An improved window for ring imaging Cerenkov detectors

Calcium fluoride crystal is used as a window in RICH (ring imaging Cerenkov) detectors because of its vacuum ultraviolet transmission down to 125.7 nm. This permits triethylamine (TEA) photosensitive gas to be used in the detector. The crystal would be more useful if larger, single crystals could be achieved. A crystal growing furnace was designed, based upon the concepts of a vertical slab geometry, to produce large, single crystals. These concepts are embodied in a furnace which produces calcium fluoride crystals 35 cm square by 7.5 cm thick. These ingots are an optimal geometry for the fabrication of RICH detector windows. Surface strengthening experiments indicate that large-area calcium fluoride windows can be made much more reliable than historic, nominal stress-at-failure values would predict. It is noted that maximum transmission of Cerenkov radiation in the vacuum ultraviolet spectral region can be ensured with special purification technology. Very pure calcium fluoride crystals show increased resistance to forming color centers after exposure to several megarads of ionizing radiation