SIMPLE TECHNIQUE TO INCREASE THE LINEARITY AND FIELD-OF-VIEW IN POSITION-SENSITIVE PHOTOMULTIPLIER TUBES

Crossed anode wire position sensitive photomultiplier tubes (PS-PMTs) detect the location of a light source and provide the X and Y axis coo rdinates of the event. These coordinates are typically generated using Anger logic, where a resistor chain divides the current flow into two signals for each coordinate (X+, X- & Y+, Y-). In the standard readou t, identical resistor values are used across the entire resistor chain . While this arrangement provides a linear readout in the central port ion of the photomultiplier face, the readout is non-linear and sometim es even double valued near the edges of the PS-PMT due to the truncati on of the charge beyond the last anode wire. To counter this effect, w e have increased the value of the resistance near the ends of each res istor chain in order to compensate for the charge lost beyond the anod e wires. Measurements were made using a Hamamatsu R-3941 PS-PMT couple d to a pixellated EGO matrix of cut crystals with a 2 mm pitch in each direction. After changing the end resistors, the usable field-of-view increased by 39%. This simple modification should enhance the operati on of PS-PMTs in applications such as positron emission mammography, a nd small animal PET imaging.