Forbidden Transitions
Beta decays can be classified according to the L-value of the emitted radiation. When L > 0, the decay is referred to as "forbidden." Nuclear selection rules require high L-values to be accompanied by changes in nuclear spin (J) and parity (π). The selection rules for the Lth forbidden transitions are:
where Δπ = 1 or -1 corresponds to no parity change or parity change, respectively. The special case of a 0+ → 0+ transition (which in gamma decay is absolutely forbidden) is referred to as "superallowed" for beta decay, and proceeds very quickly by this decay route. The following table lists the ΔJ and Δπ values for the first few values of L:
| Forbiddenness | ΔJ | Δπ |
|---|---|---|
| Superallowed | 0+ → 0+ | no |
| Allowed | 0, 1 | no |
| First forbidden | 0, 1, 2 | yes |
| Second forbidden | 1, 2, 3 | no |
| Third forbidden | 2, 3, 4 | yes |
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