Page "Stable isotope" Paragraph 41

The positivity of energy release in these processes means that they are allowed kinematically ( they do not violate the conservation of energy ) and, thus, in principle, can occur.

They are not observed due to strong but not absolute suppression, by spin-parity selection rules ( for beta decays and isomeric transitions ) or by the thickness of the potential barrier ( for alpha and cluster decays and spontaneous fission ).