Optical bleaching in situ for barium fluoride crystals

This report presents results of optical bleaching for barium fluoride crystals to be used to construct a precision electromagnetic calorimeter at future hadron colliders. A practical technique for in situ bleaching by illuminating crystals with light from a UV lamp through an optical fiber is established. The light attenuation length of current production BaF2 crystals can be set to around 200 (160) cm under 400 (130) rad/h by using 4.3 (1.1) mW bleaching light from a mercury (xenon) lamp through a ∅0.6 mm fiber. The color center dynamics model proposed in our early publication [1] has also been verified. The technical approach described in this report can be used for other candidate crystals to maintain an adequate light attenuation length in a radiation environment.