Improvement of the longitudinal uniformity in the response of long BGO detectors

Abstract Painting black the crystal close to the photomultiplier end significantly improves the uniformity. Use of a photomultiplier whose photocathode is slightly narrower than the cross section of the crystal also improves the uniformity. Both results may be interpreted in terms of the important role of the total internal reflection of light at the crystal surface. If the above two techniques are employed, the uniformity of the signal output in a typical BGO crystal of 10 × 10 × 200 mm 3 is ± 10% over 10–190 mm from the the photomultiplier end and ± 2.2% over 50–190 mm.