Development of a Bonner Sphere Neutron Spectrometer from a Commercial Neutron Dosimeter

TL;DR

This paper presents the development of a neutron spectrometer based on Bonner Spheres, extending a commercial neutron dosimeter with new shells and a spectral unfolding algorithm, enabling accurate neutron spectrum measurements from thermal to 20 MeV.

Contribution

The paper introduces a novel Bonner Sphere neutron spectrometer built from a commercial dosimeter, including a new spectral unfolding code using genetic algorithms.

Findings

Calibration uncertainty of ±8.6%

Background neutron count rate of 1.57×10⁻³ s⁻¹

Effective spectral unfolding with the NSUGA code

Abstract

Bonner Spheres have been used widely for the measurement of neutron spectra with neutron energies ranged from thermal up to at least 20 MeV. A Bonner Sphere neutron spectrometer (BSS) was developed by extending a Berthold LB 6411 neutron-dose-rate meter. The BSS consists of a $^{3}$He thermal-neutron detector with integrated electronics, a set of eight polyethylene spherical shells and two optional lead shells of various sizes. The response matrix of the BSS was calculated with GEANT4 Monte Carlo simulation. The BSS had a calibration uncertainty of $\pm 8.6\%$ and a detector background rate of $(1.57 \pm 0.04) \times 10^{-3}$ s$^{-1}$. A spectral unfolding code NSUGA was developed. The NSUGA code utilizes genetic algorithms and has been shown to perform well in the absence of a priori information.