Improvement of Scintillation Performance in Large Size Yttrium Doped BaF_2 Crystals

Barium fluoride (BaF_2) crystal with sub nanosecond scintillation has attracted a broad interest in the HEP community. One crucial issue is its slow scintillation component with 600 ns decay time, which causes pile-up. Our previous studies show that the slow component can be suppressed effectively by yttrium doping. In this work, a 160 mm long BaF_2 ingot with 5% yttrium doping was grown at BGRI, and was used to cut a 100 mm crystal and several small samples. Their optical and scintillation properties were measured at Caltech. The results confirm that yttrium doping effectively suppresses the slow component while maintaining its ultrafast light and light response uniformity unchanged. This is encouraging for a BaF_2 :Y crystal based ultrafast calorimeter for Mu2e-II.