Spectroscopic performance of a Sr co-doped 3” LaBr 3 scintillator read by a SiPM array

LaBr3:Ce crystals represent a key element for high-resolution gamma-ray detectors based on indirect conversion. Recent developments in crystal technologies have brought to the market Sr2+ co-doped 3" scintillators with a light yield 28% higher than standard LaBr3 crystals. Concurrently, high-density silicon photo-multipliers (SiPMs) have enabled the possibility of solid-state readout of these crystals, providing state-of-the-art energy resolution and wide dynamic range. In order to assess the performance of a solid-state readout of a large LaBr crystal, we developed a low-noise readout instrument based on a 12 x 12 array of 6 mm x 6 mm NUV-HD SiPMs produced by FBK. The array of photo-detectors was coupled to a custom-developed 8-channel ASIC with automatic gain switching named GAMMA. Detector biasing, data acquisition, processing and transfer are performed by a compact microcontroller-based unit. The resulting instrument was tested with two different crystals: a standard 3" LaBr3:Ce3+ and the Sr2+ co-doped version. Thanks to the low-noise performance of SiPMs and of the electronics it has been possible to investigate the ultimate noise performance of the crystal and compare the results between the two crystals. A 3.4% FWHM energy resolution at the Cs-137 662 keV photopeak was measured with the standard LaBr3 crystal, while it improved to 2.6% at 662 keV with the co-doped crystal. Beyond presenting the unprecedented spectroscopic performance of the 3" co-doped crystal, this work is a further demonstration of the SiPM consolidating technology suitability to replace photo-multiplier tubes (PMT) for demanding applications in nuclear physics and astrophysics.