Nuclear recoil identification in a scientific charge-coupled device

Abstract

Charge-coupled devices (CCDs) are a leading technology in direct searches for dark matter because of their eV-scale energy threshold and micrometer-scale spatial resolution. Recent studies have also highlighted the potential for using CCDs to detect coherent elastic neutrino-nucleus scattering. 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 defect cluster 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 Am⁹Be neutron source, we demonstrate >93% identification efficiency for nuclear recoils with energies >150 keV, where the coincident ionization events were confirmed to be nuclear recoils due to their topology. The technique remains fully efficient down to 90 keV, decreasing to 50% at 8 keV and reaching (6±2)% between 1.5 and 3.5 keV. Irradiation with a ²⁴Na gamma-ray source does not result in any detectable defect clusters, with the fraction of electronic recoils with energies <85 keV that are spatially correlated with defects <0.1%.