Radiation damage of CsI(Tl) crystals above 103 rad

CsI(Tl) crystals of 1×1×10 cm3 were exposed to 60Co γ rays with various doses in the range from 103 to 106 rad. The pulse height reduction, measured with a 137Cs isotope placed at the small end of the crystal opposite to a photomultiplier, varied from 28% at 1.58×103 rad to more than 85% at 1.83×106 rad. The radiation damage continued to be significantly saturated between 103 and 105 rad. Irradiation caused absorption of light at short wavelengths, mainly below 600 nm, making strongly irradiated crystals reddish. No sizable spontaneous recovery was observed. It was found that the damage quickly (within hours) recovers to a considerable extent by exposure to sunlight.