Evaluation of multi-channel PMTs for readout of scintillator arrays

We have evaluated two Philips multi-channel photomultiplier tubes (MC-PMTs) as position-sensitive photon detectors for reading out small scintillator arrays. The 64 channel XP1702 MC-PMT has a standard glass entrance window while the XP1722 model has a fibre-optic entrance window designed to reduce inter-channel crosstalk. Scientillator arrays are coupled typically to these MC-PMTs using optical fibers. In this paper, we present measurements of inter-channel gain variation, pixel-to-pixel crosstalk, single photoelectron spectra, MC-PMT linearity and sensitivity to magnetic fields. For both the XP1702 and XP1722 models, the maximum-gain variation over all 64 channels is typically 3:1, while the difference between any two adjacent channels is less than 2:1. Crosstalk to the nearest neighbor is 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. The output of the 10th dynode (position signal) remains linear up to 400 photoelectrons at a gain of 4 × 10⁶ using a Philips VD107 voltage divider. Performance in a magnetic field was found to depend strongly on the orientation of the tube relative to the field. Images obtain 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 XP1722 MC-PMT.