Design and simulation of a three-detector instrument for accurate neutron H*(10) and energy distribution calculation

Abstract

Many efforts have been made to improve the energy response of neutron rem meters to match the fluence to ambient dose equivalent conversion coefficients(DCC) accurately. In this work, a design of a portable instrument based on three thermal neutron detectors is proposed and simulated. The detectors are embedded in different depth along the cylindrical high density polyethylene moderator and their count rates are recorded simultaneously. Two different H*(10) calculation methods have been introduced. One is linear correction method based on the combination of detector responses to approximate the DCC curve provided in ICRP74. The other is spectrum unfolding method which unfold the neutron spectrum at the measuring point for further H*(10) calculation. Monte Carlo simulations have been applied to calculate the response matrices of the three detectors. And standard neutron spectra are used to evaluate the performance of the instrument. As the results indicate, linear correction method is accurate and stable for H*(10) estimation, with mean relative prediction error of 8.45% for tested spectra. Spectrum unfolding method is not stable and have large H*(10) estimation error in some cases, but it can simultaneously derive neutron energy distributions with acceptable approximate degree. The instrument is highly suitable for H*(10) measurement for one-directional parallel neutron beam as well as isotropic neutron source with determined location. And by applying an absorber shell or detector arrays, the angular influence for the instrument is effectively improved to obtain more accurate H*(10) estimation results.