Counting Calories : Studies of Energy Loss in a Segmented Calorimeter

Abstract: The Gamma-ray Large Area Space Telescope (GLAST) is a space mission with anticipated launch in 2007. The aim is to study the gamma-ray sky in the energy range 10 keV – 300 GeV. The GLAST electromagnetic calorimeter is 8.6X0 deep and has a total of 1536 CsI(Tl) crystals each measuring 32.62.x2.67x1.99 cm3. The crystals are read out at both ends with PIN photodiodes.As a part of the quality control procedure during crystal production, samples from all CsI(Tl) boules were irradiated with gamma-rays from a 60Co source. The average decrease in light yield was found to be (13.4±0.7)% after 100 Gy, 5 times the expected dose during a 5 year mission. The correspondence between the radiation damage of boule samples and the full-sized crystals were examined. A full-size crystal was also irradiated with a 180 MeV proton beam, with a decrease in light yield of (22±5)% after a dose of 175 Gy. The induced radioactivity of the crystal was also studied and a number of radioactive nuclei could be identified.Detailed simulations of electron- and photon-induced showers in a segmented CsI calorimeter have been performed in the energy range 1 – 100 GeV, and for incident angles 0°, 30° and 60°. The energy distributions in the 1.99 cm (1.08X0) thick layers were found to change systematically along the shower, varying little with initial energy. Three probability distributions have been fitted to the data: negative binomial, log-normal and Gaussian distributions. The energy fluctuations generated from non-perpendicular incident were similar to those for perpendicular incidence.Two energy reconstruction methods were also studied, shower profile fitting and leakage correction. The latter gave better energy resolution, 5.1% and 7.6% (6.6% and 9.1%) for 1 GeV and 10 GeV electrons (photons) in the segmented 8.6X0 calorimeter. A third method, using a maximum likelihood method is also discussed.