Characterization of Large Frustum CsI(Tl) Crystals for the ${\rm R}^{3}{\rm B}$ Calorimeter

Studies with different bi-frustum CsI(Tl) crystal samples from several providers, coupled to large area avalanche photodiodes (Hamamatsu S8664-1010 2CH), specifically developed for this project, were performed as part of the R&D program for the design of the R
³
B calorimeter (CALIFA). Particular attention is given to the energy resolution optimization for CsI(Tl) crystal samples with realistic dimensions. The results obtained with Large Area Avalanche Photo-Diode (LAAPD) are very promising and suggest CsI(Tl) as a suitable candidate for the Barrel part of the calorimeter.