Lecture10NuclearPotentialsandRadioactiveDecay_001

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Lecture 10: Nuclear Potentials and Radioactive Decay I. Nuclear Stability and Basic Decay Modes A. Schematic Representation : Synthesis Equilibration Decay X + Y + Energy Z A * Z ( 10 20 s) ( ~ 10 16 10 20 s Composite Nucleus (Activated Complex) B. Stable Nuclei 1. N/Z composition: Does not change with time peak of <BE> curve Kinetic vs. Thermodynamic stability; detection limit 10 20 y 2. Total: 266 At least one stable nucleus for all Z=1 83 EXCEPT 43 Tc and 61 Pm C. Radioactive Nuclei 1. Definition: A nucleus that SPONTANEOUSLY alters its neutron/proton composition or energy state FIRST-ORDER RATE PROCESS RADIOACTIVE DECAY IS IDENTICAL WITH AN ELEMENTARY UNIMOLECULAR DISSOCIATION IN CHEMISTRY. ( A B + C) Contrast with: nuclear reactions – n/p changes induced by collisions, 2 nd order NUCLEAR REACTIONS HAVE THE SAME FORM AS AN ELEMENTARY BIMOLECULAR CHEMICAL REACTION (A + B C + D) 2. Half-life: t 1/2 Definition: The length of time required for one-half the nuclei in a sample to disintegrate (decay): N = N 0 e  t ; = 0.693 t 1/2
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3. Primary Decay Modes a. Alpha Decay: 2 4 He emission b. Beta Decay: neutron proton conversion -1 0 e ; specifies nuclear origin SAME PARTICLE e specifies atomic origin c. Gamma Decay: 0 0 , photon emission = nuclear origin ; x-ray, uv, visible, ir = atomic/molecular origin d. Exotic decay modes: fission, protons, neutrons, 14 C, etc. 4. Radioactivity in Nature (t 1/2 10 8 y) a. U–Th Decay series 92 238 U (4.5 10 9 y) 6 8 82 206 Pb (24.1%) A = 4n + 2 92 235 U (7.1 10 8 y) 4 7 82 207 Pb (22.1%) A = 4n + 3 90 232 Th (1.4
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