Distance-dependency of the D2O-moderated 252Cf spectrum and influence on the calibration factors

Abstract

The D2O-moderated 252Cf is a non-point neutron source used as a calibration facility for measuring devices. The Bonner spheres spectrometer (BSS) is one of the common instruments for neutron spectrometry. Due to having a non-point source, the neutron spectrum depends on distance. On the other hand, because of the detector and source size, there is a deviation in BSS reading from that expected according to the inverse square law. In the present study, the Monte Carlo simulations were applied to investigate the effect of beam divergence on the response of BSS. The average energy of neutrons reaching the surface of the BSS was dependent on distance, and its value increases with increasing distance. Subsequently, the response of the BSS under plane-parallel beams was also dependent on the distance. Thus, for 2″, 3″, and 5″ spheres, with increasing distance, the response decreased, and for larger spheres, it was the opposite. The calculated geometry correction factors for 12″, 15″ and 18″ spheres were less than one at all distances; however, for other spheres, at closer distances they were higher than one, and gradually reduced to below the one by increasing the distance.