Study of Environmental Effects on Photo-MultiplierTubes for the X-ray Polarimetry Mission XL-Calibur

Abstract: XL-Calibur is a balloon-borne hard X-ray polarimetry mission with goals to analyze celestial objects too small and distant to be imaged. A crucial part of XL-Calibur is the anti-coincidence shield which through active collimation reduces unwanted hits when gathering data. The active collimation is done with a scintillator in combination with photo-multiplier tubes (PMTs). The PMTs are sensitive to external factors such as magnetic fields and varying temperatures. This report seeks to test and characterize the effects of these on the PMTs. The temperature tests are conducted by measuring the output while temperature-cycling in a light-tight vacuum thermal chamber. The magnetic-field tests are conducted in a light-tight box while applying magnetic fields of varying strength. This report finds a negative correlation between the PMT gain and temperature. As the PMTs are expected to be below room temperature during the fight, this effect is actually beneficial to their gain. This report also finds that magnetic fields can greatly impact the gain of the PMTs, which has led to the design decision of wrapping the PMTs in layers of magnetic shielding for the flight.