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Feb_22_2010-1

# Feb_22_2010-1 - Chapter 24 Nuclear Reactions and Their...

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Chapter 24 Nuclear Reactions and Their Applications

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Beta decay – is a more general class of radioactive decay that includes three types: β - decay, β + emission, and electron capture. β + emission: Positron emission – occurs through a process in which a proton in the nucleus is converted into a neutron, and a positron is expelled. 1 1 p 1 0 n + 0 1 β 11 6 C 11 5 B + 0 1 β Electron (e-) capture (EC): This occurs when the nucleus interacts with an electron in an orbital from a low atomic energy level. The net effect is that a proton is transformed into a neutron. 1 1 p + 0 -1 e 1 0 n + 0 1 β 55 26 Fe + 0 -1 e 55 25 Mn + h ν (x-rays and neutrinos)

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Sample Problem 24.1 Writing Equations for Nuclear Reactions PLAN: SOLUTION: PROBLEM: Write balanced equations for the following nuclear reactions: (a) Naturally occurring thorium-232 undergoes α decay. (b) Zirconium-86 undergoes electron capture. Write a skeleton equation; balance the number of neutrons and charges; solve for the unknown nuclide. A = 228 and Z = 88 232 90 Th 228 88 Ra + 4 2 He (a) 232 90 Th A Z X + 4 2 He 86 40 Zr + 0 -1 e 86 39 Y A = 86 and Z = 40 (b) 86 40 Zr + 0 -1 e A Z X
Nuclear Stability and the Mode of Decay The Band of Stability - Key factors determine the stability of a nuclide: the # of neutrons (N), and protons (Z) and their ratio, that we calculate from (A-Z)/Z. Also influencing the stability is the total mass of the nuclide. - Figure (next page) shows the # of neutrons vs. # of protons for all stable nuclides. Important points: • The points form a narrow band of stability that gradually curves above the N = Z line. • Very few stable nuclides exist for N/Z <1; One is 3 2 He. • Many nuclides with N = Z are stable, such as 4 2 He, 12 6 C, 16 8 O, etc. The heaviest of these is 40 20 Ca. For lighter nuclides, one neutron for each proton (N = Z) is enough to provide stability. • The N/Z ratio gradually increases as Z increases. Examples include 56 26 Fe (N/Z = 1.15), 107 47 Ag (N/Z = 1.28), 209 83 Bi (N/Z = 1.52)

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Stability and Nuclear Structure Electrostatic forces between protons would break nucleus apart if not for the presence of an attractive force called the strong force . This strong force exists between all nucleons – protons and neutrons.

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Feb_22_2010-1 - Chapter 24 Nuclear Reactions and Their...

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