Fisica Dos

45
Applications of Nuclear Physics
CHAPTER OUTLINE
45.1 45.2 45.3 45.4 45.5 45.6 45.7 Interactions Involving Neutrons Nuclear Fission Nuclear Reactors Nuclear Fusion Radiation Damage Radiation Detectors 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 efficiently rob a fast-moving neutron of kineticenergy as they scatter it. Once the neutron is slowed down, a hydrogen nucleus can absorb it in the reaction n + 1 H → 2 H. 1 1 The excitation energy comes from the binding energy of the extra nucleon.

Q45.2

*Q45.3 Answer (c). The function of the moderator is to slow down the neutrons released by one fission so that they can efficiently cause more fissions. Q45.4 The advantage of a fissionreaction is that it can generate much more electrical energy per gram of fuel compared to fossil fuels. Also, fission reactors do not emit greenhouse gases as combustion byproducts like fossil fuels—the only necessary environmental discharge is heat. The cost involved in producing fissile material is comparable to the cost of pumping, transporting and refining fossil fuel. The disadvantage is that some ofthe products of a fission 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 lastfor 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/c2 we estimate 5 × 1013 J/9 × 1016 m2/s2 ≈ 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 fission product nuclei.Q45.7 The products of fusion reactors are generally not themselves unstable, while fission reactions result in a chain of reactions which almost all have some unstable products. For 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 asample spend in close proximity, the more nuclei will fuse—hence the confinement time is a factor. Fusion of light nuclei to a heavier nucleus releases energy. Fission of a heavy nucleus to lighter nuclei releases energy. Both processes are steps towards greater stability on the curve of binding energy, Figure 44.5. The energy release per nucleon is typically greater for fusion, and this process isharder to control.
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Q45.8

Q45.9

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Q45.10 Advantages of fusion: high energy yield, no emission of greenhouse gases, fuel very easy to obtain, reactor cannot go supercritical like a fission reactor, low amounts of radioactive waste. Disadvantages: requires high energy input to sustain reaction, lithium and helium are scarce, neutrons released by reaction causestructural damage to reactor housing. *Q45.11 Answer Q1 > Q2 > Q3 > 0. Because all of the reactions involve 108 nucleons, we can look just at the change in binding-energy-per-nucleon as shown on the curve of binding energy. The jump from lithium to carbon is the biggest jump, and next the jump from A = 27 to A = 54, which is near the peak of the curve. The step up for fission from A = 108 to A = 54 issmallest. Both of the fusion reactions described and the fission reaction put out energy, so Q is positive for all. Imagine turning the curve of binding energy upside down so that it bends down like a cross-section of a bathtub. On such a curve of total energy per nucleon versus mass number it is easy to identify the fusion of small nuclei, the fission of large nuclei, and even the alpha decay of...