Measurement of the coincidence response of very thin BGO crystals

More than half the 511 keV photons in BGO crystals undergo Compton scattering at least once prior to photoelectric interaction within the detector. In a PET scanner, this can result in mis-positioning of annihilation events. As crystal dimensions are made smaller, the fraction of mis-positioned events increases. We have studied the coincidence aperture function (CAF) of 25 mm×10 mm BGO crystals with thicknesses varying from 1 mm to 3 mm in steps of 0.5 mm. By sandwiching the active crystal between two other BGO crystals not in contact with the photomultiplier, we have studied the effect of Compton scatter upon CAF. By allowing the annihilation photons to be incident along the 25 mm axis and the 10 mm axis, we have studied the effect of detector depth upon the CAF, with and without Compton scatter. The CAF increases linearly with crystal width, ranging from 1.3 mm crystal. The lines joining the FWHM of the CAFs as a function of crystal width appear to converge indicating that no further improvement in resolution can be achieved by reducing crystal width. The CAFs for 10 mm and 25 mm long crystals without Compton scatter is essentially the same. The presence of neighbouring crystals results in an increase in the CAF which is greater for long crystals compared with short ones of the same width. Using 1 mm wide and 10 mm long crystals sandwiched between two uncoupled BGO crystals, we have achieved spatial resolution of 1.64 mm FWHM for a PMT separation of 34 cm. Recent experiments with PMT separation of 11.5 cm have yielded 1.23 mm FWHM CAF in the same setup