The Influence Of The Shape Of BGO Scintillators To The Energy Resolution

Energy resolution Thesis Supervisor: Shuhua Zhou, Professor of physicsThe Hyperball is a HpGe detector system and it was designed and built by the cooperation between China and Japan. BGO scintillators are used for Compton suppression in the HpGe detector system for gamma spectrum measurement. To meet the special needs in the experiment three irregular shapes of BGO scintilllators have been used in each HpGe detector system in the Hyperball.For checking the quality of the BGO scintillators, the energy resolution of 72 BGO scintillators has been measured with both the collimated and uncollimated l37Cs Y 梤ay sources at different position. Some special phenomena have been found, and studied in detail.In the measurement we found that the energy resolution of irregular BGO scntillators was worse than the regular one when uncollimatedsource was used.We postulated the irregular shaPe of the BGO scintillators caused theenegy peaks aPpeared at different position in the energy spectrUm whenthe gamma rays were incident in different directions. The resulted peakwas the overlaPping of these peaks and then widened the FWHM. Inorder to check this postUlation, we used collimated 137Cs source to do thecontrast experiment. Comparing the results, we fOund that the energyresolution of all scintillators became much better. Furthermore we foundthat the energy resolution of irregular BGO scintillators is better than theregular ones When the collimated source was used. In addition, the peakarea was calculated and comPared, we found the total counts of the peakarea of irregular BGO scintillators are lager than the regular ones.The reason is that the refractive index of the BGO scintillators israther large and the total reflection haPpens easily. So some light willreflect along a closed zigzag line and could not escaPe from the regularcrystal. Contrary, the light inside the irregular crystal will reach the PMTeasily and the PMT will collect more energy so the resolution is better.Consequently, the shaPe of the BGO scintillators affects theresolution obviously, so it should not be neglected in the designing anddeveloping new BGO counters or other scintillation detectors.