Intrinsic energy resolution and light yield nonproportionality of BGO

The intrinsic energy resolution and nonproportionality of the light yield as a function of gamma ray energies, in the energy range of 14 keV to 1.33 MeV, were studied for small BGO (Bi4Ge3O12) crystals at room and liquid nitrogen (LN2) temperatures. The study showed that the intrinsic resolution of BGO and the light yield nonproportionality, as a function of energy do not depend on the crystal temperature. High light outputs of 14000 +/- 300 electron-hole pairs and energy resolution of 6.5% +/- 0.2% for 662 keV gamma rays were measured with the 9 mm diameter, 4 mm thick crystals, coupled to large area avalanche photodiodes and cooled down to LN2 temperature. Special attention was paid to analyzing the energy resolution of the escape peaks, which were well separated from the full-energy peaks due to the good energy resolution of BGO at LN2 temperatures and the energy of bismuth KX-rays. The intrinsic energy resolution of the BGO crystal for escape peaks does not show deviations from analysis of total energy absorption photopeaks. This is in spite of the fact that the contributions of X-rays and Auger electron cascade in creation of escape peaks are much smaller than in full-energy peaks. In the small volume crystal, mainly electrons produced in the photoelectric absorption create the escape peaks.