Experimental Validation of a Gamma Detector Based on 3" LaBr3:Ce:Sr Read Out by SiPMs in an Extended Energy Range

The GAMMA detector, a high-dynamic-range, high-resolution, gamma-ray spectrometer based on a 3" co-doped lanthanum bromide (LaBr
3
:Ce:Sr) crystal (73 ph/keV conversion efficiency, 25 ns decay time) coupled to 144 NUV-HD Silicon Photomultipliers (SiPMs) from Fondazione Bruno Kessler, has been validated in an extended energy dynamic range (80 keV - 15.1 MeV) showing state-of-the-art energy resolution along the entire range thanks to the automatic gain switching feature (Adaptive Gain Control, AGC) of the system. This is, to our knowledge, the first time that SiPMs are employed to measure such high energy, together with a wide dynamic range and excellent energy resolution (2.7% at 662 keV). The pixelated nature of the surface detector, combined with Machine Learning algorithms, can be exploited to reconstruct the position of first interaction of the gamma rays inside the scintillator crystal. In previous measurements made with a
¹³⁷
Cs collimated source, a spatial resolution of about 1 cm has been obtained. This position sensitivity capability has thus been exploited to compensate for relativistic Doppler broadening effects at which gamma rays at 15.1 Mev, emitted by carbon nucleus moving at relativistic speed (v/c ~ 0.05), are subjected. This made it possible to improve the FWHM resolution of the peak by approximately 15%, which is consistent with what is theoretically expected.