Assessment of new photosensors for fast timing applications with large scintillator detectors

The measurement of absolute nuclear transition probabilities is a very sensitive tool to study the structure of the atomic nucleus. Direct access to transition rates can be achieved via the lifetime of the nuclear levels de-populated in radioactive decay. The Advanced Time-Delayed (ATD) method, or Fast Timing, is a well-established technique to measure lifetimes down to a few ps. The development of the technique was based on the use of BaF₂ detectors, but a recent major breakthrough occurred with the introduction of LaBr₃(Ce) crystals, uniting excellent time response with much superior energy resolution than BaF₂ crystals. Relatively large LaBr₃(Ce) cylindrical detectors of typically 1.5"×1.5" are employed for fast timing, in combination with fast 2-inch photomultiplier tubes from Photonis such as the linear focused 8-stage XP20D0. Another option for a 2-inch fast phototube is the 8-stage Hamamatsu R9779, whose timing properties have already been tested with small LSO crystals. New possibilities are also offered by novel photosensors such as silicon photomultipliers, which are intrinsically fast. In this work we have investigated the performance of the Hamamatsu R9779 photomultiplier tube, and the viability of CeBr₃ crystals for fast-timing applications.