Radiation Measurements 50 (2013) 141e144
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Experimental study of the response of radon track detectors with solid absorbers as radiators D. Pressyanov*, S. Georgiev, I. Dimitrova, K. Mitev Faculty of Physics, St. Kliment Ohridski University of Sofia, 5 James Bourchier Blvd., 1164 Sofia, Bulgaria
h i g h l i g h t s < Detection properties of “radon film badges” (absorber þ track detector) are studied. < The response of detectors in different packages is similar. < The experimental results are in agreement with the theoretical model estimates.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 29 November 2011 Received in revised form 22 April 2012 Accepted 26 April 2012
A new concept for passive radon monitors e “radon film badges” has been recently proposed. It uses plastic radiators of high radon absorption ability, coupled with external solid state nuclear track detector (SSNTD). First experiments and theoretical modeling revealed a remarkable potential for practical applications. In this report we present results of a dedicated experiment in which detectors were coupled with different absorbers and placed in a variety of different easy-to-carry packages. The results confirmed that radon can freely diffuse in the packages. The response of the monitors with different absorbers was determined and a good agreement with the theoretical model was found. This strengthen the basis for further research and development of this new methodology. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: 222 Rn Plastic absorbers Alpha-tracks Theoretical model Track detector
1. Introduction A new concept for radon monitors was recently proposed by Tommasino et al. (2009); Tommasino (2010); Tommasino et al. (2010). The concept is to couple a solid state nuclear track detector (SSNTD) with external foil that absorbs radon and serves as a radiator. The pioneering experiments, presented by Tommasino et al. (2009) revealed great potential for practical applications. These new dosemeters are remarkable with their compactness and can easily be fitted in many different objects like personal film or TLD dosimeters, document cases, staff-officer badges etc. That would allow convenient radon-film-dosemeters to be constructed by inserting the monitors in objects that a person can easily (or usually) carry. This possibility lies on the assumption that radon diffuses freely inside these objects. This assumption is reasonable, however it has not been previously verified. A theoretical model that describes the response of such radon monitors (Pressyanov, 2011) was developed recently. If the basic
* Corresponding author. Tel./fax: þ359 2 8687009. E-mail address:
[email protected]fia.bg (D. Pressyanov). 1350-4487/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2012.04.021
properties of the absorber and the SSNTD are known, the model allows to estimate the dosemeter response, respectively the range of measurable radon concentrations, and to evaluate the applicability of the dosemeters in different conditions. Despite that the preliminary results suggested the feasibility of the model, more comprehensive experimental validation is needed. The objective of this paper is to present dedicated experimental work aiming to: Confirm the assumptions that radon diffuses freely inside different objects, suitable for radon-film-dosemeters holders, and that the response of the dosemeters does not depend on the holders’ construction. Validate the theoretical model and draw conclusions about its feasibility by comparison with experimental results.
2. Materials and methods In present experiments two types of absorbers were made from polycarbonate foils of high radon absorption ability. The first
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Fig. 1. Illustration of exposure geometry of LR-115/II detector with monolithic absorber (M-absorber) of 250 mm thick Makrofol DE foil. Radon enters freely from both sides and decreases in depth due to radioactive decay. On the right the activity concentration (CA) profile is shown.
type (Fig. 1) was monolithic absorber (M-absorber) from Makrofol DE 1e4 foil (Makrofol is a product of Bayer AG, Leverkusen, Germany) with thickness 250 mm. Several detectors were also exposed with 500 mm and 700 mm thick foil. The second type (Fig. 2) was composite absorber (C-absorber) made from four leaves of Makrofol N foils of thickness 20 mm. Makrofol N has somewhat higher solubility of radon than Makrofol DE (Tommasino and Tokonami, 2011). In the present work the solubility is quantified by the dimensionless partition coefficient K defined as the ratio of the concentrations in the polycarbonate at its surface and the ambient concentration. The absorbers were fixed on Kodak-Pathe LR-115/II detectors as shown in Figs. 1 and 2. No glue was used, so that radon can diffuse between the foils and the detector. These new radon monitors can be made very compact and easy to use with standard personal dosemeter badges, labels, envelopes etc. In the conducted experiments detectors with different absorbers were placed in a variety of different packages e film and TLD personal dosemeters, conference badges etc. (Fig. 3). In nine of the packages a couple of detectors were arranged e with M-absorber and C-absorber. The designed dosemeters were exposed to known radon activity concentrations using the set-up shown in Fig. 4. After the exposure the LR-115/II detectors were etched with 10% NaOH at 60 C for 120 min. The tracks were counted visually by an optical microscope Nikon Eclipse E200.
3. Results and discussion One of the major benefits of the approach proposed by Tommasino et al. (2009) is the compatibility of the radon-filmbadges with variety of available holders in which they can be placed. The first goal of our experiments was to check experimentally that the basic assumption of the approach is valid: i.e. that radon can freely reach the absorber in all cases when the radon detectors are not hermetically packed. The results showed no systematic differences between detectors with identical absorbers placed in different packages (Fig. 5). The standard deviation (SD) in the individual response is 10%. That confirms our assumption and allows various design of holders with the only strict restriction that they are not hermeticaly packed. After the first pioneering works of Tommasino et al. (2009); Tommasino (2010); Tommasino et al. (2010) attention was focused on different materials that have high radon absorption ability and may serve as radiators. A theoretical model has been recently published (Pressyanov, 2011), that can be useful on the design step of these new monitors. The model estimates the sensitivity and response of radon-film-badges at different temperatures and for different absorber thickness. One of the goals of the present study was to verify experimentally this model. In the present experiments we have used two types of polycarbonate material: Makrofol DE and Makrofol N. The partition coefficient K for Makrofol DE has
Fig. 2. Exposure geometry with composite absorber (C-absorber) made of 4 Makrofol N foils with thickness 20 mm. Radon enters freely from both sides of each foil and its concentration profile is almost constant. The total thickness of 4 foils is greater than the range of the emitted alpha-particles thus adding more foils would not increase the signal.
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been studied in two independent series of experiments (Pressyanov, 2009; Pressyanov et al., 2011) which gave coherent value of about K ¼ 25 10% at room temperature. For Makrofol N Tommasino et al. (2009) reported value of K ¼ 40 5. We have used these as input values in our model calculations. For the case of M-absorber, the other input quantity of the model is the diffusion pffiffiffiffiffiffiffiffi ffi length (LD ¼ D=l, where D is the diffusion coefficient of radon in the polycarbonate material, and l is the decay constant of 222Rn). As in the present experiments M-absorbers were made only of Makrofol DE, we have used the value of the diffusion length LD ¼ 58 mm from previous experimental studies of this material (Pressyanov, 2009). The comparison between theoretical values and experimental results revealed that in all cases the experimental response is about 10% higher than the model estimates. Albeit this difference is within the range of the uncertainties of both experimental and model values, it is systematic, so we cannot exclude a real bias. One possible reason for this can be the difference between the real solubility and the value used in the model. As the present experiments were not conducted under controlled temperature, a difference between the average temperature during exposure and the temperature to which the model value is related can explain this bias.
Fig. 3. (A) Photo of empty badges and envelopes, used in experiments. (B) Badges and envelopes with radon films inside. In each package a SSNTD with M-absorber and/or Cabsorber is placed e in some cases several different absorbers are used.
Fig. 4. A scheme of the experimental set-up used for exposure of the dosemeters. Radon was supplied by a certified radium source. The activity concentration of radon in the system was monitored by an AlphaGuard radon monitor.
Fig. 5. Response of detectors with identical absorbers placed in different packages. The lines indicate mean 1 SD interval. (A) with 250 mm thick M-absorber, (B) with Cabsorbers.
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4. Conclusion The response of radon-film-dosemeters based on external radon absorbers and SSNTDs placed in different convenient packages is studied experimentally. The results show similar response for dosemeters in different packages and confirm that radon diffuses freely inside them. The response is higher when composite absorbers made of 4 20 mm Makrofol N foils are used. The general compatibility between experimental and theoretical results shows that the theoretical model could be used for optimization of the design and estimate of the response of radon-film-dosemeters. Experimental and theoretical research demonstrates, that radonfilm-badges can be constructed with different kind of absorbers and absorber material, thus covering wide range of 222Rn concentrations. Acknowledgments This research was funded by DIAL Ltd., Bulgaria. We are grateful to Dr. Luigi Tommasino for providing Makrofol N foils. Fig. 6. Response of detectors with M-absorbers of different thickness e 250 mm, 500 mm and 700 mm. The results confirm the theoretical prediction (Pressyanov, 2011) that the response in this range of thicknesses does not depend on the thickness. This is due to the similar profile of radon concentrations in the first 65 mm below the foils surface. All alpha-particles that reach the detector are emitted from this layer.
Another theoretical prediction that was checked experimentally was that for M-absorbers, the response does not depend on the thickness of the absorber for thicknesses greater than 250 mm. As seen in Fig. 6 this is confirmed experimentally (in the range of thicknesses 250e700 mm). The response of detectors with C-absorbers is about 3 times higher than that of the detectors with M-absorbers (Fig. 5). This could be explained with the better source geometry (Fig. 2) as well as with the higher K of radon in Makrofol N than that in Makrofol DE. Summarizing the correspondence between the model and the experimental results, we can conclude that the theoretical model is reliable and can be used for the design of radon-film-badges and data analysis.
References Pressyanov, D., 2011. Modeling response of radon track detectors with solid absorbers as radiators. Radiat. Meas. 46, 357e361. Pressyanov, D., 2009. Modeling a 222Rn measurements technique based on absorption in polycarbonates and track-etch counting. Health Phys. 97, 604e612. Pressyanov, D., Mitev, K., Dimitrova, I., Georgiev, S., 2011. Solubility of krypton, xenon and radon in polycarbonates. Application for measurement of their radioactive isotopes. Nucl. Instrum. Meth. Phys. Res. A 629, 323e328. Tommasino, L., Tommasino, M.C., Viola, P., 2009. Radon-film-badges by solid radiators to complement track detector-based radon monitors. Radiat. Meas. 44, 719e723. Tommasino, L., 2010. Radon film-badges versus existing passive monitors based on track etch detectors. Nukleonika 55, 549e553. Tommasino, L., Tommasino, M.C., Espinosa, G., 2010. Radon filmbadges based on radon-sorption in solids. A new field for solving long-lasting problems. Rev. Mex. De Fisica S56, 1e4. Tommasino, L., Tokonami, S., 2011. Four passive sampling elements (quatrefoil)-II. Film badges for monitoring radon and its progeny. Radiat. Prot. Dosim. 145, 284e287.