Measurements of time-dependent neutron spectra from low-temperature ice block.

Abstract

The time-dependent neutron spectra from the surface of an externally pulsed finite block (156 mm diameter, 160 mm high) of ice were measured at 86, 35, 26 and 16 K over an energy range of 0.0006~0.02 eV and for durations of time extending from 100 to beyond 600 ,us following fast neutron injection.It was found that, at 86 and 38 K, asymptotic equilibrium distribution was established already around 400 ps after neutron pulse injection, whereas at 26 K, even after 800 f1S, the average neutron energy of the measured spectrum was still slightly higher than that of Maxwellian distribution corresponding to the moderator temperature, and further at 16 K, 900 fJ.S after neutron pulses, the difference between the averages of measured and Maxwellian distribution remained at about 50%.These results should usefully serve test in neutron scattering kernels, as well as studies for improving methods of calculation.