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

TL;DR

This study analyzes cosmic-ray hits on space-based Si:As IBC detectors, revealing their rate, amplitude distribution, and spread, with implications for space detector operation and noise mitigation.

Contribution

It provides the first detailed characterization of cosmic-ray interactions with Si:As IBC detectors in space, including hit rates, amplitude distributions, and effects on pixel spread.

Findings

Hit rate correlates with proton flux but is over twice expected.

Hit amplitudes follow Landau distribution, mostly below noise thresholds.

Pixel spread influenced by geometry, scattering, and interpixel capacitance.

Abstract

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.