Inelastic neutron scattering study of the vibration frequencies of hydrogen in calcium dihydride

Abstract

We report incoherent inelastic neutron scattering measurements on commercially available standard grade CaH 2 (∼95% pure). This compound is of interest because of a current research programme on the transmutation of long-lived nuclear wastes. CaH 2 is one possible material that could be used for localised moderation of the neutron spectrum in the PHENIX Fast Reactor, to optimise the core for the transmutation of higher actinides using dedicated moderated targets. The particular advantage of CaH 2 is that it is believed to be relatively stable in liquid sodium. For the case of ZrH 2, modelling existing experimental core data requires the use of the MCNP Monte Carlo code using the detailed IINS cross-sections for ZrH 2 from the ENDF-B/VI file. No such data exist for CaH 2, hence the need for the present experiment. The measurements used the IN1 Be-Filter instrument at the ILL going from 15 to 200 meV, at both 20 °C and 10 K. The data at 20 °C show evidence of acoustic modes at about 20 meV, a strong feature at 40 meV due to Ca(OH) 2 impurities and two strong bands between 70 and 100 meV and between 110 meV and 140 meV—each split into three distinguishable peaks. This is consistent with the crystal structure, which has two H sites, one a distorted octahedral site (lower band) and the other a distorted tetrahedral (upper).