Development of a Mobility Analyzer for Studying the Particle

Chapter 26 Development of a Mobility Analyzer for Studying the Particle-Producing Phenomena Related to Radon Progeny 2

Downloaded by AUBURN UNIV on September 29, 2017 | http://pubs.acs.org Publication Date: February 5, 1987 | doi: 10.1021/bk-1987-0331.ch026

Lisa M. Kulju1, Kai-Dee Chu, and Philip K. Hopke3 1Department of Civil Engineering and Institute for Environmental Studies, University of Illinois, Urbana, IL 61801 2Department of Nuclear Engineering and Institute for Environmental Studies, University of Illinois, Urbana, IL 61801 Departments of Civil Engineering and Nuclear Engineering and Institute for

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Environmental Studies, University of Illinois, Urbana, IL 61801 The determination of the activity size distribution of the ultrafine ions is of particular interest due to their influence on the movement and deposition of Po-218. These ultrafine ions are the result of radiolysis and their rate of formation is a function of radon concen tration, the energy associated with the recoil path of Po-218, and the presence ofH2Ovapor and trace gases such as SO . A joint series of experiments utilizing a mobility analyzer, the separate single screen method, and the stacked screen method were conducted to examine the activity size distribution of the ultrafine mode. The results obtained from the mobility analyzer are dis cussed in this paper. Radon-222, a decay product of the naturally occuring radioactive element uranium-238, emanates from soil and masonry materials and is released from coal-fired power plants. Even though Rn-222 is an inert gas, its decay products are chemically active. Rn-222 has a a half-life of 3.825 days and undergoes four succesive alpha and/or beta decays to Po-218 (RaA), Pb-214 (RaB), Bi-214 (RaC), and Po-214 (RaC ). These four decay products have short half-lifes and thus decay to 22.3 year Pb-210 (RaD). The radioactive decays products of Rn-222 have a tendency to attach to ambient aerosol particles. The size of the resulting radioactive particle depends on the available aerosol. The attachment of these radionuclides to small, respirable particles is an important mechanism for the retention of activity in air and the transport to people. The ionic charge, diffusivity, and electrical mobility associated with these small radioactive particles are three parame ters controlling plateout. A particle can acquire an electrical charge by a number of mechanisms which promote the transfer of electrons to and from the particle surface, therefore producing a negatively and positively charged particle, respectively. With the decay of Rn-222, an alpha particle and Po-218 are formed. As these 2

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0097-6156/87/0331-0357$06.00/0 © 1987 American Chemical Society

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

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two p a r t i c l e s move i n o p p o s i t e d i r e c t i o n s , e l e c t r o n s a r e s t r i p p e d from the Po-218 by the d e p a r t i n g a l p h a p a r t i c l e o r by the r e c o i l motion. The i o n r e g a i n s most o f i t s e l e c t r o n s a s i t s l o w s t o r e s t , such t h a t SQ% o f t h e t i m e , Po-218 e x i s t s a s a s i n g l y charged, p o s i t i v e i o n . The r e m a i n i n g 12% o f the t i m e , Po-218 o c c u r s a s t h e n e u t r a l s p e c i e s ( W e l l i s c h , 1913; P o r s t e n d o r f e r and Mercer, 1979). These radon daughter i o n s have e l e c t r i c a l m o b i l i t i e s i n t h e same range as o r d i n a r y a t m o s p h e r i c i o n s and t h e i o n s can be d i v i d e d i n t o a number o f groups where each group has a p a r t i c u l a r m o b i l i t y ( N o l a n , 1916; M c C l e l l a n d and Nolan, 1926). B r i c a r d e t aJL (1966) r e p o r t s measurements f o r f i v e d i s t i n c t m o b i l i t y groups w i t h v a l u e s i n t h e range o f 0.4-2.2 cm s V f o r small radioactive ions i n a i r . Electrical Mobility I n p a s t s t u d i e s , t h e m o b i l i t y spectrum o f s m a l l r a d i o a c t i v e i o n s i n a i r was determined by t h e u t i l i z a t i o n o f two d i f f e r e n t s p e c t r o m e t e r s : a Z e l e n y s p e c t r o m e t e r and an E r i k s o n s p e c t r o m e t e r . I n t h e Z e l e n y s p e c t r o m e t e r , a i r e n r i c h e d w i t h radon o r t h o r o n i s drawn t h r o u g h a chamber and t h e m o b i l i t y o f t h e r e s u l t i n g i o n s i s determined by drawing the i o n s from t h e chamber i n t o a Z e l e n y - t u b e , a c y l i n d e r w i t h a n e g a t i v e l y charged e l e c t r o d e p o s i t i o n e d down t h e c e n t e r . The i o n s a r e s e p a r a t e d by t h e i r e l e c t r i c a l m o b i l i t i e s and the d i s t r i b u t i o n o f r a d i o a c t i v i t y a t t a c h e d t o t h e c e n t r a l w i r e i s used t o deduce t h e m o b i l i t y spectrum o f t h e s m a l l r a d i o a c t i v e i o n s . The a l p h a a c t i v i t y i s measured by t h e Renoux Method (Renoux, 1961) where t h e w i r e i s c u t i n t o e q u a l l e n g t h segments t h a t a r e p l a c e d between two ZnS s c i n t i l l ators. I n s t u d i e s by B l a n c e t a l ^ _ (1963), two m o b i l i t y groups i n a i r were observed by t h e u t i l i z a t i o n o f a Z e l e n y s p e c t r o m e t e r s u g g e s t i n g t h e e x i s t e n c e o f two t y p e s o f i o n s . The m o b i l i t y spectrum o f Po-216 and Pb-212 i o n s as d e t e r m i n e d by B l a n c (1963) s u g g e s t s t h a t t h e m o b i l i t y peaks a t 0.82 + 0 . 0 3 c m s " V " and 1.65 + 0 . 0 5 α η 3 ~ ' ν " c o r r e s p o n d t o a e r o s o l s t h a t c a r r y one and two e l e c t r i c a l c h a r g e s . I n the E r i k s o n s p e c t r o m e t e r ( E r i k s o n , 1922), t h e m o b i l i t y i s determined from the d i s t r i b u t i o n o f a c t i v i t y d e p o s i t e d on a n u c l e a r e m u l s i o n i n a p a r a l l e l p l a t e c o n f i g u r a t i o n r a t h e r t h a n onto a c o l l e c t i n g e l e c t r o d e as i n t h e Z e l e n y s p e c t r o m e t e r . The two p a r a l l e l , n u c l e a r p h o t o g r a p h i c p l a t e s a r e p o s i t i o n e d such t h a t t h e s e n s i t i v e s u r f a c e i s o r i e n t e d toward t h e i n t e r i o r . Experiments performed by B r i c a r d e t a l . (1966) u t i l i z e d a i r e n r i c h e d i n radon o r t h o r o n t o i n s u r e a s i g n i f i c a n t number o f t r a c k s on t h e p h o t o g r a p h i c p l a t e . The e x i s t e n c e o f two m o b i l i t y peaks was observed where the broad band had m o b i l i t i e s i n t h e range o f 0.3 t o 1.1 cm s " V~ and the narrow band had a m o b i l i t y peak c e n t e r e d around 2.1 cm s " V" . M o b i l i t y s p e c t r a u s i n g a n E r i k s o n s p e c t r o m e t e r and a Z e l e n y s p e c t r o m e t e r show f o u r m o b i l i t y peaks which i n d i c a t e f o u r d i f f e r e n t groups o f i o n s (Fontan » t 1969). The m a j o r i t y o f t h e s e e x p e r i m e n t s were performed under unknown c o n d i t i o n s i n terms o f the c o n c e n t r a t i o n s o f t r a c e gases i n the a i r and h u m i d i t y l e v e l s . W i t h unknown r e a c t i o n c o n d i t i o n s , the t r u e c h e m i c a l n a t u r e o f t h e m o l e c u l e s i s hard t o e l u c i d a t e . T h e r e f o r e , t h e r e i s a need f o r a w e l l c o n t r o l l e d system. 2

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Design A d i f f u s i o n chamber ( F i g u r e 1) o r i g i n a l l y d e s i g n e d f o r n e u t r a l i z a t i o n r a t e s t u d i e s by Chu and Hopke (1985) has been m o d i f i e d t o be an



26. KULJU ET AL.

Development of a Mobility Analyzer

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Figure 1.

The d i f f u s i o n chamber.


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RADON AND ITS DECAY PRODUCTS

Erikson-type spectrometer. The system i s composed o f two p a r a l l e l s t a i n l e s s s t e e l p l a t e s and two T e f l o n p l a t e s t h a t form a r e c t a n g u l a r chamber. Both ends o f the chamber a r e designed t o h o l d f i l t e r s which a r e a b l e t o c r e a t e a homogeneous f l o w f o r the gas as i t e n t e r s t h e chamber. A u n i f o r m e l e c t r i c f i e l d i s o b t a i n e d by c o n n e c t i o n t o a DC h i g h v o l t a g e . A c o n t i n u o u s m o n i t o r i n g process i s f e a s i b l e because a custom f a b r i c a t e d r e c t a n g u l a r s i l i c o n d e t e c t o r i s mounted i n t o the ground p l a t e . T h i s d e t e c t o r has been f a b r i c a t e d t o be as f l a t as p o s s i b l e a c r o s s i t s f a c e and t o have i t s s u r f a c e a t ground p o t e n t i a l . Thus, i t s presence i n the ground p l a t e does not p e r t u r b the e l e c t r i c field lines. The gas i s drawn through one opening f o l l o w e d by two g l a s s f i b e r f i l t e r s thus p e r m i t t i n g o n l y the radon and t e s t gas i n t o the chamber. P a r t o f the Po-218 formed i n the chamber w i l l d e p o s i t on the w a l l s by d i f f u s i o n . To i n s u r e the absence o f p l a t e d - o u t Po-218 a l p h a p a r t i c l e s i n the d e t e c t o r , the d i s t a n c e between t h e two m e t a l p l a t e s was designed t o be 6.0 cm. T h i s d i s t a n c e was chosen because Po-218 a l p h a p a r t i c l e s have a range o f 4.6 cm i n a i r and n i t r o g e n . Thus, a 6.0 cm d i s t a n c e i s l a r g e enough so t h a t no counts can come from the bottom p l a t e . T h i s d i f f u s i o n chamber was m o d i f i e d t o p r o v i d e a u n i f o r m f l o w from two channels a t the e n t r a n c e , one f o r the f i l t e r e d room a i r and the o t h e r f o r the gas from the radon chamber. T h i s m o d i f i e d m o b i l i t y a n a l y z e r i s s c h e m a t i c a l l y shown i n F i g u r e 2. The p r e s s u r e heads a r e a d j u s t e d so t h a t the gas v e l o c i t i e s , v, a r e the same i n both chan n e l s . An a d j u s t a b l e v e r t i c a l e l e c t r i c f i e l d , E, i s p r o v i d e d through the a n a l y z e r so t h a t charged p a r t i c l e s a r e drawn toward the d e t e c t o r l o c a t e d a t χ cm from the e n t r a n c e . With the known d i s t a n c e , d, between the r a d o n - l a d e n gas channel and the d e t e c t o r i m p l a n t e d p l a t e , the m o b i l i t y c a n then be d e t e r m i n e d from Κ = (vd)/(Ex)

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R e s u l t s and D i s c u s s i o n As p r e v i o u s l y mentioned, past s t u d i e s used n o n - f i l t e r e d a i r w i t h unknown c o n c e n t r a t i o n s o f t r a c e gases a t unknown r e l a t i v e h u m i d i t i e s . A l s o , many o f the s t u d i e s used p l a s t i c a g i n g chambers t h a t may have i n t r o d u c e d v o l a t i l e monomers i n t o the a i r . These unknown f a c t o r s a r e i m p o r t a n t t o determine i n o r d e r t o f u l l y u n d e r s t a n d the n a t u r e o f t h e u l t r a f i n e p a r t i c l e mode. A c c o r d i n g t o the c l a s s i c a l thermodynamic t h e o r y o f i o n c l u s t e r f o r m a t i o n (Coghlan and S c o t t , 1983), t h e r e l a t i v e h u m i d i t y and t r a c e gases w i l l a f f e c t the e x i s t e n c e o f c o n d e n s a t i o n n u c l e i . Megaw and W i f f e n (1961) observed an i n c r e a s e i n n u c l e i f o r m a t i o n w i t h the presence o f s u l f u r d i o x i d e . I n o r d e r t o examine the p r o c e s s o f u l t r a f i n e p a r t i c l e f o r m a t i o n , a j o i n t s e r i e s o f e x p e r i m e n t s were conducted a t the Denver Research C e n t e r o f the U.S. Bureau o f Mines. I n the Denver radon chamber, t h e a c t i v i t y s i z e d i s t r i b u t i o n o f the u l t r a f i n e mode was measured u s i n g the m o b i l i t y a n a l y z e r designed by Chu and Hopke (1985), the s e p a r a t e s i n g l e s c r e e n method (Holub and Knutson, 1987), and the s t a c k e d s i n g l e s c r e e n method (Holub and Knutson, 1987) f o r v a r i o u s r e l a t i v e h u m i d i t i e s and f o r v a r i o u s c o n c e n t r a t i o n s o f SO The r e s u l t s



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Development of a Mobility Analyzer

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Development of a Mobility Analyzer for Studying the Particle

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