Upload
others
View
2
Download
0
Embed Size (px)
Citation preview
P-1b-42
1
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.
P-1b-42
2
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
P-1b-42
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 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
P-1b-42
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
P-1b-42
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 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)
P-1b-42
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.
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).
P-1b-42
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, 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).