A Review and Outlook for the Removal of Radon-Generated Po-210 Surface Contamination

TL;DR

This paper reviews methods for removing radon progeny, especially Po-210, from detector surfaces, emphasizing chemical and electrochemical techniques to improve surface cleaning for low-background experiments.

Contribution

It provides a comprehensive review and new insights into the chemical and electrochemical processes affecting Po-210 removal from copper surfaces.

Findings

Oxidizing agents influence Po ion stability in etching solutions.

Electropolishing effectively removes Po atoms from surfaces.

Chemical conditions can be optimized to maximize Po removal efficiency.

Abstract

The next generation low-background detectors operating deep underground aim for unprecedented low levels of radioactive backgrounds. The deposition and presence of radon progeny on detector surfaces is an added source of energetic background events. In addition to limiting the detector material's radon exposure in order to reduce potential surface backgrounds, it is just as important to clean surfaces to remove inevitable contamination. Such studies of radon progeny removal have generally found that a form of etching is effective at removing some of the progeny (Bi and Pb), however more aggressive techniques, including electropolishing, have been shown to effectively remove the Po atoms. In the absence of an aggressive etch, a significant fraction of the Po atoms are believed to either remain behind within the surface or redeposit from the etching solution back onto the surface. We explore the chemical nature of the aqueous Po ions and the effect of the oxidation state of Po to maximize the Po ions remaining in the etching solution of contaminated Cu surfaces. We present a review of the previous studies of surface radon progeny removal and our findings on the role of oxidizing agents and a cell potential in the preparation of a clean etching technique.