Measuring Polonium-218 Diffusion-Coefficient Spectra Using Multiple

Chapter 25 Measuring Polonium-218 Diffusion-Coefficient Spectra Using Multiple Wire Screens

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2

R. F.Holub1and E. O. Knutson 1Geophysics Division, Denver Research Center, U.S. Bureau of Mines, Department of the Interior, Denver, CO 80225 2Environmental Measurement Laboratory, Department of Energy, New York, NY 10014 The Bureau of Mines, Denver Research Center and the Department of Energy, Environmental Measurement Laboratory, developed through parallel efforts, two c l o s e l y related techniques for the measurement of Po (RaA) d i f f u s i o n coefficient spectra. This work was prompted by reports in the past 5 years indicating that the diffusion coefficient of unattached Po may vary due to various physical and chemical factors in different environments. The diffusion coefficient is

218

218

important because it affects the amount and s i t e of Po deposition in the respiratory t r a c t .

218

Our multiple screen techniques are adapted from George's 1972 single screen technique for measuring the unattached Po activity. The Environmental Measurements Laboratory technique uses three screens in parallel, the Bureau of Mines three (or four, in a later version) in series. After drawing a sample, the alpha activity on the front face of each screen is counted with a 2-pi scintillation counter and a factor we have determined by experiment is applied to determine the total activity on each screen. The diffusion coefficient spectrum is then determined by applying a variant of the theory by Cheng and Yeh. Results to date indicate that, except in situations where there is intense aerosol formation, the spectrum is peaked at about 0.08 ± .01cm/sec. 218

2

218

Each liter of air normally contains a few atoms each of Po, Pb, Bi and Po, which are the short-lived decay products of the radioactive noble gas radon. When inhaled, these atoms can be deposited on the lining of the respiratory tract, causing irradiation of the tissue due to further radioactive decay. This irradiation accounts for about one half of the average persons dose

214

211+

21H

This chapter not subject to US. copyright. Published 1987, American Chemical Society

In Radon and Its Decay Products; Hopke, Philip K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

25.

HOLUB AND KNUTSON

218

Measuring Po Diffusion-Coefficient Spectra

341

from n a t u r a l r a d i a t i o n , and i t i s b e l i e v e d t h a t t h i s does c o n t r i b u t e t o the i n c i d e n c e of lung cancer ( S i n n a e v e , 1984). I n some underground mines and i n some homes, t h e c o n c e n t r a t i o n of radon and i t s decay p r o d u c t s can become h i g h enough to c o n s t i t u t e a s e r i o u s h e a l t h c o n c e r n . T h i s concern m o t i v a t e d our r e s e a r c h . B o t h polonium n u c l i d e s are a l p h a e m i t t e r s and t h e r e f o r e of p a r t i c u l a r c o n c e r n . I n h e a l t h p h y s i c s i t i s customary to d i f f e r e n t i a t e between a t t a c h e d and u n a t t a c h e d Po: the f o r m e r , u s u a l l y the l a r g e r of the two c o n s i s t s of P o atoms a t t a c h e d to a i r b o r n e p a r t i c l e s which a r e c o p i o u s l y p r e s e n t i n v i r t u a l l y e v e r y atmosphere; the l a t t e r c o n s i s t s of a P o atom or i o n , f r e q u e n t l y surrounded by s e v e r a l dozen m o l e c u l e s o f a c o n d e n s i b l e species p r e s e n t i n the a i r . The purpose o f t h i s paper i s t o p r e s e n t a new method f o r measuring the s i z e p r o p e r t i e s o f t h e s e u n a t t a c h e d Po clusters. I n 1984 our two l a b o r a t o r i e s began t o d e v e l o p a m u l t i p l e s c r e e n t e c h n i q u e f o r making amount and s i z e measurements on Po. The work proceeded i n d e p e n d e n t l y a t f i r s t , l e a d i n g to d i f f e r e n t c o n f i g u r a t i o n s of s c r e e n s . I n c r e a s i n g l y , the v a l u e o f c o l l a b o r a t i o n became apparent and t h i s l e d t o a one-week c o l l a b o r a t i v e experiment a t the BOM l a b o r a t o r y , September 23-27, 1985. T h i s paper c o v e r s e s s e n t i a l l y a l l of our work on s c r e e n s t o d a t e . 2 1 8


2 1 8

2 1 8

2 1 8

2 1 8

P r i o r Work P r i o r Work on D i f f u s i o n C o e f f i c i e n t s . B e g i n n i n g w i t h C h a m b e r l a i n and Dyson ( 1 9 5 6 ) , s e v e r a l i n v e s t i g a t o r s have measured and r e p o r t e d d i f f u s i o n c o e f f i c i e n t s f o r r a d i o a c t i v e heavy m e t a l atoms o r i o n s i n a i r . The Chamberlain - Dyson v a l u e o f 0.054 c m / s e c , measured f o r P b , i s s t i l l w i d e l y used f o r b o t h P b and P o . Busigin et. a l ( 1 9 7 9 ) , who have reviewed the e x i s t i n g d a t a and r e p o r t e d new measurements, contend t h a t the d i f f u s i o n c o e f f i c i e n t of P o can v a r y from 0.001 to 0.1 cm?/sec. P o s s i b l e gas-phase c h e m i c a l r e a c t i o n s were mentioned t h a t might cause these l a r g e changes. G o l d s t e i n and Hopke (1985) found t h a t the d i f f u s i o n c o e f f i c i e n t of P o c o u l d be " a d j u s t e d " i n the range 0.03 t o 0.08 cm /sec by c o n t r o l l i n g the admixed t r a c e g a s e s . The d i f f u s i o n c o e f f i c i e n t was used as an index of the degree of e l e c t r i c a l n e u t r a l i z a t i o n o f Po ions. A p p r o a c h i n g from another d i r e c t i o n , S i n c l a i r et a l (1978) and K n u t s o n et a l (1984) r e p o r t t h a t d i f f u s i o n b a t t e r y measurements o f r a d o n daughter a e r o s o l - s i z e d i s t r i b u t i o n s o f t e n show a s m a l l peak w h i c h c o u l d be i n t e r p r e t e d as the u n a t t a c h e d f r a c t i o n . I t s p o s i t i o n would i n d i c a t e d i f f u s i o n c o e f f i c i e n t s from 0.0005 to 0.05 cm /sec. Q u i t e r e c e n t l y Raes (1985) a p p l i e d the c l a s s i c a l t h e o r y o f homogenous n u c l e a t i o n o r i g i n a l l y d e v e l o p e d by B r i c a r d et a l (1972) t o atmospheres c o n t a i n i n g SO2, H2O and P o i o n s . Depending on the H 0 and SO2 c o n c e n t r a t i o n s , i o n s c o u l d grow t o a q u a s i - s t a b l e c l u s t e r which would evaporate upon e l e c t r i c a l n e u t r a l i z a t i o n , o r t o a l a r g e r s i z e which would s u r v i v e n e u t r a l i z a t i o n . I m p l i c i t i n the r e s u l t s d i s c u s s e d above, e s p e c i a l l y the G o l d s t e i n and Hopke and the Raes p a p e r s , i s the f a c t t h a t the Po d i f f u s i o n c o e f f i c i e n t can be m u l t i - v a l u e d . Therefore, 2

2 1 2

2 1 2

2 1 8

2 1 8

2 1 8

9

2 1 8

2

2 1 8

2

2 1 8


342

RADON AND ITS DECAY PRODUCTS


e x p e r i m e n t a l i s t s s h o u l d be p r e p a r e d t o d e a l w i t h a spectrum r a t h e r than a s i n g l e d i s c r e t e v a l u e f o r the d i f f u s i o n c o e f f i c i e n t s . P r i o r Work on Wire S c r e e n s . The concept o f u s i n g w i r e s c r e e n s i n the measurement o f r a d i o a c t i v e a i r b o r n e atoms o r i o n s d a t e s back a t l e a s t t o B a r r y ( 1 9 6 8 ) , who developed a method f o r measuring t h e amount o f r a d i o i o d i n e by d r a w i n g a i r through a s c r e e n , and then p e r f o r m i n g a r a d i o a s s a y o n the s c r e e n . I o d i n e atoms, h a v i n g a r e l a t i v e l y h i g h d i f f u s i o n c o e f f i c i e n t , d i f f u s e r e a d i l y from the a i r s t r e a m t o the s u r f a c e o f t h e w i r e s . I o d i n e atoms t h a t impinge on the w i r e s have a h i g h p r o b a b i l i t y o f s t i c k i n g due t o t h e i r c h e m i c a l a c t i v i t y . B a r r y p r e s e n t e d d a t a on the c o l l e c t i o n e f f i c i e n c y o f copper s c r e e n s f o r i o d i n e . F o l l o w i n g B a r r y , James e t a l (1972), and Thomas and H i n c h l i f f e (1972) i n v e s t i g a t e d the use o f w i r e screens f o r c o l l e c t i n g Po atoms o r i o n s . E x p e r i m e n t s were done i n the absense o f a e r o s o l p a r t i c l e s , y i e l d i n g c o l l e c t i o n e f f i c i e n c y as a f u n c t i o n o f s c r e e n dimensions and f a c e v e l o c i t y . I n f o r m a t i o n was developed on the f r a c t i o n o f d e p o s i t e d α-activity t h a t c o u l d be counted from the f r o n t and back s i d e s o f t h e s c r e e n s . George (1972) s e t t l e d o n a 60 mesh s t a i n l e s s s t e e l w i r e s c r e e n operated a t a f a c e v e l o c i t y o f 10-20 cm/sec as a means o f measuring the amount o f u n a t t a c h e d P o . Using f r e s h P o and no a e r o s o l , he found t h a t α-counting the f r o n t face o f the s c r e e n y i e l d e d 5 0 % o f the counts o b t a i n e d by α-counting a f i l t e r sample. I n t e s t s w i t h a c o n d e n s a t i o n n u c l e u s c o u n t e r he found t h a t the s c r e e n c o l l e c t e d o n l y a few p e r c e n t o f a e r o s o l p a r t i c l e s . T h i s l e d t o a s t a n d a r d t e c h n i q u e f o r measuring the unattached f r a c t i o n : t o sample s i m u l t a n e o u s l y on a 60 mesh s c r e e n and a f i l t e r and α-count b o t h ; extract P o d a t a from b o t h by m o d i f i e d Tsovoglou o r e q u i v a l e n t t e c h n i q u e ; m u l t i p l y the s c r e e n r e s u l t by 2 and d i v i d e by the r e s u l t from the f i l t e r . I n 1975 t h e r e was a new development i n the use o f w i r e s c r e e n s ; S i n c l a i r and Hoopes (1975) d e s c r i b e d a d i f f u s i o n b a t t e r y ( f o r measuring the p a r t i c l e s i z e o f a e r o s o l s ) made o f v e r y f i n e 635-mesh s t a i n l e s s s t e e l s c r e e n . An e m p i r i c a l e q u a t i o n was developed f o r t h e c o l l e c t i o n e f f i c i e n c y . T h i s d i f f u s i o n b a t t e r y has become one o f the s t a n d a r d t e c h n i q u e s i n a e r o s o l measurements. L a t e r , S i n c l a i r e t a l (1978) d e s c r i b e d a s c r e e n d i f f u s i o n b a t t e r y c o n f i g u r a t i o n s u i t e d f o r measuring the a c t i v i t y - w e i g h t e d s i z e d i s t r i b u t i o n o f radon daughter a e r o s o l s . Cheng and Yeh (1980) g r e a t l y f a c i l i t a t e d the use o f w i r e screens by showing t h a t a t h e o r y developed f o r f i b e r f i l t e r s a l s o worked f o r p r e d i c t i n g a e r o s o l p e n e t r a t i o n through 635-mesh w i r e s c r e e n s . The e q u a t i o n i s : 2 1 8

2 1 8

? 1 8

2 1 8

2

3

Ρ = exp (-A P e ~ / 1 where

(1)

β

Ρ p e n e t r a t i o n of screen Pe - 2a U/D a « w i r e r a d i u s (cm) U f l o w v e l o c i t y (cm/sec) D • d i f f u s i o n c o e f f i c i e n t o f p a r t i c l e (cm /sec) β

2


25.

HOLUB AND KNUTSON 10·56 ^^e*

1

π(1ποι)α


h

β

218

Measuring Po Diffusion-Coefficient Spectra f o r p a r a l l e l s t a g g e r e d c y l i n d e r model,

4.52

f o r f a n model,

2.51-3.35

for real

thickness of screen

volume o f s o l i d

4 msc

t o t a l volume

ir^hp

mass o f s c r e e n (g) s d » s c r e e n diameter (cm) ρ s » s c r e e n d e n s i t y (g cm"

343

filter. (cm)

u

£

u

s

3

)

The c o n s t a n t B, which i s a f u n c t i o n o f g e o m e t r i c arrangement, i s g i v e n by 2

l / 3

BC » 2.9(-0.5 I n α + α-0.25α - 0 . 7 5 ) * f o r p a r a l l e l s t a g g e r e d c y l i n d e r model, BF - 2.7 Β - 1.5-2.0

f o r f a n model, for real f i l t e r .

U s i n g t h i s e q u a t i o n , one c o u l d c a l c u l a t e t h e e f f e c t o f changing the s c r e e n geometry o r the f a c e v e l o c i t y . L a t e r papers by the same a u t h o r s showed by experiment t h a t the t h e o r y works w e l l f o r a v a r i e t y o f s c r e e n s and f a c e v e l o c i t i e s . P a r t i c l e s i z e s from .015 t o 0.5μ were t e s t e d . S c h e i b e l and P o r s t e n d o r f e r (1984) have shown i n d e p e n d e n t l y t h a t the Cheng-Yeh e q u a t i o n works w e l l down to particle size 4 nm. Samuelsson (1984) r e i n t r o d u c e d w i r e s c r e e n s as a means o| measuring the amount of unattached P o . He assumed 0.05 cm /sec f o r the d i f f u s i o n c o e f f i c i e n t , t h e n a p p l i e d the Cheng-Yeh e q u a t i o n to c a l c u l a t e the amount caught on the s c r e e n . An experiment l i k e t h a t o f James e t a l (1972) was done t o d e t e r m i n e α-activity c o u n t a b l e from the f r o n t f a c e o f the s c r e e n . F o r a 200 mesh s c r e e n at f a c e v e l o c i t y comparable t o our f a c e v e l o c i t i e s , Samuelsson found t h i s amount to be 80% of t h a t f o r the same amount of m a t e r i a l d e p o s i t e d on a f i l t e r . T h i s agreed w i t h t h e James e t a l (1972) r e s u l t f o r comparable f a c e v e l o c i t y . I n c o n c l u s i o n , i t i s c l e a r t h a t no s i z e s p e c t r a c o u l d be measured by the above t e c h n i q u e s f o r u n a t t a c h e d daughters and t h a t a b e t t e r t e c h n i q u e must be developed. a

2 1 8

New Work On

Screens

T e s t of the Cheng-Yeh E q u a t i o n A g a i n s t Old Data. As a l r e a d y mentioned, the Cheng-Yeh e q u a t i o n seems t o be w e l l e s t a b l i s h e d f o r a e r o s o l p a r t i c l e s i n the s i z e range 4 t o 200 nm. (A m o d i f i c a t i o n , the C h e n g - K e a t i n g - K a n a p i l l y e q u a t i o n , i s v a l i d f o r l a r g e r s i z e s . ) As a p r e l i m i n a r y s t e p towards s m a l l e r s i z e , we have t e s t e d the e q u a t i o n a g a i n s t t h e e x i s t i n g d a t a o f B a r r y (1968), James e t a l (1972) and Thomas and H i n c h l i f f e ( 1 9 7 2 ) . One o f the a u t h o r s , EOK, i s f o r t u n a t e t o have a c c e s s t o the o r i g i n a l T h o m a s - H i n c h l i f f e d a t a books; o t h e r d a t a p o i n t s were read o f f graphs a p p e a r i n g i n the c i t e d In Radon and Its Decay Products; Hopke, Philip K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

344


papers. Samples of the o r i g i n a l Thomas-Hinchliffe screens were weighed and measured to calculate the s o l i d fraction, α· For the other authors, we estimated α from the information given, mainly wire diameter and mesh s i z e . The best o v e r a l l f i t between the Cheng-Yeh equation and pooled data was obtained f o r a d i f f u s i o n c o e f f i c i e n t of 0.05 cm /sec for 218p (other values were t r i e d ) . The f i t , although not perfect, was remarkably good i n view of the very broad range of conditions (10-fold range i n both face v e l o c i t y and mesh size) covered by the pooled data. We conclude that the existing data generally support the Cheng-Yeh equation. 2


Q

Elementary Theory of the Front-to-Back q-count Ratio for Wire Screens. The use of wire screens for measurement of unattached f r a c t i o n of P o hinges on two factors. It i s necessary to know: how much P o deposits on the screen (as a function of d i f f u s i o n c o e f f i c i e n t and other parameters) and how this deposit i s distributed around each wire. The l a t t e r point i s important because α-particles from the decay of P o on the back of a wire cannot penetrate the wire to be counted from the front. Experiments to investigate these are discussed l a t e r . 2 1 8

2 1 8

2 1 8

Although the Cheng-Yeh equation appears to be adequate for describing t o t a l deposition on a screen, i t gives no information about the exact s i t e of deposition. To get some insight into the s i t e of deposition, we make use of an equation developed by Friedlander (1977): e 9π ^ layl

V

1

(APe) /

m

0 "

3

L45X

1

l

2

sin / xl

n o Q

/

3

where the l e f t hand i s the gradient of concentration (to which deposition rate i s proportional) at the point χ on the surface of the wire, and A Pe η»

s

β

β

a constant (value not needed i n our calculation) the Peclet number, defined e a r l i e r concentration f a r from the cylinder surface

xl * x/a, distance of the point on the surface from the front of the cylinder, measured i n units of cylinder radius, a 1

χ - / sin /

2

χ 1 .dxl

(3)

Figure 1 shows the geometry. S t r i c t l y speaking, equation 2 applies only to single cylinders i n slow cross flow, for point p a r t i c l e s with n e g l i g i b l e deposition and with the thickness of boundary layer much less than the radius of the cylinder. In order to study the implications of Equation 2, i t was evaluated at 80 points i n the range xl 0 to n. At xl • 0, L'Hospltal's rule from calculus was needed. For larger x l , Equation 3 was evaluated for each xl using trapezoid rule numerical integration, y i e l d i n g values f o r use i n Equation 2. It was found that the rate of deposition i s the highest for xl near zero, diminishing to zero at xl - π . s


25.

HOLUB AND KNUTSON

218


Direction of air f l o w

345

Scintillator

(I)


(2) COUNTING Deposition geometry

(I)

Region

of

alpha's,

(2)

R e g i o n of alpha's,

GEOMETRY forward

directed

angle = -rr-x,

radians

backward directed a n g l e = X| r a d i a n s .

F i g u r e 1. I l l u s t r a t i o n o f t h e geometry a s p e c t s f o r c a l c u l a t i n g the f r o n t - t o - b a c k a c t i v i t y r a t i o . To c a l c u l a t e c o u n t i n g e f f i c i e n c i e s f o r a l p h a p a r t i c l e s , a c o u n t i n g geometry f a c t o r i s needed as a f u n c t i o n o f x l . F i g u r e 1 i l l u s t r a t e s geometry f a c t o r s based on a s i m p l e , s i n g l e - c y l i n d e r model f o r the s c r e e n . We assume t h a t a l p h a p a r t i c l e e m i s s i o n i s i s o t r o p i c and t h a t a l l a l p h a p a r t i c l e s e m i t t e d w i t h a f o r w a r d component o f v e l o c i t y w i l l r e a c h a s c i n t i l l a t o r p l a c e d ( a f t e r s a m p l i n g ) on t h e f r o n t o f t h e s c r e e n . From t h i s i t f o l l o w s t h a t the geometry f a c t o r f o r c o u n t i n g from t h e f r o n t o f t h e s c r e e n i s ( x l -πφ)/π· By t h e same r e a s o n i n g , t h e geometry f a c t o r f o r a s c i n t i l l a t o r p l a c e d on t h e back s i d e o f t h e s c r e e n i s x l / a . By combining t h e c o u n t i n g geometry f a c t o r s w i t h t h e d e p o s i t i o n p a t t e r n s ( f r o m E q u a t i o n 2 ) , u s i n g an 80 p o i n t t r a p e z o i d r u l e , and i n t e g r a t i n g t h e s e two geometry f a c t o r s over t h e d e p o s i t on t h e c y l i n d e r y i e l d s 0.59 and 0.41 as t h e a l p h a a c t i v i t y c o u n t a b l e from t h e f r o n t and back, r e s p e c t i v e l y . The t h e o r e t i c a l f r o n t - t o - b a c k r a t i o i s t h e r e f o r e 1.44. In f a c t , t h e r e i s u s u a l l y a s u b s t a n t i a l drop i n Po c o n c e n t r a t i o n on p a s s i n g through t h e s c r e e n , so t h e d e p o s i t i o n on the w i r e s o f a s c r e e n I s u s u a l l y n o t n e g l i g i b l e . One would expect t h a t t h e b a c k - s i d e d e p o s i t i o n would be reduced, l e a d i n g t o a l a r g e r f r o n t - t o - b a c k r a t i o . T h i s i s p a r t i a l l y borne o u t by t h e experiments, discussed next. 2 1 8

New Measurement o f F r o n t - t o - B a c k R a t i o and L o s s e s . T a b l e 1 shows t h e p h y s i c a l c h a r a r a c t e r i s t i c s o f t h e s c r e e n s used i n t h i s work - two s e t s o f t h r e e screens f o r BOM and one s e t f o r EML. (BOM has r e c e n t l y added a f o u r t h s c r e e n t o each s e t . ) Note t h a t t h e two BOM s e t s have w i d e l y d i f f e r e n t c h a r a c t e r i s t i c s . We have found t h a t i t i s e s s e n t i a l t o measure p h y s i c a l p r o p e r t i e s c a r e f u l l y . The s o l i d f r a c t i o n was determined i n each case from t h e mass and t h i c k n e s s o f c i r c l e s c u t t o an a c c u r a t e l y known s i z e ( u s i n g a l a t h e i n t h e case o f BOM). The t h i c k n e s s was measured w i t h a micrometer. The mass was determined w i t h s t a n d a r d d e v i a t i o n about 0.5% (from t h r e e w e i g h i n g s ) . The w i r e d i a m e t e r and s c r e e n s ' mesh dimensions were measured from photomicrographs and from t h e s p e c i f i c a t i o n s g i v e n by m a n u f a c t u r e r s . The f r o n t - t o - b a c k measurement w i l l be d e s c r i b e d below. In Radon and Its Decay Products; Hopke, Philip K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987.


346

T a b l e 1.

Numerical Mesh ( i n " )


1

Screen

Characteristics

Solid Fraction

Counting Character istics F/B*

Measured Thickness

Mass p e r U n i t area

(cm)

g/cm

.0229 .0191 .0140

.052 .0431 .0296

.121 .0994 .0750

.298 .258 .319

2.45 2.55 2.62

.0089 .0036 .0020

.0188 .0067 .0050

.0569 .0152 .0131

.342 .287 .346

2.9 4.0 7.2

.016 .010 .004

.0356 .0259 .0135

.102 .063 .029

.36 .308 .275

2.55 2.88 3.35

Wire Diameter (cm)

2

α

BOM - Set 1 40 ( s s ) * * 60 ( c ) 80 ( s s ) BOM - Set 2 1*0 ( c ) 325 ( s s ) 635 ( s s ) ΕML Set ( s s ) 60 100 200

*At face, v e l o c i t y » 18-21 cm/sec, SL - 1.07 ± 0.09% p e r s c r e e n , u s i n g the f i r s t method ( p . 7) *F/B » 2.34 ± .32 and SL » 1.22 ± 0.08 per t h r e e s c r e e n s , u s i n g t h e second method ( p . 7) ss

a

stainless steel,

c • copper



25.

HOLUB AND KNUTSON

218


347

U n l i k e the p r e v i o u s BOM work, where many s c r e e n s were sampled c o n s e c u t i v e l y ( H o l u b , 1984) the two s e t s o f s c r e e n s were exposed and measured s i m u l t a n e o u s l y i n s e r i e s . The s c r e e n s were mounted on b r a s s r i n g s 1 mm t h i c k and the whole s t a c k l o a d e d i n t o an adapted f i l t e r h o l d e r . The s c r e e n s were p l a c e d g o i n g from lower c o a r s e s t t o f i n e s t mesh so t h a t f i r s t c a t c h e s the s m a l l e s t s i z e c l u s t e r s and so on, a s shown i n f i g u r e 2. C o r r e c t mounting o f the f i l t e r ( t h e l a s t i n the s t a c k ) was checked by measuring a p r e s s u r e drop a c r o s s the probe to d e t e c t any l e a k . I n o r d e r to t e s t the r e l i a b i l i t y o f the method the s e t s c o n s i s t e d of 40, 60, 80 and 120, 325, 635 mesh s c r e e n s w i t h the f i l t e r s a t the end of each s e t . The f a c e v e l o c i t y was kept around 18 cm/sec and the r a d i o a c t i v i t y on the s c r e e n s and t he f i l t e r s were measured u s i n g s t a n d a r d equipment (ZnS s c i n t i l l a t o r , PM t u b e , a u t o m a t i c c o u n t e r s ) by means o f the m o d i f i e d T s i v o g l o u method. The radon d a u g h t e r s measured were g e n e r a t e d from about 1000 p c i / 1 of radon w i t h v a r i o u s l e v e l s o f the c o n d e n s a t i o n n u c l e i (CN), h u m i d i t i e s and t r a c e gas c o n c e n t r a t i o n s . I n p e r f o r m i n g the measurements o f one s e t o f s c r e e n s a f t e r a n o t h e r , the u s u a l assumption o f c o n s t a n c y o f the main parameters o f the chamber was made ( H o l u b , 1984; D r o u l l a r d e t a l , 1984) so t h a t the r e s u l t s from a p p r o p r i a t e c o n s e c u t i v e measurements can be compared. T h i s assumption was proven c o r r e c t by r e p r o d u c i b i l i t y o f the r e s u l t s . I n o r d e r t o d e t e r m i n e the e f f i c i e n c y o f the s c r e e n s , the f r o n t - t o - b a c k r a t i o (F/B) o f a c t i v i t i e s and a l o s s f a c t o r (SL) by w h i c h one m u l t i p l i e s t o c o r r e c t f o r l o s s i n the s c r e e n s ( t h e a l p h a p a r t i c l e s absorbed i n a s c r e e n t h a t cannot be d e t e c t e d e i t h e r d u r i n g the f r o n t o r back measurements, see f i g u r e 1) have t o be d e t e r m i n e d . E x p e r i m e n t a l l y , t h i s was accomplished by two methods: The f i r s t method c o n s i s t e d of two measurements made s i m u l t a n e o u s l y s i d e - b y - s i d e , one w i t h one s c r e e n wrong s i d e o u t . The r e s u l t s f o r the F/B r a t i o s a r e shown i n f i g u r e 3. The e r r o r b a r s a r e t h a t from the c o u n t i n g s t a t i s t i c s o n l y because the o t h e r e r r o r s a r e n e g l i g i b l e . A g a i n assuming the c o n s t a n c y o f a l l r e l e v a n t parameters i n the chamber d u r i n g the experiment the l o s s f a c t o r s on one s c r e e n p l u s f i l t e r system were SL 1.07 ± 0.09. Any mesh s i z e dependency o f the l o s s f a c t o r s c o u l d not be a s c e r t a i n e d because o f the l a r g e e r r o r . The dependency o f F/B on mesh s i z e , however, i s e x p l i c i t and caused by the f a c t t h a t each s c r e e n i s s a m p l i n g the same ( s m a l l ) s i z e c l u s t e r s . These r e s u l t s , i n f a c t a r e not i n disagreement w i t h the r e s u l t s of method //2 i n d i c a t i n g t h e i r s i z e dependency. The r e a s o n s f o r F/B 500 mesh b e i n g h i g h e r t h a n 635 a r e not known. The second method on which most d a t a i n f i g u r e s 4a and 4b a r e based, c o n s i s t s o f making two c o n s e c u t i v e measurements, one of w h i c h had the f u l l s c r e e n s s e t (3 i n these experiments) r i g h t - s i d e - o u t w i t h r e s p e c t t o the f l o w d i r e c t i o n through the probe ( t h e f r o n t measurement) and the o t h e r had the s c r e e n s e t wrong-side-out w i t h r e s p e c t t o the f l o w d i r e c t i o n . Repeated p a i r s o f measurements (about 20) were made and the r a t i o found to be F/B = 2.34 ± .32 and the l o s s e s SL « 1.22 ± .08. Note t h a t 1.22 i s a p p r o x i m a t e l y e q u a l t o 1.07 , where 1.07 i s the s i n g l e s c r e e n SL f a c t o r . The apparent 58

3

American Chemical Society Library 1155 18th St., N.W.

In Radon and Its Decay Products; Hopke, Philip K.; Washington, D.C. 20036 ACS Symposium Series; American Chemical Society: Washington, DC, 1987.

In Radon and Its Decay Products; Hopke, Philip K.; ACS Symposium Series; American Chemical Society: Washington, DC, 1987. 0

Figure 2. BOM stacked screens probe. used f o r tight f i t

The rubber rings between screens are


00

Η

α

§

w Εζ -ο

α

> ο H

S

Ό

>

*5

00

25. HOLUB AND KNUTSON

218


349


14 13 F/g= F r o n t

I2|

500

II

and

to

back

ratio

635

mesh

have

double

pattern

500

10

Others

single

pattern

9 8 6 3

" > >7

ί

6

J400

5| 4

[80

3 2| I

1

ΙΟ"

10'-6

10" TOTAL

WIRE

10" AREA x THICKNESS, cm

10 5

F i g u r e 3. The f r o n t - t o - b a c k a c t i v i t y r a t i o as measured by method 1 as f u n c t i o n o f t h e t o t a l w i r e s u r f a c e a r e a times t h e t h i c k n e s s o f the s c r e e n . The numbers by t h e p o i n t s a r e mesh s i z e p e r i n c h . The e r r o r b a r s a r e c a l c u l a t e d from c o u n t i n g s t a t i s t i c s . The r e a s o n f o r 500 mesh h a v i n g h i g h e r F/B t h a n 635 mesh i s not u n d e r s t o o d .


10"

350


100 ρ

BOM, both sets, average of 6 measurements E M L , average of 4 measurements

i

CN< c m "

Same spectrum finer size

3


RH

Measuring Polonium-218 Diffusion-Coefficient Spectra Using Multiple

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