Pulse propagation experiments of thermal neutrons in graphite

Pulse propagation experiments of thermal neutrons were carried out in a graphite system to investigate the diffusion and the thermalization processes of neutrons in graphite. The principle of the thermal pulse source used was based on the distinct difference in the slowing down and the thermalization times between paraffin and graphite. From the Fourier transformed data of the measured propagating thermal pulses were derived the inverse attenuation length and the phase lag per unit length as functions of frequency. Peaks and caves distinctly observed in the gain and the phase lag curves at various distances from the source surface in the range of frequency above 2,000 rad/sec were attributed to the existence of a continuum eigen value region, due to cold neutron penetration into the frequency dependent attenuation. The diffusion and diffusion cooling coefficients were derived into the range below this frequency. The results (reduced to 20℃, 1.6 g/cm³ density) are: D₀ = (2.16 ± 0.02) x 10⁵ cm²·sec^-1, C₀ = (2.8 ± 0.3) x 10⁶ cm⁴·sec^-1. The results of die away experiment using the same graphite under critical buckling are: D₀ = (2.21 ± 0.08) x 10⁵ cm²·sec^-1, C₀ = (2.6 ± 1.2) x 10⁶ cm⁴·sec^-1. This agreement is obtained by rejecting the data in the continuum region, and the present values are closer to the more recent theoretical values than derived from experiments by other authors. (author)