Evaluation of Hamamatsu R5900 series PMTs for readout of high-resolution scintillator arrays

Two types of Hamamatsu R5900 series photomultiplier tubes (PMTs) have been evaluated as position-sensitive photon detectors for reading out arrays of small scintillator elements. Both PMTs have metal-channel dynode structures and compact dimensions of 26×26×20 mm³. Model R5900-C12 is a position sensitive PMT (PS-PMT) with photoelectrons shared among 12 plate anodes arranged in a crossed pattern such that there are six anodes each in the X and Y directions. Model R5900-M64 is a multi-channel PMT (MC-PMT) with 64 individual channels and low inter-channel crosstalk. Scintillator arrays can be coupled to these PMTs either directly or by using optical fibers. In this paper, we present measurements of single photoelectron spectra, pulse linearity, positioning response, inter-channel gain variation and pixel-to-pixel crosstalk for these PMTs. We have also measured their positioning response in reading out high-resolution lutetium oxyorthosilicate (LSO) scintillator arrays. Across a sample of 30 R5900-M64 MC-PMTs which were manufactured recently, the maximum gain variation over all channels is less than 3:1, while the difference between any two adjacent channels is less than 2:1. With a tapered voltage divider, the output of the anode signal at a gain of 10⁶ remains linear up to 200 photoelectrons for the R5900-M64. For the R5900-C12, the summed anode signal remains linear up to 400 photoelectrons when irradiated through a 2 mm diameter aperture. Images obtained with a Na-22 source irradiating an array of 1×1×10 mm³ LSO crystals coupled by an optical fiber bundle to the R5900-C12 or the R5900-M64 show excellent crystal identification.