A study of transit time variation in the PMT with a gain programmable voltage divider for a TOF PET

In the front-end circuit of a Positron Emission Tomography (PET) camera, adjusting the photomultipliers (PMT) gain by changing the voltage distribution in dynodes instead of changing the total high voltage is more convenient to realize, and this method is better in signal-to-noise-ratio (SNR) than using a variable-gain-amplifier (VGA) in the signal path which may induce extra noise. In this study, we revised the previous voltage divider design to achieve wider gain adjustable range which relaxes the PMT gain-spread requirement to reduce the PMT cost. An accurate test bench was used to measure the PMT transit time variation as a function of the relative gain. From the test result, within the adjustable gain range, the PMT transit time variation can be as large as several hundreds of picoseconds; and this is non-ignorable in the Time-of-Flight (TOF) applications. Hence, a time correction is required. A look-up-table (LUT) from the transit time variation vs. PMT relative gain curve is used to correct this transit time variation with the gain adjustment simultaneously without performing the time-consuming entire system timing recalibration.