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The usability of wood as a volume trap for the purpose of retrospective radon exposure assessment
Abstract
The possibility of using wood samples as a retrospective radon monitor was investigated. Retrospective radon monitors are based on the analysis of the enhanced 210Po content of the bulk of the sample as a result of radon exposure. Several wood samples in different sizes and shapes were tested for their radon penetrability and the speed at which radon can diffuse throughout the material. Also, the volume ratio of air to solid material and the natural occurring 210Po background were determined. It was seen that only in fairly exceptional cases wood can be used as an acceptable volume trap. This is mostly due to the high and variably occurring natural 210Po background in wood samples with respect to the expected increase in radon-related 210Po due to common radon concentrations in dwellings.
... This volume trap method has been used in surveys of high radon dwellings in Norway, Germany, and Serbia. In these field surveys, it has proved to be a viable field retrospective technique (Paridaens and Vanmarcke, 1999). ...
... This volume trap method has been used in surveys of high radon dwellings in Norway, Germany, and Serbia. In these field surveys, it has proved to be a viable field retrospective technique (Paridaens and Vanmarcke, 1999). ...
... An obvious, easy to find candidate, seemed to be wood. Although wood complies with the first two conditions, it was shown (Paridaens 1999) that the high and variable 210 Po background renders wood unsuitable as a volume trap. Another important factor in choosing a suitable material for a volume trap is the ease with which its volume can be determined. ...
Approximately 200 volume traps were retrieved from dwellings in various radon prone areas in Europe. They were analysed for the purpose of retrospective radon assessment. Emphasis is put on specific problems encountered when using field samples as opposed to laboratory exposed samples. It was seen that in very dusty circumstances, direct penetration of radon decay products from the outside to the centre of the volume traps calls for extra caution. Rinsing the samples is proposed as a solution and was tested in field and laboratory conditions, showing good results. An attempt was made to give an assessment of the achievable accuracy of the method. Where possible, the volume trap retrospective results were compared with contemporary measurements or to retrospective results from surface traps. The overall impression is that although volume traps are sometimes hard to find in the field, the high reliability of the results makes it well worth the effort.
The techniques for estimating radon exposures to man in retrospect will be overviewed. The focus will be on radon (222Rn) issues connected to exposures in dwellings. Potential retrospective radon metrologies are based on alpha damage in plastics, trapping of 210Pb within porous objects, and alpha recoil implantation into hard surfaces. The use of plastics is hampered by the short exposure time and problematic reconstruction of exposure geometry, handling habits, etc. The strength of the porous volume trap approach is that the growth rate of 210Pb is not disturbed by the complex indoor behaviour of airborne short-lived decay products. The most common radon retrospective approach today is based on the natural implantation of 210Pb atoms into glass or other hard indoor surfaces. Large-scale applications of the glass method have been many since the development of in-situ detectors for non-destructive determination of implanted activities. The implantation depth following a single alpha decay is about 50 nm in vitreous glass and this short range makes the surface trap approach sensitive to disturbances from contaminants on the surface.
Background: In epidemiological studies investigating radon-related health risk, the radon exposures are often derived from contemporary short-term radon measurements. Recent radon measurements, taken over a period of 1 year in dwellings, are often casually extrapolated and taken to be representative for periods spanning several decades. By simply considering the well-known variability of radon concentrations in dwellings and its sensibility to trivial factors such as living habits or minor changes in the dwelling itself, an amount of skepticism concerning the reliability of this practice should arise. In mining areas, this variability is already well acknowledged since mining conditions, such as ventilation of mine shafts, flooding of mine galleries, etc., are known to be able to change radon concentrations in dwellings built on top of mine shafts for over more than one decade. Methods: This study was conducted in stable rural communities in Yugoslavia. Present radon concentrations were compared to an indicator of past radon concentrations, that is the 210Po activity, fixed on glass surfaces or in the bulk of voluminous materials. Results: In less than 50% of the cases, the past and present radon concentrations agree within a factor of 2. Conclusions: This comparison indicates that even in stable rural communities, deriving past radon concentrations by extrapolating present ones is not a particularly good practice.
A method for retrospectively assessing the average radon concentration in a dwelling, for a period as long as 40 years or more, is described. It is based on the free penetration of radon gas into bulky spongy materials called volume traps, and subsequent trapping of radon decay products inside them. This leads to a build-up of 210Pb, a radon decay product with a half life of 22.3 years, which reaches an equilibrium with the alpha emitter 210Po, indicative of the average radon concentration over the exposure period. The stuffing of mattresses, chairs, cushions, etc. can be considered a good volume trap. A chemical separation procedure is described, transforming the volume trap into a 210Po alpha source, whose activity is determined through alpha spectroscopy. The technique has been applied in over 300 samples, retrieved from dwellings in Serbia, Germany, Norway and Sweden. The technique has proven to be very robust and reproducible. In cases where radon data were available during the exposure period of the volume trap, a remarkable agreement was found. The best volume traps appeared to be polyester sponges found in many stuffings. The major drawback is the destructive nature of the method, its major advantage is the direct relationship between the radon gas and the 210Po signal. The technique can be useful in epidemiological studies of the lung cancer risk from indoor radon, for assessing the long term radon exposure.
Before the May 2003 TOPOFF II exercise in Seattle, the U.S. EPA's Laboratory in Las Vegas prepared five spiked samples to be analyzed by the Washington State Department of Health's (WSDOH) Radiation Laboratory. Two of these were simulated deposition samples prepared on packaging tape. Laboratories throughout the world are investigating rapid methods for analyzing plume-borne radioactive materials. While measuring gamma emitters in environmental samples is fairly straightforward, the potential presence of alpha emitters adds complexity. The short range of alpha particles and the high degree of energy interference between nuclides usually require chemical separations and very thin mounts for alpha spectrometry. Work published on Frisch-Grid alpha counting of transuranics in soil and the commercial development of radon-rejection for air filters indicate that alpha spectrometry can be used directly on some media with success. This suggests that plume-borne material sampled from air or freshly deposited surface layers may be counted directly by alpha spectrometry, making it possible to identify both alpha and gamma emitters and determine their relative concentrations. The long-range objective is to propose a sampling method that can be used for rapid qualitative and semi-quantitative analyses using conventional radioanalytical instruments, with minimal preparation. This paper describes experimentation with this approach during a real-time exercise. All samples were prepared by spiking with (137)Cs, (241)Am, (238)Pu, and (239)Pu and presented to the WSDOH's Radiation Laboratory for analysis during TOPOFF II. The laboratory quickly identified and determined the ratios of all four contaminants on the tape samples using sequential alpha spectrometry and gamma-ray spectroscopy without chemical separations.
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