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We evaluate the hit rate of cosmic rays and their daughter particles on theSi:As IBC detectors in the IRAC instrument on the Spitzer Space Telescope. Thehit rate follows the ambient proton flux closely, but the hits occur at morethan twice the rate expected just from this flux. Toward large amplitudes, thesize distribution of hits by single-charge particles (muons) follows the LandauDistribution. The amplitudes of the hits are distributed to well below theenergy loss of a traditional ``average minimum-ionizing proton'' as a result ofstatistical fluctuations in the ionization loss within the detectors.Nonetheless, hits with amplitudes less than a few hundred electrons are rare;this places nearly all hits in an amplitude range that is readily identifiedgiven the read noises of modern solid-state detectors. The spread of individualhits over multiple pixels is dominated by geometric effects, i.e., the range ofincident angles, but shows a modest excess probably due to: (1) showering andscattering of particles; (2) the energy imparted on the ionization products bythe energetic protons; and (3) interpixel capacitance. Although this study isfocused on a specific detector type, it should have general application tooperation of modern solid-state detectors in space.

Cosmic-Ray-Related Signals from Detectors in Space: the Spitzer/IRAC Si:As IBC Devices