Light collection

EFC points approximately to the interaction point to make a tower geometry. The crystals are shaped in trapezoid and their dimensions of the front and rear surfaces are significantly different. As a result, scintillation lights generated along the crystal axes are not collected uniformly. A Monte Carlo simulation by GEANT indicates that a good uniformity of light collection is important to get a good energy resolution. Therefore, all crystals were matt-treated to achieve a uniform light collection. The largest surface of the forward crystals and the second largest surface of the backward crystals were made dim. All other surfaces were polished. The nonuniformity of light collection for various types of crystals was reduced to about 10 % level [7]. After the matt treatment, all the crystals were wrapped by a 100 $\mu$m Teflon sheet and a 20 $\mu$m aluminized Mylar sheet.