The figures show the low-energy background events distributed in our detector after application of an algorithm to correct their positions. Background events can be seen to cluster mostly at the surfaces of the detector, at high radii and at the cathode near the bottom. (The color scale is logarithmic)
Event interaction position is important for background rejection, likelihood analysis etc. Our 3D position reconstruction is based on event drift time and PMT hit patterns. However, as the drift field is not perfectly vertical, the reconstructed position at the gate does not exactly correspond to the interaction position. To get to a corrected position, a data-driven method based on the radioactive isotope Krypton-83m is developed. The idea is to utilize the radial uniformity of Krypton-83m events. Regular Krypton-83m calibrations throughout the whole science run can guarantee that we have sufficient statistics to properly correct positions for different radius, angle, depth and time. Thanks to this new position algorithm, we were able to increase the useful exposure by around 30%.