Recoilless gamma-emission from excited atomic states in europium

The Moessbauer effect of the 97-keV transition in 153Eu is known to be particularly suited for identifying very short-living atomic states of Eu produced after the electron capture decay of 153Gd by the isomer shift of the emission spectrum. In the experiments reported to date Gd2O3 had been used as source material. A more detailed analysis of the transmission spectra through Eu2O3 absorbers indicated asymmetric lines showing enhanced absorption when the emission line is shifted to higher energies. This observation led the authors to re-investigate the emission spectrum. Transmission measurements through the resonant absorber as a function of the Doppler shift of the source in respect to the absorber are given. The transmission spectrum with both source and absorber cooled to 4.2 degrees Kelvin is also given and exhibits clear asymmetry up to the highest Doppler shift, with enhanced absorption for higher energies. Ramifications for the possibility that certain excited states of Eu3+ are produced are discussed; in particular, the electronic configuration 4f65s25p6 may be responsible for the gamma emission indicating still larger isomer shifts. 7 refs, 1 fig