Radon and Its Decay Products - American Chemical Society

Chapter 21

Downloaded by UNIV OF NEW SOUTH WALES on September 18, 2017 | http://pubs.acs.org Publication Date: February 5, 1987 | doi: 10.1021/bk-1987-0331.ch021

Plate-Out of Radon and Thoron Progeny on Large Surfaces J. Bigu Elliot Lake Laboratory, Canada Centre for Mineral and Energy Technology, Energy, Mines, and Resources Canada, P.O. Box 100, Elliot Lake, Ontario P5A 2J6, Canada The radon and thoron progeny plate-out characteristics of several materials have been investigated in a large (26 m) radon/thoron test facility (RTTF). Discs (about 0.5 mm thickness, and 25 mm diameter) were made up of different materials such as metals, plastics, filter materials, fabrics, and powders. The discs were placed on the wall, and/or horizontal trays, of the RTTF for 24 hours. After exposure the materials were removed from the RTTF and their surface α-activity was measured (differential α-count, gross α-count and aspectrometry). Experiments were also conducted using specially designed Ra-226 and Th-228 reference sources. Plate-out studies were conducted preferentially at low relative humidity and low aerosol concentrations. Significant differences in the measured surface α-activity were found between different materials. This work is relevant to the determination of radon and thoron progeny deposition velocities on, and attachment rates to, large surfaces. 3

The short-lived decay products of radon and thoron are initially formed in a positively charged atomic state of great diffusivity. These decay products readily attach themselves to small surfaces such as aerosols, and to large surfaces such as walls. Attachment of particles to large surfaces is commonly referred to as plate-out. The degree of plate-out depends on environmental conditions including aerosol concentration, air moisture content, the presence of tracer gases, and most probably, on the properties of the surface of the material. Although theoretical and experimental data are available on the attachment of the radon and thoron progeny to aerosols, less is known on the attachment to walls with regards to the chemical nature and physical characteristics of the wall surface. Data on the rate of attachment or deposition, i.e., plate-out of radioactive particles on walls can be used to calculate the particle deposition velocity. Deposition rates can be determined experimentally by measuring the surface activity on some samples 0097-6156/87/0331-0272$06.00/0 Published 1987 American Chemical Society

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


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designed for the purpose. These samples u s u a l l y assume the form of small 'discs' which are attached to the surface where plate-out i s to be determined. The samples are used because plate-out measurements carried out d i r e c t l y on the surface of interest may not be p r a c t i c a l , and i n most cases quite d i f f i c u l t . Furthermore, because of p r a c t i c a l c o n s i d e r a t i o n s , a l i m i t e d number of m a t e r i a l s are used f o r t h i s purpose under the assumption that the mechanism, or extent, of deposition i s independent of the type of material used. As there i s i n s u f f i c i e n t evidence to support or deny t h i s view, work in this area is relevant. This paper deals with the plate-out c h a r a c t e r i s t i c s of a v a r i e t y of m a t e r i a l s such as metals, p l a s t i c s , f a b r i c s and powders to the decay products of radon and thoron under l a b o r a t o r y - c o n t r o l l e d conditions. In a previous paper, the author reported on measurements on the attachment r a t e and d e p o s i t i o n v e l o c i t y of radon and thoron decay products (Bigu, 1985). In these experiments, s t a i n l e s s s t e e l d i s c s and f i l t e r paper were used. At the time, the assumption was made that the s u r f a c e α - a c t i v i t y measured was independent of the chemical and physical nature, and conditions, of the surface on which the products were deposited. The present work was p a r t l y aimed at v e r i f y i n g t h i s assumption. Experimental Procedure 3

Experiments were conducted i n a l a r g e (-26 m ) radon/thoron t e s t f a c i l i t y (RTTF) designed f o r c a l i b r a t i o n purposes and s i m u l a t i o n studies (Bigu, 1984). A number of d i f f e r e n t materials were exposed i n the RTTF to a radon/radon progeny or thoron/thoron progeny atmosphere. Exposure of the m a t e r i a l s was c a r r i e d out under l a b o r a t o r y - c o n t r o l l e d c o n d i t i o n s of r a d i a t i o n l e v e l , aerosol concentration, a i r moisture content and temperature. The materials used were in the form of c i r c u l a r 'discs' of the same thickness (-0.5 mm) and diameter (-25 mm), and they were p l a c e d at d i f f e r e n t l o c a t i o n s on the w a l l s of the RTTF at about 1.6 m above the f l o o r . Other samples were p l a c e d on h o r i z o n t a l t r a y s . Samples ( d i s c s ) of d i f f e r e n t m a t e r i a l s were arranged i n sets of 3 to 4; they were placed very close to one another to ensure exposure under i d e n t i c a l c o n d i t i o n s . Exposure time was at l e a s t 24 hours to ensure s u r f a c e a c t i v i t y equilibrium, or near equilibrium, conditions. A f t e r a g i v e n exposure time, the d i s c s were removed from the RTTF and t h e i r surface α - a c t i v i t y was measured as a function of time, up to 2 hours, using standard radiation instrumentation and methods. Counting of samples began 2 min a f t e r t h e i r removal from the RTTF. For the thoron progeny, samples were counted for 15 min followed by 5 min counts with 1 min i n t e r v a l between counts. For the radon progeny two d i f f e r e n t counting routines were used, namely: 2-min counts with 1-min i n t e r v a l , and 5-min counts with 1-min i n t e r v a l . The former counting routine was intended for more precise Po-218 measurements. In a d d i t i o n , gross α-count (30 min counts) and α-spectrometry measurements were also done. The s u r f a c e α - a c t i v i t y on the samples f o r each exposed set of samples was compared. Each set of samples was exposed, and measured, a minimum of about f i v e times to improve o v e r a l l s t a t i s t i c s . Experiments were conducted i n the temperature range 19-22°C and at low r e l a t i v e humidity (10 to 40%). The aerosol concentration, mainly


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n a t u r a l atmospheric a e r o s o l s , was kept low, i.e., 5 χ 10* to 9 χ 10 cm . No attempt was made to maintain the aerosol concentration at any p a r t i c u l a r l e v e l at these low aerosol concentrations. The values a t t a i n e d were those due to c o n t r o l l e d make-up u n f i l t e r e d a i r , p u r p o s e l y l e a k i n g i n t o the t e s t f a c i l i t y used for the experiments. The aerosol concentration varied with barometric pressure. The range of r a d i a t i o n c o n d i t i o n s i n the RTTF was as f o l l o w s . Radon gas c o n c e n t r a t i o n , [Rn-222]: 40-200 pCi/L (1.48 χ 10 - 7.4 χ 10 Bq/m ; radon progeny Working Level, WL(Rn) : 20-400 mWL; thoron gas concentration [Rn-220]: 350-1700 pCi/L (1.295 χ 10 - 6.29 χ 10 Bq/m ); thoron progeny Working L e v e l , WL(Tn): 0.15-14 WL. (The square brackets are used here to denote a c t i v i t y concentration.) The materials investigated included s e v e r a l metals (copper, Cu; aluminum, A l ; s t a i n l e s s s t e e l , SS; and g a l v a n i z e d s t e e l , GS); f i l t e r m a t e r i a l s (Glass F i b e r , M i l l i p o r e and Varsopor); plastic materials (Plexiglas); 'powders' ( a c t i v a t e d carbon, talcum); and other materials such as sandpaper, emery c l o t h and several fabrics. A c t i v a t e d carbon and talcum powder samples were prepared by d e p o s i t i n g a t h i n l a y e r (-0.5 mm) of the powder on f i l t e r paper on which some binding material (glue) had been added to i t s surface to retain the powder. Further plate-out studies were conducted using radon progeny and thoron progeny r e f e r e n c e s o u r c e s , m o d e l s Rn-190 and Th-190, respectively, manufactured by Pylon E l e c t r o n i c Development (Ottawa), h e r e a f t e r r e f e r r e d to as P y l o n sources, f o r s i m p l i c i t y . These are small c y l i n d r i c a l containers ( Hopke; Radon and Its Decay Products ACS Symposium Series; American Chemical Society: Washington, DC, 1987.


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BIGU

Plate-Out of Radon and Thoron Progeny on Large Surfaces

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Radon and Its Decay Products - American Chemical Society

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