Overcoming non-uniformity limits for space-borne hard X-ray polarimeter POLAR based on scintillators and multi-anode photo-multipliers

The main performance challenge of POLAR - a novel hard X-ray polarimeter, is to achieve high statistical accuracy and large modulation factors. Both require great level of response uniformity. Though, typical sensitivity of multi-anode PMTs may vary between its channels up to the factor of two to three. Moreover, the measured signal can be further altered by light coupling quality. We investigate response equalization based on utilizing of an optical mask with light-blocking diaphragms. It is placed between scintillator bars and the PMT photo-cathode. Each PMT channel-pad is equipped with the unique diaphragm with its area related to the initial sensitivity of the corresponding channel. We present module/mask construction details, test setup and measurement results done before and after applying of the mask corrections. Applicability of the method is discussed with respect to the instrument dynamic range, low energy threshold requirements and aging effects due to the in-space utilization.