Lecture3DecayModesandLiquidDropModel_000 - 1 Lecture 3...

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1 Lecture 3 Nuclear Decay modes, Nuclear Sizes, shapes, and the Liquid drop mode l Introduction to Decay modes (continued) Gamma Decay Electromagnetic radiation corresponding to transition of nucleus from a higher excited state to a lower excited state. E E A Z A Z * Ag Ag m 110 47 110 47 ; t 1/2 = 249.8 d The ‘m’ in the superscript by the mass number denotes a “metastable state”. Notice that in gamma decay the atomic number of the parent and the atomic number of the daughter are unchanged. Sometimes a superscript g is written to denote the “ground state”. Three modes of gamma decay: a) Pure gamma emission b) Internal conversion (IC) c) Pair production (PP) a) Pure gamma emission : The gamma rays emitted by a nucleus in the gamma decay process are monoenergetic for each transition between energy levels. The gamma energies typically range from 2 keV to 7 MeV. Obviously energy must be conserved and momentum must be conserved. Thus, a small recoil energy/momentum is imparted to the daughter nucleus (can generally be ignored). Nevertheless, this energy is small so the energy of the gamma is very close to the energy of the transition. b) Internal conversion (IC): The excited nucleus de-excites by transferring its energy to an orbital electron. This energy exceeds the binding energy of that electron so consequently that electron is ejected. Xrays s ICelectron E E A Z A Z * The IC electrons are mono-energetic. Their kinetic energy is equal to the energy of the transition minus the binding energy of the electron. Internal conversion and pure gamma decay are competing processes! Because IC decay results in a vacancy in an atomic orbital in the daughter nucleus, the electrons in the daughter nucleus “shuffle down” resulting in the emission of X-rays and Auger electrons. As IC and pure gamma decay compete we can define the internal conversion coefficient ( ): ICdecay decay ;
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2 c) Pair Production : For nuclear transitions with energies greater than 1.022 MeV, it is possible for the decay energy to directly produce an electron-positron pair which is then ejected from the nucleus. Recall E=mc 2 where m= 2m e . The total kinetic energy of the pair is equal to the difference between the transition energy and 1.022 MeV (which is 2m e ). e.g O O m 16 8 16 8 ; E trans = 6.05 MeV ; t 1/2 = 7 x 10 -11 s Fission Spontaneous fission is the process in which a heavy nucleus breaks into two smaller nuclei. From the curve of <BE> we understand why this is energetically favorable.
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Lecture3DecayModesandLiquidDropModel_000 - 1 Lecture 3...

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