Semi-continuous measurement of the unattached radon decay products size distributions from 0.5 to 5 nm by an array of annular diffusion channels

A new diffusion battery was designed to make precise measurements of the indoor activity size distribution of short-lived 222Rn decay products in the size range of 0.5-5 nm. It relies on the selection of particles by five annular diffusion channels (ADCs) and on the use of a non-linear inversion method, EVE, for the reconstruction of the particle size distributions. The ADC geometry proved to allow a better selection of the particles size compared to the wire screen and the implementation of an alpha detector set in front of the sampling filter for semi-continuous activity measurements. EVE algorithm was chosen in order to take into account the uncertainties of the measurement in the deconvolution process. The diffusion battery was tested in a radon chamber, and results showed a good reproducibility of the measurements. The degree of confidence on the obtained size distributions is then discussed. Results show that the relative uncertainty on the penetration data has to be less than 10% in order to obtain a reliable size distribution. © 2007 Elsevier Ltd. All rights reserved

Les résultats des campagnes de mesures de radon et facteurs explicatifs

Des mesures de radon ont été effectuées à l’aide de détecteurs passifs dans plus de 2 500 maisons, réparties dans 36 départements français. Les 3 006 résultats obtenus indiquent une moyenne arithmétique égale à 115 Bq.m-3, et une moyenne géométrique égale à 62 Bq.m-3, avec un écart-type de 2,74. En corrigeant la distribution des données pour tenir compte de la densité de population ou de logements à l’échelle du département, les moyennes arithmétique et géométrique sont évaluées è 62 Bq.m-3 et 41 Bq.m-3 respectivement. L'analyse des données montre une influence de la nature du sous-sol prépondérante par rapport à d’autres facteurs explicatifs: niveau de la pièce dans la maison, type de pièce, matériaux de construction, ou âge de la maison

085 741 SIZE DISTRIBUTION, EQUILIBRIUM RATIO AND UNATTACHED FRACTION OF RADON DECAY PRODUCTS UNDER TYPICAL INDOOR DOMESTIC CONDITIONS

In order to characterise the behaviour of radon decay products under domestic conditions, long-term measurements were carried out from May 1997 to April 1998 in a typical dwelling located in Brittany (France). In particular, the unattached fraction and equilibrium factor were continuously measured. Moreover, the size distributions of unattached and attached radon daughters were investigated by using specific instruments implemented in the laboratory. All these experiments were carried out under different typical aerosol conditions. The results evidenced the strong influence exerted by the characteristics (concentration, size) of ambient aerosol on these different parameters. Key words: radon daughters, unattached fraction, equilibrium factor, radon daughter size distribution INTRODUCTION Radon-222, a rare natural radioactive gas, is present at different concentrations in soils and building materials. After emanation, it gives birth to the following short-lived radon decay products: Po-218, Pb-214, Bi-214 and Po-214. Due to their high diffusity, these solid particles can, then, attach to other particles present in the ambient air. It is well known that inhalation of radon daughters is responsible for about 40% of the total radiation dose received by populations. Their effect on the health depends on their behavior in indoor atmosphere. One can consider two main groups of radioactive particles: the first one called "unattached fraction" of size within 0.5 and several nanometers, while the second one corresponds to radon decay products in a larger size mode (10 -1000 nm) characterizing the "attached fraction". Unattached fraction, equilibrium factor and size distribution are the main influent parameters involved in the lung-dose calculation. Within the framework of the European program RARAD (1996RARAD ( -1999, experiments were carried out in a dwelling situated in Brittany during one year. Unattached fraction and equilibrium factor were continuously measured under different aerosol conditions. Moreover, spot measurements of unattached and attached size distributions of radon daughters were also performed. MATERIAL AND METHODS Continuous measurements of unattached fraction and equilibrium factor In order to continuously measure the unattached fraction and equilibrium ratio, we developed an original device consisting of two parallel sampling lines: an annular diffusion channel (ADC) and an open filter. As a matter of fact the diffusional behavior of the ADC geometry has been studied by analytical computation 085 Radon in the Living Environment, 19-23 April 1999, Athens, Greece 742 detector (PIPS CAM 900). Thus, only attached particles are collected. On the other hand, the open filter being also equipped with an alpha detector collects all the particles. Total and attached activity concentrations were determined by alpha spectroscopy using the method of Size distribution of unattached an attached radon daughters A granular bed diffusion battery was built in order to determine the size distribution of unattached Po-218 and Pb-214. It was composed of five granular beds, filled with glass or stainless steel balls, and a reference channel in parallel. Cut-off diameters ranged within 0.85 to 4.8 nm. The particles which were not deposited in the granular beds were collected on the downstream filters. The SDI-2001 used to collect the attached radioactive aerosol consists of an Andersen impactor followed by five granular beds in parallel After sampling periods of 5 to 10 minutes, activity concentrations of Po-218, Pb-214 and Bi-214 were determined by gross alpha counting using a modified TELS method (Hartley and Hartley, 1989). Then, the kernel matrixes of our sampling devices and the activity on each filter being known, size distributions were retrieved using RESULTS Experimental conditions Throughout the year of experimentation temperature and hygrometry remained quite stable with respective values around 20°C and 50%. Particle concentration was measured using a CNC TSI 3025, whereas the particle size distribution was determined thanks to a CNC TSI 3022 associated to a diffusion battery TSI 3040. Without sources, particle concentration was systematically below 5 000 cm-3, which is low. The mean particle concentration was 1 200 cm-3. It was greatly increased and reached 1 000 000 cm-3 in the presence of aerosols sources such as cigar and cooking smokes, or burning candles and fumigating sticks. Radon activity concentration was measured by electroprecipitation of the Po218+and ranged between 240 and 2 800 Bq.m-3 with a mean value of 1 400 Bq.m-3. The ventilation rate was estimated to 0.25 h-1 according to the method described by Unattached fraction and equilibrium factor The results of unattached fraction and equilibrium factor measurements with the aged aerosol and various aerosol sources are summarized in Radon in the Living Environment, 19-23 April 1999, Athens, Greece 085 743 With aerosol sources, the unattached fraction became negligible, i.e. below 5%, whereas the equilibrium factor was highly increased and sometimes reached values around 0.75. No unattached Pb-214 and no unattached Bi-214 were found; there was, sometimes, unattached Po-218. The time variation of unattached fraction and equilibrium factor with cigar smoke and fumigating sticks is illustrated on Nanometer size distribution of Po-218 and Pb-214 The nanometer size distributions of Po-218 and Pb-214 were found unimodal. The diameters obtained for the Po-218 with EVE and Twomey methods ranged from 0.5 to 1.25 nm, whereas geometric standard deviation (GSD) varied between 1.2 and 1.4 and 1.15 and 1.3, respectively. The EVE and Twomey algorithms gave mean diameters of 0.8 and 0.95, respectively. A typical example of the nanometer Po-218 size distribution is presented on The results obtained for Pb-214 and Po-218 were comparable. As a matter of fact, the EVE algorithm gave diameters within 0.55 and 1.25 nm with a GSD between 1.2 and 1.35 whereas Twomey gave diameters in the range 0.55-1.5 nm with GSD between 1.15 and 1.4. A typical example of nanometer Pb-214 size distribution is displayed on Attached size distributions The attached size distributions of radon daughters were also investigated under different aerosol conditions. No difference was observed between the radon daughters, so only the results concerning the potential alpha energy concentration (PAEC) will be presented hereafter. With the aged aerosol, the attached size distribution was unimodal with an accumulation mode around 190 nm and a GSD of 1.64. As more than 70% of experiments gave results with accumulation mode between 180 and 200 nm, that is only a difference of 20 nm the aged aerosol could be considered as stable. The size distributions obtained with cigar smoke and fumigating sticks were also unimodal. With the former, accumulation mode diameters ranged from 200 to 260 nm with a mean diameter of 222 nm and a GSD of 1.6. With the latter, accumulation modes varied between 240 and 280 nm with a mean diameter of 255 nm and a GSD of 1.57. These results confirmed those previously obtained during an inter-comparison campaign carried out in the same dwelling Some changes were observed with cooking smoke and burning candles. The former produced either unimodal or bimodal distributions. When the distributions were unimodal, the accumulation modes were between 160 and 205 nm with a mean diameter of 195 nm and a GSD of 1.6. In 63% of the cases, bimodal distributions were retrieved. The nucleation mode was around 35 nm with a GSD of 1.76 and accounted for 11% of the attached activity. Eighty nine percent of the attached activity fell within an accumulation mode around 200 nm with a GSD of 1.72. 085 Radon in the Living Environment, 19-23 April 1999, Athens, Greece 744 CONCLUSION In order to characterize the radon decay products under typical living conditions measurements were carried out over one year. They provided us with good quality information on unattached fraction, equilibrium factor and radon daughters size distribution. We noticed that the nature, i.e. concentration, size, of ambient aerosol exerted an important influence on parameters such as unattached fraction, equilibrium factor and radon daughters size distribution. This study provided us with a large database useful for a better characterization of uncertainties in dose-exposure calculations. In the future, a more precise and better characterization of the charged fraction of radon aerosol will be required to get a better understanding of the different cluster modes mentioned in the literature. ACKNOWLEDGMENT

Measurement of the unattached radon decay products with an annular diffusion channel battery

This chapter describes a new method to determine the size distribution of short-lived radon decay products of nanometer size. To eliminate certain problems induced by the instruments classically used, a new device has been built. It consists of five Annular Diffusion Channels of different lengths equipped with a reference filter; they operate in parallel. During sampling, the nanometer-range particles are partly trapped by diffusion onto the inner channel walls, whereas the others are collected on downstream filtering membranes. A prototype ADC battery to measure size distribution of short-lived radon decay products in nanometer form was designed to have a better selectivity than current devices. From activity data measured in each unit, the size distribution of nanometer size radon daughters can be reconstructed through a non-linear inversion method, EVE. The size range of concern is within 0.3 and 5 nm and corresponds to diffusion coefficients in the range 0.2–22 mm2 s−1

Prediction of areas presenting a high radon exhalation potential: A new methodology based on the properties of geological formations and soils

A research program carried out since 1997 produced a methodology for predicting areas with a strong potential for radon exhalation at the soil surface. This methodology is based on a quantification of the Rn exhalation rate, from a precise characterization of the main local geological and pedological parameters that control the radon source and its transport to the soil/atmosphere interface. It combines a cross mapping analysis of parameters used in a Geographic Information System with a model of the vertical transport of Rn by diffusion trough the soil. This code (TRACHGEO) calculates the radon flux density at the surface as a function of the properties of the rock and the soil. This approach is validated in 4 typical areas with different geological contexts, starting from in situ measurements of radon fluxes and of radon concentrations in dwellings. A lithogeochemical classification of the geological formations as a function of their U contents and their confrontation to Rn level measurements demonstrate the primordial influence of the U content of the basement on Rn exhalation. This study leads to an initial map of the exhalation potential by assigning a potential class to each lithogeochemistry. Nevertheless, in situ radon measurements reveal a high spatial variability on uraniferous lithologies. Tests made by the TRACHGEO tool show the need to take account of spatial heterogeneity of soils (in addition of geochemistry) to improve the mapping resolution. The TRACHGEO forecasts explain the variability of the Rn exhalation on a larger scale