Gamma-ray spectroscopy with LaBr3:Ce scintillation crystal coupled to an ultra high quantum efficiency PMT

Hamamatsu has recently developed a new PMT, R7600U-200, with Ultra BiAlkali (UBA) photocathode (QE=42% @380 nm), replacing it in a BiAlkali (BA) PMT R7600U (QE=22% @380 nm). To evaluate the improvement in energy resolution, we coupled a LaBr3:Ce scintillation crystal, with size Ø12.5mm × 12.5 mm, to a UBA and to a BA R7600 PMT (22% Q.E.@ 380 nm). Moreover, the lanthanum crystal was coupled to a Hamamatsu R6231MOD, chosen as gold standard (30% Q.E.@380 nm). We utilized also a YAP and LSO scintillation crystals, in order to study the behavior of new photocathode with lower light emission than lanthanum tri-bromide The results from LaBr3:Ce crystal show an energy resolution improvement with UBA photocathode of about 20% in the energy range 32–1332 keV with respect to R7600, close to what expected from the improved quantum efficiency. This result is confirmed also with other two scintillation crystals. Unexpectedly the UBA results with Lathanum tri-bromide are in agreement with ones from R6231 PMT, thought its lower QE. This result is probably due to the different collection efficiency of first dynode stage related to different dynode structure of tubes.