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Depth dose, dose equivalent and quality factor for reactor neutron spectra.

The distributions of absorbed dose, dose equivalent, and quality factor in the depth of a cylindrical phantom are calculated for reactor neutron spectra under a variety of shield conditions. The calculations are reported for 12 neutron spectra, which include three experimental critical assemblies-HPRR, VIPER and VINCA-and light water reactor neutrons leaking through various shields such as Al, Pb, Fe. Be, graphite, concrete, etc. The spectral dependence of H(n, y ) dose, total absorbed dose, dose equivalent, and quantity factor are discussed. For spectra degraded by low-Z materials without capture, the dose at a depth of 1 cm persisted even up to 5 cm; the effective quality factor for the spectra studied ranged from 5 to 9.7 at a depth of 1 cm. The latter remained practically constant (7.2) at all depth for light water shields (60cm). In comparison, a rapid decrease was observed for spectra leaking through high-Z materials. INTRODUCTION USING Monte-Carlo methods (Au68; NCRP71) we can calculate distribution of absorbed dose, dose equivalent, and quality factor in the depth of a cylindrical phantom for broad, parallel beams of monoenergetic neutrons. These calculations have been extended to the Cf-252 neutron spectrum (Jo71) and the Health Physics Research Reactor spectrum at Oak Ridge National Laboratory (Jo74). Murthy et al. (Mu74) computed the values of absorbed dose, dose equivalent, and quality factor for three volume elements representing the surface, center, and back of the cylindrical phantom for a few degraded fission neutron spectra. Most of the neutron spectra used by Murthy et al. were obtained by threshold detectors that may not represent the actual spectra from these assemblies. A collection of depth dose curves giving the distribution of absorbed dose, dose equivalent, and quality factor for a large number of fission neutron spectra, obtained under a variety of fission source and shield conditions, will be extremely useful in deriving specific organ doses and assessing relative hazards. A compendium of neutron spectra likely to be encountered in criticality accidents has been prepared by Ing and Makra (In77) under an IAEA research contract. We have initiated a program for calculating depth dose data for this compendium of spectra. This paper, however, gives the absorbed dose, dose equivalent and quality factor data for 12 extremely degraded neutron spectra with different kinds of shield materials. Three experimental critical assemblies with widely varying neutron leakage spectra HPRR, VIPER and VINCA are also included in this study. METHODS The reactors considered for this computation are listed in Table 1. The HPRR asse’mbly has relatively undegraded spectrum with a small 1/E component (Ha72). The VINCA assembly has predominantly a 1/E spectrum. The VIPER assembly has a highly softened spectrum in the fast neutron region with most of the flux between 1 keV and 1 MeV (De76).