# What allows 'forbidden transitions' to eventually occur?

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.

• quantum tunneling – 0tyranny 0poverty Nov 11 '17 at 5:23