Experimental evaluation of the Geant4-calculated response functions of a Bonner sphere spectrometer on monoenergetic neutron sources

Abstract

The response function of a Bonner sphere spectrometer (BSS) plays a crucial role in deriving reliable energy distributions of neutron fluence from the experimental count of each sphere by using unfolding techniques. When using Monte Carlo codes to calculate the responses of a BSS, the factors of Bonner spheres descripted in the codes, e.g., material density and geometrical parameters, will influence the accuracy of the calculated responses. Therefore, experimental evaluation of the calculations using neutron sources is essential for establishing the response matrix. A Monte Carlo code Geant4 was used to calculate the responses of a BSS based on a 3He gas counter. The uncertainties of the calculated responses were analyzed after considering some factors, including the air gap between the gas counter and spherical moderators, the mass density of moderators, and the 3He gas pressure in the counter. Good agreement was found between the calculated responses (within uncertainties) and the experimental results on six ISO recommended monoenergetic neutron sources with energies from 144 keV to 14 MeV. To evaluate the capability of the Geant4 code in establishing the response matrix of a BSS, the calculated responses before and after experimental evaluation were illustrated and compared with the results obtained by using the Monte Carlo code MCNP5 for the BSS response calculation. These comparison results provide primary support of the effectiveness of the Geant4 code in establishing the response matrix of a BSS.