Measurements have been made of the propagation of intense neutron pulses in a graphite assembly of transverse dimensions 137.2~ 127.6 cnP. It was found that, by scattering the pulsed beam and by proper positioning of the detector, the dispersion parameters could be measured over a relatively wide frequency range without significant contamination by higher space and energy modes. The resultsshowed that the Haywood-Page phonon model gives an adequate description of neutron wave interaction with graphite over a fairly wide frequency range. From the observed dispersion, parameter values of (2.092 & 0.006) x 105 cm2 s-1 for the diffusion coefficient and (34.1 I: 0.7) x lo5 cm4 s-1 for the diffusion cooling coefficient were inferred at a graphite density of 1.685 g cm-3, These figures agree with those obtained earlier on the same type of graphite using a pulse decay technique. The value obtained for the diffusion cooling coefticient is more precise than any reported previously.

A pulsed neutron beam from the thermal column of a reactor has been used to study neutron pulse propagation in graphite. A peculiar irregularity (an anomalous peak occurring rather earlier than the main peak) has been observed in the shape of the propagating pulse. The anomalous peak fist appears at long distances from the source and its relative magnitude increases quickly with distance. This phenomenon does not appear to have been reported elsewhere. Observations of the effect on the anomaly of changes in source conditions, source separation, and transverse buckling suggest that it is attributable to interference between sub-Bragg and thermal neutron waves in the frequency range around 2500 rad s-1, and not to direct penetration by cold neutrons. The observed phenomenon is very sensitive to source conditions and experimental configuration.

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