Investigations of Neutron Moderation with a Pulsed Source

The slowing-down times in water, toluene, and heavy water were obtained from measurements of capture-gamma-ray counting rates as a function of time after injection of a neutron burst. The times to the 1.46-eV resonance in indium were 0.75 ± 0.5 μsec, 1.5 ± 0.3 μsec, and 4.0 ± 1.0 μsec for the three moderators. The corresponding times to a 0.4-eV energy in cadmium were 1.75 ± 0.5 μsec, 3.25 ± 0.3 μsec, and 10.5 ± 1.0 μsec, respectively. Time-gated pulse-height spectra measurements in a large liquid scintillation detector were made to separate fast- from thermal-neutron interactions by taking advantage of slowing-down-time spectrometry. Steady-state pulse-height spectra measurements in water and in water plus indium illustrated the application of prompt-gamma-ray analysis to determination of capture rates.