The nuclear matrix element is given by: where an operator (H beta) converts the initial nuclear state Psi 0f into the final state Psi 0f and the subscript total implies we are including the entire final state, i.e. including daughter nucleus, emitted electron and anti - neutrino etc). If nuclear structure effects are very significant, this can influence the matrix element. If this was the case, would you expect comparative half - lives to vary? Take, as an example, a case where nuclear structure effects result in a small value of Mp; how would you expect this to change the half - life, if at all? Remember what the matrix element essentially 'means'.