A crosstalk and non-uniformity correction method for the Compact Space-borne Compton Polarimeter POLAR

Abstract

POLAR is a space-borne Compton polarimeter designed to measure linear polarization of 50 keV -- 500 keV gamma-rays arriving from prompt emission of gamma-ray bursts (GRBs). Plastics scintillator bars are used as gamma-ray detectors and Multi-anode photomultipliers (MAPMTs) are adopted for readout of scintillation photons. Crosstalk phenomenon is an inherent property of the MAPMT based detectors. It smears recorded energy depositions over multiple channels making correction of non-uniformities and energy calibration more difficult. Crosstalk and non-uniformity corrections are necessary to properly extract polarization degrees of GRBs. We studied influence of the crosstalk on recorded energy depositions based on laboratory measurements. The relation between genuine and recorded energy depositions is deduced from an introduced physical model. Based on it, both the crosstalk and non-uniformities can be described with a single matrix obtained during calibrations with mono-energetic gamma-rays. It allows for proper correction of the measured GRB spectra. Validity of the model is verified with dedicated experimental tests.