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21 Nuclear Chemistry Solutions to Exercises = = 82.05 = d Check. Nₜ is given to 1 sig fig, so 8 X (70 - 90) days is a more correct representation of the time frame for decay. 21.54 Radioisotopes used as diagnostic tools are introduced into the body and carried to the point where imaging or some other diagnostic data is needed. We want the decay products of these radioisotopes to leave the body and do as little damage as possible on the way. Gamma rays are penetrating radiation and can escape the body more easily than other radioactive decay products. Also, gamma rays leaving the body can be easily detected using scintillation counters. This is particularly important when imaging is the goal of the procedure. Alpha emitters are never used as diagnostic tools because alpha particles are ionizing but do not move easily through the body. Trapped inside the body, alpha particles initiate the ionization of water, which ultimately produces free radicals that disrupt the normal operation of cells. 21.55 21.56 Enriched uranium is a uranium-containing substance that has had its natural amount of uranium-235 increased. It is different from natural uranium because it contains more than 0.7% uranium-235. 21.57 The control rods in a nuclear reactor regulate the flux of neutrons to keep the reaction chain self-sustaining and also prevent the reactor core from overheating. They are composed of materials such as boron or cadmium that absorb neutrons. 21.58 (a) A moderator slows neutrons so that they are more easily captured by fissioning nuclei. (b) Water is the moderator in a pressurized water generator. (c) Graphite is used as a moderator in gas-cooled reactors, and D₂O is used in heavy- water reactors. 21.59 (a) + (b) + 21.60 Analyze/Plan. Use conservation of A and Z to complete the equations, keeping in mind the symbols and definitions of various decay products. Solve. (a) + (b) 21.61 (a) Analyze/Plan. At these temperatures, assume the reaction occurs between nuclei rather than atoms. From Table 21.7, the nuclear mass of 2 He is 4.00150. The nuclear mass of is simply the mass of a proton, 1.007276467 amu. Note that ie is a positron, which has the same mass as an electron, 5.4857991 X 10⁻⁴ amu. Calculate the difference in mass between product and reactant nuclei, and the energy released by this mass change. Do the calculation in terms of moles and grams, rather than nuclei and amu. 1 mol amu = 1 g. Solve. 660