RADON EQUILIBRIUM MEASUREMENT IN THE AIR *

Transcription

1 RADON EQUILIBRIUM MEASUREMENT IN THE AIR * SOFIJA FORKAPIĆ, DUŠAN MRĐA, MIROSLAV VESKOVIĆ, NATAŠA TODOROVIĆ, KRISTINA BIKIT, JOVANA NIKOLOV, JAN HANSMAN University of Novi Sad, Faculty of Sciences, Department of Physics, Novi Sad, Serbia, sofija@df.uns.ac.rs Received November 15, 2012 This paper presents the exact method for radon equilibrium determination by gamma spectrometry measuring of radon progeny concentrations in the air. The method is based on simultaneous sampling of air through the filter paper and alpha spectrometry measurement of radon activity concentration in the air. This paper derived a mathematical formula to calculate the initial concentrations of radon progenies 218 Po, 214 Pb and 214 Bi in the air at the start of sampling based on the detected count rate of post radon gamma lines in the sample of filter paper. Such a model containing the radioactive decay corrections during the time of sampling, cooling and measurement can be applied in other nuclear analysis where the half-life of the source has the same order of magnitude as the available recording time. Key words: radon equilibrium, gamma spectrometry, radon progeny, decay corrections. 1. INTRODUCTION Equilibrium factor between radon and short lived progenies is very important for dose assessment from inhalation of radon and it must be determined in each radon monitoring. According to the ICRP recommendations [1], for a factor of equilibrium can be assumed value of 0.4, however, since this factor depends largely on environmental conditions (hours and mode of ventilation, humidity etc. [2]) it is necessary to develop a method of measuring the progeny concentrations in order to calculate equilibrium factor according to the formula [3]: EEC Rn F = (1) C Rn EEC = 0.105C C C (2) Rn * 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. S140 S147, Bucharest, 2013

2 2 Radon equilibrium measurement in the air S141 where EEC Rn is radon equilibrium equivalent concentration; C Rn, C 1, C 2 and C 3 are the activity concentrations (in Bqm -3 ) for 222 Rn, 218 Po, 214 Pb and 214 Bi, respectively. Once equilibrium factor is determined a dose equivalent from inhalation of radon can be calculated by the formula: [ ] 0 r d Dose nsv = C ( ε +ε F) O (3) where C 0 is the mean annual radon activity concentration in Bq/m 3, ε r (0.17 nsvh -1 per Bqm -3 ) and ε d (9 nsvh -1 per Bqm -3 ) are dose conversion factors for radon and its short-lived progeny respectively, F is the equilibrium factor between radon and its short-lived progeny (F = 0.4 UNSCEAR and ICRP recommended), and O is the occupational factor (time spent indoors by average European, O= 0.7x8.76x10 3 h). In the literature, most often methods for measuring the equilibrium factor are based on the detection of gross alpha activities that bring a great deal of uncertainty and error. The main purpose of suggested method will be the validation and quality control of portable alpha spectroscopy systems that exist since many years and can perform in situ alpha spectrometry measurements in a filter. In this paper, we propose a method for simultaneous alpha spectrometry measurement of the activity concentration of radon in the air and determining of radon progeny activity concentrations by gamma spectrometry measurement of the filter paper. The advantage of gamma spectroscopy method is accurate and fast determination of radionuclide activities and problem of long cooling time and transport to the laboratory could be avoided by portable HPGe spectrometry systems. 2. METHOD OF MEASUREMENTS Radon activity concentration was measured by alpha spectrometer RAD-7, manufactured by Durridge Company, USA (fig.1) which enables continual and direct reading of radon concentration in air and therefore it is suitable for such simultaneous measuring. High volume air sampler F&J model DHFV 1SE with fiber glass filter paper of high collection efficiency of ε = 98% were used for aerosol sampling. The pump flow velocity was adjusted to value of v = m 3 /s. During the experiment radon short lived daughters attached to aerosols were collected on the fixed filter paper. After the suction the filter paper was approximately homogenous packed to the cylindrical geometry and gamma spectrometry measured in ten successive measurements in duration of 1000 s, with time of 120 s elapsed to first measurement. Gamma spectrometry measurement were performed by HPGe low level detector, Canberra manufacturer, type GX10021 with extended range from 6 kev to 3 MeV in original lead shield with wall thickness of 15 cm. Relative detector efficiency is 100% (equivalent to absolute efficiency of 3"x 3" NaI(Tl) detector on 1332 kev gamma line).

3 S142 Sofija Forkapić et al. 3 Fig.1 AIR Sampler - DHFV-1SE, F&J Speciality Products Inc., USA with glass fiber filter paper e=98% (left) and alpha spectrometer RAD-7 Durridge Company, USA (right). Fig. 2 HPGe detector 100% relative efficiency, Canberra manufacturer, extended range type GX METHOD ALGORITHM FOR RADON PROGENIES DETERMINATION IN THE AIR AT THE START OF SUCTION In order to connect the results of gamma spectrometry measurements of the filter paper and the radon progeny concentrations in the air at the start of suction (C 1 ( 218 Po), C 2 ( 214 Pb), C 3 ( 214 Bi)), it is necessary to take into account decay corrections during the time of suction t U, cooling time t H after the suction but

4 4 Radon equilibrium measurement in the air S143 before the measurements and during the measurements t M, because these periods of time are not negligible relative to short life times of radon daughters ( 218 Po 3 min, 214 Pb 26.8 min and 214 Bi 19.9 min). Fig. 3 Radioactive decay scheme of radon 222 Rn and the progenies of interest for gamma spectrometry measurements indexed as 1, 2 and 3. Fig. 4 Method algorithm for radon progenies determination. After the time t U elapsed from the beginning to stopping of suction the number of radon progeny atoms 218 Po, 214 Pb and 214 Bi collected on the filter paper N 1, N 2 and N 3 change according to differential equations (4-6) if one consider the approximation that on the beginning of suction there were no radon progenies on the filter paper N 1 (0) = 0, N 2 (0) = 0, N 3 (0) = 0: 1 U = Cvε λ N (4) U = C vε+λ N λ N (5) U = C vε+λ N λ N (6)

5 S144 Sofija Forkapić et al. 5 After the suction of the air through the filter paper the radon progeny atoms captured on the filter paper decay during the cooling and measuring time and the number of atoms have changed in accordance with the equations: 1 = λ N (7) =λ N λ N (8) =λ N λ N (9) The initial conditions for this system of equations were obtained by solving the first system of differential equations (4) in Matematica program using the values of constant parameters: λ 1 = s -1, λ 2 = s -1, λ 3 = s -1, v = m 3 /s, ε = 0.98 and t U = 1320s: N 1 (0)= C 1 (10) N 2 (0)= ( C C 2 ) (11) N 3 (0)= x10 9 ( C x 10-9 C x10-8 C 3 ) (12) Now it could be solved the system of equations (7 9) and obtained the number of not decayed nuclei of radon progenies on the filter paper N' 1 (t), N' 2 (t), N' 3 (t). The number of decayed nuclei of 214 Pb, and 214 Bi respectively during the measurement could be connected with the gamma spectrometry detected results for measured filter paper: N d =Nr (13) ε d p γ where N r is the number of decayed nuclei during the time of measurement t M, N d -area under the photopeak, ε d - photopeak detection efficiency and p g - γ-ray emission probability. Detected decays are actually the difference between the not decayed nuclei after the cooling time t = t H and not decayed nuclei after the time of measurement t = t H + t M : N r = N (t = t H ) N (t = t H + t M ) (14) where N (t = t H ) is the number of not decayed nuclei after the cooling time, and N (t = t H + t M ) - is the number of not decayed nuclei after the cooling time and measurement.

6 6 Radon equilibrium measurement in the air S RESULTS OF MEASUREMENTS Gamma spectrometry measurements were performed by low level HPGe relative efficiency of 100%. The time of measurements was 1000 s. The measurements were successive repeated ten times. Calibration curve for cylindrical geometry (d = 72 mm, h = 30 mm), density g/cm 3, matrix filter paper is presented on Figure 5. Fig.5 Calibration curve for filter paper in cylindrical geometry. Experimental obtained areas under the photopeaks of kev ( 214 Pb) and kev ( 214 Bi) are listed in Table 1. t H [s] Table 1 Experimental results for ten successive measurements of filter paper t M [s] t H +t M [s] Spectrum code N d (351.9 kev) ε d =0.0425; p γ =0.371 N d (609.3 kev) ε d =0.0264; p γ = BSOFPMF BSOFPMF BSOFPMF BSOFPMF BSOFPMF

7 S146 Sofija Forkapić et al. 7 Table 1 (continued) BSOFPMF BSOFPMF BSOFPMF BSOFPMF BSOFPMF Applying the method algorithm the next equations with unknown variables C 1, C 2 and C 3 were derived: N r ( 214 Pb)=5.76 C C 2, for t H =120 s and t M =1000 s (15) N r ( 214 Pb)=7.17 C C 2, for t H =1120 s and t M =1000 s (16) N r ( 214 Bi)= 5.23 C C C 3, for t H =120 s and t M =1000 s (17) This system of three equations with three unknown variables is possible to solve and solutions are C 1 =356 m -3, C 2 =19785 m -3 and C 3 =28446 m -3 respectively. Therefore the activity concentrations of radon progeny in the air at the start of sampling were calculated: A 1 ( 218 Po)=λ 1 C 1 =1.4(5) Bq/m 3 ; A 2 ( 214 Pb)=λ 2 C 2 =8.50(15) Bq/m 3 and A 3 ( 214 Bi)=λ 3 C 3 =16.50(15) Bq/m 3. Measurement uncertainties of A 1, A 2 and A 3 were estimated only from peak area statistical uncertainties. And finally according to measured radon activity concentration in the air of C Rn = 20±4 Bq/m 3 the equilibrium factor between radon and progenies was estimated to F = 0.54(11) which is in good agreement with experimental results and studies form the literature. 5. CONCLUSION The typical F value of 0.4 recommended by the United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), and the International Commission on Radiological Protection (ICRP) could lead to an error in the estimation of radon doses to the lung. This paper presents preliminary results for the radon equilibrium determination based on gamma-spectrometry measurements of radon progenies - gamma emitters collected on filter paper. Obtained activity concentrations of radon progenies in the air: 1.4(5) Bq/m 3 for 218 Po, 8.50(15) Bq/m 3 for 214 Pb and 16.50(15) Bq/m 3 for 214 Bi. Correlated to simultaneous measured radon concentration in the air by RAD7 radon detector 20±4 Bq/m 3, the estimated radon equilibrium factor yields 0.54(11).

8 8 Radon equilibrium measurement in the air S147 The discrepancy of the obtained radon progeny concentrations requires a complex analysis of their behavior in the atmosphere, in terms of attaching to the aerosols, deposition and discharging. A detailed examination of this method is planning on a large number of measurements, as well as the optimization of sampling times, cooling and measurement. The problem of radon progeny saturation on the filter paper during the suction will also be considered in future research. The biggest estimated measurement uncertainty for the 218 Po activity concentration indicates the strong dependence of this activity on 214 Pb and 214 Bi measured gamma spectrometry rates. Acknowledgements. The authors acknowledge the financial support of the Ministry of Education and Science of Serbia, within the projects No and No REFERENCES 1. International Commission on Radiological Protection, Recommendations of the International Commission on Radiological Protection, ICRP Publication 60 (Anals of the ICRP 21 (1991)). 2. K. Jilek, J. Thomas and L. Tomašek, First results of measurement of equilibrium factors F and unattached fractons f p of radon progeny in Czech dwellings, NUKLEONIKA 55(4), 2010, S.Y.Y. Leung, D. Nikezic, K.N. Zu, Passive monitoring of the equilibrium factor inside a radon exposure chamber using bare LR 115 SSNTDs, Nuclear Inst. and Methods in Physics Research A 564 (2006)