On optical bleaching of barium fluoride crystals

Barium fluoride (Baf2) crystals were considered by the GEM Collaboration, to be used in constructing a precision electromagnetic calorimeter at the SSC [1] . The key issue of using this crystal at the SSC is its radiation resistance [2] . An Expert Panel was assigned by the SSC laboratory to review the radiation damage problem of BaFz. The Panel met in December 1991 and again in January 1992 following a site visit to the Shanghai Institute of Ceramics (SIC) and Beijing Glass Research Institute (BGRI) by selected Panel members and the GEM Spokesmen. The principal conclusion of the Panel's February 1992 report is "there is no apparent fundamental reason why BaF2 cannot be made radiation hard" [3] . The conclusion from the BaF2 Panel is based upon existing data obtained by that time . Some highlights are summarized below: The BaF2 radiation damage caused by photons shows clear saturation . Both transmittance and light output measured after irradiation do not degrade further after initial dosage of a few tends krad to 100 krad, depending on the quality of the BaF2 crystals ; (figs . 12 and 14 of ref . [21) . There is no permanent damage caused by either photons or neutrons . All damage recovers in full after an annealing at 500°C for three hours; (figs . 12 and 17 of ref. [2]). There is very slow spontaneous annealing of the transmittance at 220 nm of BaF2 under room tempera-