On December 9, 2021, the imaging x-ray polarimetry explorer (IXPE) observatory was launched, carrying three x-ray polarimeters based upon the gas pixel detector (GPD). These devices measure the photoelectron’s ionization track following absorption of an x-ray, from which the photoelectron’s initial direction (correlated to the polarization position angle) is determined. Here we describe a method for event-by-event correction of pixel-by-pixel gain variations, which are found to be +/-20%, by comparing the charge in each pixel with the average at the same relative position inside tracks of the same shape. Using the large dataset acquired during on-ground calibration of the IXPE detectors, we have individually calibrated each of the 300×352 pixels of each detector’s ASIC. We discuss the performance improvements obtained using this method, which may be relevant to other instruments that detect individual events through images.
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