SM_chapter45

SM_chapter45 - 45 Applications of Nuclear Physics CHAPTER...

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45 Applications of Nuclear Physics CHAPTER OUTLINE 45.1 Interactions Involving Neutrons 45.2 Nuclear Fission 45.3 Nuclear Reactors 45.4 Nuclear Fusion 45.5 Radiation Damage 45.6 Radiation Detectors 45.7 Uses of Radiation ANSWERS TO QUESTIONS Q45.1 The hydrogen nuclei in water molecules have mass similar to that of a neutron, so that they can efF ciently rob a fast-moving neutron of kinetic energy as they scatter it. Once the neutron is slowed down, a hydrogen nucleus can absorb it in the reaction nH H . 1 2 +→ 1 1 Q45.2 The excitation energy comes from the binding energy of the extra nucleon. *Q45.3 Answer (c). The function of the moderator is to slow down the neutrons released by one F ssion so that they can efF ciently cause more F ssions. Q45.4 The advantage of a F ssion reaction is that it can generate much more electrical energy per gram of fuel compared to fossil fuels. Also, F ssion reactors do not emit greenhouse gases as combustion byproducts like fossil fuels—the only necessary environmental discharge is heat. The cost involved in producing F ssile material is comparable to the cost of pumping, transporting and reF ning fossil fuel. The disadvantage is that some of the products of a F ssion reaction are radioactive—and some of those have long half-lives. The other problem is that there will be a point at which enough fuel is spent that the fuel rods do not supply power economically and need to be replaced. The fuel rods are still radioactive after removal. Both the waste and the “spent” fuel rods present serious health and environmental hazards that can last for tens of thousands of years. Accidents and sabotage involving nuclear reactors can be very serious, as can accidents and sabotage involving fossil fuels. *Q45.5 Answer (b). In m = E / c 2 we estimate 5 × 10 13 J/9 × 10 16 m 2 /s 2 6 × 10 –4 kg = 0.6 g. *Q45.6 Answer (d). We compute 235 + 1 – 137 – 96 = 3. All the protons that start out in the uranium nucleus end up in the F ssion product nuclei. Q45.7 The products of fusion reactors are generally not themselves unstable, while F ssion reactions result in a chain of reactions which almost all have some unstable products. Q45.8 ±or the deuterium nuclei to fuse, they must be close enough to each other for the nuclear forces to overcome the Coulomb repulsion of the protons—this is why the ion density is a factor. The more time that the nuclei in a sample spend in close proximity, the more nuclei will fuse—hence the conF nement time is a factor. Q45.9 ±usion of light nuclei to a heavier nucleus releases energy. ±ission of a heavy nucleus to lighter nuclei releases energy. Both processes are steps towards greater stability on the curve of binding energy, ±igure 44.5. The energy release per nucleon is typically greater for fusion, and this process is harder to control.
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SM_chapter45 - 45 Applications of Nuclear Physics CHAPTER...

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