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Responses of Low Pressure Andersen Sampler for Collecting Substrates

K.Yamasaki1, Y.Yamada2, K.Miyamoto2 and M.Shimo2

1Research Reactor Institute, Kyoto University

Noda, Kumatori-cho, Sennan-gun, Osaka, 590-0494, Japan

2Division of Radiotoxicology and Protection, National Institute of Radiological Sciences

4-9-1, Anagawa, Inage, Chiba, 263-8555, Japan

INTRODUCTIONSince differing sized aerosol deposits at different location in the lung, the size distribution of inhaled aerosol-

attached 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.

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INSTRUMENT DESCRIPTIONThe 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 0.056

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

R/min, with weight of 19 kg.

　Table 1. Design and operational parameters of the low pressure Andersen sampler.

Stagenumber

Diameter ofnozzle(㎝)

Numbers ofnozzles

Pressure(mmHg)

Jet velocity(cms-1)

50% cut-offdiameter(µm)

0 0.212 98 107 9.5

1 0.121 229 141 6.2

2 0.093 229 238 4.2

3 0.073 229 380 2.9

4 0.054 229 706 1.8

5 0.036 229 1587 0.95

6 0.025 229 3291 0.51

7 0.025 134 5625 0.38

L-1 0.025 110 -75 7603 0.30

L-2 0.025 80 -195 12674 0.20

L-3 0.025 80 -350 17465 0.13

L-4 0.025 110 -550 24799 0.056

Condition (1) Temperature : 23℃ (2) Flow rate : 22.2Rmin-1

TEST OF SIZE SELECTIVE CHARACTERISTICSMeasurements 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 226Rn 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 1×104 Bq per m3 was sampled from the sampling port equipped to the

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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 1×104 particles per cm3 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 0.065 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 222Rn 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 218Po. After decay correction, the number fractions of the radioactive particles were found

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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

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: 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 substratesDOS Carnauba wax

GMD (µm) GSD GMD (µm) GSD

(1) SUS (2) SUS ＋ Grease (3) SUS ＋ Polyethylene (4) T60A20 (5) Membrane (6) 2500QAT-UP

0.260.280.270.350.280.42

1.511.461.461.571.401.83

0.210.210.210.320.250.38

1.581.541.651.441.591.51

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)

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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.

CONCLUSIONThe 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., 591-594, 1993.

(2) J.Porstendörfer, Properties and behavior of radon and thoron and their decay products in the air, J. Aerosol

Sci., 25, 219-263, 1994.

(3) J.Porstendörfer, Radon : Measurements related to dose, Environment International, 22, Suppl. 1, S563-583,

1996.

(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, 167-170

(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, 73-78 (1998).

(6) S.B.Solomon and T.Ren, Characterization of indoor airborne radioactivity, Radiat. Prot. Dosim. 45, 323-327

(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,

79-84 (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, 604-624 (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, 574-576 (1998).

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(10) N.Ito and A.Mizohata, Concentration variation of atmospheric sulfate observed at Sakai, Osaka from 1986

to 1995, J. Aerosol Res., Jpn., 13, 343-353 (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, 87-100, (1978).

(15) W.E.Ranz and J.B.Wong, Impaction of dust and smoke particles on surface and body collection, Ind. Engng

Chem., 44, 1371-1381 (1952).

(16) N.P.Vaughan, The Andersen impactor calibration, wall losses and numerical simulation, J. Aerosol Sci.,

20, 67-90 (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, 434-446 (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, 67-

72 (1998).