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. 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): DetectorMDA (Bq m -3 ) Saturation level (corresponding to 4000 cm -2 ) (Bq m -3 ) CDs at 83 µm CDs at 117 µm

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 Rn and Rn 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 = 34 ± 9 Bq m Rn = 830 ± 400 Bq m -3 House 2: 222 Rn = 75 ± 19 Bq m 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 …