Long-Lived Charge Trap Characterization for Rare-Event Searches with Skipper-CCDs

Abstract

Skipper Charge-Coupled Devices (Skipper-CCDs) are detectors with sub-electron resolution, allowing them to play a critical role in rare-event searches. A major challenge in experiments using these detectors is understanding and mitigating low-energy backgrounds which impact the single-electron rate. One potential component of the single-electron rate could arise from single-electron charge traps caused by lattice defects. In this work we use a new in-situ technique for generating uniform charge known as ``pixel-pumping'' across the CCD to study single-electron traps in a SENSEI sensor placed underground in the MINOS cavern at Fermilab, which enables sensitivity to long-lived traps with emission times $>10^3$ seconds, a regime that was previously not accessible. We fully characterize 1787 traps and use this information to estimate their contribution to the single-electron background in SENSEI. Through simulation, we find the characterized population of traps does not significantly impact the single-electron rate. We further identify the energies and cross-sections for traps to which this analysis is sensitive, and compare this with the values that could impact the single-electron rate. This analysis probes a large fraction of this region. However, we also see that a sufficiently numerous hidden population could contribute significantly to the single-electron rate, and new measurement techniques will have to be devised in order to fully rule out this population.