Measurement of Neutron Slowing Down Time in Light Water, Ice, Paraffin and Santowax

Abstract

Measurements of the neutron slowing down times in light water, ice, paraffin and santowax have been made by pulsed neutron technique. Bursts of D-T neutrons of 0.1 μsec width were generated in moderator cubes of 40×40×40 cm³. The slowing down neutrons were detected by bare as well as energy-selective filter-covered BF₃ counters, and analyzed with a 256-channel time analyzer. Slowing down times in the moderator were determined by interpreting the in-crement of the difference of events between the two counters as attributable to the fraction of the neutrons slowing down at the time of measurement below the cut-off energy of the filter.
The measured slowing down times below 0.63 and 0.43 eV agreed well with theoretical values on the 0°K free gas model. On the other hand, the measured values below 0.20 eV were found to be appreciably greater than the theoretical, which would appear to indicate that he effect of thermal agitation and chemical binding come into play at this range of energy.