RESPONSE OF A RADON CHARCOAL CANNISTER TO CLIMATIC AND RADON VARIATIONS IN THE INTE RADON CHAMBER. A. Vargas, X. Ortega, I.

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1 RESPONSE OF A RADON CHARCOAL CANNISTER TO CLIMATIC AND RADON VARIATIONS IN THE INTE RADON CHAMBER A. Vargas, X. Ortega, I. Serrano Institut de Tècniques Energètiques (INTE), Universitat Politècnica de Catalunya (UPC) Avda. Diagonal, 647, Barcelona, Spain arturo.vargas@upc.es Abstract. Charcoal canisters are commonly used to measure radon concentration for periods of 2-7 days. The radon absorption and desorption process is known to be sensitive to several environmental parameters such as temperature, relative humidity and radon concentration fluctuation. In the present study a set of exposures under different environmental conditions were done in the INTE radon reference chamber in order to evaluate the response of a canister, designed by the INTE, to relative humidity, temperature and radon concentration. A 3-day period was chosen for the 11 exposures carried out at relative humidities, temperatures and radon concentrations ranging from %, ºC and Bq m -3, respectively. In each run 6 canisters were exposed in order to achieve a good level of statistical resolution. The climatic response analysis shows that for a constant temperature of 20 ºC, radon is absorbed about 25 % more efficiently at a relative humidity of 30 % than at a relative humidity of 80 %. For a constant humidity of 50 %, radon is absorbed about 50 % more efficiently at 10 ºC than at 30 ºC. Since the increase in the canister mass during exposure is due to water absorption, a correction in the calibration factor has been estimated for different relative humidities by measuring the weight increment, before and after exposure. The response of the detector at different, but constant, radon concentration levels shows no influence on the calibration factor value. However, when radon concentration fluctuated during the 3- day period, the weight of the first day compared with the total exposure period is about 15%, that of the second day is 30 % and that of the last day 55%. 1. Introduction The charcoal adsorption technique is widely used to measure indoor radon concentrations for periods of 2-7 days. Such short-term measurement are commonly carried out in order to provide both costeffective and rapid results, which allow decisions to be made about radon protection, as indicated in the EPA document "Protocols for Radon and Radon Decay Product Measurements in Homes". During the exposure time radon is continually adsorbed and desorbed [1]. The adsorption and desorption process depends on several factors. The most important is the air humidity since charcoal adsorbs water and radon atoms have fewer sites to be adsorbed, as described by Scarpitta and Harley [2]. Furthermore, if temperature increases, radon adsorbtion decreases as stated by Ronca-Battista and Gray [3] for the EPA-type canister. Since the rate of radon diffusion into the charcoal depends on the difference in air radon concentration and charcoal, the technique does not uniformly integrate radon concentrations during the exposure period. The weight of the last few days of the measurement period is greater than for the first few days. In the present study a set of exposures under different environmental conditions were done in the INTE radon reference chamber in order to evaluate the canister response to relative humidity, temperature and radon concentration. 2. Materials and methods The radon canister detector developed by the INTE radon group contains 90 g of Chemviron SCII activated charcoal (12 x 30 mesh) coated with a 50-g Silica Gel drier-barrier. The diameter of the canister is 11 cm. Figure 1 shows a scheme of the canister and a photograph. The amount of adsorbed radon in the activated charcoal is measured by counting the gamma pulses from 267 kev to 685 kev, a region which comprises the energy of the Pb-214 (295 kev, 352 kev) and Bi-214 (609 kev) in a NaI(Tl) scintillation spectrometer. Figure 2 shows a typical γ-spectrum obtained with this system. 1

2 FIG. 1. Scheme and photograph of the INTE canister. FIG. 2. Spectrum obtained with a NaI gamma detector from an exposed canister. The radon retention in the charcoal can be described by the following equation: where d N (t) C = K λ N (t) (1) d t λ N (t) is the number of radon atoms in the charcoal during the exposure period (T); λ is the radon decay constant in s -1 ; C is the activity mean radon concentration in air during the exposure period in Bq m -3 ; K is the calibration factor and represents the mean adsorbtion rate for radon during the exposure period per unit air radon concentaration. Expressed in Bq s -1 per Bq m -3 or m 3 s -1. It is important to point out that the calibration factor, K, depends on the period of exposure and on environmental parameters such as temperature, relative humidity and radon concentration fluctuation. Once the canister is closed, after the exposure period, the number of radon atoms in the charcoal decay according to the expression: 2

3 d n (t) = λ n (t) (2) d t where n (t) is the number of radon atoms in the charcoal after the exposure period. After approximately three hours the canister is closed, so that equilibrium between radon and its daughters is achieved in the charcoal. The γ-spectrum in the NaI detector can then be carried out in order to determine the radon concentration. The net γ-pulses obtained during the counting period, NC γ, can be obtained by the following equation: where γ t + t t NC = ε λ n (t) dt (3) NC γ is the total γ-count minus the background count; t is the time from the end of the exposure period and the start of the counting period or decay time; t is the counting period; ε is the detection efficiency. The background is estimated by the spectra obtained with an empty canister. The background estimation was 40.4 counts per minute. The decay time, t, should be as low as possible in order to obtain the best statistical resolution. A 30- minutes period was chosen for the measurement period, t. The efficiency was determined by measuring γ-spectra with a reference canister. This canister is similar to that used for radon concentration field measurement, but was filled with Ra-226 solution of known activity. Afterwards, the canister was carefully sealed in order to prevent radon atom leakage. After approximately 30 days the radium activity is in equilibrium with the radon and its progeny. The counts obtained with the reference canister are then used for calculating the efficiency: NC γ ε = (4) A ref where A ref is the Ra-226 reference activity in the canister expressed in Bq. The Ra-226 solution was obtained from the Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT). The reference canister was filled with Bq and an expanded uncertainty of 5 % (k=2) for the Ra-226 was estimated. The initial condition for equation (1) is N(0)=0 and for equation (2) n(0)=n(t). Equations (1), (2) and (3) can then be solved, and the calibration factor obtained from the following equation: 2 NC γ λ K = (5) ε C λ T λ t λ t ( 1 e ) ( 1 e ) e In order to study the value of the calibration factor, the canisters were exposed to controlled environmental conditions in the INTE radon chamber. The radon calibration chamber is 2.91 x 2.91 x 2.30 m (19.5 m 3 ) with a separate chamber (1.94 x 0.97 x 2 m) leading into the main chamber. The walls are made of 2 mm thick stainless steel welded sheets so that they are air-tight. In two of the four walls there are access ports in order to take air samples, ventilate the chamber and for the necessary electrical connections. 3

4 The reference instrument for radon concentration measurement in the chamber is based on alpha spectrometric measurement of 218 Po which is collected electrostatically on a Canberra A AB passivated implanted planar silicon (PIPS) detector. The sensitive volume of the instrument is a 8360 cm 3 glass sphere covered internally with silver. The detector inside the sphere is electrically isolated from the silver. A potential of 8 kv is applied between the detector surface and the silver generating an electrostatic field that moves the charged 218 Po to the detector surface. The traceability of the measuring quantity is referred to the Physikalisch-Technische Bundesanstalt (PTB). Under typical environmental calibration conditions within the radon chamber, an expanded uncertainty of roughly 5 % (k=2) for radon concentration is usually estimated. A complete description of the system and its traceability can be found in Vargas et al. [4]. The control system for radon concentration inside the chamber continuously regulates the exhalation rate (0-256 Bq min -1 ) and the chamber ventilation airflow-rate (0-6 m 3 h -1 ). The radon generation source is a dry powder material containing 2101 kbq 226 Ra, which is enclosed in the source container (model RN-1025 manufactured by Pylon Electronics). 100 % of the radon gas produced escapes from the dry powder source material. The generated radon activity is transported by an airflow, the moisture of which was previously removed by silica gel. Afterwards, the airflow containing radon activity is divided by means of two mass-flow controllers: one part is fed into the chamber and the other is released to the open atmosphere. This system reports a high level of radon concentration stability, which allows stable activity concentrations during calibration exposures. A wide range of radon concentration can be controlled within the radon chamber, from approximately 50 Bq m -3 to more than Bq m -3. Temperature in the radon chamber is regulated using a commercial refrigeration unit with electrical heating. Humidifier and dehumidifier devices modify the relative humidity and PC software continuously controls both the temperature and relative humidity. Temperature is maintained between ± 1 ºC for the 10 ºC to 40 ºC range, and relative humidity is maintained between ± 2.5 % from 15 % to 95 %. Particle size distribution is measured with a 3070 electrostatic classifier from the TSI company. A HEPA filter in a recirculating duct system cleans the air. Moreover the ventilation ducts have their own HEPA filter. Particles are generated by an atomizer with an oil or salt solution. The particle control permits the control of the radon progeny concentration, which is measured by a sampling unit. In order to evaluate the influence of relative humidity, temperature and radon concentration fluctuation, 11 exposures inside the radon chamber were carried out under different environmental conditions ranging from 340 Bq m -3 to 8020 Bq m -3 radon concentration, from 9 ºC to 30 ºC temperature and from 30 % to 83 % relative humidity. In each run 6 detectors were exposed for statistical accuracy. The detectors were placed, for a period of 3-days on a sieved tray, located at the centre of the chamber, where radon activity is uniform. 3. Results and discussion In table 1 the different environmental conditions for each exposure are presented. The calibration factor, K, has been estimated from equation (5) for each exposure. Assuming that the most important uncertainty contributions are the radon concentration in the chamber and the Ra-226 reference activity, then the expanded uncertainty for the calibration factor is less than 10 % for a coverage probability of approximately 95 %. The effect of the different environmental conditions on K is analyzed in the following sections. Exposures 1, 2 and 3 were carried out in order to evaluate the influence of the radon concentration. Using exposure 2, 4 and 5 the influence of the relative humidity was studied. Temperature influence was analyzed using exposures 2, 6 and 7. Finally, exposures 8, 9, 10, and 11 are divided in two 4

5 periods, one at a low radon concentration and the other at a high concentration, in order to study the dependence of the calibration factor on radon concentration fluctuation. Table 1. Environmental conditions in the radon chamber for each exposure and the calibration factor estimation. The exposure in (*) was carried out in a room where the environmental conditions were controlled. Number Exposure period (days) Radon concentration (Bq m -3 ) Temperature (ºC) Relative Humidity (%) Calibration Factor K 10-6 (m 3 s -1 ) (*) (*) (*) (*) Radon concentration effects The calibration factor for exposures 1,2 and 3 are similar indicating that the radon adsorption rate does not depend on the radon concentration level Relative humidity effects Figure 3 shows the influence of the relative humidity on the calibration factor. Radon concentration and temperature are almost the same for each exposure, while the relative humidity varies from 30 % to 80 %. Due to the water absorption by the charcoal, there are fewer sites available for the radon atoms and the radon adsorption rate decreases when the humidity increases. During the exposure period the mass increases due to water absorption inside the charcoal canister, which is directly correlated to the air relative humidity. The calibration factor can then be corrected by weighing the canister before and after exposure. Figure 4 shows the calibration factor correction due to the relative humidity effect when the temperature is 20 ºC with an exposure period of three days. 5

6 FIG. 3. Calibration factor dependence on relative humidity. FIG. 4. Calibration Factor correction for different mass increases. 6

7 An exponential equation can be fitted to the data from figure 4. The following equation was obtained in order to correct the calibration factor: m K = e (6) where m is the mass increase during the exposure period in grams Temperature effects It is well known that temperature affects the adsorption rate onto carbon. In table 1 it is clearly shown that temperature affects the radon adsorption rate. Exposures 2,6 and 7 verify that the collection efficiency of the charcoal decreases as temperature increases. The difference in the calibration factor in the temperature range from 10 ºC to 30 ºC at a constant value of 50 % for relative humidity and 3000 Bq m -3 radon concentration is approximately 50 %, which can be seen in figure 5. FIG. 5. Calibration factor dependence on temperature Varying radon concentration effects Exposures 8, 9, 10 and 11 show the effect of varying radon concentration on the calibration factor. From these data it can be estimated that the weight of the first day compared with the total is about 15 %, that of the second is 30 % and that of the last day 55 %. Taking into account these weighs, the calibration factor estimation for exposures 8,9,10 and 11 are , , and respectively, which give a difference of less than 3.5 % in relation to the expected calibration factor. 7

8 Since radon concentration fluctuation is not known during the calibration period, it is important to consider that an error can be made if the possible fluctuation is not taken into account in the uncertainty evaluation. It is therefore appropriate for radon concentration measurements carried out with canisters that the radon concentration is maintained as constant as possible by closing windows, doors and other variable ventilation systems. Moreover, the measurement site indoors should be closed for at least 24 hours prior to exposure in order to stabilize the radon concentration and to get a maximum value. 4. Conclusions Our own radon measurement system using charcoal canisters has been analyzed. The system shows a high level of repeatability, is a rugged detector system and is low cost. However some consideration should be taken into account in order to get reliable measurements. Eleven exposures have been carried out in the INTE radon chamber in order to evaluate the response of the canister to radon concentration level, relative humidity, temperature and radon concentration fluctuations. The radon concentration level does not affect the radon adsorption rate. The effect of relative humidity, due to water adsorption, can be avoided by correcting the calibration factor as a function of the canister mass increase during the exposure period. The temperature also affects the radon adsorption rate. It is difficult to know the temperature during the exposure period, and if temperature is quite different to standard environmental conditions, then it is recommendable to measure the temperature or to incorporate a higher uncertainty value in the radon concentration according to figure 5. For the canister the optimum exposure period is 3 -days, since the contribution of the first day is still 15 %. The canister does not make a real integration. It was estimated that the weight of the first day compared with the total is about 15 %, that of the second is 30 % and that of the last day 55 %. A solution that improves the adsorption rate consists in incorporate a diffusion barrier [5]. At the moment, in order to minimize this effect, it is recommendable to close windows and doors during and 24-hours prior to exposure. Finally it is important to point out that for each charcoal material and canister configuration the calibration factor is different. Thus, a verification of the calibration factor should be carried out each time the laboratory acquires charcoal. Acknowledgements- The authors thank Vicente Blasco for his collaboration in the preparation and development of exposures in the radon chamber. References 1. Cohen, B.L., Cohen, E.S., Theory and practice of radon monitoring with charcoal adsorption. Health Phys. 50: , (1986). 2. Scarpitta, S.C., Harley, N.H., Adsorption and desorption of noble gases on activated charcoal: II. 222 studies in a monolayer and packed bed. Health Phys. 59: , (1990). 3. Ronca-Battista, M., Gray, D., The influence of changing exposure conditions on measurements of radon concentration with the charcoal adsorption technique. Radiat. Prot. Dosim. 24: , (1988). 4. Vargas, A., Ortega, X., Martín Matarranz, J.L., Traceability of radon-222 activity concentration in the radon chamber at the technical University of Catalonia (Spain). Accepted for publishing in Nuclear Instruments and Methods in Physics Research, Section A, (2004). 5. George, A.C., Weber, T., An improved passive activated C collector for measuring environmental 222 in indoor air. Health Phys. 58: , (1990). 8