  We evaluate the hit rate of cosmic rays and their daughter particles on the
Si:As IBC detectors in the IRAC instrument on the Spitzer Space Telescope. The
hit rate follows the ambient proton flux closely, but the hits occur at more
than twice the rate expected just from this flux. Toward large amplitudes, the
size distribution of hits by single-charge particles (muons) follows the Landau
Distribution. The amplitudes of the hits are distributed to well below the
energy loss of a traditional ``average minimum-ionizing proton'' as a result of
statistical 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 identified
given the read noises of modern solid-state detectors. The spread of individual
hits over multiple pixels is dominated by geometric effects, i.e., the range of
incident angles, but shows a modest excess probably due to: (1) showering and
scattering of particles; (2) the energy imparted on the ionization products by
the energetic protons; and (3) interpixel capacitance. Although this study is
focused on a specific detector type, it should have general application to
operation of modern solid-state detectors in space.
