How can metastable isotopes exist?

[21joanna12]

I thought I would ask this question in the quantum mechanics section as I presume it has something to do with QM, like the reason why alpha and beta decay do not happen immediately but instead there is a half life for this decay. My understanding is that with alpha decay, the strong nuclear force means that the energy required for the alpha decay is more than the nucleus has available, so you need quantum tunnelling for this to occur, which has a definite probability of happening. This gives rise to a half-life. When it comes to beta minus decay, I think that the nucleus is in a higher energy state when you have the extra proton due to repulsion of protons, and the nucleus does not have sufficient energy to go into this higher energy state classically, but with quantum tunnelling it can do this. Once it has gotten into this 'higher energy state', it actually becomes a lower energy state due to the Pauli exclusion principle. With regards to beta plus decay, I presume that the energy barrier is due to the Pauli exclusion principle but it drops into a lower energy state once beta minus decay has occurred due to the smaller force of electrostatic repulsion. But with metastable isotopes I am have absolutely no idea. I just don't understand what could be preventing an isotope from emitting a photon to drop into a lower energy state.

Please correct me if something I have said is completely wrong! Thank you in advance for any replies! :)

 

[mfb]

Metastable isotopes offen have unsual spin states - emitting a single photon to reach another state is "problematic" in terms of angular momentum conservation. To find another analogy here, the photon might have to get so much angular momentum it cannot come from inside the nucleus.