Fast Luminescence and Absorption in BGO and SiO 2
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ABSORPTION IN BGO AND SiO2
VLADIMIR GAVRILOV * and STANISLAV CHERNOV Institute of Physics, Latvian Academy of Sciences, Riga, Latvia
ABSTRACT The spectra and kinetics of the fast luminescence and optical absorption of Bismuth Germanate (BGO) after excitation by single high-power nanosecond and subnanosecond electron pulses were studied. The short-lived optical absorption of the center responsible for BGO scintillations is assigned. The cause for the low scintillation yield in BGO is discussed in comparison with analogous measurements in crystalline and amorphous silicon dioxide. The ultrafast (decay time < 50 ps) luminescence is observed and it is concluded that it can be connected to intraband transitions. INTRODUCTION Orthogermanate bismuth crystals (BGO) are widely used as scintillation detector. This material has a lower yield of scintillation than traditional detectors based on alkali halide crystals, but excels over them in other properties important for detection of radiation. In contrast with scintillators based on alkali halides, in which the process of scintillation formation has been thoroughly studied [1], this process in BGO is far from clear. There is not even an opinion concerning the nature of the centers responsible for the main luminescence band with maximum observed at 495 nm [2,3]. The application of the technique of time-resolved luminescence and absorption spectroscopy (as shown in alkali halide scintillators) can be used to obtain information that is extremely important for understanding the mechanism by which scintillations are produced in BGO. SAMPLES AND EXPERIMENTAL PROCEDURE Four types of crystals were used: No.1 - grown by the Chochralski method with a small temperature gradient; No.2 - fused and grown by Chochralski method; No.3 - fused and grown by directional crystallization; and No.4 - grown by a hydrothermal method. Their absorption spectra show that samples 1 and 2 are transparent up to 320 nm, while No.3 has weak absorption from 350 nm and No.4 exhibits strong absorption from 430 nm to the UV region. For some comparative experiments specimens of synthetic crystalline quartz of high purity and vitreous silica of type Ill were used. • Current address: 5885 Muldrow Rd., Sacramento, CA 95841, U.S.A. 303 Mat. Res. Soc. Symp. Proc. Vol. 348. ©1994 Materials Research Society
The crystals were irradiated by single pulses from a GIN-600 high-intensity electron accelerator. The beam parameters were 300 keV, 20 A/cm- 2 , and 5 ns. The fast-decaying luminescence and transient absorption were recorded through a monochromator with 18-ELU-FM multipliers and an S7-19 oscilloscope. To carry out the measurment of kinetics in the subnanosecond time region, the GIN-600 accelerator with special transformer- "sharpener" was used. It made it possible to obtain electron pulses 50ps long. The luminescence was recorded in this case with an AGAT-SF1 streak-camera. The measurement technique and experimental setups are described in more detail in Ref.[ 1]. RESULTS The luminescence spectra measured at liq
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