Survey of the properties of BGO crystals for the Extreme Forward Calorimeter at BELLE

We check the scintillation light yield of all BGO crystals used for the Extreme Forward Calorimeter (EFC) at BELLE. The percentage spread of the distribution is only 6%. The nonuniformity of the light yields measured lengthwise for the forward EFC is close to 13% and is ∼0% for the backward EFC. The test crystals, two per ingot, from mass-production are subjected to the radiation hardness test. The results obtained at BINP and NTU are in good agreement. Some EFC crystals, selected randomly, show the light yields drop about 25–50% after receiving 1 krad dose and remain stable afterwards. There is little sensitivity to the rate that the 1 krad dose is received, since the recovery process is very slow, in the order of days to weeks.