The First Radon Map of Vojvodina Sofija Curcic, Istvan Bikit, Jaroslav Slivka, Ljiljana Conkic, Miroslav Veskovic, Natasa Todorovic, Ester Varga, Dusan Mrdja Department of Physics, Faculty of Sciences, University of Novi Sad, Trg Dositeja Obradovica 4, 21 000 Novi Sad, Serbia and Montenegro E-mail: bikit@im.ns.ac.yu Abstract: Radon is a naturally occurring radioactive gas. Radon is the alpha emitter and decays to short-lived daughters ( 218 Po, 214 Pb, 214 Bi and 214 Po). Radon partially decays in material where it has been generated and partially moves rapidly by concentration-driven diffusion into the open air. The presence of determined radon activity concentration in the atmosphere is the natural contamination. Sources of radon in flats are: soil under and round the object, building s material and water and gas used in household. Radon is entering the home by emanation from walls, floor and ceiling, through underlying soil and from things and materials that are in the room. Underlying soil is the main source of indoor radon. However, building s materials also may contribute to elevated indoor radon concentration, especially in combination with low ventilation. In this paper the results of the first radon mapping in Vojvodina are presented Indoor radon activity concentration in air has been measured at the whole area of the province Vojvodina (on about 1000 locations) by plastic track detectors CR-39. On the base of the obtained results, the average indoor activity concentrations of 222 Rn (A M ) for individual municipalities and for the whole province of Vojvodina were estimated. Almost 20% of the measurements are over the 200 Bq/m 3 and 4% of the measurements are significantly elevated indoor radon concentrations. The dependence of indoor radon on the location and type of the houses and flats is discussed. Indoor radon activity concentration in air has been measured in Novi Sad in about 200 houses and flats. By measuring gamma-activity of radon daughters, radon activity concentration was determined to be 50 Bq/m 3. 1. Introduction Radon is a naturally occurring radioactive gas. It is a colorless and odorless noble gas, 7,5 times weighted than the air. Uranium 238 U decays through several different isotopes. When it reaches radium 226 Ra, it decays to radon 222 Rn and undergoes a change of state to a gas. Radon partially decays in material where it has been generated and partially moves rapidly by concentration-driven diffusion into the open air. Sources of radon in flats are: soil under and round the object, building s material and water and gas used in household. Indoor radon concentration depends on: 226 Ra content in soil and building materials (BM), moisture content in soil and BM, potential of diffusion in soil and BM, surface area and isolation quality of structures contacting with soil, building floor, air ventilation in the building, weather conditions and indoor-outdoor temperature and pressure differences [3]. The base of developing the public indoor radon politics is a definition of risks connected with the exposition of population to radon and its daughters in the living rooms. The actions that are conducted in order to reduce radon activity concentrations in flats and offices suppose the establishment of national recommendations, directives or laws about maximal permitted radon concentrations [4,5]. BM causing high concentration have been used in several countries. In some cases these are materials of natural origin (i.e., granite or alum shale concrete), and in other cases they are by-products from different industries (by-product gypsum, waste rock from mining etc.). Serbia and Montenegro has advisory reference levels for internal BM: 226 Ra< 200 Bq/kg; 232 Th<300 Bq/kg; 40 K<3000 Bq/kg; all 1
antropogenic radionuclides< 4000 Bq/kg. Maximum permitted annual indoor dose due to gamma radiation from BM is 1 msv, which corresponds to the following activity index : 226 Ra/200 + 232 Th/300 + 40 K/3000 + all antropogenic radionuclides/4000 < 1 (1) where 226 Ra is radium activity concentration in Bq/kg; 232 Th is thorium activity concentration in Bq/kg; 40 K is potassium activity concentration in Bq/kg. 2. Methods The method of the adsorption on activated charcoal canister was applied [6] in some measurements of radon activity concentrations. The canister gamma activity (the activity of 214 Bi and 214 Pb radon daughters) was measured by means of high resolution germanium and NaI(Tl) scintillation spectrometers. The efficiency of the detectors was determined using the EPA 226 Ra Reference source. The canisters were exposed for two days. The typical time between the end of the exposion and the beginning of the measurement was about two hours. Gamma spectrometric measurements were performed with high resolution HPGe gamma spectrometer with nominal efficiency 22%, was placed in another schielding chamber with iron walls 25 cm thick. In order to achieve 5% statistical accuracy at 100 Bq/m 3 the time of measurement was usually 1 hour. In our measuring chamber the radon levels are very low (less than 5Bq/m 3 ) so radon fluctuations could not affect the results of most measurements. The latest measurements were performed by CR-39 track detectors on about 1000 locations in spring 2003 in order to make the first radon map. The exposure time was 90 days. CR-39, which is a clear 1 cm 2 stable plastic sensitive to the tracks of alpha particles, is the most widely used and accurate detector for radon measurements. The etching, evaluating and counting processes were performed in Hungary by Radosys Company. 3. Results The results of the previous measurement of agricultural soil are presented in Table I. As can be noticed, the average concentrations of detected radionuclides were found at natural environmental level. The radionuclide 137 Cs was identified in all soil samples.the large standard deviation and the large difference between the minimum and maximum 137 Cs activity concentrations show typical features of a man-made contaminant. This radionuclide originates from the accident of the nuclear power plant 'Lenin' in Chernobyl in 1986. Due to 30 years half-life of this radionuclide, it will be relocated, washed out and redistributed. However it will be present for a long time in the Vojvodina ecosystem. Table I The average Ā av, minimal A (min) and maximal A (max) concentration of radionuclides in agricultural soil [7] radionuclide Ā av [Bq/kg] A (min) [Bq/kg] A (max) [Bq/kg] 137 Cs 12±9 1.1 55.0 238 U 51±9 24.0 72.0 226 Ra 39±7 19.7 51.0 232 Th 53±8 22.0 64.0 40 K 554±92 238 730 2
On Fig.1 results of measurements of indoor radon concentrations in Novi Sad flats by charcoal cannisters are presented. The results of the statistical analysis of about 180 measurements are listed in TableII. FIG. 1. Frequency distribution of radon activity concentrations in Novi Sad flats during period 1992-2003 Table II Maximum (A max ), minimum (A min ) and average indoor radon activity concentration A av with standard deviation σa in Novi Sad flats radionuclide A max A min A AV σa 222 Rn 391 2 50 69 The results of the latest measurements of indoor radon concentrations by CR-39 performed in spring 2003 are presented on Figure 2 and in Table III. Indoor radon activity concentration in air has been measured at the whole area of the province Vojvodina (on 1000 locations). On the base of the obtained results, the average indoor activity concentrations of 222 Rn (A AV ) for individual municipalities (Table III) and for the whole province of Vojvodina were estimated. In Vojvodina the average indoor radon activity concentration yields 144 Bq/m 3 (Table IV). FIG. 2. Distribution of indoor radon activity concentrations measured by CR-39 during the period December 2002 March 2003 3
Table III Statistical data for radon activity concentration for individual municipalities in Vojvodina No. Municipality A AV σ n the number of measurements Amin Amax 1. Novi Sad 133 115 86 10 445 2. Pancevo 119 103 50 26 668 3. Sombor 157 129 48 37 726 4. Subotica 71 72 49 11 449 5. Zrenjanin 160 140 53 24 893 6. Sremska Mitrovica 164 118 40 19 463 7. Vrsac 98 68 33 15 385 8. Backa Palanka 147 91 32 17 378 9. Indjija 173 183 29 18 792 10. Kikinda 138 99 32 20 306 11. Ruma 218 140 28 26 566 12. Stara Pazova 239 182 29 16 707 13. Kula 152 111 24 47 437 14. Vrbas 155 100 27 15 377 15. Alibunar 104 136 22 15 627 16. Backa Topola 139 71 18 54 319 17. Becej 116 74 30 24 309 18. Kanjiza 99 70 20 30 286 19. Kovin 173 154 20 28 622 20. Nova Crnja 168 78 16 49 364 21. Novi Becej 102 93 10 31 323 22. Odzaci 108 72 24 30 305 23. Senta 124 68 20 17 303 24. Secanj 203 105 21 30 415 25. Sid 192 144 20 16 438 26. Titel 116 72 13 21 281 27. Zabalj 181 172 10 53 658 28. Zitiste 100 90 10 20 323 29. Ada 103 119 10 2 402 30. Apatin 67 73 3 19 151 31. Bac 152 99 9 23 372 32. Backi Petrovac 160 118 15 28 380 33. Bela Crkva 258 205 9 58 408 34. Beocin 200 157 9 45 454 35. Coka 154 65 10 73 264 36. Irig 114 147 10 33 521 37. Kovacica 105 66 5 26 198 38. Mali Idjos 166 188 10 41 234 39. Novi Knezevac 117 60 9 26 202 40. Opovo 125 87 8 57 317 41. Pecinci 163 84 11 46 338 42. Plandiste 111 65 7 43 212 43. Srbobran 150 91 10 43 303 44. Sremski Karlovci 149 136 8 30 453 45. Temerin 94 98 11 14 372 4
Table IV Maximum (A max ), minimum (A min ) and average indoor radon activity concentration A AV with standard deviation σa in Vojvodina houses measured by CR-39 during period December 2002 March 2003 A AV σ n the number of measurements Amin location Amax location 144 120 968 2 Ada 893 Zrenjanin 4. Discussion The mean value of indoor radon activity concentration (Table II) obtained by charcoal canisters is (50 ± 69) Bq/m 3. Only 5 % of the results exceed the 200 Bq/m 3 value accepted as an intervention level in Serbia and Montenegro. The highest results are obtained for flats on ground level. The owners of this flats and houses were advised how to solve the radon build-up problem. The lognormal distribution obtained proves the random nature of the radon build-up in the flats investigated. The target locations of the newest investigations of indoor radon concentrations by CR-39 track detectors were old farmer houses in the suburbs region. The results of this measurement (Table IV) yield the mean value of 144 Bq/m 3 with the standard deviation of 120 Bq/m 3. Almost 20% of the measurements are over the 200 Bq/m 3 and 4% of the measurements are significantly elevated indoor radon concentrations. The results are above the expected values for the plains districts, despite the normal concentrations of 238 U, maybe due to underground waters. The results are above the expectations for the low-land districts and show that dominant sampling in new city flats can seriously underestimate the radon buildup problem. 5. Conclusions The results obtained confirm that radioecological problems at home building are not neglibile even in flat agricultural region. With detailed measurements and carefull selection satisfactory building materials can be found, just slightly contributing to the radon build-up problem. However bad ground isolation in the houses will cause significant radon build-up even on soils with moderate uranium content. Acknowledgment The authors acknowledge the financial support of the Ministry of Science, Technology and Developing of Serbia, in the frame of the project Nuclear Spectroscopy and Rare Processes (No 1859) and the City Development Institute of Novi Sad. References 1. Andrejeva, O.S., Natural and Depleted Uranium, Atomizdat, Moscow (1979) (in Russian) 2. Kikoin, I.K., Tables of Physical Constants, Atomizdat, Moscow (1976) (in Russian) 3. United Nations Scientific Committee on Effects of Atomic Radiation (UNSCEAR), Ionizing radiation: Sources and biological effects. Exposures to radon and thoron and their decay products. Report to General Assembly, with annexes, 141-210, (1982) 4. EC, 1997: Radiation Protection 88. Recommendations for implementation of Title VII of the European Basic Safety Standards concerning significant increase in exposure due to natural radiation 5
sources. European Commission. Office for Official Publications of the European Commission. Radiation Protection Series. 5. Radon Legislation and National Guidelines. SSI report. Swedish Radiation Protection Institute. No 99: July 1999. ISSN 0282-4434 6. U.S. Environmental Protection Agency. National Residential Radon Survey: Summary Report. EPA-402-R-92-004, (1992) 7. I.Bikit, J. Slivka, D. Mrdja, N. Zikic-Todorovic, S. Curcic, E. Varga, M. Veskovic, Lj. Conkic: Simple Method for Depleted Uranium Determination, Japanese Journal of Applied Physics (JJAP), 5269-5273, Tokyo (2003) 6