Nuclear internal conversion between bound atomic states

We present experimental and theoretical results for rate of decay of the (3/2)+ isomeric state in $^{125}$Te versus the ionic charge state. For charge state larger than 44 the nuclear transition lies below the threshold for emission of a K-shell electron into the continuum with the result that normal internal conversion is energetically forbiden. Rather surprisingly, for the charge 45 and 46 the lifetime of the level was found to have a value close to that in neutral atoms. We present direct evidence that the nuclear transition could still be converted but without the emission of the electron into the continuum, the electron being promoted from the K-shell to an other empty bound state lying close to the continuum. We called this process BIC. The experimental results agree whith theoretical calculations if BIC resonances are taken into account. This leads to a nuclear decay constant that is extremely sensitive to the precise initial state and simple specification of the charge state is no longer appropriate. The contribution to decay of the nucleus of BIC has recently been extended to the situation in which the electron is promoted to an intermediate filled bound state (PFBIC) with an apparent violation of the Pauli principle. Numerical results of the expected dependence of PFBIC on the charge state will be presented for the decay of the 77.351 keV level in $^{197}$Au