Missing gamma lines and matrix-element hindrance

An absent otherwise-fast gamma transition can support a spin assignment that forbids it. It is not an absolute selection rule: a nuclear reduced matrix element may be small even when angular momentum and parity allow the mode. Meaningful limits require detection thresholds, branching normalization and competing-channel information. Assuming ordinary unhindered strengths can yield a useful conventional assignment, but that assumption must be distinguished from strict observed-line constraints.

If a cascade determines only absolute spin differences relative to a known reference spin, reflection of all inferred spins around that reference can preserve the same multipole pattern. Nonnegative-spin constraints, independently known spins, angular correlations or polarization measurements may break the ambiguity. A set of line energies and lowest-multipole labels need not do so.