Count-rate dependent sensitivity in position-sensitive photomultiplier tubes

A significant count-rate dependence of mean scintillation-pulse amplitude has been observed in prototype small gamma cameras employing NaI(Tl) crystals coupled to Hamamatsu R2487 and R3941 position-sensitive photomultiplier tubes (PSPMTs). The magnitude of the effect is position dependent. Within each camera´s 56 mm×56 mm field-of-view, as the scintillation rate is increased from 500 to 10,000 counts per second, the response to 140 keV gamma emissions from a collimated source of Tc-99m increases by approximately 2-8% in the R2487 and 1-6% in the R3941. The count-rate dependent sensitivities of both tubes were investigated by varying the rate and position of LED pulses directed at the face of the tubes with a 1 mm diameter optical fiber. Within the specified effective area of the R2487, the increase in response from a 500 Hz rate to a 100 kHz rate varied from 1.5% in the center to 20% at one edge; corresponding increases in the R3941 were only 1% and 8%. In both cases, most of the response increase occurred between 500 Hz and 10 kHz. Similar LED tests on conventional photomultiplier tubes resulted in no more than 2% increases