Study of D₂O Ice as a Cold-Neutron Source

The leakage flux from an 18 × 18 in. cylinder of D₂O with a beam of pile neutrons incident at its center has been studied at D₂O temperatures from 22° to 293°K. Intensities through beryllium and graphite filters, as well as indium foil transmissions, have been measured to determine cold-neutron fractions and neutron temperatures for the emerging spectra. The results of these measurements show that large volumes of D₂O ice can be useful as low-temperature moderators in reactors. The percentage of leakage neutrons with λn ≥ 3.95 Å is 21% at 22°K, a 20-fold increase over the fraction at 293°K, and about twice the value at 100°K. The neutron temperature of the leakage spectrum, calculated from the transmission data assuming a Maxwellian distribution, decreases with moderator temperature, reaching a value of about 75° for D₂O at 22°K. An abrupt increase in the fraction of cold neutrons is observed at the D₂O freezing point, which appears to reflect a change in the transport rather than the moderating properties of the D₂O, due to a decrease in the cross section for long-wavelength neutrons.