Proton in-beam test of a R 3 B CsI ( Tl ) prototype

The R 3 B calorimeter/spectrometer CALIFA[1] consists of 4000 CsI(Tl) crystals of 7 different shapes in a densely packed geometry. The prototype, produced at Lund University, belongs to the polar angular region 40°80°. It has 3x5 crystals, each with 10x30 mm 2 front area, 15x42 mm 2 back area, 10x10 mm 2 readout area and a total length of 130 mm. The extension of the detector part (100-130 mm) was used as light guide. 44-51% of the primary light was collected on the readout area in this geometry. The crystals were supplied by Amcrys-H Ltd. (Ukraine). On delivery, all surfaces were highly polished which results in a smooth exponential drop of the light output along the main axis from front area to back area. By applying a special technique of polishing/lapping the side areas, the uniformity, (LOmax LOmin)/LOaverage, was adjusted to be better than 0.7%. Two types of photosensors have been used – standard photodiodes (PD) which were used in the proton tests while avalanche photodiodes (APD) of Hamamatsu S8664-1010 type were used for gamma tests. Bench tests of the crystals using radioactive sources were carried out with the APDs operating at T = 22°C. The result is that the energy resolution in the range 0.569 2.50 MeV, can be approximated by the relation,