a simple approach to measure the radon equilibrium ...

Radiation Protection Dosimetry Advance Access published April 10, 2014 Radiation Protection Dosimetry (2014), pp. 1–4

doi:10.1093/rpd/ncu064

A SIMPLE APPROACH TO MEASURE THE RADON EQUILIBRIUM FACTOR F FROM AIR FILTER GROSS BETA COUNTING L. M. Panero1,2,*, G. Arman1, M. Caccia1, D. Cavallo1, E.M. Chiaberto2, V. Chmill1, M. Faure Ragani3 and M. Magnoni2 1 Universita` degli Studi dell’Insubria, 22100 Como, Italy 2 Dipartimento Tematico Radiazioni Ionizzanti, ARPA Piemonte, 10015 Ivrea, Italy 3 ARPAValle d’Aosta, 11020 Saint Christophe, Italy *Corresponding author: [email protected]

INTRODUCTION Radioactive secular equilibrium between radon and its short-lived progeny is reached in about 3 h: 222Rn (3.82 d) a! 218Po (3.05 min) a! 214Pb (26.8 min) b! 214Bi (19.9 min) b! 214Pb (164 ms) a! 210Pb (22.3 y) that corresponds to an equilibrium factor F ¼ 1. The concentration of the Radon progeny can be significantly affected by environmental parameters (e.g. humidity, ventilation, aerosol concentration, aerosol size distribution, surface to volume ratio, air flows and air conditioning systems), eventually resulting in F values well below 1. In dosimetric models (HRTM – ICRP)(1), the internal alpha dose for inhalation and deposition of radon progeny into the human respiratory tract can be estimated according to the formula E ¼ CRn texp F DCF ;

ð1Þ

where E is the radon effective dose (mSv), CRn is the radon activity concentration (Bqm23), texp is the exposure time (h), F is the equilibrium factor and DCF is the dose convertion factor (nSv/(Bqhm23)). The ‘dosimetric approach’ based on Equation (1) has been proposed as a standard method for radon dose assessment in the ICRP Publication 115 (2010), supporting the development of methods for the measurement of F and DCF in different conditions. Both F and DCF are functions of environmental parameters: F is affected by humidity, ventilation rate and aerosol number concentration while DCF depends on progeny’s unattached fraction, aerosol size distribution and on breathing path and rate. Experimentally, they may strongly vary from homes to workplace to underground environments. The retained value of 0.4 for

dwelling in normal conditions is a good approximation for F in most cases; however, it may be found to vary from 0.3 to 0.8 DCF may change in a more complex way not only with environmental parameters but also with the breathing mode. This paper reports a simple method for the calculation of the equilibrium factor based on the measurements of the radon concentration and on the gross beta counting, presuming a simplified room model. The aim of the study was the development of a method applicable to field surveys. MATERIALS AND METHODS Radon in air Continuous measurements of the radon concentration were performed with an AlphaGuard PQ2000(2) and with a Radim 5B unit (3) operated in a diffusion mode on an hourly base. Radon progeny sampling The airborne progeny particles were collected on different filter substrates by pumping air through a total aerosol sampling head (d ¼ 37 mm) at a constant flow of 9 l min21 for 30 min with a Leland Legacy personal sampler (SKC)(4).The chosen filter was Millipore AA 0.8 mm (mixed cellulose esters d ¼ 0.8 mm – th ¼ 150 mm) known to minimise the losses due to self-absorption(5,6). Filter activity measurements The filter activity was measured by gross alpha/beta counting with a Berthold LB770 gas proportional

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The internal alpha dose assessment (ICRP 65, 1994) depends on the value of the equilibrium factor F. This parameter is generally not measured and a mean conventional value ranging between 0.4 and 0.6 is assumed, possibly leading to a significant bias in the dose assessments. In this paper, a method for the measurement of F is reported, based on the total aerosol sampling on filters and immediate gross beta counting of 214Pb and 214Bi activities. Measurements are interpreted according to a Raabe–Wrenn and Jacobi simplified room model, leading to an estimate of the individual airborne activities of short-lived radon daughters, the removal parameter and the equilibrium factor. The method was laboratory tested and validated and it is being qualified by field surveys in different indoor environmental conditions.

L.M. PANERO ET AL.

counter (Ar 90% CH4)(7). The instrument integrates lead shielding and a veto counter to reduce the counting rate from background and features detection limits of 12 mBq for a 241Am and 22 mBq for b 90 Sr/90Y for 1 h sampling. The instrument in use was calibrated with 241Am and 90Sr/90Y standard sources. Gamma spectrometry was performed with a low background HPGe detector calibrated with standard sources and the proprietary Gamma Vision software. The method for F calculation The equilibrium factor F is the ratio of the equilibrium equivalent concentration and the radon gas concentration(8,9):

F¼

EEC i¼1 ; C0 C0

;

ð2Þ

where f1 ¼ 0.105, f2 ¼ 0.516 and f3 ¼ 0.380 are weighting coefficients and Ci are the airborne activity concentrations (C0 ¼ 222Rn, C1 ¼ 218Po, C2 ¼ 214Pb and C3 ¼ 214Bi). There are several approaches to evaluate F based on the alpha/beta ratio or on the time behaviour of the gross beta activity for the progeny. The former does not require the measurement of the radon concentration, but it is affected by the uncertainties of the a activity(10,11), due to selfabsorption and to the very fast decay of 218Po (t1/2 ¼ 15 min)(12,13). The latter, adopted here, presumes the measurement of the radon concentration but is expected to be robust. For the airborne progeny concentration, a simplified Jacobi –Porstendorfer(14,15) room model was assumed, presuming only ventilation and plateout effects air

dNi air li Niair lrem Niair ; ¼ li1 Ni1 dt

ð3Þ

where Niair are the atom concentrations in air [m23] for i ¼ 1–4 (218Po, 214Pb, 214Bi and 214Po), li are the corresponding decay constants [s21], whereas lrem is a gross removal rate [s21] that summarises the diverse processes affecting the progeny, mainly the air exchange and plateout onto the surfaces; N0 stands for 222 Rn . Steady-state conditions (i.e. constant radon concentration), solving Equation (3) the radon progeny activity concentrations Ci (Ci ¼ liNi) are given by Ci ¼

li Ci1 : li þ lrem

l1 l1 l2 þ f2 l1 þ lrem l1 þ lrem l2 þ lrem l1 l2 l3 þ f3 ; l1 þ lrem l2 þ lrem l3 þ lrem

F ¼ f1

fi Ci

ð4Þ

In this paper lrem is assumed the same for all radon daugthers, knowing that actually it can be different depending on the size of radionuclides. For example,

showing that F is independent of the radon concentration and is a function of the unknown parameter lrem. The activity concentrations of a radon progeny in air, depending on lrem, are the input to the Raabe– Wrenn(18) model, describing the experimental beta activity collected onto the filter. The Raabe–Wrenn model is based on the following first-order equations: dNifilter filter li Nifilter ; ¼ Ri þ li1 Ni1 dt

ð6Þ

where Nifilter are the atoms collected on the filter and Ri is the rate of collection [s21] given by Ri ¼ 1 Niair w;

ð7Þ

where 1 is the collection efficiency, assumed equal to 1 for the Millipore filter, and f [m3 s21] is the flow rate. The Raabe–Wrenn model describes the progeny evolution on the filter during the sampling and the measurement phase, as shown in an exemplary illustration in Figure 1. Following this model, the evaluation of F was performed measuring the radon concentration before sampling and taking the gross beta activity vs. time. The only free model parameter is the removal rate lrem and is determined by a x2 minimisation with respect to the experimental data. The calculation of F follows then immediately from Equation (5); it is calculated from the removal rate (lrem) and not from the progeny concentrations. Therefore, the error of F (sF) is obtained by expression (8), where the total derivative of F respect to lrem is evaluated in the lrem that minimises x2. The error of the removal rate (slrem) depends on the concavity

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3 P

the J-P model considers the ventilation rate constant for two fractions but distinguishes between the unattached and attached fraction. This distinction leads to the introduction of others unknown parameters, such as the plateout of unattached and attached fraction. Some investigations report plateout for unattached fraction 100 times greater than plateout for attached one(8). The chosen approach is a simplification, considering that the unattached fraction represents not .10 % of the total activity in air(16) and the condition (4) can be assumed to give an upper limit for the concentration values(8). Therefore, this method provides preventive values for F. The equilibrium factor is described by equation(17):

SIMPLE APPROACH TO MEASURE THE RADON EQUILIBRIUM FACTOR Table 1. F and lrem values with different radon concentrations and environmental conditions (S, sampling site). x2/n.d.f.

N

CRn [Bq m23]

lrem [h21]

F

1 1 1 1 1 2 2 2 2 2

0.15 1.75 0.72 0.50 0.41 0.23 0.23 0.10 0.47 0.23

24 18 12 12 12 12 12 12 12 10

288+26 261+21 250+18 200+14 156+12 108+8 107+9 105+7 100+8 99+8

0.64+0.13 0.65+0.12 1.00+0.13 1.00+0.12 0.95+0.13 1.30+0.17 1.10+0.17 1.00+0.16 1.60+0.20 1.55+0.18

0.66+0.08 0.64+0.07 0.54+0.06 0.54+0.05 0.55+0.06 0.47+0.04 0.51+0.04 0.54+0.04 0.42+0.04 0.42+0.04

The per cent errors of F are 9 –12 %. N represents the number of experimental points.

Figure 3. Model vs. measured gross beta curve.

Figure 2. Validation of gross beta activity measurements by gamma spectrometry (HPGe detector).

of the parabola x2 in function of lrem, as the Marquardt algorithm provides for, and on the radon concentration error. dF ð8Þ slrem : sF ¼ dlrem

Validation by beta and gamma comparison The F calculation method described above can give a correct result provided that the beta activity is not biased by the difference of the beta spectra. In fact the instrument was originally calibrated with a 90 Sr/90Y standard source featuring a spectrum

significantly different from the beta emitters of a radon progeny (214Pb and 214Bi). The beta counting rate was validated by a cross comparison with gamma counting on the same filter, using the g emission lines of 214Pb and 214Bi. Data are reported in Figure 2, showing that the two data sets are statistically compliant (x2/n.d.f. ¼ 0.5).

MEASUREMENTS AND RESULTS A series of measurements were carried out over a set of twin couples of Millipore AA 0.8 filters sampled in different radon concentrations and ventilation conditions. The samples were taken in two different rooms: a first underground level storage room of 90 m3 volume with natural ventilation (S ¼ 1 in Table 1) and a first floor storage room (S ¼ 2 in Table 1), with the ventilation system and same volume.

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Figure 1. Expected gross alpha and beta curves: model of the activity collected on the filter during the sampling time (growing curves) and the experimental counting measurements (decaying curves).

S

L.M. PANERO ET AL.

In Table 1, the results of these surveys are presented. In all cases a very good agreement between expected and measured beta activities was found as shown by the low x2/n.d.f. values. An example of the good fitting of our experimental data is given in Figure 3. CONCLUSIONS A new procedure to measure F and the room removal rate lrem was developed and qualified. The method is based on gross beta counting of radon progeny collected on air filters and radon gas measurements. It is expected to be robust and flexible for field surveys also with portable beta counters.

Authors are grateful to Stefano Bertino, Luca Bellina e Giuliana Garbarino (ARPA Piemonte) for the contribution in the HPGe spectrometry and alpha/beta counting and spectrometry instruments and for the useful comments and suggestions. FUNDING This study was undertaken in the frame of the RADICAL Project (RADon: Integrated Capabilities of Associated Labs), approved within the INTERREG Cooperation Program 2007–2013 Italy – Switzerland. REFERENCES 1. ICRP 2010. Lung cancer risk from Radon and Progeny and statement on Radon. Publication 115, Ann. ICRP40(1). 2. Alphaguard http://saphymo.de//download/ag_fb_gb_ 05_2011.pdf. 3. Radim 5B Jirˇ´ı Plch M.Eng. - SMM – Prague http:// plch-smm.com/radim5b.php.

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ACKNOWLEDGMENTS

4. Leland Legacy Sample Pump http://www.skcinc.com/ pumps/100-3000.asp. 5. Busigin, W., van der Vooren, Antoon. and Colin, R. P. Collection of radon daughetrs on filter media. Am. Chem. Soc. 14(5), 533– 536 (1980). 6. Nazaroff, W. M. A residential radon daughter monitor based on alpha spectroscopy. PhD thesis, University of California, (1980). 7. Berthold LB770 https://www.berthold.com/en/rp/lb770-10-channel-low-level-counter. 8. Swedjemark, G. A. The equilibrium factor F. Health Phys. 45, 453–462 (1983). 9. UNSCEAR, Sources and effects of ionizing radiation. Report to the general assembly, with annexes, (2000). 10. Katona, T., Kanyar, B. et al. Determining 222Rn daughter activities by simultaneous alpha- and beta-counting and modeling. J. Radioanal. Nuc. Chem. 272, 69– 74 (2007). 11. Papp, Z. An intercomparison between gross a counting and gross b counting for grab-sampling determination of airborne radon progeny and thoron progeny. Nucl. Instrum. Methods. 558, 569–575 (2006). 12. Martz, D. E. Analysis of atmospheric concentration of RaA, RaB and RaC by Alpha Spectroscopy. Health Physics. 26, 131– 138 (1969). 13. Jonassen, N. and Hayes, E. E. The measurement of low concentrations of the short-lived Radon-222 daughters in the air by alpha spectroscopy. Health Physics. 26, 104– 110 (1974). 14. Jacobi, W. Activity and potential alpha energy of radon and Radon daughters in different air atmospheres. Health Physics. 22, 441– 450 (1972). 15. Porstendorfer, J., Wicke, A. and Shraub, A. The influence of exhalation, ventilation and deposition processes upon the concentration of Radon Thoron and their decay products in room air. Health Phys. 34, 465– 473 (1978). 16. Porstendorfer, J. Environ. Int. 22(Supp. 1), S563–S583 (1996). 17. Planinic, J. and Faj, Z. Equilibrium factor and dosimetry of Rn by a nuclear tract detector. Health Physics. 59, 349– 351 (1990). 18. Raabe, O. G. and Wrenn, M. E. Analysis of the activity of radon decay product samples by weighted least square. Health Physics. 17, 593– 605 (1969).