Understanding UCN production in solid D2: The generalized density of states measured via inelastic neutron scattering

The dynamics of solid deuterium [H. Nielsen, et al., Phys. Rev. B 7 (1973) 1626] (sD2) is studied by means of inelastic thermal neutron scattering (coherent and incoherent scattering) at different temperatures and para-ortho ratios. In this paper, the results for the generalized density of states (GDOS) are presented and discussed. The measurements were performed at the thermal time-of-flight (TOF) instrument IN4 at the ILL Grenoble, and at the cold neutron TOF instrument TofTof at the FRM II. The GDOS contains beside the hcp phonon excitations of the sD2 the rotational transition J=0→1 and 1→2. The measured and expected intensities of these rotational excitations are strongly depended on the ortho concentration in the sD2. Above E=10meV there are still strong excitations, which are very likely smeared out higher energy optic phonons and multiphonon contributions. An empirical method of separation of the one- and multiphonon contributions to the GDOS will be presented and discussed. The impact of these measurements on the UCN production in sD2 will be shown in detail.