Measurement of the bulk radioactive contamination of detector-grade silicon with DAMIC at SNOLAB

TL;DR

This study measures and sets limits on radioactive contaminants in silicon CCDs used in dark matter detection, demonstrating the technology's potential to identify backgrounds and improve sensitivity in future experiments.

Contribution

The paper introduces a method using CCD spatial resolution to quantify and set limits on bulk radioisotope contamination in detector-grade silicon.

Findings

Measured $^{32}$Si contamination at 140 ± 30 μBq/kg.

Set upper limits on $^{210}$Pb, $^{238}$U, and $^{232}$Th contamination levels.

Demonstrated CCD capability to identify and reject spatially-correlated backgrounds.

Abstract

We present measurements of bulk radiocontaminants in the high-resistivity silicon CCDs from the DAMIC at SNOLAB experiment. We utilize the exquisite spatial resolution of CCDs to discriminate between $\alpha$ and $\beta$ decays, and to search with high efficiency for the spatially-correlated decays of various radioisotope sequences. Using spatially-correlated $\beta$ decays, we measure a bulk radioactive contamination of $^{32}$Si in the CCDs of $140 \pm 30$ $\mu$Bq/kg, and place an upper limit on bulk $^{210}$Pb of $< 160~\mu$Bq/kg. Using similar analyses of spatially-correlated bulk $\alpha$ decays, we set limits of $< 11$ $\mu$Bq/kg (0.9 ppt) on $^{238}$U and of $< 7.3$ $\mu$Bq/kg (1.8 ppt) on $^{232}$Th. The ability of DAMIC CCDs to identify and reject spatially-coincident backgrounds, particularly from $^{32}$Si, has significant implications for the next generation of silicon-based dark matter experiments, where $\beta$'s from $^{32}$Si decay will likely be a dominant background. This capability demonstrates the readiness of the CCD technology to achieve kg-scale dark matter sensitivity.