V.Schmidt, P. Hamel. Radon in the Living Environment, April 1999, Athens, Greece

Transcription

1 Radon in the Living Environment, 39 MEASUREMENTS OF DEPOSITION VELOCITY OF RADON DECAY PRODUCTS FOR EXAMINATION OF THE CORRELATION BETWEEN AIR ACTIVITY CONCENTRATION OF RADON AND THE ACCUMULATED Po-0 SURFACE ACTIVITY V.Schmidt, P. Hamel Federal Office for Radiation Protection, Department of Radiation Protection, Section. Koepenicker Alle 0-30, 038 Berlin, Germany The retrospective determination of radon exposure levels in dwellings by means of the measurement of the Po-0 surface activity is subject to various certainties. These result partly from the values assumed for the equilibrium factor F and for the attached fraction f, and, more importantly, from differences in the deposition velocities of short-lived decay products of Rn-, caused by varying conditions of turbulence. In order to evaluate the actual range of the variation which occurs der German living conditions, measurements for the deposition velocity parameter were carried out in several dwellings in which increased levels of radon were present. The statistical evaluation of the measurements produced a mean deposition velocity of.7 m/h for Po-8 and 0.4 m/h for Pb-4, and a relative standard deviation in the measured values of as low as approximately 50%. This lay significantly below the certainty value expected from the literature and would seem to justify the retrospective determination of the radon exposure from Po-0 surface activity measurement for use in, for example, epidemiological studies. Keywords: radon, radon decay products, surface deposition, Po-0 measurements INTRODUCTION Alongside the measurement of Pb-0 in so-called volume traps or in whole body coting, the method of measuring Po-0 deposited on the surface of household fittings, which have been exposed over long periods of time, represents another way for the retrospective determination of radon exposure. This method assumes a quantitative relation between the surface activity of long-lived decay products and the radon concentration, or the concentration of radon decay products. The parameter used to quantify this relation is the so-called deposition velocity. This parameter may, therefore, be regarded as the calibration factor, and its determination, including confidence level, is an important prerequisite in the application of the method. The deposition velocity (v) is defined as the ratio between the particle flux j, reaching the surface and the particle concentration C at a distance from the surface at which the influence of the walls can be ignored: j( d) v = c( d) As both particle concentration and particle flux are dependent on the particle diameter d, the deposition velocity is also dependent on particle size. In this investigation the radon decay products were coarsely differentiated into free or attached and aerosol-attached fractions. The deposition velocity is thus defined here as v for the attached fraction and v att for the attached fraction. It is known that the 43

2 39 Radon in the Living Environment, deposition velocity of the attached fraction of Rn decay products is about 00 times higher than that of the attached fraction (Porstendoerfer, 994 []). The certainty of the method is to a large extent due to the dependence of the plate-out process on the turbulence parameters of the room air. It was thus considered necessary to establish the degree of variation of the deposition velocity in normal living areas, and then to assess whether this still allows a meaningful correlation of the surface activity with the mean radon concentration. The deposition velocity was measured in living areas der different conditions of use and, in particular, different heating conditions. To attain reasonable measurement accuracy dwellings with radon concentrations above 500 Bq/m 3 were selected.. In these dwellings, the deposition velocity was measured directly over a period of several days der typical conditions of use. Uncertainties from losses due to recoil and cleaning do not form a part of the study. THE RELATION BETWEEN RN- CONCENTRATION IN AIR AND THE PO-0 SURFACE ACTIVITY The surface activity of the nuclides is established in radioactive equilibrium with particle nuclide flux depositing on the surface. The surface activity for Po-8, A is equal to the particle flux j. The surface activity of Pb-4 is obtained from the Po-8 flux j plus the Pb-4 flux j and the surface activity of Bi-4 as the sum of three components j +j +j 3. Under normal plate-out conditions, the attached fraction of the nuclide Bi-4 can be ignored. If one, furthermore, assumes, that all attached decay products have the same deposition velocity, v att, the surface activity of Po-0 is given by: A Po 0 = att att att att λpb 0 texp [ v C + v C + v (C + C + C )] ( e ) 3, () equilibrium with Pb-0 being a precondition. The contribution of the attached decay products to the total surface activity can in most cases be ignored due to their much lower deposition velocity. If one tries an extended correlation to the Rn- concentration, which is far more frequently measured, then additional assumptions of mean nuclide activity ratios, attached fraction f p and Rn equilibrium factors F need to be made: t Po 0 p Rn 0 λ A v f F b C e Pb = ( ) () The factor b is a fction of the nuclide ratio, and the ratio of the deposition velocities of Po-8 and Pb-4. It may be pointed out that for these parameters deviations from assumed mean values have only a slight influence on the value for factor b. An estimation produced an certainty for the value b, given certain living conditions, of less than 0%. 44

3 Radon in the Living Environment, 39 Using equation (), error margins for the relation between C Rn and A Po-0 can be obtained. From the measured values of the quantities f p, F and v which are published in the literature, the following ranges result for these particular values. In the literature usually no distinction is made between the deposition velocities of the individual nuclides: f p F v m/h This results in an overall certainty in the geometric mean value of one order of magnitude, mainly due to the deposition velocity. For this reason the deposition velocity was investigated more closely. DESCRIPTION OF THE MEASURING METHOD For the purposes of measuring the deposition velocity, the flux of particles and the concentration of particles need to be determined in parallel. The particle concentration is derived from measurements of the activity concentrations of the individual nuclides. For the determination of the activity concentrations of the individual nuclides the MARKOV method was used. The attached and attached fractions are measured separately by drawing the air to be measured through a wire screen and an aerosol filter in sequence. Once flow-decay equilibrium has been established the particle flux can be determined directly from the surface activity. Only the flux of the Po-8 and Pb-4 attached fractions contribute to the surface activity. The surface activity deposited directly on the light tight window of a Si detector was measured α-spectrometrically. The detector window was maintained at zero potential to eliminate electrical field interference. Preliminary tests in the BfS test chamber, using post-exposure measurement, showed no differences for the deposition velocity on various surfaces of different micro-roughness (wood, aluminium and glass). Room air flow and turbulence conditions had been simulated in these tests. Po-8 and Po-4 on the surface of the detector window can be spectrometrically determined. Using peak shape analysis the contribution of the activity in the air was differentiated. Figure shows a typical alpha spectrum which was used for evaluation. The deposition velocity can be calculated separately for Po-8 and Pb-4, by means of the surface activities and activity concentrations measured der dynamic equilibrium (as previously defined). The following equations apply: v v A λ = = A c a for Po-8, and ( A4 A ) λ = = ( A A 4 c a for Pb-4 ) 45

4 39 Radon in the Living Environment, where: v i = the deposition velocity of the attached Po-8 (i=) and Pb-4 (i=), a i = the activity concentration in air of the attached Po-8 (i=) and Pb-4 (i=), c i = the particle concentration in air of the attached Po-8 (i=) and Pb-4 (i=) λi = the decay constants for Po-8 (i=) and Pb-4 (i=) A i = the surface activities for Po-8 (i=) and Po-4 (i=4) For measurements in dwellings the particle detector was integrated as well as possible into the surface on which the deposition velocity was to be measured (see figure ). Measurement spots were chosen to be representative of larger surfaces. Spacial differences in the deposition velocity, caused by the geometry of the surface to be measured, in particular by corners and edges, were not part of this investigation. For this reason measurements were carried out on surfaces at least 30 cm away from corners or edges. In this way, the influence of room (living area) parameters which affect the mixing of air, was primarily determined. The measuring certainty (σ) achieved was at a level of approximately ±0% for v and arod ±30 to ±50% for v. The possible influence of turbulence on the deposition velocity was also tested in BfS chamber where a wide range of turbulence conditions could be reproducibly simulated. The turbulence was quantified by measurements of the air velocity distributions and calculating the variance ( nd moment of the distribution). RESULTS Table summarises the measurements carried out. The chamber measurements (Table, measurement No 4,5) include deposition velocity measurements over an extended range of turbulence conditions (e.g. at extreme heating - Table measurement No.5). The measurements carried out in living areas show, taking into accot the measuring certainties, that the range of variation in the measured values is significantly lower than was gleaned from the literature. The values measured in living areas correspond with those measured in the BfS test chamber. For non-extreme turbulence conditions the values measured in dwellings lie in the very low turbulence range of the BfS chamber measurements. Despite the rather low number of measured values available, the measuring conditions investigated can be considered to be representative for normal living conditions in Germany. Deposition velocities in the range of about 0,7 to 4 m/h for Po-8 (with a mean value of,7 m/h), and 0. to 0.7 m/h (mean value of 0.43 m/h) for Pb-4, can thus be expected. Purely statistical treatment of the measured values produces a relative certainty of arod 50%. The deposition velocities for Po-8 and Pb-4 attached fractions differ significantly. The ratio between v and v averages at 4:, subject to the inferior measurement precision for the deposition velocity of Pb-4 (v ). CONCLUSION 46

5 Radon in the Living Environment, 39 The noticeable low range of deposition velocities fod in this work contrasts with the much wider ranges that were loosely assumed from the literature. This result should reduce the expected overall certainty of retrospective determination of Rn exposure from measurements of Po-0 activity on surfaces. REFERENCES [] Porstendörfer, J.Properties and behavior of radon and thoron and their decay products in the air.j. Aerosol Sci. (994), pp [] Czarwinski,R.Retrospektive Bestimmg der Exposition durch RadonBfS-Jahresbericht 996, S Retrospective Determination of Exposure due to Radon [3] Reineking, A.; Porstendörfer, J. The attached fraction of the short-lived radon decay products in indoor and outdoor environmentshealth Phys. (990) 58, pp [4] Schmidt, V.Anlagerg kurzlebiger Radonzerfallsprodukte an Oberflächen in Wohnräumen BfS- Jahresbericht 996, S

6 39 Radon in the Living Environment, Table : Summarised results characteristic measuring conditions deposition velocity No. kind of room season heating v (m/h) v (m/h) inhabited room summer -,03 0,49 inhabited room summer -,78 0,74 3 inhabited room summer -,5 0,36 5 inhabited room summer -,88 0,74 6 living-room summer low,6 0,6 7 living-room summer low 3,58 0,33 8 living-room autumn medium,49 0,46 9 living-room autumn medium,4 0,7 0 living-room spring medium,00 0,5 kitchen spring low 0,68 0,34 kitchen spring low 0,8 0,8 3 living-room spring low,36 0,55 mean value (No. -3),74 0,43 relative statistical σ certainty (in %) BfS-chamber low level of turbulence,70 0,60 5 BfS-chamber high level of turbulence 5,50, 48

7 Radon in the Living Environment, Cots / channel Energy in MeV Figure : An α-spectrum measured with a Si detector 49

8 39 Radon in the Living Environment, Wall Si-Detector Preamplifier Ampl. MCA PC A A ca. 30 cm Measuring Device for Radon- Decay- Products C C Figure : Measuring arrangement 50