Nuclear Recoil Identification in a Scientific Charge-Coupled Device

TL;DR

This paper introduces a novel method to identify nuclear recoils in CCDs by detecting lattice defects, significantly improving dark matter search sensitivity through event-by-event discrimination.

Contribution

The authors develop and demonstrate a technique for nuclear recoil identification in CCDs based on spatial correlation with lattice defects, achieving high efficiency at relevant energies.

Findings

>93% identification efficiency for nuclear recoils above 150 keV

Technique remains effective down to 90 keV, with decreasing efficiency at lower energies

Electronic recoils show no significant defect correlation, confirming method specificity

Abstract

Charge-coupled devices (CCDs) are a leading technology in direct dark matter searches because of their eV-scale energy threshold and high spatial resolution. The sensitivity of future CCD experiments could be enhanced by distinguishing nuclear recoil signals from electronic recoil backgrounds in the CCD silicon target. We present a technique for event-by-event identification of nuclear recoils based on the spatial correlation between the primary ionization event and the lattice defect left behind by the recoiling atom, later identified as a localized excess of leakage current under thermal stimulation. By irradiating a CCD with an $^{241}$Am$^{9}$Be neutron source, we demonstrate $>93\%$ identification efficiency for nuclear recoils with energies $>150$ keV, where the ionization events were confirmed to be nuclear recoils from topology. The technique remains fully efficient down to 90 keV, decreasing to 50$\%$ at 8 keV, and reaching ($6\pm2$)$\%$ at 1.5--3.5 keV. Irradiation with a $^{24}$Na $\gamma$-ray source shows no evidence of defect generation by electronic recoils, with the fraction of electronic recoils with energies $<85$ keV that are spatially correlated with defects $<0.1$$\%$.