The construction and performance of the CsI hodoscopic calorimeter for theGLAST beam test engineering module

We describe the design, construction, and performance of the CsI hodoscopic calorimeter of the GLAST beam test engineering module (BTEM), a full-size prototype of one of the 16 towers of the GLAST large-area telescope, approv ed by NASA to be launched in 2005. The calorimeter is composed of 80 CsI cr ystals organized in a hodoscopic arrangement with eight layers of ten cryst als. The crystals are read out with PIN photodiodes at both ends. Light tap ering along the length of the crystals produces an asymmetry in the light m easured at the ends. The asymmetry is used for interaction positioning alon g the length of each crystal. The major design goals included the demonstra tion of a mechanical design to survive launch into space with minimal passi ve material, low power electronics with a dynamic range of similar to5 x 10 (5), and digital data acquisition with < 20 mus dead time per event. We wil l describe the design and give results from the analysis of the beam test i n winter 1999/2000 at the Stanford Linear Accelerator Center, where the BTE M was tested in gamma, positron, and hadron beams.