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

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 surface. If the above two techniques are employed, the uniformity of the signal output in a typical BGO crystal of 10 x 10 x 200 mm3 is +- 10% over 10-900 mm from the photomultiplier end and +- 2.2% over 50-190 mm. (orig.)