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