High Spin Suppression of Decay
The most common mechanism for suppression of gamma decay of excited nuclei, and thus the existence of a metastable isomer for the nucleus, is lack of a decay route for the excited state that will change nuclear angular momentum (along any given direction) by the most common amount of 1 quantum unit of spin angular momentum. Such a change is necessary to emit a gamma photon, which has a spin of 1 unit in this system. Integral changes of 2,3,4, and more units in angular momentum are possible (the emitted photons carry off the additional angular momentum), but changes of more than 1 unit are inhibited by about 5 orders of magnitude in rate for every additional unit of spin change larger than 1, that the emitted gamma ray must carry. The highest known spin change of 8 units occurs in the decay of Ta-180m, which suppresses its decay by a factor of 1035 from that associated with 1 unit, so that instead of a natural gamma decay half life of 10−12 seconds, it has a half life of more than 1023 seconds, or at least 3 x 1015 years, and thus has yet to be observed to decay.
Read more about this topic: Nuclear Isomer
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