Development of Yttrium-Doped BaF2 Crystals for Future HEP Experiments

Because of their ultrafast scintillation with subnanosecond decay time, barium fluoride (BaF2) crystals have attracted broad interest in the high energy physics and nuclear physics communities. One crucial issue, however, is its slow scintillation component with 600-ns decay time, which causes pile-up in a high rate environment. Previous studies show that the slow component can be suppressed effectively by rare earth doping. In this paper, we report investigations on a set of Phi 18 x 21 mm(3) BaF2 cylinders doped with different yttrium levels grown at Beijing Glass Research Institute (BGRI), from which the optimized yttrium doping level was determined. A Phi 40 x 160 mm(3) BaF2 ingot with 5 atomic % (at.%) yttrium doping was consequently grown at BGRI and was used to cut one 25 x 25 x 100 mm3 crystal and several thin slices. Their optical and scintillation properties were measured at Caltech. The results show that yttrium doping effectively suppresses the slow component while maintaining its ultrafast light unchanged. Research and development will continue to develop large-size BaF2: Y crystals with improved optical quality for a fast BaF2: Y crystal calorimeter for Mu2e-II.