- Email: [email protected]
On indoor radon contamination monitoring with plastic nuclear track detectors: The influence of plate-out effect
Nucl. Tracks Radiat. Meas., Vol. 15, Nos. 1-4, pp. 539-542, 1988 Int. J. Radiat. Appl. Instrurn., Part D Printed in Great Britain
0191-278X/89 $3.00 + .00 ~ 1989 Pergamon Press pie
ON INDOOR R A D O N CONTAMINATION MONITORING WITH PLASTIC N U C L E A R T R A C K DETECTORS: THE INFLUENCE OF P L A T E - O U T EFFECT Giulio Bigazzi 1 , J u l i o C . H a d l e r N. 2 and S e r g i o R . P a u l o 2 iIstituto di G e o c r o n o l o g i a e G e o c h i m i c a Isotopica, 56100 Pisa, Italy 2Instituto de F I s i c a "Gleb Wataghin", Campinas, SP, B r a z i l
UNIC~MP,
CNR,
13100
A b s t r a c t - It was p e r f o r m e d m e a s u r e m e n t of the radon-222 and daughters a l p h a - a c t i v i t y inside a glass r e c i p i e n t employing SSNTDs. The results of these m e a s u r e m e n t s i n d i c a t e no negligible i n f l u e n c e of radon d a u g h t e r s plate-out on the surfaces b o t h of m a t e r i a l s s u r r o u n d i n g the detectors and of the detectors themselves. In this w o r k some aspects of the influence of the p l a t e - o u t e f f e c t (in absence of ventilation) w e r e studied. K E Y W O R D S
-
Radon-222 and d a u g h t e r s detection; free from v e n t i l a t i o n .
p l a t e - o u t effect;
conditions
i. I N T R O D U C T I O N Effects of the presence of Rn-222 and d a u g h t e r s atoms in i n d o o r atmospheres are s t u d i e d since many years. To e s t i m a t e the lung dose o r i g i n a t e d from Rn-222 and d a u g h t e r s i n h a l a t i o n the a l p h a - a c t i v i t y f r o m Rn-222, P o - 2 1 8 and Po-214 d e s i n t e g r a t i o n in the air is u s u a l l y estimated el,ployingsolid state n u c l e a r track detectors. It is k n o w n 1'2 that radon d a u g h t e r atoms in the alr has a fraction of free atoms and o t h e r a t t a c h e d to aerosols and also that the two component parts of these fractions can d e p o s i t on surfaces of indoor materials. This d i s t u r b s the h o m o g e n e i t y of radon d a u g h t e r distribution in the air and as c o n s e q u e n c e p r o d u c e s difficulties to transform % r a c k s / c m 2 on SSNTDs in a l p h a - a c t i v i t y p r e s e n t in the air. 2. E X P E R I M E N T A L P R O C E D U R E A glass r e c i p i e n t of 5.75 liters c o u n t a i n i n g a a t m o s p h e r e of Rn-222 and d a u g h t e r s o r i g i n a t e d from the e m a n a t i o n of a 3 m i c r o - C i Ra-226 source have been e m p l o y e d in SSNT~s r e s p o n s e m e a s u r e m e n t s . The inner radius of this recipient is 8.6cm and the t o t a l area of i n n e r w a l l s is 1 , 6 0 0 c m 2. In o r d e r to test the h o m o g e n e i t y of a l p h a e m i s s o r s / c m 3 i n s i d e this r e c i p ~ ent s o l i d state n u c l e a r track detectors (CR-39 - A m e r i c a n Acrylics; LR-II5 type II and K0 Ilford n u c l e a r emulsions) w e r e exposed there at d i f f e r e n t places during about one week. The c h e m i c a l d e v e l o p m e n t of the p l a s t i c d e t e c t o r s e m p l o y e d by us is: CR-39 sheets e t c h e d in a 6 . 2 5 N NaOH s o l u t i o n at (70.0 ± 0 . 5 ) 9 C for 400min.; LR-II5 sheets in a 2.5N N a O H s o l u t i o n at (60.0 ± 0.5)9C for 100min. (in these conditions, the tracks in the samDleS of LR-II5 have very regular edges 3 and few ones (<10%) p e r f o r a t e the s e n s i b l e layer of this detector). It was o b s e r v e d a s y s t e m a t i c t e n d e n c y that detectors placed near the central region of the r e c i p i e n t d e t e c t e d more alpha tracks than those juxtap o s e d to the walls. This m e a n t that the alpha-activity was not homogeneo u s l y d i s t r i b u t e d inside the recipient. The r e l a t i o n s h i p b e t w e e n the n u m b e r of a l p h a - e m i s s o r s p e r u n i t a r y volume and the d i s t a n c e of this v o l u m e to the w a l l s was investigated, firstly, exposing inside the r e c i p i e n t a strip of CR-39 of 17.2cm in lenght t o u t c h i n g two d i a m e t r i c a l l y o p p o s e d p o i n t s on the walls. If the n u m b e r of alpha e m i s s o r s / c m 3 was h o m o g e n e o u s l y d i s t r i b u t e d inside the recipient, it w a s e x p e c t e d less t r a c k s / c m 2 on the r e g i o n of strip near the w a l l s than on its central region w h e r e the solid angle for alpha-particle r e g i s t r a t i o n is not l i m i t e d by the p r e s e n c e of the w a l l s . It was obs e r v e d that, inside the limits of the e x p e r i m e n t a l errors, the number of t r a c k s / c m 2 was n e a r l y c o n s t a n t on the w h o l e strip. M a k i n g t h e o r e t i c a l c a l c u l a t i o n s taking into a c c o u n t the e m p l o y e d exposition g e o m e t r y it was checked that (inside the limits of the experimental
539
540
G. BIGAZZI et al.
errors) the decrease of the s o l i d angle for a l p h a - p a r t i c l e r e g i s t r a t i o n near the walls was compensated by an e x t r a a l p h a - a c t i v i t y c o n s t i t u t e d by radon daughters atoms p l a t e d - o u t on the walls. To investigate the p l a t e - o u t e f f e c t on the performance of the SSNTDs, sheets of LR-II5, CR-39 and n u c l e a r e m u l s i o n s w e r e e x p o s e d at d i f f e r e n t distances , d, from the walls. All the d e t e c t o r s r e m a i n e d w i t h its back sides turned towards the walls. The f i g u r e 1 shows track d e n s i t i e s ( b a c k g r o u n d already subtracted) on sheets of n u c l e a r e m u l s i o n (l-a), CR-39 (l-b), LR-II5 (l-c) exposed to the Rn-222 e m a n a t i o n d u r i n g 5.9 days and on sheets of CR-39 (l-d) exposed during 7.8 days. It's easy to o b s e r v e that all detectors placed near the recipient w a l l s p r e s e n t less t r a c k s / c m 2 t h a n far w a l l ones. 3. E X P E R I M E N T A L DATA A N A L Y S I S Our experimental r e s u l t s can o n l y be u n d e r s t o o d if it is supposed that a fraction of the daughters of r a d o n - 2 2 2 p r e s e n t e d in the air p l a t e - o u t on the w a l l s of the employed r e c i p i e n t . In o t h e r words, the r e c i p i e n t w a l l s behave as an "absorver" of radon d a u g h t e r s . T h e r e f o r e the a l p h a - a c t i v i t y from radon daughters in the air d e c r e a s e s in the w a l l surroundings producing less tracks/cm 2 on the d e t e c t o r s p l a c e d there than on t h o s e p l a c e d far from the walls. The LR-II5 response is less "disturbed" by the p r e s e n c e of the w a l l s than the response of the o t h e r d e t e c t o r s . This fact is expected because the LR-II5 has a low energy limit of r e g i s t r a t i o n to a l p h a - p a r t i c l e s . At our etching conditions a l p h a - p a r t i c l e s emitted at low d i s t a n c e s f r o m this detec tor do not produces o b s e r v a b l e tracks, t h e r e f o r e a LR-II5 p l a c e d on the walls only detect a l p h a - p a r t i c l e s emitted at a certain d i s t a n c e from the w a l l s where the p l a t e - o u t i n f l u e n c e can be low. The decrease of track d e n s i t y o b s e r v e d on the detectors exposed occurs only at distances to the w a l l s less than 2cm. This " m a x i m u m effective distance" for the influence of the radon d a u g h t e r s p l a t e - o u t on the r e c i p i e n t w a l l s is e x p e c t e d to be a f u n c t i o n of the m e a n v e l o c i t y of the radon daughters in the air (~). Since t h e y are m o v i n g w i t h a mean v e l o c i t y ~ (and are placed, in a time t ± 0 in a D o i n t w h e r e d is the d i s t a n c e to the walls) it is n e c e s s a r y a mean t i m e (~) to their collision w i t h some point of the walls. If this t is h i g h w h e n it's c o m p a r e d w i t h the m e a n lifes of the radon daughters there's no s i g n i f i c a n t e f f e c t of r e m o t i o n of radon d a u g h t e r s by deposition on the r e c i p i e n t w a l l s (we b e l i e v e that this occurs, in our condi tions, for values of d e x c e d i n g 2cm). W i t h regard to ~, since a f r a c t i o n of the Rn-222 d a u q h t e r atoms moves in the air a t t a c h e d to a e r o s o l s p a r t i c l e s I ~ w i l l be a f u n c t i o n of the v e l o c i t y d i s t r i b u t i o n both of the a e r o s o l s as of the free radon d a u g h t e r atoms. In this m a n n e r from figure 1 we c o n c l u d e d that, in our c o n d i t i o n s , the radon d a u g h t e r p l a t e - o u t on the r e c i p i e n t w a l l s causes a "deplection" on the a l p h a - a c t i v i t y w h i c h is s i g n i f i c a n t at d i s t a n c e s f r o m the walls under the "maximum e f f e c t i v e d i s t a n c e " . Radon d a u g h t e r p l a t e - o u t on d i f f e r e n t m a t e r i a l s Samples of CR-39 w e r e e x p o s e d j u x t a p o s e d to sheets of approx. 70cm 2 of some d i f f e r e n t m a t e r i a l s (glass, wood, aluminium, r i g i d PVC, paper and g l a s e d tile). The ratio of the t r a c k d e n s i t y o b s e r v e d on these CR-39 samples to the track d e n s i t y o b s e r v e d on CR-39 samples e x p o s e d in the central region of the r e c i p i e n t (background a l r e a d y subtracted) is s h o w e d in table 1 for 3 different experiences. TABLE EXPERIENCE
RIGID PVC PAPER WOOD GLASED TILE GLASS ALUMINIUM
0.66~0.05 0.62±0.05 0.63±0.05 0.62±0.05 0.66±0.05 0.73±0.05
1
1 EXPERIENCE
0.73±0.06 0.74±0.06 0.68±0.05 0.74±0.06 0.60±0.06 0.71±0.05
2
EXPERIENCE
J
0.57±0.05 0.63±0.05 0.63±0.05 0.58±0.05 0,71±0.06
tracks/cm 2 All CR-39 s a m p l e s j u x t a p o s e d to some m a t e r i a l p r e s e n t e d less This result than the CR-39 e x p o s e d in the central region of the recipient. daughter i n d i c a t e that, in p r i n c i p l e , any m a t e r i a l w i l l b e h a v e as a radon detectors "absorver" influencing the a l p h a tracks r e g i s t r a t i o n of the p l a c e d in their s u r r o u n d i n g s . "Self-plate-out" Like any o t h e r m a t e r i a l the surfaces of the SSNTDs t h e m s e l v e s can also b e h a v e as an " a b s o r v e r " of the radon d a u g h t e r s p r e s e n t in the air. M e a s u r e m e n t s of the n u m b e r of the Rn-222 d a u g h t e r s p l a t e d - o u t on differ-
INDOOR RADON MONITORING
541
~7 o x 6 c~ ~5 4¸ NUCLEAR EMULSION! texp = 5,9days I(~ )
3.5. 3.2
I
2.9
I
2.6
2.3 I 2.0 CR-39 texp = 5.gdays
(b)
1.6 1.4 1.2~
I
t
t
1.0 LR-II5 texp = 5.9days
(C)
4.4, 4.2, 4.0" 3.8 3.6 3.4 3.2 3.0
t
texD = 7.8days i
!
2
o
!
4
!
I
6
!
!
8
.
p
d (cm)
F i g u r e i: track d e n s i t i e s on S S N T D s e x p o s e d at d i f f e r e n t distances , d, to the r e c i p i e n t walls.
H'~ 1 5 : Z / 4 , - J J
542
G. BIGAZZI et al.
ent materials (the same w h i c h w e r e u t i l i z e d to o b t a i n the table l)and on CR-39, LR-II5 and n u c l e a r e m u l s i o n d e t e c t o r s w e r e effected. T h e s e materials and d e t e c t o r samples s t a y e d inside the 5.75 liters r e c i p i e n t for about one week. When the recipient was o p e n e d n u c l e a r e m u l s i o n sheets were rapidly juxtaposed to those m a t e r i a l s (including the detectors) during 4 h o u r s to detect alpha-particles o r i g i n a t e d f r o m decay of the radon d a u g h t e r s platedout on their surfaces. In some p r e l i m i n a r m e a s u r e m e n t s , it is o b s e r v e d that all the emulsions (including those p l a c e d on CR-39, LR-II5 and emulsion sheets~ present track d e n s i t i e s b e t w e e n 200 and 800 tracks/cm 2 (background already subtracted). This r e s u l t i n d i c a t e that radon d a u g h t e r atoms plateout on the detector s u r f a c e s as w e l l as on the s t u d i e d m a t e r i a l surfaces. Although this p r e l i m i n a r r e s u l t could be u t i l i z e d to c a l c u l a t e the n u m b e r of radon daughter atoms p l a t e d - o u t on the m a t e r i a l s and detectors, we can't e s t i m a t e the radon d a u g h t e r d e p o s i t i o n rate b e c a u s e we d o n ' t k n o w yet the fraction of radon d a u g h t e r atoms a t t a c h e d to a e r o s o l s just b e f o r e the recipient opening. Even in cases that d e t e c t o r s can be e x p o s e d far f r o m any m a t e r i a l object the radon and daughter m o n i t o r i n g is not free of p l a t e - o u t effects: The radon daughters deposition on the d e t e c t o r s u r f a c e s ("self-plate-out") produces a deplection on the a l p h a - a c t i v i t y present in the air in the surroundings of the detectors. In this m a n n e r the r e l a t i o n s h i p between the track density p r e s e n t e d by a S S N T D e x p o s e d in a certain indoor atmosphere and the total a l p h a - a c t i v i t y in this a t m o s p h e r e can't b e o b t a i n e d supposing a h o m o g e n e o u s spatial d i s t r i b u t i o n of a l p h a e m i s s o r s in the air. The total track density observed on the surface of a SSNTD w i l l be a sum of three parcels: i) The number of t r a c k s p r o d u c e d on the d e t e c t o r s u r f a c e by Rn-222 a l p h a - a c t i v i t y which is h o m o g e n e o u s l y d i s t r i b u t e d in the air; 2) the number of tracks p r o d u c e d by P o - 2 1 8 and P o - 2 1 4 a l p h a - a c t i v i t y in the air w h i c h is "deplected" in the d e t e c t o r s u r r o u n d i n g s (due to the p r e s e n c e there of material, objects i n c l u d i n g the d e t e c t o r s t h e m s e l v e s ) ; 3) the n u m b e r of tracks p r o d u c e d by the Po-218 and Po-214 a c t i v i t y p l a t e d - o u t on the d e t e c t o r surface during the e x p o s i t i o n time. 4. C O N C L U S I O N S In conditions free f r o m v e n t i l a t i o n it is o b s e r v e d t h a t radon daughters p l a t e - o u t effect can not be n e g l e c t e d w h e n SSNTDs are e m p l o y e d to estimate the Rn-222 and d a u g h t e r s a c t i v i t y in air. In short the e x p e r i m e n t a l results o b t a i n e d here can be s u m m a r i z e d as: i) From table 1 it can be seen that SSNTDs e x p o s e d j u x t a p o s e d to the surfaces of m a t e r i a l o b j e c t s p r e s e n t less t r a c k s / c m 2 than far ones; 2) D e t e c t o r s p l a c e d u n d e r a " m a x i m u m e f f e c t i v e d i s t a n c e " from a given material object are "disturbed" by radon d a u g h t e r s p l a t e - o u t on this object; 3) The "self-plate-out" also i n f l u e n c e s m e a s u r e m e n t s of a l p h a - a c t i v i t y of radon and d a u g h t e r s p r e s e n t in the air. In cases w h e r e the s e l f - p l a t e - o u t is high also is h i g h the d e p l e c t i o n on the a l p h a - a c t i v i t y of the radon daughters in the air s u r r o u n d i n g the detector. It is k n o w n that the v e n t i l a t i o n i n c r e a s e s the i n d o o r radon d a u g h t e r dep o s i t i o n rate 2. In p l a c e s w h e r e the v e n t i l a t i o n is a f a c t o r to be c o n s i d e r e d b o t h the p l a t e - o u t as the s e l f - p l a t e - o u t s h o u l d e x e r c e a i n f l u e n c e more sign i f i c a n t than in our c o n d i t i o n s . Radon daughters can d e p o s i t on any material. This fact indicates that some care must be taken in d e t e c t i o n of Rn-222 and d a u g h t e r s a c t i v i t y in the air even in cases that o t h e r d e t e c t o r s (different of SSNTDs) are employed. ACKNOWLEDGEMENTS The authors express t h e i r g r a t i t u d e to F A P E S P - S P - B r a z i l and to B i l a t e r a l A g r e e m e n t C N P q - B r a z i l / C N R - I t a l y w h i c h f i n a n c e d p a r t i a l l y this paper. REFERENCES 1. W . J a c o b i - H e a l t h Physics, voi.22, p p . 4 4 1 - 4 5 0 (1972). 2. K.D.Cliff, A . D . W r i x o n , B . M . R . G r e e n and J . C . H . M i l e s - H e a l t h Physics, voi.45, n92, pp. 323-330 (1983). 3. G . B i g a z z i , J . C . H a d l e r , A . L . F . M a r q u e s and S . R . P a u l o - N u c l e a r Tracks, vol.12, n.l-6, pp. 713-716 (1986).
0191-278X/89 $3.00 + .00 ~ 1989 Pergamon Press pie
ON INDOOR R A D O N CONTAMINATION MONITORING WITH PLASTIC N U C L E A R T R A C K DETECTORS: THE INFLUENCE OF P L A T E - O U T EFFECT Giulio Bigazzi 1 , J u l i o C . H a d l e r N. 2 and S e r g i o R . P a u l o 2 iIstituto di G e o c r o n o l o g i a e G e o c h i m i c a Isotopica, 56100 Pisa, Italy 2Instituto de F I s i c a "Gleb Wataghin", Campinas, SP, B r a z i l
UNIC~MP,
CNR,
13100
A b s t r a c t - It was p e r f o r m e d m e a s u r e m e n t of the radon-222 and daughters a l p h a - a c t i v i t y inside a glass r e c i p i e n t employing SSNTDs. The results of these m e a s u r e m e n t s i n d i c a t e no negligible i n f l u e n c e of radon d a u g h t e r s plate-out on the surfaces b o t h of m a t e r i a l s s u r r o u n d i n g the detectors and of the detectors themselves. In this w o r k some aspects of the influence of the p l a t e - o u t e f f e c t (in absence of ventilation) w e r e studied. K E Y W O R D S
-
Radon-222 and d a u g h t e r s detection; free from v e n t i l a t i o n .
p l a t e - o u t effect;
conditions
i. I N T R O D U C T I O N Effects of the presence of Rn-222 and d a u g h t e r s atoms in i n d o o r atmospheres are s t u d i e d since many years. To e s t i m a t e the lung dose o r i g i n a t e d from Rn-222 and d a u g h t e r s i n h a l a t i o n the a l p h a - a c t i v i t y f r o m Rn-222, P o - 2 1 8 and Po-214 d e s i n t e g r a t i o n in the air is u s u a l l y estimated el,ployingsolid state n u c l e a r track detectors. It is k n o w n 1'2 that radon d a u g h t e r atoms in the alr has a fraction of free atoms and o t h e r a t t a c h e d to aerosols and also that the two component parts of these fractions can d e p o s i t on surfaces of indoor materials. This d i s t u r b s the h o m o g e n e i t y of radon d a u g h t e r distribution in the air and as c o n s e q u e n c e p r o d u c e s difficulties to transform % r a c k s / c m 2 on SSNTDs in a l p h a - a c t i v i t y p r e s e n t in the air. 2. E X P E R I M E N T A L P R O C E D U R E A glass r e c i p i e n t of 5.75 liters c o u n t a i n i n g a a t m o s p h e r e of Rn-222 and d a u g h t e r s o r i g i n a t e d from the e m a n a t i o n of a 3 m i c r o - C i Ra-226 source have been e m p l o y e d in SSNT~s r e s p o n s e m e a s u r e m e n t s . The inner radius of this recipient is 8.6cm and the t o t a l area of i n n e r w a l l s is 1 , 6 0 0 c m 2. In o r d e r to test the h o m o g e n e i t y of a l p h a e m i s s o r s / c m 3 i n s i d e this r e c i p ~ ent s o l i d state n u c l e a r track detectors (CR-39 - A m e r i c a n Acrylics; LR-II5 type II and K0 Ilford n u c l e a r emulsions) w e r e exposed there at d i f f e r e n t places during about one week. The c h e m i c a l d e v e l o p m e n t of the p l a s t i c d e t e c t o r s e m p l o y e d by us is: CR-39 sheets e t c h e d in a 6 . 2 5 N NaOH s o l u t i o n at (70.0 ± 0 . 5 ) 9 C for 400min.; LR-II5 sheets in a 2.5N N a O H s o l u t i o n at (60.0 ± 0.5)9C for 100min. (in these conditions, the tracks in the samDleS of LR-II5 have very regular edges 3 and few ones (<10%) p e r f o r a t e the s e n s i b l e layer of this detector). It was o b s e r v e d a s y s t e m a t i c t e n d e n c y that detectors placed near the central region of the r e c i p i e n t d e t e c t e d more alpha tracks than those juxtap o s e d to the walls. This m e a n t that the alpha-activity was not homogeneo u s l y d i s t r i b u t e d inside the recipient. The r e l a t i o n s h i p b e t w e e n the n u m b e r of a l p h a - e m i s s o r s p e r u n i t a r y volume and the d i s t a n c e of this v o l u m e to the w a l l s was investigated, firstly, exposing inside the r e c i p i e n t a strip of CR-39 of 17.2cm in lenght t o u t c h i n g two d i a m e t r i c a l l y o p p o s e d p o i n t s on the walls. If the n u m b e r of alpha e m i s s o r s / c m 3 was h o m o g e n e o u s l y d i s t r i b u t e d inside the recipient, it w a s e x p e c t e d less t r a c k s / c m 2 on the r e g i o n of strip near the w a l l s than on its central region w h e r e the solid angle for alpha-particle r e g i s t r a t i o n is not l i m i t e d by the p r e s e n c e of the w a l l s . It was obs e r v e d that, inside the limits of the e x p e r i m e n t a l errors, the number of t r a c k s / c m 2 was n e a r l y c o n s t a n t on the w h o l e strip. M a k i n g t h e o r e t i c a l c a l c u l a t i o n s taking into a c c o u n t the e m p l o y e d exposition g e o m e t r y it was checked that (inside the limits of the experimental
539
540
G. BIGAZZI et al.
errors) the decrease of the s o l i d angle for a l p h a - p a r t i c l e r e g i s t r a t i o n near the walls was compensated by an e x t r a a l p h a - a c t i v i t y c o n s t i t u t e d by radon daughters atoms p l a t e d - o u t on the walls. To investigate the p l a t e - o u t e f f e c t on the performance of the SSNTDs, sheets of LR-II5, CR-39 and n u c l e a r e m u l s i o n s w e r e e x p o s e d at d i f f e r e n t distances , d, from the walls. All the d e t e c t o r s r e m a i n e d w i t h its back sides turned towards the walls. The f i g u r e 1 shows track d e n s i t i e s ( b a c k g r o u n d already subtracted) on sheets of n u c l e a r e m u l s i o n (l-a), CR-39 (l-b), LR-II5 (l-c) exposed to the Rn-222 e m a n a t i o n d u r i n g 5.9 days and on sheets of CR-39 (l-d) exposed during 7.8 days. It's easy to o b s e r v e that all detectors placed near the recipient w a l l s p r e s e n t less t r a c k s / c m 2 t h a n far w a l l ones. 3. E X P E R I M E N T A L DATA A N A L Y S I S Our experimental r e s u l t s can o n l y be u n d e r s t o o d if it is supposed that a fraction of the daughters of r a d o n - 2 2 2 p r e s e n t e d in the air p l a t e - o u t on the w a l l s of the employed r e c i p i e n t . In o t h e r words, the r e c i p i e n t w a l l s behave as an "absorver" of radon d a u g h t e r s . T h e r e f o r e the a l p h a - a c t i v i t y from radon daughters in the air d e c r e a s e s in the w a l l surroundings producing less tracks/cm 2 on the d e t e c t o r s p l a c e d there than on t h o s e p l a c e d far from the walls. The LR-II5 response is less "disturbed" by the p r e s e n c e of the w a l l s than the response of the o t h e r d e t e c t o r s . This fact is expected because the LR-II5 has a low energy limit of r e g i s t r a t i o n to a l p h a - p a r t i c l e s . At our etching conditions a l p h a - p a r t i c l e s emitted at low d i s t a n c e s f r o m this detec tor do not produces o b s e r v a b l e tracks, t h e r e f o r e a LR-II5 p l a c e d on the walls only detect a l p h a - p a r t i c l e s emitted at a certain d i s t a n c e from the w a l l s where the p l a t e - o u t i n f l u e n c e can be low. The decrease of track d e n s i t y o b s e r v e d on the detectors exposed occurs only at distances to the w a l l s less than 2cm. This " m a x i m u m effective distance" for the influence of the radon d a u g h t e r s p l a t e - o u t on the r e c i p i e n t w a l l s is e x p e c t e d to be a f u n c t i o n of the m e a n v e l o c i t y of the radon daughters in the air (~). Since t h e y are m o v i n g w i t h a mean v e l o c i t y ~ (and are placed, in a time t ± 0 in a D o i n t w h e r e d is the d i s t a n c e to the walls) it is n e c e s s a r y a mean t i m e (~) to their collision w i t h some point of the walls. If this t is h i g h w h e n it's c o m p a r e d w i t h the m e a n lifes of the radon daughters there's no s i g n i f i c a n t e f f e c t of r e m o t i o n of radon d a u g h t e r s by deposition on the r e c i p i e n t w a l l s (we b e l i e v e that this occurs, in our condi tions, for values of d e x c e d i n g 2cm). W i t h regard to ~, since a f r a c t i o n of the Rn-222 d a u q h t e r atoms moves in the air a t t a c h e d to a e r o s o l s p a r t i c l e s I ~ w i l l be a f u n c t i o n of the v e l o c i t y d i s t r i b u t i o n both of the a e r o s o l s as of the free radon d a u g h t e r atoms. In this m a n n e r from figure 1 we c o n c l u d e d that, in our c o n d i t i o n s , the radon d a u g h t e r p l a t e - o u t on the r e c i p i e n t w a l l s causes a "deplection" on the a l p h a - a c t i v i t y w h i c h is s i g n i f i c a n t at d i s t a n c e s f r o m the walls under the "maximum e f f e c t i v e d i s t a n c e " . Radon d a u g h t e r p l a t e - o u t on d i f f e r e n t m a t e r i a l s Samples of CR-39 w e r e e x p o s e d j u x t a p o s e d to sheets of approx. 70cm 2 of some d i f f e r e n t m a t e r i a l s (glass, wood, aluminium, r i g i d PVC, paper and g l a s e d tile). The ratio of the t r a c k d e n s i t y o b s e r v e d on these CR-39 samples to the track d e n s i t y o b s e r v e d on CR-39 samples e x p o s e d in the central region of the r e c i p i e n t (background a l r e a d y subtracted) is s h o w e d in table 1 for 3 different experiences. TABLE EXPERIENCE
RIGID PVC PAPER WOOD GLASED TILE GLASS ALUMINIUM
0.66~0.05 0.62±0.05 0.63±0.05 0.62±0.05 0.66±0.05 0.73±0.05
1
1 EXPERIENCE
0.73±0.06 0.74±0.06 0.68±0.05 0.74±0.06 0.60±0.06 0.71±0.05
2
EXPERIENCE
J
0.57±0.05 0.63±0.05 0.63±0.05 0.58±0.05 0,71±0.06
tracks/cm 2 All CR-39 s a m p l e s j u x t a p o s e d to some m a t e r i a l p r e s e n t e d less This result than the CR-39 e x p o s e d in the central region of the recipient. daughter i n d i c a t e that, in p r i n c i p l e , any m a t e r i a l w i l l b e h a v e as a radon detectors "absorver" influencing the a l p h a tracks r e g i s t r a t i o n of the p l a c e d in their s u r r o u n d i n g s . "Self-plate-out" Like any o t h e r m a t e r i a l the surfaces of the SSNTDs t h e m s e l v e s can also b e h a v e as an " a b s o r v e r " of the radon d a u g h t e r s p r e s e n t in the air. M e a s u r e m e n t s of the n u m b e r of the Rn-222 d a u g h t e r s p l a t e d - o u t on differ-
INDOOR RADON MONITORING
541
~7 o x 6 c~ ~5 4¸ NUCLEAR EMULSION! texp = 5,9days I(~ )
3.5. 3.2
I
2.9
I
2.6
2.3 I 2.0 CR-39 texp = 5.gdays
(b)
1.6 1.4 1.2~
I
t
t
1.0 LR-II5 texp = 5.9days
(C)
4.4, 4.2, 4.0" 3.8 3.6 3.4 3.2 3.0
t
texD = 7.8days i
!
2
o
!
4
!
I
6
!
!
8
.
p
d (cm)
F i g u r e i: track d e n s i t i e s on S S N T D s e x p o s e d at d i f f e r e n t distances , d, to the r e c i p i e n t walls.
H'~ 1 5 : Z / 4 , - J J
542
G. BIGAZZI et al.
ent materials (the same w h i c h w e r e u t i l i z e d to o b t a i n the table l)and on CR-39, LR-II5 and n u c l e a r e m u l s i o n d e t e c t o r s w e r e effected. T h e s e materials and d e t e c t o r samples s t a y e d inside the 5.75 liters r e c i p i e n t for about one week. When the recipient was o p e n e d n u c l e a r e m u l s i o n sheets were rapidly juxtaposed to those m a t e r i a l s (including the detectors) during 4 h o u r s to detect alpha-particles o r i g i n a t e d f r o m decay of the radon d a u g h t e r s platedout on their surfaces. In some p r e l i m i n a r m e a s u r e m e n t s , it is o b s e r v e d that all the emulsions (including those p l a c e d on CR-39, LR-II5 and emulsion sheets~ present track d e n s i t i e s b e t w e e n 200 and 800 tracks/cm 2 (background already subtracted). This r e s u l t i n d i c a t e that radon d a u g h t e r atoms plateout on the detector s u r f a c e s as w e l l as on the s t u d i e d m a t e r i a l surfaces. Although this p r e l i m i n a r r e s u l t could be u t i l i z e d to c a l c u l a t e the n u m b e r of radon daughter atoms p l a t e d - o u t on the m a t e r i a l s and detectors, we can't e s t i m a t e the radon d a u g h t e r d e p o s i t i o n rate b e c a u s e we d o n ' t k n o w yet the fraction of radon d a u g h t e r atoms a t t a c h e d to a e r o s o l s just b e f o r e the recipient opening. Even in cases that d e t e c t o r s can be e x p o s e d far f r o m any m a t e r i a l object the radon and daughter m o n i t o r i n g is not free of p l a t e - o u t effects: The radon daughters deposition on the d e t e c t o r s u r f a c e s ("self-plate-out") produces a deplection on the a l p h a - a c t i v i t y present in the air in the surroundings of the detectors. In this m a n n e r the r e l a t i o n s h i p between the track density p r e s e n t e d by a S S N T D e x p o s e d in a certain indoor atmosphere and the total a l p h a - a c t i v i t y in this a t m o s p h e r e can't b e o b t a i n e d supposing a h o m o g e n e o u s spatial d i s t r i b u t i o n of a l p h a e m i s s o r s in the air. The total track density observed on the surface of a SSNTD w i l l be a sum of three parcels: i) The number of t r a c k s p r o d u c e d on the d e t e c t o r s u r f a c e by Rn-222 a l p h a - a c t i v i t y which is h o m o g e n e o u s l y d i s t r i b u t e d in the air; 2) the number of tracks p r o d u c e d by P o - 2 1 8 and P o - 2 1 4 a l p h a - a c t i v i t y in the air w h i c h is "deplected" in the d e t e c t o r s u r r o u n d i n g s (due to the p r e s e n c e there of material, objects i n c l u d i n g the d e t e c t o r s t h e m s e l v e s ) ; 3) the n u m b e r of tracks p r o d u c e d by the Po-218 and Po-214 a c t i v i t y p l a t e d - o u t on the d e t e c t o r surface during the e x p o s i t i o n time. 4. C O N C L U S I O N S In conditions free f r o m v e n t i l a t i o n it is o b s e r v e d t h a t radon daughters p l a t e - o u t effect can not be n e g l e c t e d w h e n SSNTDs are e m p l o y e d to estimate the Rn-222 and d a u g h t e r s a c t i v i t y in air. In short the e x p e r i m e n t a l results o b t a i n e d here can be s u m m a r i z e d as: i) From table 1 it can be seen that SSNTDs e x p o s e d j u x t a p o s e d to the surfaces of m a t e r i a l o b j e c t s p r e s e n t less t r a c k s / c m 2 than far ones; 2) D e t e c t o r s p l a c e d u n d e r a " m a x i m u m e f f e c t i v e d i s t a n c e " from a given material object are "disturbed" by radon d a u g h t e r s p l a t e - o u t on this object; 3) The "self-plate-out" also i n f l u e n c e s m e a s u r e m e n t s of a l p h a - a c t i v i t y of radon and d a u g h t e r s p r e s e n t in the air. In cases w h e r e the s e l f - p l a t e - o u t is high also is h i g h the d e p l e c t i o n on the a l p h a - a c t i v i t y of the radon daughters in the air s u r r o u n d i n g the detector. It is k n o w n that the v e n t i l a t i o n i n c r e a s e s the i n d o o r radon d a u g h t e r dep o s i t i o n rate 2. In p l a c e s w h e r e the v e n t i l a t i o n is a f a c t o r to be c o n s i d e r e d b o t h the p l a t e - o u t as the s e l f - p l a t e - o u t s h o u l d e x e r c e a i n f l u e n c e more sign i f i c a n t than in our c o n d i t i o n s . Radon daughters can d e p o s i t on any material. This fact indicates that some care must be taken in d e t e c t i o n of Rn-222 and d a u g h t e r s a c t i v i t y in the air even in cases that o t h e r d e t e c t o r s (different of SSNTDs) are employed. ACKNOWLEDGEMENTS The authors express t h e i r g r a t i t u d e to F A P E S P - S P - B r a z i l and to B i l a t e r a l A g r e e m e n t C N P q - B r a z i l / C N R - I t a l y w h i c h f i n a n c e d p a r t i a l l y this paper. REFERENCES 1. W . J a c o b i - H e a l t h Physics, voi.22, p p . 4 4 1 - 4 5 0 (1972). 2. K.D.Cliff, A . D . W r i x o n , B . M . R . G r e e n and J . C . H . M i l e s - H e a l t h Physics, voi.45, n92, pp. 323-330 (1983). 3. G . B i g a z z i , J . C . H a d l e r , A . L . F . M a r q u e s and S . R . P a u l o - N u c l e a r Tracks, vol.12, n.l-6, pp. 713-716 (1986).