INTERCOMPARISON BETWEEN RADON PASSIVE AND ACTIVE MEASUREMENTS AND PROBLEMS RELATED TO THORON MEASUREMENTS *

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1 INTERCOMPARISON BETWEEN RADON PASSIVE AND ACTIVE MEASUREMENTS AND PROBLEMS RELATED TO THORON MEASUREMENTS * B. BURGHELE, C. COSMA Faculty of Environmental Science and Engineering, Babes-Bolyai University, Cluj-Napoca, Romania, burghele.bety@ubbcluj.ro Received November 15, 2012 Researchers around the world had already established that radon and thoron are a hazard to the human health. Different techniques and methods are applied in order to measure them as accurately as possible. In Romania, most measurements earlier conducted were focused on using grab-sampling measurements to establish the two gasses decay progeny activity concentration; however, recently, there has been an increased interest in using passive devices together with active devices for long-term measurements. In order to get more accurate knowledge about the real radon and thoron indoor concentration in dwellings as well as in public buildings and work places, a new survey was carried out in 15 locations within and surrounding Birlad Town, Vaslui County. Passive methods including two types of discriminative detectors based on CR 39 chips as well as active measurements, such as RAD7 were used for this survey. During this survey, it was also pointed out the continuous difficulty in measuring thoron, due to its short half-life and faults in the measuring system. Key words: radon, thoron, passive devices, active measurements. 1. INTRODUCTION It goes without saying that radon ( 222 Rn) represents a hazard to the human health [1] and that along the years researchers around the world have developed numberless methods and devices to measure it to estimate its contribution to the total natural dose received by the population and to find remediation possibilities [2]. Shortly thereafter was discovered that along with radon came thoron ( 220 Rn), a decay product of thorium ( 232 Th); thoron concentration in some cases exceeded [3] that of radon and therefore, could cause overestimations of the radon contribution to the natural dose. Moreover, thoron interferes with some of the radon detection devices [4]; consequently providing an overestimation of the effective dose received by the general population. Due to its short half-life (56 seconds) thoron * 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. S56 S61, Bucharest, 2013

2 2 Intercomparison between radon passive and active measurements S57 measurements often encounter certain difficulties whether if there are used passive or active methods, i.e. due to the distance from the wall and floor, time of measurement, diurnal and seasonal variations. The present survey included dwellings, schools and public institutions; for each location a standard questionnaire was filled in order to collect data regarding building materials, year of construction, occupational habit, geological background and the dweller s status (smoker or non-smoker). It was also taken into consideration that all measurements were taken during spring-summer period [5]. 2. MATERIALS AND METHODS In order to measure radon activity concentration more thoroughly, two types of passive detectors and one active device were used in the present survey. Also, there were used one passive detector and one active device to investigate the thoron activity. All passive devices are represented by nuclear track detectors based on CR-39 chips, provided by Radosys Ltd. Budapest, namely RSK, used to measure radon and RADUET, used to measure both radon and thoron activity concentration. A Durridge RAD7 device was used in both cases for active measurements. In case of passive detectors the protocol was to deploy each device in a high occupancy location (more than 4 hours spent inside) during a period of 90 to 100 days; each pair of detectors were placed onto present cupboards, tables or other furniture within 30 to 50 cm from the wall. After the exposure, all devices were collected and brought to the laboratory where were chemically etched in 6.25 M solution for 4 ½ hours at 90 C. Thusly prepared the CR-39 plates are read using a Radosys microscope which provides the track density number on the chips. According to the exposure period and the track density found on the chips the exposure and the activity concentration of radon and thoron can be calculated. The RAD7 system used for active measurements is based on spectral analysis of alpha emitters such as radon and thoron but also their decay progenies. In the present study it was used to measure the emission from the building material, floors, indoor activities at different distances from the walls and indoor diurnal measurements. The protocol used for emission was 5 measurements in Sniff mode, with 10 minutes cycle time. 3. RESULTS During the survey, 15 locations within and surrounding Birlad Town of Vaslui County were investigated with passive detectors, 6 of which were also submitted to active measurements. The selected communes and the specified town are marked in Fig. 1b, they represent 8% of the total area and approximately 20%

3 S58 B. Burghele, C. Cosma 3 population living in Vaslui County, Fig. 1a. Two communes had only one set of detectors while the rest had two detectors each; three sets of detectors were used for the city. (a) Fig. 1 The investigated area represented by: (a) Vaslui County, Romania and (b) 7 communes located in the south-west of the county. The plot of all data obtained for radon measurements can be seen in Figure 2. It can be observed that in case of radon, the activities obtained with RADUET are higher compared to RSK and RAD7 diurnal measurements. It is to be specified that both types of detectors were places in the same location, hence, large differences between the two gas concentrations of radon were not expected. Standard deviation of repetitive measurements with our microscope concluded in a range for track densities of (RSK) and (RADUET) for radon and for thoron. However, there is yet no explanation for the high differences between indoor concentrations (from 1 to 4 times higher values for RADUET compared to RSK) obtained during this test. Upon further investigation it was pointed out that among the reasons for these differences most relevant proved to be the age of the detectors [6, 7] since the RSKS detectors were older than 2 years compared to RADUET detectors or misplacement of detectors by the owners. The thoron activity concentration was found to be slightly higher than radon in two of the investigated dwellings, location 5 and 9, as plotted in Fig. 3. Due to the fact that thoron has a half-life of 55.6 second it cannot travel as far from its source as radon can before decaying. Thoron measurements performed with the help of RAD7 indicated that there is a high activity in the vicinity of the walls and low activity towards the centre of the room, Fig. 4. Moreover, emission measurements in one adobe house pointed out that the walls (building material) represent the source of thoron and the floor (filling) of radon with average activity concentrations from walls of 395±96 Bq m -3 for thoron and 181±45 Bq m -3 for radon while from the floor were obtain 66±43 Bq m -3 and 435±80 Bq m -3 respectively. Further investigation is needed in order to identify why the activity of thoron is higher in some adobe houses and low in other buildings constructed with the same material. It is commonly observed that compared to that of radon gas, a much smaller fraction of the thoron gas in soil ever reaches the interior of a (b)

4 4 Intercomparison between radon passive and active measurements S59 building [8]. At a distance of 1m from the wall the thoron activity concentration appears to be fairly constant and independent of the distance from the floor. However, thoron gas can still be a hazard since its progeny 212 Pb (half-life of 10.6 hours) has more that enough time to accumulate within the indoor air. Fig. 2 The different activity concentrations of radon obtained with different measuring systems. Fig. 3 The different activity concentrations of thoron obtained with different measuring systems.

5 S60 B. Burghele, C. Cosma m from the wall on the floor 50 cm from the wall 50 cm above the floor 1m from the wall Radon Thoron Concentration (Bqm -3 ) m from the wall 2m from the wall 1.5m from the wall cm from the wall : : : : : : :24 Time Fig. 4 Variations of thoron concentration with the distance from the walls and floor measured with RAD7. For this text were taken into study 11 dwellings, 2 schools and 2 public buildings. The construction age of the buildings varies between 150 to 7 years old, 80% being built in the second part of the XX th century. Adobe as specified in Table 1, represents the building material used for 60% of the locations, including those with higher thoron activity. In order to calculate the equilibrium equivalent concentration (EECRn) of radon and thoron (EECTn) two different equilibrium factors [9] between the gas and its short-life progeny were used, 0.4 for radon EEC and 0.02 for thoron EEC and the average activity concentration for each location and gas. The resulted arithmetic mean per gas shows an activity concentration of 129 Bqm -3 for radon and 53 Bq m -3 for thoron. It is to be enhanced that the AM obtained for radon is quite close to the average value of 126 Bqm -3, calculated for Romania [2]. Location index and building materials Table 1 Measurements of radon and thoron with different investigating methods and for different types of building materials Radon concentration RADUET RSK RAD7 EECRn Thoron concentration RADUET RAD7 EECTn 1 adobe 218±1 85± ± adobe 289±1 126± ± adobe 125±1 37± ± red bricks 226±4 93± ± adobe and 177±4 46± ±9 4.5

6 6 Intercomparison between radon passive and active measurements S61 mortar 6 red bricks 208±6 63± ± red bricks 430±8 88± ± red bricks and gypsum 265±4 106± ±9 1.9 board 9 adobe 126±3 48± ± adobe 123±1 41± ± red bricks 255±3 68± ± adobe 180±3 62± ± adobe 235±2 80± ± adobe and gypsum board 189±1 107± ± BCA (cellularexpanded concrete) 232±2 74± ± AM+SD 219 ± ± ± ± ± ± ± CONCLUSIONS After this survey, only one location, namely a school built 150 years ago with red bricks presented a radon activity concentration higher than the reference level for radon indoor [10], respectively 430 Bq m -3 measured with RADUET. An average concentration between the two types of passive detectors decreases this activity under the action limit. The rest of the data were bellow this reference level making this region, until further research, a rather safe place to live when it comes to radon and thoron. These measurements represent the first survey using passive methods for investigating both radon and thoron gas within this region of Romania. Acknowledgements. This paper was realised with the support of POSDRU CUANTUMDOC DOCTORAL STUDIES FOR EUROPEAN PERFORMANCES IN RESEARCH AND INOVATION ID79407 project funded by the European Social Found and Romanian Government. REFERENCES 1. Stiefer P.S., Weir B.R., Health risk attributable to environmental exposure, Radon. Journal of Hazardous Materials 39, , A. Cucoş (Dinu), C. Cosma, T. Dicu, R. Begy, M. Moldovan, B. Papp, D. Niţă, B. Burghele, C. Sainz, Thorough investigations on indoor radon in Băiţa radon-prone area (Romania), Science of the Total Environment. Vol. 431, pp , 1 August J. Chen, D. Moir, T. Pronk, T. Goodwin, M. Janik and S. Tokonami, An update on thoron exposure in Canada with simultaneous 222Rn and 220Rn measurements in Fredericton and Halifax. Rad. Prot. Dosim., Vol. 147, No. 4, pp , 2011.

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