Population of excited electronic levels by beta decay, and its influence on Moessbauer source spectra

In a M\"ossbauer source experiment the β decay produces a large initial population of the excited electronic levels of the M\"ossbauer source atom. It is shown that this causes such excited levels to contribute more prominently in the source experiment than in the corresponding absorber experiment. These contributions can provide a particularly sensitive method for studying relaxation rates among the excited electronic levels. A detailed discussion and model calculations are given for dilute paramagnetic $4f$ impurities in a metal.