High Sensitivity Passive Radon Detector for Measuring Radon in Low-background Underground Nuclear/Particle Physics Laboratories

Strict radon control is highly desirable in the deep underground low background nuclear/particle physics facilities. Ideally,
²²²
Rn levels should be kept below 1 Bq.m
^-3
to reduce the undesirable background contribution of radiation by
²²²
Rn and its progeny. So far the commercially available radon monitors cannot provide sensitivity in this range and unique
²²²
Rn detection systems should be constructed. In this report we propose highly sensitive passive
²²²
Rn monitor with high total detection area and low intrinsic background that can reach the required sensitivity. It consists of a large number of "detector elements". One detector element consists of two DVD halves that face a radon absorber (Makrofol N foils) with their inner surface. The polycarbonate material of the commercial DVDs has very good track etch properties and the intrinsic background of their inner surface is very low (1.1 ± 0.3 cm
^-3
) and can be further reduced by thermal annealing and special counting procedures that eliminate surface defects. The presented pilot experimental results and sensitivity assessment show that with the proposed monitor quantitative
²²²
Rn measurements are possible at a level of 20 mBq.m
^-3
or less. The main application of such radon monitor is foreseen for the purposes of radon control in the unique very low-background deep underground nuclear/particle physics facilities, where
²²²
Rn is mitigated to sub Bq.m
^-3
levels.