Exploration of Methods to Remove Implanted $^{210}$Pb and $^{210}$Po Contamination from Silicon Surfaces

TL;DR

This study investigates various cleaning techniques, including heat, wet-chemistry, and plasma methods, to effectively remove long-lived radioactive contaminants $^{210}$Pb and $^{210}$Po from silicon surfaces used in rare-event physics detectors.

Contribution

It demonstrates for the first time that high-temperature baking can effectively eliminate implanted $^{210}$Pb and $^{210}$Po contamination from silicon surfaces.

Findings

1200°C bake achieves near-perfect removal of contamination

Etching methods can be highly effective with sufficient depth

Multiple cleaning approaches are recommended for different detector phases

Abstract

Radioactive contaminants on the surfaces of detector components can be a problematic source of background events for physics experiments searching for rare processes. Exposure to radon is a specific concern because it can result in the relatively long-lived $^{210}$Pb (and progeny) being implanted to significant subsurface depths such that removal is challenging. In this article we present results from a broad exploration of cleaning treatments to remove implanted $^{210}$Pb and $^{210}$Po contamination from silicon, which is an important material used in several rare-event searches. We demonstrate for the first time that heat treatments ("baking") can effectively mitigate such surface contamination, with the results of a 1200 $^{\circ}$C bake consistent with perfect removal. We also report results using wet-chemistry and plasma-based methods, which show that etching can be highly effective provided the etch depth is sufficiently aggressive. Our survey of cleaning methods suggests consideration of multiple approaches during the different phases of detector construction to enable greater flexibility for efficient removal of $^{210}$Pb and $^{210}$Po surface contamination