The Phoswich Detectors

Each detector is composed by two crystals of NaI(Tl) and CsI(Na) optically coupled according to the PHOSWICH (PHOSphor sandWICH) configuration ([Knoll, 1989]). The NaI(Tl) and CsI(Na) crystals are 3 mm and 50 mm thick, respectively. The scintillation light produced in each phoswich is collected through a quartz light pipe into a PMT EMI D-611 (fig. ). The detector window of each PDS unit is made of beryllium (1500 $\mu$m), Silicone rubber (89 $\mu$m), aluminized Kapton ($0.1$ $\mu$m Al and 76 $\mu$m Kapton) and 100 $\mu$m Teflon film.

Thanks to the different decay constants of the crystals ($0.25$ $\mu$s for the NaI(Tl) and $0.6$ $\mu$s and $\sim 7$ $\mu$s for the CsI(Na)), it is possible to distinguish between three possible event classes: the events detected only in the NaI(Tl), those only in the CsI(Na) and those detected in both. Because the NaI(Tl) plays the role of X-ray detector, only the events belonging to the first class described above are accepted, while the others are rejected.