Measurements of Time Dependent Energy Spectra of Neutrons in a Small Graphite Assembly

Abstract

The time-dependent energy spectra of neutrons have been measured in a small 30 x 3 0 x 30 cm3 graphite assembly by means of the linac-chopper method, with a view to establishing experimental evidence that there is no asymptotic spectrum in such a small assembly, and in order to study the non-asymptotic behavior of neutrons.The arrangement of a polyethylene pre-moderator adjacent to the assembly made the measurements possible with the improvement obtained thereby of the neutron counting statistics. It was indicated from calculation that the presence of the premoderator had little effect-at least above the Bragg cut-off energy-on the evolution in time of the energy spectra of neutrons in the graphite assembly.T h e experimental results indicated very probable disappearance of asymptotic spectra, and revealed significant enhancement of trapping at Bragg energies with the lapse of time. This is consistent with the results of pulsed neutron experiments in small assemblies conducted by Takahashi et al., and falls in line with de Saussure's approximation.The spectra in the graphite assembly showed significant space dependence, the spectra becoming harder with increasing distance from the pre-moderator. This hardening may be attributed to the relatively faster propagation of higher energy neutrons.

Measurements of time-dependent neutron spectra from low-temperature ice block.

“…The data-acquisition system has already been described elsewhere ( 9 ) and the following description will be limited to a summary recapitulation. The time interval between fastneutron burst and mid-point of chopper opening, which determines the delay time, is controlled by phasing the linac and chopper.…”

“…INTRODUCTION using this combination of pulsed neutron source and chopper< 2 )- ( 9 ). Study of the changes in time shown by pulsed fast neutrons in various media is useful for investigations on the process of neutron thermalization in moderating material.…”