High energy gamma-ray spectroscopy with LaBr3 scintillation detectors

Lanthanum bromide scintillation detectors produce very high light outputs within a very short decay time (typically ~20 ns) which means that high instantaneous currents can be generated in the photocathode and in the dynode chain of the photomultiplier tube (PMT). The net result is that signal saturation can occur long before the recommended PMT biasing conditions can be reached. In search of an optimised light readout system for LaBr3, we have tested and compared two different PMT configurations for detection of up to 15 MeV gamma-rays. This range was chosen as being appropriate for gamma-ray remote sensing and medium energy nuclear physics applications. For the scope we have conducted experiments at two facilities, the Laboratori Nazionali del Sud (LNS) in Catania, Italy [1] and the High Intensity Gamma-ray Source (HIγS) at Triangle University Nuclear Laboratory, in Durham, North Carolina, USA [2]. The PMT configurations we have tested are: 1) a standard dynode chain operated under-biased; 2) a 4-stage reduced chain operated at nominal inter-dynode bias. The results are that shortening the number of active stages, as in configuration 2), has advantages in preserving energy resolution and avoiding PMT saturation over a large energy range. Still the use of an under-biased PMT, configuration 1), can be considered a satisfactory solution, at least in the case of PMTs manufactured by Photonis. The results of this study will be used in support of the gamma-ray and neutron spectrometer (MNGS) on board of BepiColombo, the joint ESA/JAXA mission to Mercury, scheduled for launch in 2014.