determine the Concept An element with such a high Z value (the Coulomb- repulsion force is proportional to Z 2 ) would either fission spontaneously or decay almost immediately by emission (see Figure 40-3). 7 Why is a moderator needed in an ordinary nuclear fission reactor? 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. 8 Explain why water is more effective than lead in slowing down fast neutrons. 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....
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This note was uploaded on 02/28/2012 for the course PHYSICS 122 taught by Professor Goussiou during the Spring '12 term at University of Washington.
- Spring '12