Investigating the energy resolution of arrays of small scintillation crystals

Arrays of small scintillation crystals are being used increasingly for high-resolution imaging applications in nuclear medicine. Although the degree of pixellation now available is high for some scintillation materials, this spatial-resolution is often achieved at the expense of degraded energy-resolution due to the lower, and more variable, light-collection efficiency. The energy-resolution of a detector is, however, especially important in nuclear medicine where events that have been scattered in the body need to be rejected efficiently. The light-output from a range of CsI(Tl) arrays was measured in terms of the number of photoelectrons detected, using a hybrid photodiode. This data, used in conjunction with the measured energy-resolution and an estimate of the intrinsic energy-resolution of CsI(Tl), was used to assess the magnitudes of the various contributions to the overall energy-resolution of these detectors. This information suggested that there is an opportunity to improve their energy-resolution by carefully choosing the geometry and reflector material to minimise the variance in the light collection. If the nature of this variance is understood sufficiently well, there maybe an opportunity to apply a similar post-processing technique to that which has dramatically improved the performance of other standard scintillation detectors. This possibility depends on the use of either the uniform quantum efficiency of the M-HPD photocathodes or a monolithic array of PIN diodes. This would ensure that no additional or indeterminate variance in the light-collection is introduced. Measurements made using fine BGO arrays for higher-energy applications and columnar grown CsI(Tl) for X-ray imaging will also be presented.