We have investigated the response of 10 micron cutoff HgCdTe 1024 × 1024 pixel arrays, grown by Teledyne Imaging Sensors (TIS) on CdZnTe substrates, to ionizing radiation in the form of galactic cosmic ray secondaries near sea level, primarily muons, and natural radiation. The arrays are optimized for use in space observatories, such as the proposed NEOCam mission, so response to ubiquitous cosmic rays is crucially important. We analyzed 2000 6-second integration samples, for each pixel in each array, to characterize their response to ionizing radiation. Muons and other ionizing radiation leave 'footprints' in the data, visible as a sudden 'jump' in signal in the time series, which affects a cluster of pixels simultaneously. We investigated 4 key properties of these radiation hits: the number of pixels affected in each cluster, the charge generated by the event, the detector noise directly after the hit, and the responsivity of the pixel before and after each hit. The responsivity (plus dark current), given by the slope of the time series, was unaffected by the radiation hits. Likewise the correlated-double-sampling read noise was unaffected by the hits. The total charge generated was in reasonable agreement with that expected from 2-4 GeV muon hits. 8-12 pixels were typically affected by single hits, irrespective of the thickness of the CdZnTe substrate (800, 48 and zero microns). This cluster size was significantly larger than that observed in a 2.5 micron cutoff array from TIS, but similar to that shown by a 5 micron cutoff device from TIS tested for response to energetic protons for the JWST mission.