Localisation of gamma-ray interaction points in thick monolithic CeBr3 and LaBr3:Ce scintillators

Abstract

Localisation of gamma-ray interaction points in monolithic scintillator crystals can simplify the design and improve the performance of a future Compton telescope for gamma-ray astronomy. In this paper we compare the position resolution of three monolithic scintillators: a 28×28×20mm3 (length×breadth × thickness) LaBr3:Ce crystal, a 25×25×20mm3 CeBr3 crystal and a 25×25×10mm3 CeBr3 crystal. Each crystal was encapsulated and coupled to an array of 4×4 silicon photomultipliers through an optical window. The measurements were conducted using 81keV and 356keV gamma-rays from a collimated 133Ba source. The 3D position reconstruction of interaction points was performed using artificial neural networks trained with experimental data. Although the position resolution was significantly better for the thinner crystal, the 20mm thick CeBr3 crystal showed an acceptable resolution of about 5.4mm FWHM for the x and y coordinates, and 7.8mm FWHM for the z-coordinate (crystal depth) at 356keV. These values were obtained from the full position scans of the crystal sides. The position resolution of the LaBr3:Ce crystal was found to be considerably worse, presumably due to the highly diffusive optical interface between the crystal and the optical window of the enclosure. The energy resolution (FWHM) measured for 662keV gamma-rays was 4.0% for LaBr3:Ce and 5.5% for CeBr3. The same crystals equipped with a PMT (Hamamatsu R6322-100) gave an energy resolution of 3.0% and 4.7%, respectively.