Neutron Thermalization Studies in Graphite

The neutron diffusion and integral scattering properties of graphite have been measured by the pulsed-neutron-source technique. Measurements were performed on assemblies with geometric bucklings from 0.0036 to 0.0151 cm⁻². The time of attainment of asymptotic neutron-energy spectra was determined from transmission measurements. Results from these latter measurements indicated that the minimum thermalization time, for the assemblies investigated, was ≈ 2 msec. Asymptotic ‘average’ neutron velocities, measured both internally and externally, were obtained as a function of assembly size. Results indicated that an asymptotic neutron spectrum did not exist for assemblies with B² ≈ 0.015 cm⁻² at a time 2 msec after the neutron pulse. A value of the diffusion cooling coefficient, C = 45.6 ± 4.7 × 10⁵ cm⁴/sec was derived from the measured ‘thermal’ neutron decay constants.