Study of the thermal neutron activation of a gamma-ray detector for BNCT dose monitoring

We present a study on the neutron activation of a gamma-ray detector for a BNCT-SPECT dose imaging system. The detector is based on a LaBr3(Ce+Sr) scintillator crystal, coupled with a matrix of Silicon Photomultipliers (SiPMs), read by a dedicated electronics system. This detector has successfully demonstrated to be capable to identify the 10B compounds when irradiating borated vials with thermal neutrons. However, a background signal around 478 keV was detected, suggesting the activation of the detector itself. This study aims to determine the origin of this background signal by simulating the two main parts of the detector, which are the crystal and electronic boards, in order to assess their contribution to the background signal. The results of the FLUKA simulations show that the neutron capture reactions on both the crystal and electronic boards cause a relevant background nearby the BNCT signal, thereby limiting the detector's sensitivity. To address this issue, a customized cadmium shielding has been developed. This solution was tested at the TRIGA Mark II research nuclear reactor of Pavia University, where experimental measurements and corresponding FLUKA simulations proved its effectiveness.