Decay Chain - Beta Decay Chains in Uranium & Plutonium Fission Products

Beta Decay Chains in Uranium & Plutonium Fission Products

Since the heavy original nuclei always have a greater proportion of neutrons, the fission product nuclei almost always start out with a neutron/proton ratio significantly greater than what is stable for their mass range. Therefore they undergo multiple beta decays in succession, each converting a neutron to a proton. The first decays tend to have higher decay energy and shorter half-life. These last decays may have low decay energy and/or long half-life.

For example, uranium-235 has 92 protons and 143 neutrons. Fission takes one more neutron, then produces two or three more neutrons; assume that 92 protons and 142 neutrons are available for the two fission product nuclei. Suppose they have mass 99 with 39 protons and 60 neutrons (yttrium-99), and mass 135 with 53 protons and 82 neutrons (iodine-135), then the decay chains can be found in the tables below.

Nuclide Half-life
99Y 1.470(7) s
99Zr 2.1(1) s
99mNb 2.6(2) min
99Nb 15.0(2) s
99m2Mo 0.76(6) µs
99m1Mo 15.5(2) µs
99Mo 2.7489(6) d
99mTc 6.0058(12) h
99Tc 2.111(12)E+5 a
99Ru stable
Nuclide Half-life
135I 6.57(2) h
135Xe 9.14(2) h
135Cs 2.3(3)E+6 a
135Ba stable

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... always have a greater proportion of neutrons, the fission product nuclei almost always start out with a neutron/proton ratio significantly greater than what is stable for their ... Therefore they undergo multiple beta decays in succession, each converting a neutron to a proton ... The first decays tend to have higher decay energy and shorter half-life ...

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