The Characteristics of a High Intensity 24 keV Iron-Filtered Neutron Beam

The spectrum and dose characteristics have been determined for a high intensity, iron-filtered, 24 keV, neutron beam facility recently installed in the PLUTO reactor at Harwell. The neutron energy spectrum of the beam, in the energy range 10 keV-1.5 MeV, was measured with the reactor at low power using a high resolution spectrometry system based on hydrogen proportional counters. The neutron fluence and dose rates were derived from these measurements, and scaled to full power using a 235U fission counter as a monitor of the beam. Activation neutron detectors were irradiated in the beam, with the reactor at full power, to estimate the fluence rate of neutrons outside the energy range of the hydrogen counter spectrometer. Spatial profiles of the neutron and gamma ray dose rates across the beam were determined using arrays of 6LiF (TLD-600) and 7LiF (TLD-700) thermoluminescence dosemeters (TLDs). The gamma ray dose rate in the beam was monitored with 7LiF TLDs. The hydrogen counter spectrometer and activation neutron detector measurements showed that 24 keV neutrons account for 93% of the fluence rate and 79% of the neutron dose rate in the beam, with only 2% of the fluence rate and a negligible fraction of the dose rate outside the range of the spectrometer. At full power, during the reactor cycle in which these measurements were made, the neutron fluence rate was 2.00 x 107 cm-2.s-1 (±12%), equivalent to a dose rate in water of 205 mGy.h-1 (±13%). The estimated systematic uncertainties on these values are given at 95% confidence limits. A small additional random uncertainty (±1.5% limits) arises from fluctuations in reactor power. The additional dose rate arising from the accompanying gamma rays in the beam was found to be 30 mGy.h-1 (±3%, systematic uncertainty; ±14%, random uncertainty; both values are 95% confidence limits). There is a ±20% variation in the neutron and gamma ray dose rates across the beam, which is ~50 mm in diameter.