Measurements of Neutron Slowing Down Time in Graphite by Capture Gamma-Ray Detection

摘要

The neutron slowing down time in graphite was measured by means of capture γray detection. Bursts of fast neutrons produced by D-T reaction were injected into a graphite pile. Detection was by NaI(T1) scintillator covered by metallic foil of either In, Cd or Gd, serving as resonance absorber. The slowing down time was determined from the maximum point of the reaction rate registered by this counter and presented in the form of time spectrum. The values thus determined were 19.2±0.6, 39±3, 43.0±0.7, 46.1±1.0, 47.5±0.8, 55±2 and 85±6 μs, for 1.58, 0.48, 0.44, 0.40, 0.36, 0.30 and 0.17 eV respectively. These values agree well with calcu-lated based on crystal model. The time response of the resonance detector also was in reasonable agreement with calculation. The effect of the medium being finite and that of detector position on the observed slowing down time were estimated. From this experiment, it became clear that the process of the neutron slowing down shows some definite influence of the crystal effect of graphite and is explained pretty well by Williams' model.