Editing Nuclear isomer (section)

==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, 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 ''ħ'' in the [[Spin (physics)|spin]] angular momentum. This change is necessary to emit a gamma photon, which has a spin of 1 unit in this system. Integral changes of 2 and more units in angular momentum are possible, but the emitted photons carry off the additional angular momentum. Changes of more than 1 unit are known as [[forbidden transition]]s. Each additional unit of spin change larger than 1 that the emitted gamma ray must carry inhibits decay rate by about 5 orders of magnitude.<ref>Leon van Dommelen, ''[http://www.eng.fsu.edu/~dommelen/quantum/style_a/ntgd.html Quantum Mechanics for Engineers] {{Webarchive|url=https://web.archive.org/web/20140405004809/http://www.eng.fsu.edu/~dommelen/quantum/style_a/ntgd.html |date=5 April 2014 }}'' (Chapter 14).</ref> The highest known spin change of 8 units occurs in the decay of <sup>180m</sup>Ta, which suppresses its decay by a factor of 10<sup>35</sup> from that associated with 1 unit. Instead of a natural gamma-decay half-life of 10<sup>−12</sup> seconds, it has yet to be observed to decay, and is believed to have a half-life on the order of at least 10<sup>25</sup> seconds, or at least {{val|2.9|e=17}} years.

Gamma emission is impossible when the nucleus begins in a zero-spin state, as such an emission would not conserve angular momentum.{{Citation needed|date=July 2019}}