# Creation of Nuclear Isomers

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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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.