Test of LAAPDs Coupled to CsI(Tl) Crystals for the CALIFA R3B/FAIR Calorimeter

Several studies with different Large Area Avalanche Photo-Diodes (LAAPDs) coupled to ad-hoc bi-frustum shaped CsI(Tl) crystals have been carried out as a part of the R&D program for the CALIFA R3B/FAIR calorimeter. CALIFA, which is designed for the detection of light charged particles and gamma-rays in a wide energetic domain, has very stringent requirements. We report in this work our studies on the energy resolution for gamma-rays using LAAPDs as photosensors. One of the factors affecting the energy resolution is the matching between the APD active area and the crystal exit face. We present in this paper a procedure for characterizing APDs based on the measurement of their contribution to the energy resolution. The results obtained are very promising and suggest that a solution based on CsI(Tl) crystals coupled to LAAPDs could be suitable for, at least, part of the CALIFA calorimeter.