I have been studying the uses of Tc-99m in nuclear medicine, but am confused about what actually causes Tc-99m to eventually decay to Tc-99. Logically it makes sense that it would eventually decay to the more stable isotope, but what mechanism eventually allows the 'forbidden transition' to occur? Background knowledge I have so far: In the general case of any quantum mechanical system, if the angular momentum of an excited isomer is significantly different to that of its isomer at ground state, the system may lack a decay route that allows it to emit gamma radiation. In order to make an isomeric transition to ground state, technetium-99m has an angular momentum that requires a nuclear spin change greater than one quantum unit (ℏ). Such an integral change is necessary for the emission of a gamma photon, but is defined as a ‘forbidden transition’, as it presents significant potential energy barriers that delay gamma emission.
Usually what happens is that we make approximations when calculating transition amplitudes. We normally apply some kind of perturbation theory. In some order of approximation we then arrive at forbidden transition rules but these are only forbidden to the approximation we are working but they may be some higher order terms that are not zero that contribute to the transition rates.