Evaluation of multi-channel PMT's for readout of scintillator arrays

We have evaluated two Philips multi-channel photo-multiplier tubes (MC-PMT's) for use as position sensitive photon detectors for reading out scintillator arrays. We are interested in the use of such a detector for medical imaging applications with relatively low light output scintillators such as bismuth germanate (BGO). The 64 pixel XP1702 model MC-PMT has a standard glass entrance window whereas the XP1722 has a fiber-optic entrance window designed to reduce crosstalk. Scintillator arrays are typically coupled to the PMT's using fiber-optics. In this evaluation, parameters such as inter-channel gain variation, crosstalk, photoelectron spectrum and linearity were measured. For both the XP1702 and XP1722, the maximum gain variation over all 64 channels is less than 3:1, while the difference between any two adjacent channels is less than 2:1. Crosstalk to the nearest neighbor is around 10% for the fiber optic input window tube compared with 20% for the glass input window tube and is relatively insensitive to the size and numerical aperture of the optical fiber used. Dynode linearity range is maintained up to a signal level of 400 photoelectrons. Images obtained with a 511 KeV source irradiating an 8 × 8 array of 2 × 2 × 10 mm BGO crystals coupled by optical fiber to these tubes show excellent crystal identification, particularly using the lower crosstalk XP1722 MC-PMT.