Development of Rn-222 Emanation Sources with Integrated Quasi 2 pi Active Monitoring

In this work, a novel approach for the standardization of low-level Rn-222 emanation is presented. The technique is based on the integration of a Rn-222 source, directly, with an alpha-particle detector, which allows the residual Rn-222 to be continuously monitored. Preparation of the device entails thermal physical vapor deposition of (RaCl2)-Ra-226 directly onto the surface of a commercially available ion implanted Si-diode detector, resulting in a thin-layer geometry. This enables continuous collection of well resolved alpha-particle spectra of the nuclei, decaying within the deposited layer, with a detection efficiency of approximately 0.5 in a quasi 2 pi geometry. The continuously sampled alpha-particle spectra are used to derive the emanation by statistical inversion. It is possible to achieve this with high temporal resolution due to the small background and the high counting efficiency of the presented technique. The emanation derived in this way exhibits a dependence on the relative humidity of up to 15% in the range from 20% rH to 90% rH. Traceability to the SI is provided by employing defined solid-angle alpha-particle spectrometry to characterize the counting efficiency of the modified detectors. The presented technique is demonstrated to apply to a range covering the release of at least 1 to 210 Rn-222 atoms per second, and it results in SI-traceable emanation values with a combined standard uncertainty not exceeding 2%. This provides a pathway for the realization of reference atmospheres covering typical environmental Rn-222 levels and thus drastically improves the realization and the dissemination of the derived unit of the activity concentration concerning Rn-222 in air.