Bismuth Germanate as a High-Energy Gamma-Ray Detector

Abstract

For the past several years the majority of experiments that detected gamma-rays above 10.0 MeV involved large volume NaI detectors with anticoincidence shields surrounding them. These shields serve two purposes: first, they veto cosmic ray events, and second, they reject gamma-ray events that do not deposit their total energy in the crystal. Recently bismuth germanate (Bi/sub 4/Ge/sub 3/O/sub 12/), or BGO, scintillators have been developed with many properties that may make them an attractive alternative to NaI in many applications. In this particular experiment the authors detect gamma rays following high-energy neutron capture using a white neutron source. The requirements of detector system are that it have good energy and time resolution, high efficiency, and low sensitivity to neutron backgrounds. Since the authors require an array of detectors to simultaneously measure the angular distribution of the radiation along with the excitation function, NaI detectors with anticoincidence shields seem impractical. They therefore decided to construct a detector system consisting of five 7.6-cm-long by 7.6-cm-diam-BGO scintillators spanning an angular range from 140 to 45 degrees in the reaction plane. A table lists some of the properties of BGO/sup 4/ and the values for NaI for comparison. For present application the advantages ofmore » BGO are its high average atomic number and its large density. Both these factors contribute towards improving the detector efficiency. The disadvantages of BGO are its lower light output (1/10 of NaI), its high index of refraction (n = 2.15), and its longer wavelength at maximum light output. In addition large BGO crystals often contain trapped bubbles, which scatter the light. All these factors tend to limit the resolution of the scintillator.« less