Isomerieverschiebung im Eu¹⁵¹

Abstract

The isomer shift of the 21.7 keVγ transition in Eu¹⁵¹ has been studied for various divalent and trivalent europium compounds using the Mössbauer technique. Theγ energy is lower by up to 1.1·10⁻⁶ eV in divalent compounds than in trivalent compounds. Using data from atomic spectroscopy it is estimated that the electron density at the nucleus is larger by about 1.9·10²⁶cm⁻³ for the configuration 4f ⁶ than for 4f ⁷ due to different shielding of thes ² shells. The difference of the mean square radii of the 21.7 keV state and the ground state is thenδ〈r²〉=+0.03 fm².

The measured isomer shift between trivalent and metallic europium and the relatives-electron densities in the rare earth metals measured by the positron annihilation rates are used to establish a calibration scheme for isomer shifts in rare earth metals. This calibration scheme is used to deduce a changeδ〈r ²〉=+0.005₅ fm² between the 26 keV state and the ground state of Dy¹⁶¹.