RETROSPECTIVE RADON AND THORON MEASUREMENTS BY HOME STORED CDs/DVDs-RESEARCH AND PRACTICE Dobromir Pressyanov University of Sofia “St. Kliment Ohridski” Bulgaria
CD-method for retrospective 222 Rn measurements • Based on radon absorption in the polycarbonate material of CDs/DVDs and etching tracks at certain depth (>76 m) beneath the surface; • Proposed in 2001 and since 2007 regularly applied for practical measurements; • To date: > 1000 measured CDs/DVDs, incl. for retrospective measurements in > 400 dwellings in Bulgaria
Key new feature: use of “detectors” that are already available in (almost) any dwelling
Etching tracks from absorbed 222 Rn: at >76 m beneath the surface.
A CD ready for electrochemical etching
CDs etching equipment
Automatic counting by computer scanner
Experimental correlation…
Methodology: reliability and precision of the measurements: • Background in “new” disks: 2 – 40 cm-2; Problem houses (>200 Bq m-3) can be detected with < 1 y exposure. • Possibility for a posteriori individual calibration and temperature correction.
Calibration laboratory
Influence of: • Pressure: not significant effect (<5%); • Humidity: not significant effect (<7%); • Temperature: bias that can be corrected a posteriori; • Cigarette smoke and dust deposition: not significant effect (<5%); • Plate-out: no contribution to tracks at >80 m (<<1%); • CD/DVD storage in jewel cases: no effect;
Useful range of the method (for CDs/DVDs exposed for 10 y): Detector MDA (Bq m-3) Saturation level (corresponding to 4000 cm-2) (Bq m-3) CDs at 83 µm 2. 9 3000 CDs at 117 µm 4. 0 4100
The method is already applied for measurements in: • Air (including retrospective measurements by home stored CDs/DVDs); • Water (including participation in recent intercomparison with USA laboratories); • Soil-gas.
What is new in the strategy? • “Traditional surveys”: 1) To distribute detectors in the targeted group; 2) To collect and analyze them. The exposure time is well known. • “CD/DVD” based surveys: 1) Use detectors that are available practically in any dwelling; 2) To obtain and analyze the disks. To estimate the exposure time.
Comparison with conventional measurements in the past
CD/DVD-based measuring campaigns in Bulgaria
Summary • There is a running methodology for CDs/DVDs analysis that is productive (up to 30 disks/day); • Uncertainty of a single measurement is within 20%, with <10% achievable; • In large scale surveys based on personal communication, >30% of the contacted people can be expected to participate. This percentage is expected to be higher if public information campaigns are organized.
New developments (2011)-combined retrospective measurements of radon and thoron
The key concept is to measure the net track density (n) in two depths: n (thoron) = n(69 µm) - n(80 µm), where 1. 2
Combined retrospective measurements of 222 Rn and 220 Rn Thoron detection window
Results – DVDs in jewel cases. Individual a posteriori calibration will be necessary for better precision
Practical impact … on radon mitigation strategy House 1: 222 Rn House 2: 222 Rn = 34 ± 9 Bq m-3 220 Rn = 830 ± 400 Bq m-3 = 75 ± 19 Bq m-3 220 Rn = 3400 ± 1600 Bq m-3 If these houses are tested with radon detectors that are also sensitive to thoron, wrong decision to mitigate against radon can be made, and expenses wasted!!!
Thank you …
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