The Use of Threshold Detectors to Determine Changes in a Fast Neutron Energy Spectrum with Depth in a Phantom

Abstract

Experiments have been carried out with threshold detectors in a tissue equivalent phantom, to determine the effect of the phantom on the energy spectrum of a broad and a narrow beam of fast neutrons. Detectors were chosen with activation thresholds covering the required energy range (about 0-19 MeV), and also to give sufficient induced activity for a reasonable irradiation time. Discs of detector materials were irradiated at various depths within the phantom, up to 30 cm. The decay curve for each was determined, and compared with that of the detectors irradiated in air. Results indicate that the beam hardens slightly at increasing depths. Spectral change is slightly greater for the narrow beam than for the broad beam, but in either case insufficient to cause significant change in R.B.E.