Responses of Low Pressure Andersen Sampler for Collecting Substrates

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1 Responses of Low Pressure Andersen Sampler for Collecting Substrates K.Yamasaki 1, Y.Yamada 2, K.Miyamoto 2 and M.Shimo 2 1 Research Reactor Institute, Kyoto University Noda, Kumatori-cho, Sennan-gun, Osaka, , Japan 2 Division of Radiotoxicology and Protection, National Institute of Radiological Sciences 4-9-1, Anagawa, Inage, Chiba, , Japan INTRODUCTION Since differing sized aerosol deposits at different location in the lung, the size distribution of inhaled aerosolattached radon progeny is important in determining the lung dose (1,2,3). Direct measurements of the activity size distribution of the environmental radon progeny are performed with a low pressure cascade impactor (4,5), a multichannel graded wire screen diffusion battery (6,7,8), and their combination system (9). A cascade impactor is a very useful instrument for measuring the size distribution of the aerosol particles in various fields such as the environmental pollution (10), the health physics and the atmospheric electricity. Cascade impactors which have been operated at normal pressure were restricted to the minimum size selection of 0.4 µm in diameter. Low pressure cascade impactors are improved to select the minimum size from 0.02 to 0.06 µm in diameter with operating at the reduced pressure of few hundred Pascals. Several researchers apply some types of low pressure cascade impactors (Andersen, Berner, Davis, MOUDI etc.) to measure the activity size distribution of the radon progeny in the environment (4,5,11,12). In spite of their careful use, their nonideal behavior is not adequately known (13,14). The stage cut off points of cascade impactor are generally obtained from theoretical calculation using inertial impaction theory (15) rather than from experimental calibration (16,17). The usual calculation procedures assume for simplicity that all stages have identical collection characteristics and that a multi jet-multi stage impactor behaves similarly to a single jet-single stage impactor. Effects of factors such as jet Raynolds number, inlet condition, and the surface nature of collecting substrates are generally ignored. Some of important factors which affect the reliability of impactor data are the wall loss, particle bounce, break up, electrostatic attraction, and the surface nature of collection substrates (13,14,17). For example, if the impactor will be collecting liquid or sticky particles, then particle bounce will not be a problem and nearly any substrates can be used on the impaction plate. On the other hand, if the particles are solid and may bounce, then an adhesive layer such as grease or oil may have to be applied to the substrate. Complicated natures of these factors are not amenable to theoretical treatments, but by developing the understanding through experimental means, one can minimize their negative effects on impactor performance. We have reported the calibration procedures of size selective characteristics of a low pressure Andersen sampler for some collecting substrates using several kinds of aerosol-attached radon progeny with polystyrene latex particles as solid particulate carriers (5). This report describes the calibration procedures using DOS (dioctyl sebacate) particles as liquid particulate carriers and Carnauba wax particles as solid particulate carriers. 1

2 INSTRUMENT DESCRIPTION The low pressure Andersen cascade impactor (LP-20-RS, Tokyo Dylec Co. Ltd.) is a round multi jet-multi stage impactor which has twelve stages, and samples at a flow rate of 22.2 liters/min with the reduced pressure of -550 mmhg at the last stage. The aerodynamic cut off diameters, corresponding to particle diameter collected with the efficiency of 50 %, are evaluated with theoretical calculation to be 9.5, 6.2, 4.2, 2.9, 1.8, 0.95, 0.51, 0.38, 0.30, 0.20, 0.13 and µm for each stage. Particles larger than 0.38 µm are sampled at normal atmospheric pressure using first 8 stages, corresponding to the Andersen normal type impactor. Four additional stages, operating at reduced pressures of -75 to -550 mmhg, divide smaller particles. The back up filter collects particles smaller than those collected by the last stage. Table 1 lists the design and operational parameters of the tested Andersen low pressure cascade impactor. The impactor is made of the aluminum, and cylindrically shaped, with a diameter of 10 cm, standing 30 cm high, and connected to a vacuum pump (OFD-150W, Satoh Vacuum Instruments Co. Ltd.) with displacement power of 150 /min, with weight of 19 kg. Table 1. Design and operational parameters of the low pressure Andersen sampler. Stage number Diameter of nozzle( cm ) Numbers of nozzles Pressure (mmhg) Jet velocity (cms -1 ) 50% cut-off diameter(µm) L L L L Condition (1) Temperature : 23 (2) Flow rate : 22.2 min -1 TEST OF SIZE SELECTIVE CHARACTERISTICS Measurements of the size distribution of the aerosol attached test radon progeny were carried out using the radon that chamber system (Fig.1) installed in NIRS (18). The system is consisted of a radon gas source, an aerosol generator for carrier aerosols, an aging chamber, an aging and mixing chamber, an exposure chamber for animal experiments and a charcoal bed for radon gas trap. Soil containing 226 Rn was used for the radon gas source. The radon gas that emanated from soil usually circulated through the aging chamber with volume of 100 liters. The aerosol attached test radon progeny was formed by mixing with carrier aerosols in the aging and mixing chamber (1020 liters). The test radon progeny about Bq per m 3 was sampled from the sampling port equipped to the 2

3 aging and mixing chamber. The carrier liquid aerosols were produced by a condensation aerosol generator (SLG270, Topas GmbH) using DOS (dioctyl sebacate) as the aerosol material. On the other hand, the carrier solid aerosols were produced by an evaporation-condensation aerosol generator (MAGE, Coop. LAVORO E AMBIENTE) using Carnauba wax as the aerosol material. The particle size and concentration were controlled by changing the heating temperature for vaporization and the flow rate of saturator air through the generator. The size range of the produced carrier particles was 0.1 µm to 0.7 µm (aerodynamic diameter) with GSD (geometric standard deviation) less than 1.4. The particle concentration was over particles per cm 3 for all aerosol sizes. The concentration and size distribution of the carrier aerosols in the aging and mixing chamber were continuously monitored by a laser particle counter (PMS, model : HS-LAS) for larger than µm. Collecting substrates that was examined in this study were : (1) uncoated clean stainless steel plate (SUS), (2) Dow Corning silicone oil or grease coated stainless steel plate (SUS+Grease), (3) polyethylene sheet covered stainless steel plate (SUS+Polyethylene), (4) membrane filter (TM-1, Advantec-Toyo Corp.), (5) teflon binder glass fiber filter (T60A20, Pallflex Products Corp.), (6) quartz fiber filter (2500QAT-UP, Pallflex Products Corp.). The test chamber was left at least 4 hrs because the correlation between 222 Rn and its progeny reached to the radioactive equilibrium after the injection of the carrier aerosols. The aerosol attached test radon progeny were sampled on each stage of the impactor for two minutes with a flow rate of 22.2 liters per minute. The alpha particles from the radon progeny which was collected on the collecting substrate were sequentially measured with a ZnS(Ag) scintillation counter for 30 seconds after 20 minutes waiting for the decay of 218 Po. After decay correction, the number fractions of the radioactive particles were found 3

4 for each stage of the impactor. Since the size distribution was close to a log-normal distribution, the data of the cumulative fraction vs. the aerodynamic diameter were plotted on a logarithmic probability section paper. The smooth line through those data points was used to find the geometric median aerodynamic diameter (GMD), and geometric standard deviation (GSD). Fig. 2. Cumulative activity size distribution on the DOS particles (CMD:0.26µm, GSD:1.4) for various collecting substrates. Figure 2 shows the typical cumulative activity size distributions on the DOS particles (CMD : 0.26 µm, GSD 4

5 : 1.4) labeled with radon progeny for several collecting substrates. The relatively small variation of the observed size distributions may be originated the fact that the DOS particle is a liquid particle. It is considered that the effects of the particle bounce and reentrainment are minimized by the use of liquid particles, but the results with solid particles may differ significantly from those obtained by using liquid particles (5). Table 2 shows the experimental values of GMD and GSD obtained by assuming log-normal size distributions on the DOS particles and the Carnauba wax particles for several collecting substrates. This table shows that the stainless steel plates (1)~(3) with different surface natures have a nearly equal collection characteristics, but filters (4)~(6) have different ones owing to their surface natures for both particles. Carnauba wax particle was produced as an Table 2. Parameters of the activity size distributions on the DOS particles (CMD:0.26µm, GSD:1.4) and Carnauba wax particles (CMD:0.21µm, GSD:1.4) for various collecting substrates. Collecting substrates DOS Carnauba wax GMD (µm) GSD GMD (µm) GSD (1) SUS (2) SUS + Grease (3) SUS + Polyethylene (4) T60A20 (5) Membrane (6) 2500QAT-UP example of typical solid particles. But this table shows that Carnauba wax particle has a same nature to the liquid DOS particle for particle collection by impactors. Carnauba wax particle may have a highly sticky nature. From these results, it is concluded that the silicone grease coated stainless steel plate is one of the best substrate for the size selective collection of the radon progeny having unknown nature owing to the carrier aerosols. On the other hand, it was concluded that the interstage wall loss was negligibly small for the measurements of the size distribution of radon progeny from a comparison between the total activity collected on the filter and the sum of the activity collected on each stage. Fig. 3. Correlation between AMD(HS-LAS) and AMD(LPAS) 5

6 The theoretically induced size selective characteristics of the low pressure Andersen sampler were experimentally checked using DOS liquid test radon progeny aerosols having different diameters of 0.1 µm to 0.7 µm which were produced by the condensation aerosol generator. The result is shown in Fig.3. Here, AMD (HS- LAS) is the aerodynamic median diameter which is transfered using attachment theory (2) from the count median diameter (CMD) and GSD measured by the laser particle counter (HS-LAS), with considering the corrections of the refractive index and the density of the DOS aerosols. In spite of the poor data, it is clear that the experimental size selective characteristics of the sampler differ from theoretical one. Thus, a cascade impactor may need an appropriate calibration procedure including the interstage characteristics for determining the accurate size distribution. CONCLUSION The size selective performance of a low pressure Andersen sampler was examined experimentally using DOS liquid aerosols and Carnauba wax solid aerosols labelled with radon progeny. The best size selective performance was obtained when silicone grease coated stainless steel plates were used for the collecting substrates. It was found that a cascade impactor might need an appropriate calibration procedure including the interstage characteristics for determining the accurate size distribution. REFERENCES (1) K.Yamasaki, K.Okamoto and T.Tsujimoto, Unattached fraction and the size distribution of the radon progeny in air of a nuclear facility, Proc. Asia Cong. Radiat. Prot., , (2) J.Porstendörfer, Properties and behavior of radon and thoron and their decay products in the air, J. Aerosol Sci., 25, , (3) J.Porstendörfer, Radon : Measurements related to dose, Environment International, 22, Suppl. 1, S , (4) G.Butterweck, J.Porstendörfer, A.Reineking and J.Kesten, Unattached fraction and the aerosol size distribution of the radon progeny in a natural cave and mine atmospheres, Radiat. Prot. Dosim. 45, (1992). (5) K.Yamasaki, Size selective performance of low pressure Anderson sampler and its application to activity size distribution of radon progeny, Radon and thoron in the human environment, A.Katase and M.Shimo, eds., World Scientific, (1998). (6) S.B.Solomon and T.Ren, Characterization of indoor airborne radioactivity, Radiat. Prot. Dosim. 45, (1992). (7) Y.Jin, L.Xu, G.Fang, M.Shimo and W.Zhuo, Size distribution of atmospheric radioactive aerosol and its measurement, Radon and thoron in the human environment, A.Katase and M.Shimo, eds., World Scientific, (1998). (8) E.O.Knutson, A.C.George and K.W.Tu, The graded screen technique for measuring the diffusion coefficient of radon decay products, Aerosol Sci. and Technol. 27, (1997). (9) T.Haninger, Size distributions of radon progeny and their influence on lung dose, Radon and thoron in the human environment, A.Katase and M.Shimo eds., World Scientific, (1998). 6

7 (10) N.Ito and A.Mizohata, Concentration variation of atmospheric sulfate observed at Sakai, Osaka from 1986 to 1995, J. Aerosol Res., Jpn., 13, (1998). (in Japanese) (11) T.Tsujimoto, H.Miyake et. al., Continuous measurement of the size distribution of radon progeny using drum impactor, KURRI Prog. Rep. 1995, 220, (1995). (12) K.W.Tu, I.Fisenne and A.Hutter, Short and long-lived radionuclide particle size measurements in a uranium mine, EML, U.S. DOE Report EML-588, (1997). (13) A.K.Rao and K.T.Whitby, Non-ideal collection characteristics of inertial impactors-Ⅰ. Single stage impactors and solid particles, J. Aerosol Sci., 9, 77-86, (1978). (14) A.K.Rao and K.T.Whitby, Non-ideal collection characteristics of inertial impactors-Ⅱ. Cascade impactors, J. Aerosol Sci., 9, , (1978). (15) W.E.Ranz and J.B.Wong, Impaction of dust and smoke particles on surface and body collection, Ind. Engng Chem., 44, (1952). (16) N.P.Vaughan, The Andersen impactor calibration, wall losses and numerical simulation, J. Aerosol Sci., 20, (1989). (17) V.A.Marple, K.L.Rubow and S.M.Behm, A microorifice uniform deposit impactor (MOUDI) : Description, calibration, and use, Aerosol Sci. and Technol. 14, (1991). (18) Y.Yamada, A.Koizumi, H.Yonehara, M.Shimo and J.Inaba, Prototype exposure chamber of radon for animal experiments, Radon an thoron in the human environment, A.Katase and M.Shimo eds., World Scientific, (1998). 7

Performance Characterization of A New Cam System M.J. Koskelo 1, J.C. Rodgers 2, D.C. Nelson 2, A.R. McFarland 3 and C.A. Ortiz 3

Performance Characterization of A New Cam System M.J. Koskelo 1, J.C. Rodgers 2, D.C. Nelson 2, A.R. McFarland 3 and C.A. Ortiz 3 1 CANBERRA Industries, Meriden, CT 06450 2 Los Alamos National Laboratory,