First direct proof of internal conversion between bound states

We present direct evidence for the process of internal conversion between bound atomic states (BIC) when the binding energy of the converted electron becomes larger than the nuclear transition energy. This process has been proposed as an explanation of the measured, unexpectedly short lifetime of the first excited state of ${}^{125}\mathrm{Te}$ with charge state larger than ${44}^{+}.$ We have detected the $K_{\alpha }$ x rays emitted in flight which follow the filling of the K-shell vacancy created by the bound internal conversion process, together with $\gamma$ rays from Te ions in charge states ranging between ${44}^{+}$ and ${48}^{+}.$ For ${\mathrm{Te}}^{45+}$ and ${\mathrm{Te}}^{46+},$ the comparison of the x-ray to $\gamma$-ray ratios with the theoretical calculations of the internal conversion coefficients including decay to bound atomic states, assuming Te ions in their ground electronic state, show poor agreement. The agreement becomes good if account is taken of BIC decay of excited initial states with different occupancies of the ${2p}_{1/2}$ and ${2p}_{3/2}$ subshells. In this situation, the half-life becomes sensitive to the precise initial state and simple specification of the charge state alone is no longer appropriate.