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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