SLOWING DOWN OF ATOMS WITH KINETIC ENERGIES OF W LESS-THAN-OR-EQUAL-TO 5EV IN NOBLE-GASES - EVIDENCE FOR INELASTIC INTERACTIONS

Profiles of the 264.6 keV γ-line of75As emitted after the electron capture decay of75Se have been measured using nuclear resonance fluorescence in combination with γγ-coincidences as a high resolution, variable-energy γ 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 to W∼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. © 1979 Springer-Verlag.