Radiation tests of CsI(Tl) crystals for the GLAST satellite mission

The electromagnetic calorimeter of the Gamma-ray Large Area Space Telescope (GLAST) consists of 16 towers of CsI(Tl) crystals. Each tower contains 8 layers of crystals (each 326.0 × 26.7 × 19.9 mm 3 ) arranged in a hodoscopic fashion. The crystals are read out at both ends with PIN diodes. Crystals produced by Amcrys-H are used. As a part of the quality control procedure during crystal production, samples from the uncut boules were systematically irradiated with gamma-rays from a 60Co source. The decrease in light yield due to radiation damage was measured, determining the radiation hardness of the boule. All boule samples passed the radiation hardness requirements, showing an average decrease in light yield of (14 ± 4)% after having received a dose of 200 Gy. Studies have also been carried out to verify the correspondence between the post-irradiation properties of the boule samples and the full-size crystals which are subsequently cut from the boule. A crystal log was irradiated with gamma-rays from a 60Co source and showed a decrease in light yield of (24 ± 4)% after a dose of 180 Gy. A full-size crystal was also irradiated with a 180 MeV proton beam and the radiation induced attenuation and induced radioactivity was studied. The light yield was found to have decreased with (22 ± 5)% after 175 Gy, and the dominant radioactive isotopes were identified.