On the decay of148Pm

In the frame of a soluble coupled channels model we compare the exact lifetimeτ of a resonant state with two approximate estimates for thisτ. The exactτ value is related to the exact total widthΓ byτ=?Γ ^?1. The first approximate method has been derived by Fermi. It consists in a first order perturbation calculation ofΓ. The second one is Smith's time-delay method which leads to a direct relation betweenτ and the behaviour of theS-matrix along the real energy axis. We show that the Smith's method is the most useful one.