= 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.
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