Radon Determination by Activated Charcoal Adsorption and Liquid Scintillation Measurement. Canoba, A.C.; López, F.O. and Oliveira, A.A.

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1 Radon Determination by Activated Charcoal Adsorption and Liquid Scintillation Measurement Canoba, A.C.; López, F.O. and Oliveira, A.A. Publicado en: Journal of Radioanalytical and Nuclear Chemistry, vol. 240, no. 1, p , 1999

2 RADON DETERMINATION BY ACTIVATED CHARCOAL ADSORPTION AND LIQUID SCINTILLATION MEASUREMENT Canoba, A.C.; López, F.O. and Oliveira, A.A. Nuclear Regulatory Authority Argentina ABSTRACT A passive diffusion method for the determination of radon concentration has been optimised and calibrated. The device consists of a scintillation vial containing activated charcoal, a diffusion barrier and a desiccant agent. The response to diverse atmospheric humidity and variable exposure intervals was studied. The result is a detector independent of atmospheric humidity till 7 days of exposure. The method was compared with electret detectors (US EPA) with very satisfactory results. The advantages of this method are its simplicity, low cost, low detection limit, the total automatization of the measurement and its absolutely independence of humidity to measure in a wide range of radon concentrations. INTRODUCTION The radon gas ( 222 Rn) is the most important source of natural radiation. It has been estimated that 222 Rn and its short lived decay products contribute with three quarters of the annual effective dose received by man from natural terrestrial sources and is responsible for about half of the dose from total sources 1. The greater part of this dose is derived from the inhalation of the short lived progeny, and this is especially true indoors. Because radon poses such a serious health problem, it is important to measure and mitigate indoors to reduce occupant exposure. Diverse techniques have been developed to measure the radon gas concentration in air, some of the better known methods are thermoluminicent detectors, nuclear track detectors, electret ion chambers, and continuous radon monitors. In the last years, a lot of work has been done on the measurement of radon by adsorption on activated charcoal , based on its well known property of adsorbing different gases and vapors, including radon, by means of Van Der Waal s basic principle. In most cases, the quantity of 222 Rn adsorbed is later measured by counting the gamma ray emissions of both lead-214 and bismuth-214. This is possible due to the short half lives of these progeny. The purpose of this work is to optimise and calibrate a method to determine the concentration of 222 Rn in air, by means of its adsorption on activated charcoal and its subsequent measurement in a liquid scintillation counter 11, taking advantage of the alpha-beta decay of the short lived 222 Rn decay products, once equilibrium has been reached. Water vapor is the predominant factor that interferes in the adsorption of 222 Rn on activated charcoal, as it competes with 222 Rn in its adsorption capacity This point is very important for us as in a great part of Argentine the humidity reachs very elevated values. The quantity of water vapor adsorbed is directly proportional to the relative humidity at constant temperature. The quantity of radon adsorbed decreases slightly with an increase in temperature, although this decrease is not significant in our work range temperature 6. To decrease the water vapor effect, diffusion barriers were tried, as well as different types of desiccant agents. 407

3 The advantage of this technique is that it is completely passive, of very low cost, higher efficiency and a lower detection limit compared with gamma-spectrometry methods. Finally, this measurement method can be fully automated and is completely independent of humidity till 7 days of exposure. EXPERIMENTAL Detector design The sampler used for the measurement of radon concentration in air consists of a 20 ml, low 40 K, glass scintillation vial. Two (2) grams of activated charcoal (Norit RKJ 1,5) are placed in the bottom of the scintillation vial, then a diffusion barrier consisting of a 1 cm thick synthetic sponge is inserted, four (4) grams of a particulate desiccant (silica gel) is then added to adsorb water vapor, and the lid is screwed on. Any adsorbed moisture is removed from the charcoal and the desiccant by heating them in a 120 o C oven for three hours prior to weighting the parts and assembling the vials. The relative proportions of the constituents are the result of research made to optimise the ability to adsorb the radon. By studying different combinations in weight of activated charcoal and silica gel, and the addition of a diffusion barrier, this device has proved to be an excellent method to measure radon-222. Sampling is carried out by exposing the sampler where the 222 Rn measurement is to be carried out by unscrewing the lid of the assembled glass vial for a determined period of time. The 222 Rn gas diffuses into the charcoal and is adsorbed. Once the collection period is completed, the lid is screwed back on. At the laboratory, the lid is opened and the desiccant and the diffusion barrier are removed. Ten (10) ml of a scintillation solution are added, and the vial is sealed with a silicon based adhesive. The scintillation solution is prepared by dissolving 5 g of PPO (2.5 Diphenyloxazole) and 0.05 g of POPOP (2.2-p-Phenylene-bis (5-phenyloxazole)) diluted to 1 L. with toluene. Toluene and the scintillators are of scintillation grade. To favor desorption of the radon gas from the charcoal to the scintillation solution, the vial is shaken vigorously. The vial is then centrifuged for five minutes at 2000 r.p.m. The closed vial is left for at least six hours to allow radiological and chemical equilibrium and then, it is counted using a low background liquid scintillation counter (Model Tri-Carb 2550 LL, Packard). Calibration The calibration of the method was carried out using a one cubic meter, acrylic reference chamber that contains uranium ore as the 222 Rn source. The temperature, humidity and 222 Rn concentration are controlled and monitored constinuously. The 222 Rn concentration is measured using scintillation cells 14, and counted in a scintillation cell counter (Model Ludlum 2200). To study the effect of atmospheric humidity on the response of the samplers, a series of tests were undertaken using the calibration chamber to evaluate changes in the response varying the different parameters. The samplers were exposed in quintuplicate to three different humidities: 30%, 50% and 80%, these being representative of low, medium and high humidity conditions respectively. The calibration chamber humidity was maintained constant during each set of tests. To lower the humidity, the air in the chamber was pumped through a closed circuit having desiccant drying columms until the desired value was reached. To raise the humidity values, water is evaporated inside the chamber. Humidity was measured using a hydrometer. The experiments were carried out at a constant temperature ranging from (20-22 o C) for exposure periods which varied between 17 and 170 hours. The exposed samplers were processed and measured as previously described. For each humidity condition, no fewer than 60 samplers were used to obtain the calibration curve. 408

4 The equation which was used to calculate the empirical calibration factor is: where: F = ( t e e C r D) / C n (1) F: the calibration factor (h m -3 ) t e : exposure period (hours) e: total efficiency (counts per minute Bq -1 ) C r : reference concentration of 222 Rn (Bq m -3 ) D: correction factor due to radioactive decay C n : net cpm, i.e. measured counts per minute - background counts per minute The correction factor due to radioactive decay is expressed by the following equation: D = exp -( λ t) (2) where λ is the radon radioactive decay constant and t is the time from the middle of the exposure time till the beginning of counting 2. Once the calibration factor has been calculated, the radon concentration of an unknown sample is obtained in the following way: where: C rx = F C n / ( t e e D) (3) C rx : radon concentration unknow sample (Bq m -3 ) RESULTS AND DISCUSSION Figure 1 shows the results of the calibration of the method for different exposure times and different humidity conditions. It is observed that the adsorbed concentration of radon is linear for all the studied conditions for an exposure period of up to 7 days. The linear regression curves obtained for each humidity condition (Figure 1), were analysed, and as can be seen, no significant differences between the calculated parameters for the three conditions analysed may be observed. This is fundamentally due to the combined effect of the diffusion barrier and the adequate quantity of desiccant used. Calibration factor ( h m3 ) Relative humidity values 80% 50% 30% Exposure time (hours) Figure 1. Calibration factor vs. different exposure times for different humidity conditions. 409

5 When drawing the linear regression curve including all the data (for 30%, 50% and 80% humidity), we obtain the curve shown on Figure 2, with a correlation factor of which indicates that the proposed method has an excellent response and may be used without applying any correction for atmospheric humidity while monitoring, as it is independent of the variations in the humidity. Calibration factor (F) ( h m3 ) F=A + Bt Parameters of the regression A=33,425, B=0,535 R=0, Exposure time (hours) Figure 2. Linear regression curve including all the data analysed (for 30, 50 and 80% of relative humidity) Estimation of the error and the detection limit The error in the measurement may be estimated by the following equation 7 : error(2 σ) = 2 (cpm m + cpm b ) 1/2 / (t m ½ (cpm m - cpm b )) (4) where t m is the measurement time expressed in minutes; cpm m and cpm b are the counts per minute of the sample and the background respectively. The calculated average error for all the measurements undertaken was between 5 and 10%. The lower limit of detection has been calculated using the following expression lld (cpm) = 3 (cpm b ) 1/2 / t b ½ (5) where lld is the detection limit expressed in counts per minute, cpm b and t b are the counts per minute of the background and the time, respectively. To express the lower limit of detection in concentration units (Bq m -3 ), considering the exposure conditions and taking into account equation (5), we obtain: LLD (Bq m -3 ) = (lld(cpm) F ) / ( t e e D) (6) The LLD of the developed technique was estimated to be 2 Bq m -3, for a of 24 cpm counter background and for a 60 minutes counting time, for a typical monitoring time of 72 hours. 410

6 Comparison with electrets With the object of comparing the designed samplers with another method of similar characteristics of exposure time and detection limit, a parallel test was undertaken. In 80 cases, for this purpose, both the designed samplers and electret ion chambers detectors 15 were deployed in pairs in dwellings of Buenos Aires, not only for the comparison study but also to asses with both methods the dose of the biggest population city in Argentine. The result of this intercomparison study may be observed in Figures 3, 4 and 5. Figure 3 shows the frequency distribution of the values obtained with activated charcoal. In one dwelling both vials showed a value of about 200 Bq.m -3. The values follow aproximately a log-normal distribution. The arithmetic mean was 22.0 Bq m -3 and the geometric mean was 15.0 Bq m frequency Bq m Figure 3. Frequency distribution of concentration of Rn-222 in 80 dwellings of Buenos Aires with activated charcoal detectors. Figure 4 shows the frequency distribution of the values obtained with electret ion chamber detectors. The extreme value was 230 Bq m -3. The arithmetic mean was 23.0 Bq m -3 and the geometric mean was 16.0 Bq m -3. These results are very close with the results obtained with charcoal adsorption frequency Bq m-3 Figure 4. Frequency distribution of concentration of Rn-222 in 80 dwellings of Buenos Aires with electret ion chamber detectors 411

7 It is important to say that these values are similar to those calculated in a survey during 1996 with track-etch detector where the arithmetic mean was 28 Bq m -3 for 100 dwellings analysed in the same city. We can estimate, then, the dose due to radon exposure in Buenos Aires being about 0.6 msv. y -1 with a maximun of 5.7 msv. y -1. In order to compare both detector systems (activated charcoal and electret), the ratio of the results from the parallel exposed detectors was calculated. The frequency distribution of the ratios is shown in Figure 5. The characteristic parameters are summarised in the following table: electret / charcoal mean 1.1 standard dev. 0.5 min 0.3 max 3.2 As we can infer from Figure 5, about 65% of the cases shown a ratio between frequency ,0 0,4 0,8 1,2 1,6 2,0 2,4 2,8 3,2 ratio of results Figure 5. Histogram of the intercomparison study between electret detectors and activated charcoal detectors. R represents the ratio between the radon concentration measured by electret detectors and activated charcoal detectors. Besides, both detectors were exposed in parallel, in 30 cases, in calibration chambers with radon concentration ranging from 500 Bq m -3 to Bq m -3. These results are summarised in the following table: electret / charcoal mean 1.1 standard dev. 0.3 min 0.7 max 1.7 These values show a good correlation between both methods. CONCLUSIONS A passive diffusion method for the measurement of radon concentration has been optimised and calibrated. It is based on its adsorption on activated charcoal and its subsequent measurement in a liquid scintillation counter that measures the alpha-beta decay of the short lived 222 Rn decay products once equilibrium has been reached. It is a method mid-way between time integrated sampling and 412

8 instant sampling methods. It has the advantage of being able to determine radon concentration in air from one day to several days, being then an adequate method to choose in survey monitoring. By using a diffusion barrier and an adequate quantity of desiccant, a single curve for the calibration factor versus exposure time was obtained, with a very good linear regression. This means that, we can dispose of a measurement technique absolutely independent of ambient humidity. When the performance of these detectors is compared with that of electret ion chambers, we see that there is a high correlation, with the added advantage that unlike the latter, these detectors may be used to measure very high concentrations of radon gas without losing linearity as occurs with the electrets when the lower voltage limit is reached. Another advantage to be considered is that under extreme humidity conditions (95-99%) the designed samplers maintain there linearity, while, in our experience, electrets become unstable under extreme humidity conditions. As well as the above mentioned characteristics, the designed samplers are small and very light, which makes them ideal for large scale monitoring programmes, as they are easily transported and distributed. An added advantage is that the technique is not at all sophisticated and highly skilled specialised personnel is not required. Other advantages of this method are: low cost due to the materials employed which are of common use and easily obtained commercially, the automatization of the measurement in a liquid scintillation counter, its capacity to measure a wide range of radon concentrations and its low detection limit. ACKNOWLEDGEMENTS We would like to acknowledge the assistance we received in the translation and revision of this paper from Marta Arnaud and Ana Bomben. REFERENCES 1. United Nations Scientific Committee on the Effects of Atomic Radiation. Sources and effects of ionizing radiation, UNSCEAR Report to the General Assembly with Scientific Annexes. New York: United Nations; (1993), B. L. Cohen, E. S. Cohen,. Health Phys., 45, (1983), A. C George,. Health Phys., 46, (1984), H. M. Prichard, K. A Marien, Health Phys., 48, (1985), B. L Cohen, R. Nason, Health Phys., 50, (1986), T. Ren, L. Lin,. Rad. Prot. Dos., 19, (1987), U.S. Environmental Protection Agency. NAREL Standard operating procedures for radon-222, measurement using diffusion barrier charcoal canister. Washington D.C: U.S. EPA Office Of Radiation Programme; EPA 520/ ; (1990). 8. C. S. Scarpitta, Health Phys., (1992), 62, M. C Schroeder, U. Vanags, C. T. Hess,. Health Phys., 57 vol. 1, (1989) F. Schönhofer, K. Pock, H. Friedmann,. J. Radioanal. Nucl. Chem., 193 Nº 2, (1995), H. M. Prichard, K. Marien,. Anal. Chem., 55, (1983), P. M. Pojer, J. Peggie, R. O' Brien, S. Solomon, K Wise, Health Phys., 58, (1990), C. S Scarpitta, Health Phys., 68, (1995), H. L. Lucas,.Rev. Sci. Instrum., 28, (1957), P. Kotrappa, J. C. Dempsey, J. R. Hickey, L. R. Stieff,. Health Phys., 54, (1988),

The LSC Approach to Radon Counting in Air and Water

CHAPTER 32 The LSC Approach to Radon Counting in Air and Water Charles J. Passo, Jr. and James M. Floeckher INTRODUCTION Various methods exist to monitor 222Rn in air. There are seven commonly used types