A novel approach for long-term determination of indoor 222 Rn progeny equilibrium factor using nuclear track detectors

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1 Nuclear Instruments and Methods in Physics Research A 56 (3) A novel approach for long-term determination of indoor Rn progeny equilibrium factor using nuclear track detectors K. Amgarou*, Ll. Font, C. Baixeras Grup de F!ısica de les Radiacions, Facultat de Ciencies, Department de Fisica, Universitat Aut "onoma de Barcelona, E-8193 Bellaterra, Spain Received July ; received in revised form November ; accepted 5 March 3 Abstract A detailed study of the measurement principles of airborne Rn decay products by means of nuclear track detectors (NTDs), taking into account the range of variation of the parameters influencing their concentration indoors, has shown that it is not possible for the existing methods to obtain the associated long-term equilibrium factor with an appropriate accuracy. For this reason, we have established a novel approach based on the new concept of reduced equilibrium factor, which can be obtained from the only measurement of airborne Rn and its a-emitter daughter ( 18 Po and 1 Po) concentrations using a passive, integrating and multi-component system of NTDs. We have found that the equilibrium factor has a linear dependence on the reduced equilibrium factor regardless the values taken for the rates of ventilation, of aerosol attachment and of surface deposition. By using well-controlled exposures in a reference laboratory, we have shown that the equilibrium factor values determined with our system agree with those obtained by active monitors. Finally, as a pilot test, several dosimeters were exposed in an inhabited Swedish single-family house. The results of this exposure suggest the usefulness of this method to perform routine surveys in private homes and in workplaces in order to estimate the annual effective dose received by the general public and the workers due to the presence of Rn daughters. r 3 Elsevier Science B.V. All rights reserved. PACS: 3.6; 7.57.K; 9.; 9.3; 85.5.P; 87.5.N.P Keywords: Rn progeny; Equilibrium factor; Nuclear track detectors 1. Introduction To well assess the annual radiation dose from exposure to Rn (radon) progeny in private homes and in workplaces, the information about the equilibrium-equivalent concentration (C eq ) *Corresponding author. Tel.: ; fax: address: khalil.amgarou@uab.es (K. Amgarou). integrated over long periods of time (from months to a year) is of great interest. Owing to its simplicity and cheapness, long-term measurement of indoor Rn concentration by means of a Nuclear Track Detector (NTD) enclosed by an inlet filter within a diffusion chamber, is a wellestablished method used by many laboratories [1]. The inlet filter protects the enclosed detector from exterior airborne aerosol, dust and moisture as also the Rn and/or Rn (thoron) decay 168-9/3/$ - see front matter r 3 Elsevier Science B.V. All rights reserved. doi:1.116/s168-9(3)1366-

2 K. Amgarou et al. / Nuclear Instruments and Methods in Physics Research A 56 (3) products formed outside the diffusion chamber. Furthermore, it delays the entry of Rn gas into the subsequent sensitive volume ensuring, therefore, the Rn discrimination because of its short half-life. There is a great amount of data about indoor Rn levels as a consequence of many surveys carried out around the world with passive integrating detectors []. The knowledge of this database gives only a rough estimation of the long-term equilibrium-equivalent concentration (C eq ) by assuming a mean equilibrium factor of. [3]. However, due to local and temporal fluctuations of the processes (ventilation, aerosol concentration, surface deposition, etc.) influencing Rn and its progeny concentrations, the assumption of a unique equilibrium factor for any real situation of indoor exposures can result in considerable misinterpretations of the inhalation dose actually accumulated. Existing data of indoor Rn progeny concentrations and, subsequently, of C eq correspond mainly to punctual or short-term measurements with active monitors in few sites [, 1]. These methods are very useful when the goal is to obtain values of these decay products at a specific time or to analyse their temporal evolution. Otherwise, because of their cost, size, and operating mode, they are not adequate for long-term measurement in a significant fraction of dwelling stock. Accordingly, it would be of great interest to use NTDs to determine the indoor Rn daughter concentrations along the same period of time during which the Rn concentration is obtained by the diffusion chamber.. Review of existing passive methods The first method for Rn progeny equilibrium factor determination using NTDs was proposed by Frank and Benton [15]. This method, known as conventional method, consists of using two NTDs: one enclosed within a diffusion chamber and a second one (open) facing directly the indoor air to be measured. The open detector lets the measurement of the total air a-particle concentration, C T, of Rn+ 18 Po+ 1 Po. By means of the ratio between its reading and that of the enclosed NTD, it is possible to estimate the equilibrium factor. This correlation must be done experimentally under well-defined irradiation conditions in pure Rn atmospheres. However, the conventional method, which was used by other authors under different system designs and detector configurations [16 19], offers not reliable results. Dom!anski et al. [16] specifically state its lack of precision and its limited success. The instability of its response is due mainly to the dependence of its reading on the equilibrium factor [], on the eventual presence of Rn and on the plate-out effects of Rn and/or Rn decay products above the open detector surface [1]. In addition, this method assumes that the open NTD has the same response (sensitivity) for each of the a-active Rn daughter. We have shown in Amgarou et al. [] that this assumption is not valid in general. Fleischer [3] used for Rn daughter measurement four identical NTDs, three of which are covered by different polyethylene absorber foils of mm, mm and 53.5 mm thick. The bare detector and the first one with mm absorber can register Rn+ 18 Po+ 1 Po, whereas those with mm and 53.5 mm absorbers record a-emissions from 18 Po+ 1 Po and 1 Po, respectively. With this method the a-active daughters ( 18 Po and 1 Po) are measured separately and the concentration of the 1 Pb (whose b-emissions cannot be registered by any NTD) is calculated from an inferred diffusion constant, which is chosen to be unidimensional and invariable. This constant is assumed to be the only parameter that controls the daughter distribution indoors. Nevertheless, this method fails since the behaviour of indoor Rn progeny near surfaces is influenced by many factors. For these reasons, Fleischer et al. [] put the four NTDs within a special housing box designed to provide unidimensional and uniform air diffusion throughout the passive detectors. The results of this device were, however, not satisfactory. From 7 exposures carried out in pure Rn atmospheres, result in a total disagreement and the method failed. In the other 5 exposures, the measured values by the NTDs were 36% higher than those obtained with the active method. In addition, the large dimensions of the diffusion box

3 188 K. Amgarou et al. / Nuclear Instruments and Methods in Physics Research A 56 (3) ( cm 3 ) complicate its application for Rn progeny measurements in private homes and in workplaces. Ling [5] and D.orschel and Piesch [6] suggest that, by fixing the plate-out constants as well as the airborne-unattached daughter fractions to their geometric mean values and keeping the ventilation rate as a free parameter, the equilibrium factor can be expressed as a function of the ratios of each Rn daughter concentrations to that of Rn. Identification and separation of airborne Rn and its a-emitter progeny concentration is feasible using a-spectrometry technique based on measurements of a-track geometric parameters and a detector configuration similar to that of the conventional method [7 9]. For a given eccentricity (i.e., ratio of the track axis lengths), which is dependent on the angle of incidence, the track area is inversely proportional to the energy of the emitted a-particle. In this sense, it has been shown that by a simultaneous measurement of surface track sizes and its optical property, such as average greyness or brightness, an energy resolution of B.1 MeV could be obtained [3]. Nonetheless, this technique is very tedious and need the use of automated and sophisticated track analysis systems. Other way to perform a-spectrometry with NTDs is through electrochemical etching (ECE) technique [31 3]. The basis of this technique was adopted in our laboratory to develop a former method to determine the long-term equilibrium factor from a separate measurement of airborne Rn and 1 Po concentration [33 35]. This method was expected to be much more accurate than the conventional method [33,36]. The results obtained from indoor application of this method in different private homes and workplaces located in the Barcelona area [33 35], indicate that the averaged equilibrium factor agrees with the assumed world mean value [3]. Nevertheless, the assumption of being the ventilation rate the most relevant parameter is not always a good approximation. Similarly to ventilation, the deposition and aerosol attachment processes depend strongly on the geological features, meteorological conditions, indoor characteristics and inhabitant behaviour causing fluctuations in their values that might be very important and that might lead to considerable fluctuations in the measurement. To date, long-term determination of indoor Rn progeny equilibrium factor by means of NTDs is found to be problematical. In one of the latest intercomparison of passive radon detectors organised by the Commission of the European Communities [37], out of 3 measurements carried out by 5 passive detector systems, only one result agreed with those obtained with active measuring systems. In the following, we present the theoretical basis of a new passive integrating system for the simultaneous measurement of Rn and its a- emitter progeny ( 18 Po and 1 Po) in order to obtain the corresponding long-term equilibrium factor in private homes and in workplaces. 3. Equilibrium factor determination using NTDS: theoretical basis The equilibrium factor between Rn and its decay products can be obtained as F Rn ¼ :15f 18Po þ :515f 1Pb þ :38f 1Bi ð1þ where f i is the disequilibrium degree of the airborne Rn daughter i ( 18 Po, 1 Pb or 1 Bi) and is calculated as the sum f i ¼ fi u þ fi a ; being fi u and fi a ; respectively, the disequilibrium degree of its atoms that are unattached and attached to the airborne aerosol particles. In this equation, since the half-life of the 1 Po (165 ms) is very small compared to that of the 1 Bi (19.9 min) and since the NTDs do not register the b-emissions of this last, both elements are considered as one state. Hereafter, the 1 Bi disequilibrium degree (f 1Bi )is substituted by that of the 1 Po (f 1Po ). According to Jacobi [38], the disequilibrium degrees of aerosol-attached and airborne-unattached Rn progeny within a reference room are not independent and are connected with each other via the following balance equations f u i and f a i ¼ l iðf u i 1 þ p i 1f a i 1 Þ l i þ l a þ l v þ l u d ¼ l afi u þð1 p i 1 Þl i f a i 1 l i þ l v þ l a d ðþ

4 K. Amgarou et al. / Nuclear Instruments and Methods in Physics Research A 56 (3) where l i is the decay constant of the ith daughter, p if1 is its associated recoil factor or desorption probability due to the decay of its attached precursor i 1 (p if1 ¼ :8 in the case of a- emissions and p i 1 ¼ in the case of b-emissions) [39], l a is the aerosol-attachment rate of the ith daughter to airborne aerosol particles, and l u d (la d ) is the airborne-unattached deposition rate (aerosol-attached deposition rate) on the available indoor surfaces (walls, floor, ceiling and furniture). With the only exception of the decay constants and the recoil factor that have well known and fixed values, the other parameters (l a ; l v ; l u d and l a d ) appearing in Eq. () are highly dependent on the enclosure geometry, on the aerosol concentration, on the ambient conditions as well as on the inhabitant activities and on the Heating, Ventilating and Air Conditioned (HVAC) systems used. All these effects are time-dependent and vary from one environment to another. For practical purposes, it is not possible to determine experimentally their values from direct measurements in private homes and workplaces. Hence, in this work, the indoor ventilation, aerosol-attachment and surface deposition (airborne-unattached and aerosol-attached) rates are considered as free parameters whose ranges of variation taken from Refs. [, 55] are summarised in Table 1. Under such circumstances, the disequilibrium degree of each airborne Rn decay product can be calculated as a function of these variables as follows f i ¼ f i ðl a ; l v ; l u d ; la d Þ: ð3þ Inserting these results into Eq. (1) we can write F Rn ¼ Fðl a ; l v ; l u d ; la d Þ: ðþ Table 1 Typical range of variation of the indoor ventilation, aerosolattachment and surface deposition (airborne-unattached and aerosol-attached) rates [, 55] Parameter Symbol Range of variation Ventilation l a (. ) h 1 Aerosol attachment l v (5 5) h 1 Unattached plate-out l u d (5 11) h 1 Attached plate-out l a d (.5 1.1) h 1 From these relationships and for each given run of l a ; l v ; l u d and la d values, the associated values of the Rn progeny disequilibrium degrees and the equilibrium factors are obtained. In consequence, it is possible to establish a one-to-one correspondence between the equilibrium factor and each of the Rn daughter disequilibrium degree. To analyse these correspondences, we have plotted in Fig. 1 the Rn progeny equilibrium factor as a function of f i, i.e., F Rn ¼ Fðf i Þ; assuming all the possible values for the free parameters l a ; l v ; l u d and l a d : The variation steps chosen for these free parameters were.1 h 1 for l v ; 5h 1 for l a ; 5h 1 for l u d and.5 h 1 for l a d : As the conventional method is based on the Rn progeny equilibrium factor estimation from the ratio of the open NTD reading to that of the enclosed NTD, the function F Rn ¼ Fðf T Þ; being f T ¼ 1 þ f 18Po þ f 1Po ; accounting for this method is represented in Fig. 1 as well. We can observe in this figure that there is not a clear correlation between the Rn progeny equilibrium factor and the 18 Po disequilibrium degree. The large area obtained for the curve F Rn ¼ Fðf 18Po Þ makes evidence of its high dependence with respect to the values taken by l a ; l v ; l u d and la d : Any effort attempting to estimate the Rn progeny equilibrium factor from the only measurement of the 18 Po disequilibrium degree will never succeed in giving satisfactory results independently on the type of detector used. In turn, the correlation between the Rn progeny equilibrium factor and the 1 Pb disequilibrium degree is quite good (F Rn Ef 1Pb )andis not affected by any of the above free parameters. The quite perfect correlation obtained for the function F Rn ¼ Fðf 1Pb Þ means that the Rn progeny equilibrium factor can be obtained with a good precision if a separate and accurate measurement of airborne 1 Pb concentration is performed together with that of Rn. Using this method, the knowledge of the remaining Rn decay product concentrations is not necessary and can be obviated. Unfortunately, this method cannot be adopted in this study since the NTDs are insensitive to the b-emissions of this radionuclide. In contrast to our previous finding [33] and to that obtained by D.orschel and Piesch [6], the

5 19 K. Amgarou et al. / Nuclear Instruments and Methods in Physics Research A 56 (3) Fig. 1. Rn progeny equilibrium factor as a function of the 18 Po, 1 Pb and 1 Po disequilibrium degrees and of the ratio f T assuming all the possible values for the free parameters l a ; l v ; l u d and la d : Rn progeny equilibrium factor estimation from 1 Po disequilibrium degree measurement is not a good approximation. The corresponding function, F Rn ¼ Fðf 1Po Þ; is far from being lineal and presents considerable fluctuations especially at lower values of 1 Po disequilibrium degree. These fluctuations may lead to additional uncertainties in the Rn progeny equilibrium factor estimation from the 1 Po disequilibrium degree. Supposing that there is not any uncertainty associated with the 1 Po disequilibrium degree measurement, the uncertainty for the estimation of the Rn progeny equilibrium factor is found to vary from 5% at high F Rn values (F Rn :7) to 35% at low F Rn values (F Rn :). Quasi-similar behaviour is found for the conventional method although it presents smaller fluctuations than the function F Rn ¼ Fðf 1Po Þ: The corresponding uncertainties found for this method vary from 3% at high F Rn values to 5% at low F Rn values. The higher precision found previously by our group [33] and by D.orschel and Piesch [36] for the 1 Po method in comparison with the conventional one may be true only where the attachment and the plate-out (attached and unattached) rates are fixed to their geometric mean values. However, in realistic situations, those rates present high fluctuations, specially the plate-out rate that is a controversial parameter. All the above results seem to confirm that there is no possibility for an accurate determination of the long-term Rn progeny equilibrium factor by means of NTDs. As the NTDs may register only a-particles emitted from Rn, 18 Po and 1 Po in air, we have considered convenient to define a new quantity, which we have called the reduced equilibrium factor, F Red ; and which is calculated as F Red ¼ :15f 18Po þ :38f 1Po : ð5þ By analogy, assuming all the possible values for the variables l a ; l v ; l u d and la d ; the function F Rn ¼ FðF Red Þ is plotted in Fig. to study the correlation between the two quantities. As clearly shown in this figure, the function F Rn ¼ FðF Red Þ has a similar behaviour as the function F Rn ¼ Fðf 1Pb Þ: The dependency between the two quantities F Rn and F Red is lineal and the corresponding least-square minimisation fitting gives F Rn ¼ :F Red : ðr ¼ :996Þ: ð6þ

6 K. Amgarou et al. / Nuclear Instruments and Methods in Physics Research A 56 (3) Table The comparison between the equilibrium factors obtained from the Reineking and Portend.orfer [] measurements of Rn and its progeny concentrations in different German houses and those estimated using the reduced equilibrium factor concept Airborne concentration (Bq m 3 ) Equilibrium factor Rn 18 Po 1 Pb 1 Po Measured Eq. (6) % deviation a Fig.. Rn progeny equilibrium factor as a function of the corresponding reduced equilibrium factor, F Red ¼ :15f 18Po þ :38f 1Bi ; assuming all the possible values for the free parameters l a ; l v ; l u d and la d : The values of the linear fitting parameters are given in the text. The uncertainties introduced by the fit were found to be less than.5%. To test the validity of this equation, we have taken the experimental measurements of indoor Rn and its progeny concentrations performed by Reineking and Porstend.orfer [], under realistic conditions and using active detection systems, in 1 rooms of different German houses. The results of these measurements together with the associated equilibrium factors are shown in Table. Also given in this table, are the corresponding values estimated from the reduced equilibrium factor. The global agreement between the measured equilibrium factor values and those estimated using Eq. (6) is within % and constitutes a reasonable experimental verification of the usefulness of the reduced equilibrium factor concept. As a direct consequence of this promising finding, long-term Rn progeny equilibrium factor can be estimated with the best precision if proper optimisation of experimental conditions for the 18 Po and 1 Po separate measurement by means of NTDs, regardless the 1 Pb, is performed. In this method, assumptions about ventilation, aerosol attachment and deposition (airborne-unattached and aerosol-attached) rates are not necessary a Measured Eq: ð6þ Calculated as Measured 1: In the case of Rn and its progeny, the equilibrium factor is given by F Rn ¼ :91f 1Pb þ :86f 1Bi ð7þ where f 1Pb and f 1Bi are, respectively, the 1 Pb and 1 Bi disequilibrium degrees. Similarly to Rn decay products, we have studied the oneto-one correspondence between the equilibrium factor and the disequilibrium degrees of the Rn daughters. The results obtained are given in Fig. 3. As shown in this figure, like the functions F Rn ¼ Fðf 1Pb Þ and F Rn ¼ FðF Red Þ; there is a good correlation between Rn progeny equilibrium factor and the 1 Pb disequilibrium degree. However, again because of being the 1 Pb a b-emitter,

7 19 K. Amgarou et al. / Nuclear Instruments and Methods in Physics Research A 56 (3) Fig. 3. Rn progeny equilibrium factor as a function of the 1 Pb and 1 Bi disequilibrium degrees assuming all the possible values for the free parameters l a ; l v ; l u d and la d : f u 18 Po Freuency (%) 3 1 f a 18 Po f18 Po f u 1 Pb 3 1 f a 1 Pb 3 1 f1 Pb f a 6 f 1 1 Po Po F Rn Fig.. Frequency distribution in % of fi u ; fi a ; f i and F values calculated for Rn progeny assuming all the possible values for the free parameters l a ; l v ; l u d and la d : Data of these curves have been fitted to a log normal distribution. the function F Rn ¼ Fðf 1Pb Þ cannot be applied to the NTDs. In turn, the function F Rn ¼ Fðf 1Bi Þ shows broad fluctuations and the uncertainties in the estimation of the Rn progeny equilibrium factor from the 1 Bi disequilibrium degree measurement is about 1 5%. Thus, in contrast to Rn decay products, the Rn progeny equilibrium factor cannot be estimated with a good accuracy by means of NTDs. We want to point out that the values obtained for the disequilibrium degrees and the equilibrium factor as well as the airborne-unattached and the aerosol-attached disequilibrium degrees of Rn daughters are much smaller compared to those obtained for Rn decay products. Figs. and 5 show in % the frequency histograms of airborneunattached and aerosol-attached of Rn and Rn progeny, respectively, assuming all the

8 K. Amgarou et al. / Nuclear Instruments and Methods in Physics Research A 56 (3) f a 1 Pb 3 1 f1 Pb 8 6 f a 1 Bi f1 Bi F Rn Fig. 5. Frequency distribution in % of fi u ; fi a ; f i and F values calculated for Rn progeny assuming all the possible values for the free parameters l a ; l v ; l u d and la d : Data of these curves have been fitted to a log normal distribution. Table 3 The geometric means and the range of variation of fi u ; fi a ; f i and F-values obtained for Rn and Rn progeny assuming all the possible values for the free parameters l a ; l v ; l u d and la d as well as their geometric standard deviations (s g) and the corresponding correlation factors (R ) of the fitted curves Rn progeny Rn progeny 18 Po 1 Pb 1 Po 1 Pb 1 Bi fi u. (.).3 (.1) s g ¼ 1:5 s g ¼ 1: R ¼ :9 R ¼ :98 fi a.7 (.9).3 (.7). (.5).3 (.1).1 (.) s g ¼ 1:1 s g ¼ 1:5 s g ¼ 1:7 s g ¼ 1:5 s g ¼ 1:8 R ¼ :87 R ¼ :9 R ¼ :96 R ¼ :98 R ¼ :98 f i.8 (.9).3 (.7). (.5).3 (.1).1 (.) s g ¼ 1:1 s g ¼ 1:5 s g ¼ 1:7 s g ¼ 1:5 s g ¼ 1:8 R ¼ :9 R ¼ :9 R ¼ :9 R ¼ :98 R ¼ :98 F.3 (.7).3 (.1) s g ¼ 1: s g ¼ 1:5 R ¼ :96 R ¼ :98 possible values for the free parameters l a ; l v ; l u d and l a d : Also shown in these figures are the associated disequilibrium degrees and the equilibrium factor. The curves for f1 u Po ; f1 u Pb and f1 u Bi are not included because they have a unique value equal to zero. According to Figs. and 5, the best fit of the frequency histograms is given by lognormal distribution functions. Table 3 summarises the geometric means and the ranges of variation of fi u ; fi a ; f i and F values calculated for Rn and Rn progeny assuming all the possible values for the free parameters l a ;

9 19 K. Amgarou et al. / Nuclear Instruments and Methods in Physics Research A 56 (3) l v ; l u d and la d : In this table, the corresponding geometric standard deviations and the correlation factors of the fitted curves are given as well. Only the 18 Po and 1 Pb could be found in airborneunattached state in private homes and workplaces. However, their airborne-unattached disequilibrium degrees are generally very small especially that of 1 Pb, which is practically non-existent and do not present any appreciable variation around their geometric mean values. Consequently, the airborne Rn and Rn decay products are controlled principally by the presence of indoor aerosol particles and the frequency distribution of their disequilibrium degrees are similar to that of the aerosol-attached disequilibrium degrees. Considering the geometric means only, it can be clearly seen that the disequilibrium degrees are very close to the aerosol-attached disequilibrium degrees. These last decrease from one daughter to another along the decay chain direction. The wide spread range of variation found, especially for the disequilibrium degrees as well as for the equilibrium factors of Rn and Rn progeny, indicate that the assignment of typical values to the free parameters for any realistic case of indoor environment has a large degree of uncertainty.. Principle of measurement Rn and its short lived daughters 18 Po and 1 Po emit, during their radioactive decay, a- particles with initial energies of 5.9, 6. and 7.69MeV, respectively. Separate measurement of the airborne 18 Po and 1 Po concentrations requires different a-energy responses for the detector used. However, when exposing a NTD in direct contact with air, the a-emissions of airborne Rn and its progeny are slowed down to lower energies during their path to the detector. Thus, for each a-emitter nuclide, a-particles with different energies, from MeV up to its initial energy of emission (E ), may reach the detector surface. To well optimise the experimental conditions for a separate measurement of the 18 Po and/or 1 Po concentration in air, the knowledge of the slowing down spectrum of these nuclides in air is of great interest. Taking into account that the straggling effect in the Bragg curve of a-particles (with energy of several MeV) in the air medium is of about 1%, their slowing down spectrum can be obtained from dn de p 1 for EpE ð8þ ð de=dxþ where de=dx is the stopping power of a-particle in air. Fig. 6A shows, in arbitrary units, the slowing down spectrum curve of the a-particles emitted from Rn and its decay products in air. The stopping power of the a-particles in air has been calculated using the Srim- code 1 [56]. Besides the a-particles emitted from airborne Rn and its progeny, the detector may register also the a-emissions of the Rn daughters that are plated out directly on its surface. Thus, the slowing down spectrum of Eq. (8) may be superimposed by two well-resolved peaks corresponding to the initial a-energies of the 18 Po (6. MeV) and the 1 Po (7.67 MeV). The presence of a-emitter nuclides on the detector surface even in small amounts may considerably alter its response. In addition, it is not possible to identify or reject, at the counting phase, those tracks generated by plate-out effect. As a consequence, the corresponding contribution to the detector reading must be avoided. As clearly illustrated in Fig. 6A by a coloured area, separate measurement of the airborne 1 Po concentration could be performed if definite upper and lower energy thresholds, namely an a-energy window [E min E max ], for track registration are conveniently chosen. In this case, the NTD must have a lower energy threshold, E min ; above the initial a- energy of the 18 Po (6. MeV), to avoid both Rn and 18 Po measurement as well as the plateout peak of this last. On the other hand, the upper energy threshold, E max ; should be below the initial a-energy of 1 Po (7.69MeV) to ensure a total elimination of the corresponding plate-out effect. In the case of 18 Po measurement, any probable a-energy window allowing its detection will register at least a fraction of the 1 Po a-particles as well. Likewise, since the upper energy threshold 1 Available at:

10 K. Amgarou et al. / Nuclear Instruments and Methods in Physics Research A 56 (3) Fig. 6. Slowing down spectrum in arbitrary units due to Rn (A) and Rn (B) a-emitter progeny in air and the corresponding peaks of the plate out effect on the detector surface. The coloured area shows the a-energy window response necessary for the NTDs to measure the airborne 1 Po concentration. Because of its short half-life (15 ms), the 16 Po is in secular equilibrium with its parent ( Rn) and is expected to behave like a gas in airborne-unattached form. necessary to eliminate the contribution of its plateout effect will be very close to the initial energy of Rn a-emissions, it is not feasible experimentally for the NTDs to obtain an a-energy window that ensures a complete elimination of the Rn contribution. Under such circumstance, long-term measurement of the airborne 18 Po concentration by means of NTD is viable only if the Rn and 1 Po concentrations are evaluated separately. In this way, three NTDs are needed to apply this method: one enclosed within a diffusion chamber to determine Rn concentration and a pair open NTDs with different but well-defined a-energy windows for a separate measurement of airborne 18 Po and 1 Po. We have used Makrofol disc plates, of 5 mm thick and covered with a thin (3 mm) Mylar aluminised foil, as NTDs. In the case of enclosed detector, we use the so-called FzK 3 diffusion chamber and a polyethylene filter. Given the Manufactured by Bayer-AG, Germany. 3 FzK is an abbreviation of the Forschungzentrum Karlsruhe centre, Germany, formerly known as KfK (Kernforschungzentrum Karlsruhe) centre. characteristics of this diffusion chamber [57], the a-energy window chosen for Rn is from 3. to 5. MeV. For practical reasons, the upper and lower a-energy thresholds for airborne 18 Po measurement could be chosen to be similar to that used for Rn. The experimental conditions applied to Makrofol detectors to obtain the required energy windows [3. 5.] MeV for Rn and 18 Po and [ ] MeV for 1 Po are, respectively: (1) h chemical etching (CE) at C using 6 M KOH mixed with 5% ethanol as an etchant followed by 1.5 h ECE, and () 6 h CE at C using 7.5 M KOH mixed with 5% ethanol followed by 1 h ECE. This last is performed in both cases at an alternating electric field strength of 33 kv cm 1 and 3 khz frequency. Last but not least, in the supposed presence of Rn and its progeny, we can see in Fig. 6B, that the plate-out peaks of 1 Bi and 1 Po with initial a-energies of 6.7 and 8.78 MeV, respectively, do not overlap the 18 Po and 1 Po a-energy windows. Thus, in the case of long-term measurements of airborne 18 Po and 1 Po concentrations, the open With an alcohol purity of B96%.

11 196 K. Amgarou et al. / Nuclear Instruments and Methods in Physics Research A 56 (3) NTDs used is not altered by the plate-out effect on their surface of Rn and Rn progeny. However, any eventual presence of airborne Rn and its decay products may interfere the response of the open detectors and may contribute to the overall uncertainty of our system. The estimation of this interference would be the subject of a future work. 5. Comparison with active monitors The performance of the method developed in this study with active detectors was tested at the National Radiological Protection Board (NRPB, England) centre, in which 3 sets of 5 multicomponent passive dosimeters were irradiated under different Rn exposures but at varied equilibrium factors. This facility is the European regional reference laboratory for Rn and its progeny measurements. The Rn levels to which the dosimeters were exposed ranged from B1 kbq m 3 h for the exposure 1 to B kbq m 3 h for exposure 3. The equilibrium factor is controlled by means of an aerosol generator that reduces the plate-out effect of the decay products and an electrostatic precipitator with a fan for their subsequent collection. The Rn daughters are continuously monitored using the wire-screen technique together with a back-up filter [58]. Table summarises the averaged values of Rn progeny equilibrium factor obtained with our passive integrating system and by the NRPB active monitors. The equilibrium factor values of our detection system agree with those given by the Table The comparison between the values of Rn progeny equilibrium factor obtained with our passive integrating system and those given by the NRPB active monitor Equilibrium factor This work NRPB Exposure Exposure Exposure NRPB, taking into account the associated uncertainties. The overestimation given by our system at low equilibrium factor (exposure ), where the plate-out effect is very important, is due mainly to the open detector used for airborne 1 Po measurement. This may suggest that this detector has probably recorded a fraction of the Rn daughters, which are deposited directly on its surface. In the third exposure, the levels of Rn and its decay products were extremely high (above 15 kbq m 3 h); so that, the detectors used have suffered from the problem of track saturation effect. Because of the large overall uncertainties on the equilibrium factor of our dosimeter, further investigations of the parameters affecting the measurement process are required to improve the system precision and to optimise its response under different exposure conditions. 6. Application indoor A field test of our passive integrating system was carried out in an inhabited Swedish single-family house as a result of a collaboration with Lund University. The reason for choosing a house in the North of Europe comes because of the high values of the Rn concentration found previously in this region within the frame of an European Union project in which a coordinated survey of the Lund- Kiel-Leipzig-Montpellier-Barcelona-Rome areas was carried out [59]. This house has two welldifferentiated parts: the main building and the annexes. The first one is the major part of the house, where the most used rooms are placed and has a crawl-space of about one meter depth. The annexes are in direct contact with soil and include a guestroom, a storage zone and the garage. Except the crawl-space, which is naturally ventilated, the house is isolated from exterior with all the windows and doors closed. Two sets of 5 dosimeters were exposed for one month (18 September 19October 1) in the crawl-space and in the guestroom. The Rn, 18 Po and 1 Po concentrations measured by our dosimeter as well as the estimated equilibrium factors for the crawl-space and the guest room are given in Table 5. The low

12 K. Amgarou et al. / Nuclear Instruments and Methods in Physics Research A 56 (3) Table 5 Results from indoor exposure of our passive integrating system in a Swedish house equilibrium factor value obtained in the crawlspace could be attributed mainly to ventilation. The Rn concentration found is within the range measured previously by Font et al. [59]. In the case of the guest-room, the annual effective dose, which was estimated using the conversion factor of 1.1 Sv J 1 m 3 h 1 suggested by the European council directive 96/9/EURATOM [6] and a permanency time of 7 h per year, is equal to ( ) msv y 1. The results of this exposure suggest the usefulness of our system to carry out routine surveys for Rn level measurements in private homes and in workplaces in order to estimate the associated annual effective dose received by the general public and the workers. 7. Conclusion Crawl-space Guestroom Rn (Bq m 3 ) Po (Bq m 3 ) Po (Bq m 3 ) F In this work, we have shown, taking into account the range of variation of the parameters influencing the Rn progeny concentrations indoors, that it is not possible for the existing methods using NTDs to obtain the associated long-term equilibrium factor with an appropriate accuracy, and we have established a novel approach based on the new concept of reduced equilibrium factor. This last can be obtained from the only measurement of airborne Rn and its a- emitter daughter ( 18 Po and 1 Po) concentrations. In this approach, assumptions about ventilation, aerosol attachment and deposition rates are not necessary. The passive, integrating and multicomponent dosimeter used consists of: (i) one Makrofol detector enclosed within a FzK diffusion chamber with a polyethylene filter to measure indoor Rn levels, and (ii) two Makrofol detectors that are kept in direct contact with air and that are electrochemically etched at different conditions to obtain the airborne 18 Po and 1 Po concentrations. From a test exercise, carried out under well-controlled irradiation conditions in pure Rn atmospheres, the equilibrium factor values determined with our method agree with those obtained by active methods. The results of an application indoors in an inhabited Swedish single-family house suggest the usefulness of our dosimeter to carry out routine surveys for Rn level measurements in private homes and in workplaces in order to estimate the associated annual effective dose received by the general public and the workers. However, attempts to measure Rn levels need to be performed in order to estimate the corresponding contribution to the measurement process and to provide useful and precise information concerning the general problem of indoor radiation dose assessment. The reduced equilibrium factor concept can also be considered in the design of other active or passive detectors for Rn progeny equilibrium factor measurement. Acknowledgements The authors wish to express gratitude to Dr. G. J.onsson from the Lund University and to Dr. J. C.H. Miles from the NRPB centre for allowing the exposure of our multi-component passive dosimeters. References [1] C.B. Howarth, J.C.H. Miles, Results of the NRPB intercomparison of passive radon detectors, National Radiological Protection Board (NRPB) report,. [] United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR), Sources and effects of ionizing radiation, Vol. I, United Nations Publications, New York,. [3] International Commission on Radiological Protection (ICRP), Protection Against Radon- at Home and at Work, Annals of the ICRP 65, Pergamon Press, New York, 199. [] A. Reineking, J. Porstend.orfer, Health Phys. 58 (199) 715. [5] J.E. Peter, Radiat. Prot. Dosim. 56 (199) 67.

13 198 K. Amgarou et al. / Nuclear Instruments and Methods in Physics Research A 56 (3) [6] A. Kies, A. Biell, L. Rowlinson, M. Feider, Environ. Int. (1996) S899. [7] X. Ortega, A. Vargas, Environ. Int. (1996) S19. [8] Z. Papp, S. Dar!oczy, Health Phys. 7 (1997) 61. [9] K.N. Yu, T. Cheung, Z.J. Guan, E.C.M. Young, B.W.N. Mui, Y.Y. Wong, J. Environ. Radioact. 5 (1999) 91. [1] K.N. Yu, T. Cheung, Z.J. Guan, B.W.N. Mui, Y.T. Ng, J. Environ. Radioact. 8 () 11. [11] K.N. Yu, B.M.F. Lau, Z.J. Guan, T.Y. Lo, E.C.M. Young, J. Environ. Radioact. 51 () 379. [1] N.H. Harley, P. Chittaporn, I.M. Fisenne, P. Perry, J. Environ. Radioact. 51 () 7. [13] S. Furuta, K. Ito, Y. Ishimori, Radiat. Prot. Dosim. 9 () 9. [1] K.N. Yu, B.M.F. Lau, Z.J. Guan, T.Y. Lo, E.C.M. Young, J. Environ. Radioact. 5 (1) 1. [15] A.L. Frank, E.V. Benton, Nucl. Track Det. 1 (1977) 19. [16] T. Dom!anski, W. Chruscielewski, A. Z!orawski, Radiat. Prot. Dosim. 8 (198) 31. [17] G. Somogyi, B. Parip!as, Zs. Varga, Nucl. Tracks Radiat. Meas. 8 (198) 3. [18] J. Planinic, Z. Faj, Nucl. Instr. and Meth. A 78 (1989) 55. [19] O. Sarenio, A. Guhr, Nucl. Tracks Radiat. Meas. 19 (1991) 395. [] B. D.orschel, A. Guhr, Radiat. Prot. Dosim. 9 (1993) 59. [1] C.G. Maniyan, P.P. Haridasan, A.C. Paul, K. Rudan, Radiat. Prot. Dosim. 8 (1999) 63. [] K. Amgarou, Ll. Font, D. Albarrac!ın, C. Domingo, F. Fern!andez, C. Baixeras, Radiat. Meas. 3 (1) 139. [3] R.L. Fleischer, Health Phys. 7 (198) 63. [] R.L. Fleischer, L.G. Turner, A.C. George, Health Phys. 7 (198) 9. [5] C. Ling, Nucl. Tracks Radiat. Meas. (1993) 93. [6] B. D.orschel, E. Piesch, Radiat. Prot. Dosim. 8 (1993) 15. [7] J.C. Hadler, S.R. Paulo, Radiat. Prot. Dosim. 51 (199) 83. [8] J.C. Hadler, P.J. Iunes, S.R. Paulo, Radiat. Meas. 5 (1995) 69. [9] P. Mozzo, F. Trotti, A. Temporin, M. Lanciai, F. Predicatori, F. Righetti, A. Tacconi, Environ. Int. (1996) S595. [3] M. Izerrouken, J. Skovarc, R. Ilic, Radiat. Meas. 31 (1999) 11. [31] S.A.R. Al-Najjar, A. Oliveira, E. Piesch, Radiat. Prot. Dosim. 7 (1989) 5. [3] M. Sohrabi, M. Sadeghi, Nucl. Tracks Radiat. Meas. 19 (1991) 1. [33] C. Baixeras, K. Amgarou, Ll. Font, C. Domingo, F. Fern!andez, Il Nuovo Cimento C (1999) 35. [3] C. Baixeras, K. Amgarou, Ll. Font, C. Domingo, Radiat. Prot. Dosim. 85 (1999) 33. [35] C. Baixeras, K. Amgarou, Ll. Font, C. Domingo, F. Fern!andez, Radiat. Meas. 31 (1999) 313. [36] B. D.orschel, E. Piesch, Radiat. Prot. Dosim. 5 (199) 1. [37] J.C.H. Miles, R.A. Algar, C.B. Howarth, L.M. Hubbard, S. Risica, A. Kies, A. Poffijn, Results of the 1995 European Commission intercomparison of passive radon detectors, European Commission, Directorate-General XII, Report EUR 1699, [38] W. Jacobi, Health Phys. (197) 1. [39] H. Kojima, Res. Lett. Atmos. Elec. 8 (1988) 69. [] International Commission on Radiological Protection (ICRP), Lung cancer risk from indoor exposures to radon progeny, Annals of the ICRP 5, Pergamon Press, New York, [1] D. Capra, C. Silibello, G. Queirazza, J. Environ. Radioact. (199) 5. [] J. Porstend.orfer, J. Aerosol Sci. 5 (199) 19. [3] J. Lembrechts, M. Janssen, P. Stoop, Sci. Total Environ. 7 (1) 73. [] R.C. Bruno, Health Phys. 5 (1983) 71. [5] E. Knutson, A. George, J. Frey, B. Koh, Health Phys. 5 (1983) 5. [6] A.J. Gadgil, D. Kong, W.W. Nazaroff, Radiat. Prot. Dosim. 5 (199) 337. [7] J.P. McLaughlin, B. Fitzgerald, Radiat. Prot. Dosim. 5 (199) 115. [8] H. Kojima, S. Abe, K. Fujitaka, Radiat. Prot. Dosim. 6 (1993) 13. [9] W.W. Nazaroff, D. Kong, A.J. Gadgil, J. Aerosol Sci. 3 (199) 339. [5] T. Hatori, H. Ishida, Radiat. Prot. Dosim. 55 (199) 113. [51] M. Xu, M. Nematollahi, R.G. Sextro, A.J. Gadgil, W.W. Nazaroff, Aerosol Sci. Technol. (199) 19. [5] L.T. Thatcher, W.A. Fairchild, W.W. Nazaroff, Aerosol Sci. Technol. 5 (1996) 359. [53] B.E. Leonard, Health Phys. 7 (1996) 37. [5] M. Voytchev, D. Klein, A. Chambaudet, G. Georgiev, M. Iovtchev, Radiat. Meas. 31 (1999) 375. [55] V. Schmidt, P. Hamel, Sci. Total Environ. 7 (1) 189. [56] J.T. Ziegler, J.P. Biersak, The Stopping and Range of Ions in Solids, Pergamon Press, New York, [57] M. Urban, Nucl. Tracks Radiat. Meas. 1 (1986) 685. [58] K.D. Cliff, UK National Radiological Board radon calibration procedures, J. Natl. Inst. Standards Technol. 95 (199) 135. [59] L. Font, C. Baixeras, G. J.onsson, W. Enge, R. Ghose, Radiat. Meas. 31 (1999) 359. [6] Offcial Journal of the European Communities (OJEC), Basic safety standards for the protection of the health of workers and the general public against the dangers arising from ionizing radiation, European Council Directive 96/ 9/EURATOM, OJ L159, 1996.