The GLAST Satellite Calorimeter

Abstract: The Gamma Ray Large Area Space Telescope (GLAST) is a multi-national satellite mission with aim to study the high-energy gamma ray sky. The main instrument is the Large Area Telescope (LAT) that consists of 16 towers, each housing a silicon strip precision tracker and a CsI(Tl) calorimeter. The calorimeter is made in a hodoscopic fashion from position sensitive crystal rods measuring 333.0 × 26.7 × 19.9 mm^3. Each calorimeter tower contains 8 layers of crystals (approximately 8.6 radiation lengths deep) with 12 crystals in each layer. The energy range is 30 MeV – 300 GeV. The crystal production comprised over 2000 crystals, of which 1536 build up the GLAST calorimeter and the rest are spare and test modules. All crystals had to pass an acceptance test. The quality control procedure included tests of optical and mechanical properties and radiation hardness. The test program was successfully developed and carried through within the given time frame. Light output was found to be within 10 % for more than 1800 crystals. Light output tapering could be made within the desired range for all crystals. Mechanical dimensions were held within ±0.3 mm. All data well met the design goals for GLAST. During production of flight crystals small sample crystals were irradiated using a hot 60 Co gamma source to verify the material’s radiation hardness. Correlation tests of full-sized crystals using both gamma rays and 180 MeV proton irradiation showed an accurate correspondence to the degradation of a sample crystal. The average light yield loss of a sample crystal after the specified total dose of 100 Gy was 12 %. All crystal boules were found to be well within the specified requirement of no more than 50 % light loss. An assembly of 48 crystals arranged in 8 layers of six crystals each was tested in an accelerated particle beam at CERN in order to verify the shower reconstruction capability of the GLAST LAT calorimeter. The crystals, wrap- ping material and read-out photo diodes were identical with flight specifications. The test calorimeter was calibrated with muons and shower reconstruction was performed with electron beams with energies of 20, 50, 80 and 120 GeV. The required performance of the GLAST calorimeter was specified to have an energy resolution σ of maximum 20 % (for photons) in the tested energy range, position resolution σ (for muons) better than 1.5 cm and angular resolution σ (for muons) better than 15 · cos^2 θ degrees (with θ being the off-axis angle). The analysis of the prototype calorimeter showed that the energy resolution σ could be made better than 13 %, the position resolution σ (for showers) was of the order of a few mm, both along the longitudinal axis of the crystal and transverse, and the angular resolution σ (for showers) was better than 1 degree.