The slowing down of atoms with kinetic energies ofW≲5 eV in Noble Gases: Evidence for inelastic interactions

Profiles of the 264.6 keVγ-line of⁷⁵As emitted after the electron capture decay of⁷⁵Se have been measured using nuclear resonance fluorescence in combination withγγ-coincidences as a high resolution, variable-energyγ spectrometer. The sources consisted of gaseous⁷⁵SeH₂ with different amounts of Kr or Xe. From the line profiles information has been obtained about the velocity distribution of the radioactive⁷⁵As ions after electron capture decay and about the slowing down of the⁷⁵As ions in the moderator gas. The⁷⁵As ions obtain kinetic energies of up toW～5 eV by neutrino recoil and Coulomb fragmentation of the⁷⁵SeH₂ molecules, corresponding to velocities of the order of 10⁵ cm s^?1. The slowing down of the⁷⁵As ions has been interpreted in terms of the model of colliding elastic spheres, taking into account the thermalization in an exact way. No satisfactory agreement has been obtained between measured and calculated line profiles. This result indicates that inelastic processes are of great importance in the interaction of⁷⁵As ions or atoms with the noble gas moderator atoms.