Radon in the DRIFT-II directional dark matter TPC: emanation, detection and mitigation

TL;DR

This paper investigates radon emanation from materials used in the DRIFT-II dark matter detector, develops a radon monitoring method, and demonstrates a tenfold reduction in internal radon levels through material substitution.

Contribution

It introduces a radon emanation measurement and monitoring technique for dark matter detectors and shows how material selection reduces radon background significantly.

Findings

Radon levels were reduced by a factor of ~10 through material substitution.

The detector's sensitivity to 222Rn is 2.5 μBq/l, enabling sensitive radon assay.

Radon emanation measurements agree with direct alpha spectrometry results.

Abstract

Radon gas emanating from materials is of interest in environmental science and also a major concern in rare event non-accelerator particle physics experiments such as dark matter and double beta decay searches, where it is a major source of background. Notable for dark matter experiments is the production of radon progeny recoils (RPRs), the low energy (~100 keV) recoils of radon daughter isotopes, which can mimic the signal expected from WIMP interactions. Presented here are results of measurements of radon emanation from detector materials in the 1 metre cubed DRIFT-II directional dark matter gas time projection chamber experiment. Construction and operation of a radon emanation facility for this work is described, along with an analysis to continuously monitor DRIFT data for the presence of internal 222Rn and 218Po. Applying this analysis to historical DRIFT data, we show how systematic substitution of detector materials for alternatives, selected by this device for low radon emanation, has resulted in a factor of ~10 reduction in internal radon rates. Levels are found to be consistent with the sum from separate radon emanation measurements of the internal materials and also with direct measurement using an attached alpha spectrometer. The current DRIFT detector, DRIFT-IId, is found to have sensitivity to 222Rn of 2.5 {\mu}Bq/l with current analysis efficiency, potentially opening up DRIFT technology as a new tool for sensitive radon assay of materials.