Inelastic scattering and spectral measurements of advanced cold moderator media

Inelastic neutron scattering with emphasis on low-lying modes as well as cold neutron leakage measurements were performed on four prospective advanced cold moderator materials. Employing the time-of-flight instrument SV29 and DNS at the Juelich FRJ-2 reactor, spectra were obtained from synthetic methane clathrate, tetrahydro-furane (THF) clathrate, 1,3,5-trimethyl-benzene (mesitylene) and light water ice at several temperatures between 2 and 70 K. Excitations at ±1,+2 and +3 meV were observed with synthetic methane clathrate. In mesitylene a wealth of low lying excitations were found. In the quenched phase we detected lines at 4.7, 7.2, 9.6, 13.6, 15.4, 18.4, 19.0, 23.0, 29.5 and 34.3 meV, respectively. In the annealed phase, we observed significant shifts with the majority of lines. The lowest lying lines now located at 7.0, 8.5 and 10.5 meV, respectively. In ice at T=2 K up to now unreported low-lying energy levels were found at 1.8 and 2.6 meV. An additional line at 10 meV was detected in THF clathrate. Mesitylene, synthetic methane clathrate and water ice, all at T=20 K, were tested as moderators at the Juelich spallation mock-up JESSICA. The expected gain in neutron leakage current at energies around 2 meV as compared to liquid hydrogen moderators was observed for methane clathrate and mesitylene