CR-39 SAMPLING OF INDOOR RADON IN SOUTHERN ROMANIA *

CR-39 SAMPLING OF INDOOR RADON IN SOUTHERN ROMANIA * ANGELA VASILESCU Horia Hulubei National Institute for Nuclear Physics and Engineering (IFIN), P.O.Box MG-6, RO-077125 Bucharest-Magurele, Romania, E-mail: angela@.nipne.ro Received November 15, 2012 The paper presents studies associated with the implementation of the indoor radon measurement technique using CR-39 nuclear track detectors and an automatic Radosys set-up in the Applied Nuclear Physics Department of the Horia Hulubei National Institute for Physics and Nuclear Engineering and new, in-situ, indoor radon data in various locations in South/South-East Romania. Key words: indoor radon, solid-state nuclear track detectors, CR-39. 1. INTRODUCTION The new Solid State Nuclear Track Detector (SSNTD) Indoor Radon Laboratory of the Applied Nuclear Physics Department at the Horia Hulubei National Institute of Physics and Nuclear Engineering (IFIN) employs CR-39 plastic detectors and a Radosys set-up [1]. It is mainly aimed at the regional coverage of the necessities of indoor radon measurements especially in the Southern part of Romania, but also to become a reliable member of a National Radon network. The implementation of this technique brought along a series of tests for the better understanding of this already well established method, and was doubled by indoor sampling measurements at various sites. The geographical region covered by our study was expected to be of reasonably low impact, compared to radon prone areas in the country (e.g. Crucea in Moldova, Stei in Transylvania), however, as to our present knowledge, the statistic of existing measurements for this region is low and any new data are welcome. Radon originates from the 238 U - 226 Ra natural decay series and escapes into air easily from the material in which it is formed (be it soil, rocks, water or building materials). The air we inhale (outdoors or indoors) contains radon. Average radon * Paper presented at the First East European Radon Symposium FERAS 2012, September 2 5, 2012, Cluj-Napoca, Romania. Rom. Journ. Phys., Vol. 58, Supplement, P. S311 S319, Bucharest, 2013

S312 Angela Vasilescu 2 levels are of 8 Bq/m 3 for continental outdoors, at 1m from the soil, and only 0.04 Bq/m 3 above the sea. Indoor radon concentrations reach orders of magnitude higher values, typically 12-300 Bq/m 3 up to maximum values of several thousands of Bq/m 3 in radon prone areas [2]. In temperate climate, heat conservation and reduced ventilation habits increase the radon levels, especially in the cold months, determining, e.g. for Romania, an increase of the measured levels by a factor of about 2 [3]. Published data show e.g. values in the limits of 15-1005 Bq/m 3, with an arithmetic mean of (115±48) Bq/m 3 for measurements performed in winter, spring and summer for several sites in Transylvania [4-5]. 2. EQUIPMENT AND PROCEDURE The implementation of the CR-39 SSNTD technique in IFIN was started by A. Danis with the development of a state-of-art radon probe and calibration chamber [6], and continued in our new Radon lab [7]. Presently we use Radosys equipment [1], 10x10 mm 2 CR-39 detector chips in RSKS type chambers, with 4.5 h etching in 25% NaOH at 90ºC in the Radobath RSB4 unit. The reading of the tracks and the calculation of track density is done automatically with Radometer 2000, employing routine or slightly modified counting programs for small round tracks, provided by the manufacturer. The exposure and radon volume concentration is evaluated using calibration factors provided by the manufacturer of the chips. In the routine field measurements, we used automatic reading, repeated three times, with calibration for each measurement. In the intercomparison tests we read the detector chips five to ten times and applied the Chauvenet exclusion test for outliers. We also used a modified slower reading routine, with repeated focalization on the surface of the chip, both in separate individual calibration at one reading and in a version of repeated measurement three to five times at the same focalization, in order to reduce the spread between readings. The final choice for the routines adopted for the procedures to be used in our lab was to apply the fast routine for the field measurements (repeated five times), while for intercomparison and quality control exercises we decided for the modified slow method, and check also all the other options. At each site ( point of measurement ) we installed two Radosys detectors, and for each session we kept two blank background chips, to monitor the CR-39 set used. The conditions for the experimental tests performed during the implementation of this radon measuring system will be described together with the discussion of the results.

3 CR-39 sampling of indoor radon in Southern Romania S313 3. RESULTS AND DISCUSSION In the process of evaluation of our procedures and their standardization, we performed several exercises, including comparative readings by human operator with an optical microscope, comparisons of our previous manual scanning technique [7] with the automatic Radosys system measurements, parallel counting and measurement. We also included some field measurements in our comparisons. The following evaluation tests were the most significant to define our working procedures: Intercomparison exercises with CR-39 automatic and operator reading Repeatability and reproducibility tests, and Background survey of unexposed (blank) detectors. Automatic reading is a sensitive issue in the very low density region (for background studies in particular), where various program variations have been tested, with operator supervision of the accuracy in the identification of α tracks. A problem which cannot be neglected at very low densities is the misidentification of various unavoidable defects (etching defects, impurities, scratches) as tracks, which transfer mainly into higher final values. But also some real α tracks can be lost, especially in the fast (standard) routine. If such a background correction is then applied in the lower radon concentration domain, it can lead to underestimation of the actual radon exposure. Several trial tests in state-of-art calibration vessels both in the IFIN and in the Cluj-Napoca labs and parallel reading of detector sets etched in both laboratories were performed in order to compare our results with the Environmental Radioactivity and Nuclear Dating group at Babes-Bolyai University (UBB). Both laboratories used Radosys systems, but of different generations and, accordingly, slightly different etching recipes (5h at UBB) and evaluation software versions. In the most elaborate test, we used 20 detectors, and 10 background detectors (all from one batch). The detectors were exposed in the UBB radon chamber, and then divided between the two labs (two sets of 10), together with the background detectors (5 blanks to each lab), for etching. The read-out was done for both (10+5) detector sets in both laboratories, and the final results were compared. The difference in the results obtained in the two laboratories was within 3.6%. As a general rule, the IFIN results were somewhat lower, but with better reproducibility of the track density for both etching conditions: σ n (IFIN)/σ n (UBB)= 0.28/0.34 (etch IFIN); σ n (IFIN)/σ n (UBB)=0.39/0.57 (etch UBB).

S314 Angela Vasilescu 4 The repeatability of the measurements at IFIN was tested for track density evaluation on ten repeated measurements on a set of twelve chips in the 40-50 tracks/mm 2 range, and was better than 99%. The reproducibility of the measurements was tested on two sets of ten detectors exposed in the same conditions, and it varied between 1.2-1.7%. Our first participation at an international intercomparison exercise (2011 Intercomparison exercise in field conditions Saelices el Chico, Spain) yielded good results [8]: of the three domains of exposure, our results were situated both in the low (110 kbq/m 3 ) and high (3800 kbq/m 3 ) exposure region within 1σ of the average, and within 2σ for the intermediate exposure (700 kbq/m 3 ). The final report [8] situated the IFIN results in class A (better than 10% from the overall average of the participants) for the low and high exposure and class B (better than 15%) for the intermediate exposure. This exercise used three sets of 15 detectors for the three exposure values and a set of 15 transit detectors (mounted chips), which accompanied the devices sent to Spain and back, and were etched in the same session. We also monitored this batch with four unexposed chips, for our background evaluation. This was not imposed by the intercomparison exercise, but is an internal checking routine of our detector batches. 3.1. BACKGROUND STUDIES We performed a study of the background for the CR-39 chips from two batches of detectors we used for measurements during the years 2007-2012. The background detectors were etched in each measurement session together with the field detectors, and were used in the evaluation of the background corrections. In between the sessions, the detectors were stored packaged in a freezer, they were extracted from the original packaging at the same time with the field detectors and kept wrapped firmly and packaged, in the laboratory, at room temperature. We used a minimum of two (unmounted) CR-39 chips as background detectors at each etching session. The timeline for the detectors can be reconstructed from the laboratory logbook (from arrival in the lab to unpacking and etching). During the monitored time, we used two different batches, series J (until 2010) and series T, manufactured by Radosys Ltd. Due to the fact that the lab did not fulfill yet the necessary legal conditions for authorization for third party measurements (e.g. intercomparison results, elaboration and approval of all quality assurance procedures needed), during this time, only tests and field sampling measurements were performed as research. (The number of detectors used was

5 CR-39 sampling of indoor radon in Southern Romania S315 therefore also limited.) Figure 1 shows the track density values obtained on batches J and T. The average Poisson uncertainty for background was ~19%. A batch is delivered in several sheets, packaged individually in radon free aluminized bags, with cut and marked chips glued on a foil, from which the chips are extracted with tweezers and mounted in the radon (monitor) device (or wrapped as blanks, as already mentioned). The background detectors were selected randomly from the set of unexposed chips taken for a measurement session (field or other test). The time dependence (time is correlated with the number of the detector, as the bags were opened in increasing order of the numbers) is not evident, and we see rather random local non-uniformities in the batch (more evident for the already used-up J series). Fig. 1 CR-39 background track density variation in two detector series (batches). Differences in the background densities up to 50% have been seen even within the set used in the same session. In the cases when only a pair of detector was used, the decision is difficult: the chips had to be analyzed carefully, checked by microscope, to see whether the discrepancy was not due to the program or some material or etching defects. In most cases, the differences could not be explained by the presence of an accidentally bad chip. The background results on the T series are lower and less spread.

S316 Angela Vasilescu 6 Fig. 2 Frequency distribution of background track density for the J and T CR-39 chips. Fig. 2 presents the frequency distribution of background track densities measured on both detector series. The background correction can amount in the typical (2 3 months) exposures up to 10 11 Bq/m 3, which is definitely high, representing about 10% of the warning level at 100 Bq/m 3 recommended by the World Health Organization (WHO) [9]. This aspect might need further investigation, and a reasonable decision in the acceptance of a measurement especially in the low track density region, where the background correction could be of significant impact, leading to the underestimation of the radon risk. 3.2. FIELD MEASUREMENTS As the final goal is actually the measurement in field conditions, we used CR-39 RSKS detectors in the cold season (late autumn/winter/early spring) in pairs of two, installed in various locations, in homes mainly, in Bucharest and in Ilfov county (Magurele, Ciorogarla, Copaceni, Buftea, Corbeanca), in Giurgiu (Giurgiu), Fetesti (Ialomita), Cernavoda (Constanta), Busteni (Prahova), Mogosesti, Stilpeni and Corbsori (Arges). (The location of the sites is presented on an edited Google map, see Fig. 3). Some of these sites were only temporarily inhabited, so, in those cases, the results correspond to a worst case scenario as to ventilation conditions.

7 CR-39 sampling of indoor radon in Southern Romania S317 Fig. 3 Location of field measurements by CR-39 SSNTD (black squares on an edited Google map). The measurements were performed in several campaigns, starting with 2007 to 2011. Some sites were measured several times, especially those where the home owners responded to our first results with changes. We noticed an improvement in a few cases after repairs or changing ventilation habits, but in some cases the new measurement did not show any change or even an increase above the 200 Bq/m 3 level shown in the plot in Fig. 4a. Figure 4 summarizes the data on the field measurements: the plot (a) shows the measured values for each location, and the histogram (b) presents the frequency distribution of the radon concentration in Bq/m 3. All the experimental values in the plot are averages over two detectors put in the same location with one exception, when a detector was broken by accident. The time of exposure was typically 2-3 months, and the transit times were minimized (from a few hours to a few days - less than one week). The volunteers participating in the survey were instructed to keep the devices in radon free condition, as much as possible (e.g. in open air), until they could be returned to the lab. Action levels recommended by ICRP [10] for indoor radon levels are between 200 600 Bq/m 3, and the value accepted is a political decision for every country, depending on the local geology and affordable costs. The latest recommended (WHO) warning level of 100 Bq/m 3 is also shown in Fig. 4a [9].

S318 Angela Vasilescu 8 a) b) Fig. 4 CR-39 sampling in various locations in Southern Romania, between 2007 and 2011/2012, in the cold season (a); frequency distribution of radon concentration (b). 4. CONCLUSIONS The IFIN Indoor radon lab can perform reliable measurements employing CR-39 detectors and a Radosys system.

9 CR-39 sampling of indoor radon in Southern Romania S319 Different sets of detectors can have variations of background track densities, but differences can be seen within the same set as well. With careful handling, storing and monitoring, their lifetime can be prolonged after the one stated by the manufacturer. Due to the automatic statistical counting routine, it is reasonable to evaluate whether the background correction should be taken into account at each measurement or not at all, or its impact on the result discussed separately, depending on the character of the measurement (research, accuracy evaluation of the method or routine mapping measurements). The field measurements showed reasonably low values, with a few exceptions, to confirm our expectations for the Southern part of Romania. Acknowledgements. Financial support for this work was ensured by the National Agency for Scientific Research ANCS, in part by the 2CEX06-10-78 and PN09-37-02-04 projects. The very good collaboration and useful discussions with the members of the Environmental Radioactivity and Nuclear Dating group led by Professor Constantin Cosma at the Babes-Bolyai University (Professor Cosma and Ms. Kinga Szacsvai, in particular) during the implementation of the CR-39/Radosys system at IFIN are acknowledged by the author. Thanks go also to all the volunteers participating in the field survey. REFERENCES 1. RADOSYS LTD., Technical information, http://www.radosys.com/index.htm 2. IAEA - Radiation protection against radon in workplaces other than mines, Safety Reports Series No. 33, Vienna, 2003. 3. C. Cosma, D. Ristoiu, A. Poffijn, G. Messen, Radon in various environmental samples in the Herculane spa, Cerna Valley, Romania, Env. International 22, Suppl.1 (1996) 383 388. 4. C. Cosma, K. Szacsvai, A. Dinu, D. Ciorba, T. Dicu, L. Suciu, Preliminary integrated indoor radon measurements in Transylvania (Romania), Isot. Env. Health Studies 45:3 (2009) 259 268. 5. C. Cosma, D. Ciorba, A. Timar, K. Szacsvai, A. Dinu, Radon exposure and lung cancer risk in Romania, Journ. Env. Prot. Ecol., ISSN 1311-5065, book 1 (2009) 94 104. 6. Danis et al., System for calibration of track detectors used in gaseous and solid alpha radionuclides monitoring, Rad. Meas. 34 (2001) 155 159. 7. I.A. Ghita, A. Vasilescu, Radon assessment with solid-state nuclear track detectors in Bucharest and its surrounding region, Rom. Rep. Phys. 63:4 (2011) 940 947. 8. J.L. Gutierrez-Villa et al., International Intercomparison Exercise on Natural Radiation Measurements under Field Conditions, Saelices el Chico Spain), May 2011, PubliCan, Ediciones de la Universitad de Cantabria (2012), ISBN 978-84-86116-64-4, 83 93. 9. WHO Handbook on Indoor Radon: A Public Health Perspective, WHO Press, Geneva, 2009. 10. ICRP, Protection against Radon-222 at Home and at Work, Publ.65, Pergamon Press, Oxford, 1994.