A gain-programmable transit-time-stable and temperature-stable PMT voltage divider

A gain-programmable, transit-time-stable, temperature-stable photomultiplier (PMT) voltage divider design is described in this paper. The signal-to-noise ratio can be increased by changing a PMT gain directly instead of adjusting the gain of the pre-amplifier. PMT gain can be changed only by adjusting the voltages for the dynodes instead of changing the total high voltage between the anode and the photo-cathode, which can cause a significant signal transit-time variation that cannot be accepted by an application with a critical timing requirement, such as positron emission tomography (PET) or time-of-flight (TOF) detection/PET. The dynode voltage can be controlled by a digital analog converter (DAC) isolated with a linear optocoupler. The optocoupler consists of an infra-red light emission diode (LED) optically coupled with two phototransistors, and one is used in a servo feedback circuit to control the LED drive current for compensating temperature characteristics. The results showed that a 6 times gain range could be achieved; the gain drift was < 0.5% over a 20°C temperature range; 250 ps transit-time variation was measured over the entire gain range. A compact print circuit board (PCB) for the voltage divider integrated with a fixed-gain pre-amplifier has been designed and constructed. It can save about $30 per PMT channel compared with a commercial PMT voltage divider along with a variable gain amplifier. The pre-amplifier can be totally disabled, therefore in a system with large amount of PMTs, only one channel can be enabled for calibrating the PMT gain. This new PMT voltage divider design is being applied to our animal PET camera and time-of-flight/PET research.