Effect of cold neutrons on pulse propagation in graphite

Abstract

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.

Neutron pulse propagation in graphite

“…Positive, zero and negative values have been reported. This problem, and the role of the F term in the h-expression, was discussed by Hanna (1974). It is interesting to note that although pulse decay experiments have resulted in positive and negative F values, all pulse propagation experiments have led to positive values.…”

“…After estimation of a: and .$ and their associated errors for the various frequency components, the functions (012-.$2) and 2 4 were fitted by the weighted least-squares technique to the well-known polynomials (see for example Perez and Booth 1965) a : 2 -p = c Pnw" A computer program was written to perform this fit and to evaluate the P n parameters and their associated errors. Goodness of fit criteria were used to determine the order of each polynomial (the criteria and a detailed analysis of a similar problem have been published by Hanna 1974). This last point is important since adopting the wrong order of polynomial could lead to significant errors and misleading results, Hanna (1974) reviewed such errors for pulse decay experiments.…”

“…Goodness of fit criteria were used to determine the order of each polynomial (the criteria and a detailed analysis of a similar problem have been published by Hanna 1974). This last point is important since adopting the wrong order of polynomial could lead to significant errors and misleading results, Hanna (1974) reviewed such errors for pulse decay experiments. The dependence of the deduced parameters, and of the polynomial order, on the frequency range employed was also tested.…”