Measurement of neutron spectral yield using Au foil based indigenous single sphere neutron spectrometer

Abstract

In the present work, the neutron yield and energy distribution from the (21 MeV ¹H + ^natBe) and (20 MeV ¹H + ^natTa) systems were measured using an in-house developed high density poly-ethylene based moderation assembly and Au foils as the thermal neutron detectors. These detectors were placed at different layers of the spherical moderating assembly ensuring variable thermalisation of the fast neutrons at each location. The combined setup acts as a concentric single sphere neutron spectrometer (SSNS). The major advantage of using Au foil is its superior quantitative reproducibility and possibility of multiple spectral acquisitions considering significantly large half-life of ¹⁹⁸Au (2.69 days). The response matrix of the moderation assembly along with the Au detectors placed at various depths in the moderation assembly has been evaluated using the FLUKA Monte Carlo simulations. Two separate unfolding approaches were used to construct the neutron spectra for both the reaction systems. Furthermore, an attempt has been made to study the effect of energy bins in the unfolded neutron spectral features and consequently on the resultant ambient dose equivalent values. For this purpose, a total of 5 different energy bins, viz. 31, 51, 71, 91 and 121 were tried out in the energy range between 0.1 MeV and 20 MeV for each unfolding method to generate the neutron spectra. The ambient dose equivalent estimates were arrived with the energy dependent neutron dose conversion coefficients and the estimated fluence spectra.