Time Dependent Neutron Spectra in Graphite

Abstract

An experiment is described in which the 28-Mev electron linac at Harwell was used as a neutron source to produce 1 mu sec bursts of 10/sup 11/ electrons 160 times per second. A natural uranium target was placed centrally against one face of a 60 x 62.2 x 71.1 cm graphite block, and a neutron beam extracted from a channel running into the center of the block. This beam was interrupted by a slow chopper running in synchronism with the pulses from the accelerator. In this way 100 mu sec time samples'' of the neutrons in the block were taken and their spectrum determined by the time-of-the-flight method. The instant of the time sample relative to the fast neutron pulse could be varied by an electronic delay circuit. Neutron spectra are presented for the time varying between 300 and 1000 mu sec after the pulse. These spectra differ from the Maxwellian shape but approach asymptotically to a cooled Maxwellian'' from which they are indistinguishable after 1000 mu sec. Detailed calculations of the spectra were made using an IBM 7090 computer to obtain a numerical solution of the time and energy dependent diffusion equation using a scattering kernel based on themore » scattering measurements made at Chalk River. The experimental results are also compared with spectra using a scattering kernel based on the heavy gas model with a fictitious mass of 33. (auth)« less