Nuclear Physics Ch29

Chapter 29

NUCLEAR PHYSICS

Conceptual Questions

3. A neutron is electrically neutral, so it does not feel any Coulomb force as it approaches a nucleus. It is able to penetrate the nucleus and be absorbed, increasing the energy of the nucleus and making it less stable in the process. A proton on the other hand experiences a large Coulomb repulsion from the positively charged nucleus. For this reason, a proton of the same kinetic energy would be much less likely to penetrate the nucleus and induce a nuclear reaction. 4.– decay effectively converts a neutron into a proton. Thus, a nuclide that has too many neutrons to be stable is likely to decay via –.

5. There is a mass defect due to the binding energy of atomic electrons but it is several orders of magnitude smaller than the binding energy of the nucleus and may thus be omitted without significant loss of accuracy.

6. Carbon-14 is formed when neutrons (usually produced in the upper atmosphere by cosmic rays) react with 14N nuclei in the atmosphere. The fission reactions that occur during the detonation of nuclear weapons emit neutrons, among other radioactive by-products. Neutrons released during atmospheric testing of nuclear weapons would react with 14N nuclei in the air, thereby increasing the concentration of 14C in the atmosphere. Fossil fuels are sufficiently old that any 14C they contained in the past has long since decayed. Widespread burning of fossil fuels would increase the concentrations of 12C and 13C, but not 14C. The relative abundance of 14C in the atmosphere would therefore decrease.

8. The overall reaction due to the proton-proton cycle is the same as that due to the carbon cycle except for an additional photon released in the carbon cycle. The photon has zero rest mass, however, so the rest energy of the products minus the rest energy of the reactants is the same in each case. Therefore, the total energy released is the same for both processes.

10. (a) The concentration of Rn-222 in the...