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As I understand it, if a nucleus is excited with energy exceeding its ground state, it releases energy via gamma radiation. An example would be technitium 99m, a medical tracer with a 6 hour half life and 140 keV gamma radiation. What are the characteristics of ground state nuclei allowing them the ability to be converted to metastable isomers? Can any element be converted to a metastable isomer?

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Not my particular expertise, but I suspect that the requirement for a meta-stable state is that there are only "forbidden" transitions between the excited and ground states.

Look in particular at the spin and parity of the isomer and of the products. It is necessary to conserve total angular momentum and parity, so we get a set of selection rules.

I see in the wikipedia that $^{99m}Tc$ has a spin of 1/2 while the ground state has a spin of 9/2. That implies multiple photons must be released in the reaction, and each one imposes a factor of $\alpha \approx 1/137$ to the matrix element (or $\alpha^2$ to the rate). A low rate implies a long lifetime.

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very lucid answer! –  Richard Terrett Apr 11 '12 at 10:26

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