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Application of nuclear track detectors for radon related measurements
0191-278X/89 $3.00 + .00 © 1989Pergamon Presspie
Nucl. Tracks Radiat. Meas., Vol. 15, Sos. 1-4, pp. 525--534, 1988 Int. J. Radiat. Appl. lnstrum., Part D
Printed in Great Britain
APPLICATION OF NUCLEAR TRACK DETECTORS F O R R A D O N R E L A T E D MEASUREMENTS FALAH
A. A B U - J A R A D
ENERGY RESOURCES D I V I S I O N / R E S E A R C H I N S T I T U T E KING FAHD U N I V E R S I T Y OF P E T R O L I U M AND M I N E R A L S P.O.Box 2020, D H A H R A N 31261, SAUDI ARABIA.
ABSTRACT Personal experience in the application of nuclear track detectors for radon related measurements are discussed. The paper summarizes previously published w o r k . The " C a n T e c h n i q u e " used for measuring radon emanation from building materials, walls and soil.nWorking Level Monitor" used for measuring Working levels of radon daughters in houses in a short period. "Passive radon dosimeters" used to measure radon levels in houses for long term (few months) periods. Application of nuclear track detectors for measuring the radon daughters plate-out on s u r f a c e s of mixing fan blades and walls are discussed. Uranium content of some wall papers were f o u n d t o b e 6 ppm. V a r i a t i o n of radon progeny concentration in same room is measured and supported by another study through Gas Chromatograh measurements. Independent of radon concentration on r o o m , s level in highrise buildings is concluded. Effect of Sub-floor radon emanation on radon concentration in houses is depend on wether there is sub-floor ventilation or not.
Applications, Can dosimeter, Plate-out,
KEY WORDS Technique, Working Level Monitor, Fan, paint, surfaces, floors.
Passive
Radon
INTRODUCTION Nuclear track detectors have been applied in the last few years by many authors for measuring radon and progeny activities in houses. Reviewing of all their published work in different conferences and journals is worthwhile to be done. Since too much time is needed to perform this type of work. I thought it might be useful to summarize previously published work related to radon and progeny measurements in which I have participated. For more details and method of calculations it would be worthwhile to refer to the original papers.
Different the need
TECHNIQUES, APPLICATIONS AND RESULTS techniques were established and applied in different arose. A summary of these follow:
ways
as
For long-term measurements of radon exhalation from building materials or walls a "Can technique" was developed 1. In this a track detector CR-39 was placed in u small, impervious vessel and sealed to a typical part of the wall or individual bricks, Figure 1. The d e t e c t o r was placed at 2 in from the surface of the wall or brick, hidden by a diaphragm (250 polystyrene) but freely exposed to the emergent radon. It records the decay of radon in the outer part of the can plus the decay of Po-218 and Po-214 |rlck or wall deposited on the inner walls of the can. This would reach equilibrium concentration after a week or so; hence knowing the geometry of the Fig. l. The Can Technique system and time of exposure ~1, 1 5 ~ 1 / 4 - z i
525
526
F.A. ABU-JARAD
(3 m o n t h s obtained.
or more), t h e To m e a s u r e t h e C V d/ A
equilibrium activity of emergent radon could be exhalation r a t e t h e f o l l o w i n g e q u a t i o n w a s used:
E =
T + (I/d) Where
e -dT-
(I/d)
E: C:
is the r a d o n e x h a l a t i o n p e r u n i t a r e a a n d t i m e (Bq. m -2 . h -1 ) is the integrated radon exposure as measured by the CR-39 detector ( B q .m - 3 . h ) V: i s t h e v o l u m e o f t h e c a n (m z ) d: i s t h e d e c a y c o n s t a n t of radon (h -1 ) A: i s t h e a r e a c o v e r e d b y t h e c a n ( m2 ) By s t i c k i n g three or more "Cans" to different walls of each room, a good estimation of the exhalation rate from the walls to the inside air of the room can be made. This technique applied in three different cases as follow:
The can technique was applied to compare the emanation rate from different building materials as shown in Figure 2. It shows that the hidden detectors give more tracks than the detector facing the bricks, while in the clay bricks the opposite happened. This mean that the emanation from the granite bricks escaped from the depth of the brick, while in the clay brick it emanates from the surface only. This does not correlate with the porosity measurements which indicate that the clay brick absorbed more water than the granite ones. This measurement gave the volume of the pores while it is the internal surface area which determines the amount of radon can escape. To c l a r i f y this, bricks from the two types were cut into slices with different thickness and, applying the "Can technique" for each slice. These results are shown in Figures 3 and 4 which show clearly that increasing the thickness of clay bricks does not increase the count as it does from the crushed granite.
KIO~
_7 2 ~=sooo
I
li.D"h 2~m
iE
Brick
~" .c /,O00
I
2
[]f C32
~3ooQ e~
c
e 2000
-
._c Iooo
f 3
< ~g
time
brick
Fig.
2.
~icks mode Gronfte
from crdshed Mock stone or from granite Aberdeen
Oifforent clny txlcks
CoolorlcKSl(~s
12 ~i ~0 ~SO 220 Thicknesses of brick (ram)
Fig. 3. Radon emanation for various thicknesses of crushed graint bricks. Hat~hed and plain areas as figure 2.
Radon emanation from different building materials. Case hatched areas, CR-39 detectors facing the brick surface; plain areas, CR-39 facing away from the surface.
1, case
2,
RADON RELATED MEASUREMENTS
|r~],,,o.t.~'o'
--12
527
~'
I~p~inted brick
£
200o i.I
t
O~
lsoo
o
O5
2
1
4
8
•
Fig. 5. Two Cans on the same b r i c k c o v e r the same area in two d i f f e r e n t c o n d i t i o n s : a: with u n p a i n t e d b r i c k and b: w i t h p a r t i a l l y p a i n t e d brick.
32
Thicknesll#$ @ brick (cm)
Fig. B.
4.
Radon emanation for various thicknesses Hatched and plain a r e a s as f i g u r e 2.
Effect
of
internal
wall
cover
on
radon
of
coal
emanation
slag inside
brick. houses:
The
the radon emanation z. Eleven new, bare bricks were selected from crushed granite and two cans were sealed on t h e s u r f a c e of each brick. After 79 days, the CR-39 detectors were collected and stored. Then every brick was coated with one primer coat and 4 coats of different types of paints on a l l s i d e s of the brick except the areas where the cans were sealed previously. Then, the experiments were repeated again f o r 97 d a y s ( i e . a fresh CN-39 d e t e c t o r was sealed inside each can, covering the unpainted areas). The representative diagram is shown in Figure 5. All the detectors stored and the new ones, were etched together in the same beaker. The r e s u l t s of these are shown in Table l, which shows that the number of alpha tracks on CN-39 detectors used with the partially painted bricks was more than with the bare bricks by factors ranging f r o m 3 t o 14 with an average of 7. This tends to indicate that the c o n c e n t r a t i o n of radon increases inside the p a i n t e d brick itself leading to an increase in the e m a n a t i o n from the u n p a i n t e d area u n d e r the can. The effect of paint, in general, is to reduce the radon e m a n a t i o n from the brick surface, which also lead to increase the radon concentration inside the brick. Paint (No. 9) i s e p o x y p a i n t which shows that the emanation increased under the can by a factor o f 12 w h e n i t partially painted. This means that this paint is efficient in reducing the emanation from the painted area. This was proved to be true in another experiment. Table I. Number of ~-trecks cm - z p e r 79 d a y s o n b o t h CR-39 detectors within two Cans sealed to the same unpainted areas on every brick in two different conditions:(a) with bare brick, (b) w i t h p a r t i a l l y painted b r i c k (see Fig. 5) paint
No.
1 2 3 4 5 6 7 8 9 10
(a) bare b r i c k (2.1, (3.4, (3.6, (5.9, (2.7, (6.2, (2.9, (2.3, (2.0, (2.9,
(b) p a r t i a l l y
2.1)xlO s 2.7)x103 8.4)x103 2.5)xlOS 2.1)xl0S 2.9)x10 s 4.7)x10 s 3.1)xlO s 2.4)x10 s 4.7)x103
(18.6, (20.8, (48.9, (12.1, (15,2, (23.0, (15.5, (19.6, (24.8, (18.3,
18.9)x10 s 18.O)xlO s 50.5)x10 s 9.6)x10 s 15.5)x103 23.3)x10 s 16.3)x10 s 24.9)x10 s 27.9)x10 s 23.9)x10 s
av.
3xlO 3
Av. r a d o n * concent. Bq.m -3
167
llO0
Av. e x h a l a t i o n rate Bq . h - Z . m - ~
0.06
0.40
Calculated
according
to
22x
equation
6 in
painted
lO s
reference
b/a 9, 6, 14, 2, 6, 4, 5, 9, 12, 6, 7
No 1 .
9 7 6 4 7 8 4 8 12 fi
528
F.A. ABU-JARAD
C. E m a n a t i o n f r o m e x i s t i n ~ . w a l l R i n _ d w e _ l l S n g s : T w o or three nCans" were sealed to different walls o f 30 m e a s u r e d rooms for a period of 3 months or Bore to estiaate the exhalation rate of radon from the already existing walls. The s u m m a r y o f t h e r e s u l t s as shown in Table 2. Table
2.
City
Summary o f No. o f Houses
results No. o f to the
of
wall
exhalation
Cans sealed walls
measurementsa.
Activity under C a n s ( B q . m- a )
Exhalation (Bq.m-i-h -1)
Dhahran Riyadh Jeddah
21 6 10
55 12 28
1.0 3.3 2.0
± 0.1 ± 1.2 ± 0.5
0.35 1.2 0.7
± 0.05 ± 0.5 ± 0.2
Total
37
95
1.6
± 0.3
0.6
± 0.1
Radon emanation from uranium ore: Some h o u s e s built on uranium ore on Orkeny Island/U.K., where the uranium concentration varied between 500 to 1 2 0 0 ppm 4 . T h e ~Can t e c h n i q u e N w a s a p p l i e d to find the level of radon emanation from sub-floor soil and what the effect of this is on the radon level inside houses. The radon daughters concentration inside the houses were measured by using the Working Level Monitor technique ( to be described latter in this paper). The cans were buried a t a d e p t h o f 30 cm below the soil surface in the garden of each measured house s . The radon concentration inside the Cans was found to vary from 0.26-22 KBq.m - 3 w i t h an average of 8.5 ± 7.0 KBq.m - 3 , see Table 3. The average equivalent exhalation rate was found to be 6.8 ± 5.6 KBq.m-Z.h -1If the average exhalation rate is used to estimate the total amount of radon emanated from the sub-floor soil of a house covering an area o£ soil equal t o 10 x 10 mz , t h e r a d o n e m a n a t e d from the soil under the house should b e 680 KBq . h - 1 . A s s t m i n g t h e v o l u m e o f t h e h o u s e i s 3 0 0 m31 t h e n t h e level of radon.m -3 should be around 2.5 KBq.m - 3 , taking into consideration the derived air concentration of the equilibrium-equivalent Rn-222 concentration = 1 . 5 KBq.m -3 ( I C R P ) e . Thus, the calculated value is equal to nearly double the derived limit, while the measured level of radon daughters in these h o u s e s w a s 2 . 5 mWL ( = 9 B q . m - 3 ) w h i c h i s e q u i v a l e n t to ordinary houses built in ordinary soil. The emanation from the sub-floor soil was not able to enter the houses because there was a space (about 0.5 m) u n d e r the houses and because strong winds blow throughout the year. Figure 6 shows the expected fate of emanated radon from the subsoil.
D.
Table 3. The radon concentration inside cans buried in the garden's soil of houses built the Orkeny uranium ore and their exhalation Rouse No. 1 2 3 4 5 6 7 8 9 10 11 Average
Radon conc. inside the can gBq.m -3 3.7 20.7 4.2 10.2 9.2 0.3 4.0 6.9 6.9 5.1 22.5 8 . 5 +- 7
Exhalation rate K B q . m - Z . h -1 2.8 16.4 3.3 8.1 7.4 0.2 3.0 5.5 5.5 4.1 17.9 6.75 + 5.6
on rate ~
'. i / I Fig. 6. The emanated radon from sub-soil flow with the sub-floor ventilation
RADON RELATED MEASUREMENTS
529
2. P~ ~ = . 9 _ ~ k _ ~ . . ~ ~ @ ~ The plate-out follows:
of
radon
daughters
were
studied
for
different
surfaces
as
A~_.On__m~xinK_fan__blades_~andwall__surfeces~The nuclear track detectors also used to measure the e f f e c t of a i r t u r b u l e n c e i n s i d e the r o o m s on the radon progeny. A running air m i x i n g fan was f o u n d to r e d u c e the W o r k i n g Level (WL) in air of the r o o m v-13. The r e d u c t i o n in radon d e c a y p r o d u c t s was m a i n l y due to p l a t e - o u t on i n t e r n a l s u r f a c e s in the r o o m (for m o r e details see another p a p e r in this p r o c e e d i n g ) z3. To m e a s u r e the r a t i o of p l a t e - o u t activity on the w a l l s u r f a c e s and on the fan b l a d e s CR-39 detectors were used. S e v e r a l T r a c k d e t e c t o r s w e r e s e a l e d onto the front and back face of every b l a d e of the fan u s i n g d o u b l e - s i d e d c e l l o t a p e . At the same time and for the s a m e p e r i o d o v e r w h i c h the fan was used, C R - 3 9 d e t e c t o r s w e r e exposed in d i f f e r e n t p o s i t i o n s on the walls of the room. The results of this s t u d y s h o w e d that the n u m b e r of radon d a u g h t e r s w h i c h are p l a t e d - o u t per unit a r e a on the two sides of the b l a d e s of the fan are more than those p l a t e d - o u t per u n i t a r e a on the s u r f a c e of the w a l l s by a f a c t o r of 10 on the average. B e c a u s e the s u r f a c e area of the fan blades is n e g l i g i b l e in c o m p a r i s o n of the s u r f a c e a r e a of the walls, the main d e c r e a s e in a i r b o r n e r a d o n d a u g h t e r s is due to p l a t e - o u t on the surfaces of the walls of the room. T h e p l a t e - o u t ratio d e p e n d s on the n u m b e r of aerosol p a r t i c l e s in the air of the room. It i n c r e a s e s w i t h by the r e d u c t i o n of the n u m b e r of a e r o s o l p a r t i c l e s . B. The d e ~ c ~ a t o r experiment. A n o t h e r e x p e r i m e n t was d e s i g n e d to s t u d y the p l a t e - o u t of radon d a u g h t e r s on t r a c k d e t e c t o r s s u r f a c e s T M . F i g u r e 7 shows the d e s i g n of the e x p e r i m e n t . The Track detectors were placed inside the chamber in t h r e e p o s i t i o n s ( f a c i n g upwards, downwards and s i d e w a y s a w a y from the wall). By i n j e c t i n g same a m o u n t of radon in the c h a m b e r and for the same p e r i o d , t h e o n l y v a r i a b l e was the type and n u m b e r of a e r o s o l p a r t i c l e s .
v.,._ , = - - Tj
T,
.J Warn
Fig.
nec:Jl
7. The a r r a n g e m e n t for i n t r o d u c i n g the radon into the d e s i c c a t o r w h i c h show the p o s i t i o n of the p l a s t i c as follows: a- f a c i n g upwards, b- f a c i n g d o w n w a r d s , c - f a c i n g sidways.
The experiment was repeated three times and the results are shown in Table 4. The conclusion of those experiments showed that response of detectors facing downwards and sideways are less than those facing u p w a r d s in the p r e s e n c e of s m o k e p a r t i c l e s , w h i l e all are equal in the absence of the smoke. This is explained because of less plate out in the presence of smoke particles registered by sideways and downwards d e t e c t o r s . The i n c r e a s e d r e s p o n s e on the u p w a r d s d e t e c t o r s is due to the settling of smoke particles on the surface of the detectors. The equal response of all detectors in absence of smoke is because of equal plateout on all of them without regard to the position of the detectors.
530
F.A. ABU-JARAD
Table 4. Comparison of track d e n s i t i e s o b t a i n e d under varying experimental conditions. No of Exp.
experimental conditions
I.
with smoke from KMnO4
i ii
2.
with smoke from one cigarette
3.
CR-39 cond.
Track
densities
Upwards
without smoke
on C R - 3 9
xl0 ~
detectors
(cm -z)
Downwards
Sideways
(4.8 !0.2) (0.9 ± 0 . 0 4 )
(2.5 ± 0 . 1 ) (0.2 ± 0 . 0 1 )
(2.7 ±0.1) (0.2 ±0.01)
i ii
(4.7 ± 0 , 1 ) (0.9 ±0.03)
(3.7 ± 0 . I ) (0.3 ± 0 . 0 l )
(3.6 ±0.I) (0.3 ±0.I)
i ii
(3.4 ±0.I) (0.4 ±0.02)
(3.7 ± 0 . 1 ) (0.5 ± 0 . 0 2 )
(3.7 ±0.I) (0.5 ±0.02)
(i) CR-39 detectors p l a c e d in d i f f e r e n t sides t h r o u g h the e x p e r i m e n t . (ii) new CR-39 placed on the s u r f a c e of (i) d e t e c t o r s i m m e d i a t e l y after f i n i s h i n R their e x p o s u r e to r e c o r d the p l a t e d - o u t radon p r o g e n y on its surfaces.
3.~....W.o.~.n,~...le.~.~_L_~i.to~ N u c l e a r track detectors w e r e used to m o n i t o r radon levels inside houses as a s e m i - a c t i v e technique. The d e s i g n of the w o r k i n g level m o n i t o r is shown in Figure 8. Track d e t e c t o r s LR-115 are used w i t h an a i r - s a m p l e r and glass fiber filter zs. A Known v o l u m e of air is d r a w n t h r o u g h a glass microfiber filter at a rate of 65 liter per minute. Parallel to and d i r e c t l y front of this is a disc of LR-115. This is s e p a r a t e d from the filter by a spacer ring 2 mm in t h i c k n e s s w i t h 56 r a d i a l l y d r i l l e d holes of I mm diameter. The outer d i a m e t e r of the ring is 32 mm and the inner d i a m e t e r is 25 mm. The latter b e i n g the e f f e c t i v e d i a m e t e r of both the filter and plastic. I
I II -...,~...,w . . . . . ~
I
(a)
(d)
Fig. 8:(a) C r o s s s e c t i o n a l view of W L - M o n i t o r head. (b) The a s s e m b l e d r e p l a c e a b l e unit (c) The a s s e m b l e d WL-Honitor (d) The alpha track holes in L R - 1 1 5 s h o w i n g the s e p a r a t i o n b e t w e e n e n e r g i e s 6 Mev (left) and 7.7 May (right)
RADON RELATED MEASUREMENTS
531
The filter paper, LR-115 plastic and the spacer are clamped together as a single unit. This unit can be exchanged with another unit to be used in the head of the air sampler as required, without removing the filter and the plastic from the first unit, while the activity on the filter surface decays. The assembled WL m o n i t o r and the replaceable unit are shown in Figure 8.b and 8.c. Because the plastic is not sensitive to alpha particles with energies above 4.3 Mev, for high detection-efficiency an absorber is needed in front of the detector to reduce the energy of alpha particles ( Po-218 Mev a n d P o - 2 1 4 7.68 Mev) to the registrar range. Aluainium foil thicknesses of 12 a n d 2 4 um a r e used to separate and register the above two energies respectively as shown in Figure 8.d. T h e WL i s d e t e r m i n e d according to the following equation: WL= ( 4 . 6 2 N 1 / ~ + 5 . 9 2 N z / k z ) / IO s V By c o u n t i n g Nx a n d Nz on the two halves of the plastic and knowing the total volume of air passing through the filter paper( V), and the efficiency ( k l a n d k2 ) o f the plastic for the registration of the two energies , the activity of radondaughters i n WL c a n b e c a l c u l a t e d . This technique was c o m p a r e d with the Radon Daughter Monitor (RDM) u s e d by the National Radiological Protection Board xs and also in the second CEC r a d o n a c t i v e intercomparison xv a n d s h o w e d g o o d c o r r e l a t i o n . T h e WL m o n i t o r
was applied
in
different
applications
as
follow:
A. T - ~ - ~ - m ~ a s . ~ t ~ - ~ h ~ - c ~ c ~ n . t ~ r a . t ~ i . ~ 9 ~ f - ~ d ~ - ~ a u g h t ~ r s ` - . ~ . n s . i d ~ - ~ h ~ u s e s ~ of measurements were performed inside houses in different areas t h e WL M o n i t o r . The results of these are shown in Table 5. Table
5.
Summary of
Area
measurement
with
concentration (mwl)
Birmingham/U.K Abardeen Orkney Dhahran/S.A
xs
z
# of houses
2 7 2.5
65 I0 II
7
17
house s~To study this effect, radon daughters and ventilation rooms. There was no sub-floor of this study is shown in the Table radon
6. The effect of daughters activity
H o u s e No Construction of the Floor construction
house
WL-monitor
in
different
by
Croup using
areas.
conditions
walls are normal clay bricks. walls are granite. houses built on uranium ore with sub-floor ventilation. ground floor and unoccupied.
two floor houses were chosen. T h e WL o f rate was found in ground and first floors ventilation under the houses. The results following T a b l e 6:
the subsoil under the inside the ground floor l wood conc.
2 brick cone.
house on rooms 18.
increasing
5 brick cone.
the
3 brick cone.
4 brick cone.
6 stone cone.
10.1 0.7 7.8
3.4 1.4 4.7
5.0 0.3 1.5
12.2 -
6.7 0.2 1.4
5.2 0.3 1.6
1.7 0.2 0.3
2.5 0.1 0.3
4.2 -
1.8 0.3 0.5
a.r.o.u.Dd...,..t.lo, o.r. C o n e . o f Rn d a u g h ' s . ( m W L ) Ventilation rate h -l mWL. h - 1
13.7 0.1 1.4
C o n e . o f Rn d a u g h ' s . ( m W L ) Ventilation rate h -1 nWL. h - 1
-
The ground floor results showed more concentration than the first floor. This is due to the sub-floor emanation passing directly through the solid floor to the air of the room, while in Orkney houses although the houses built on Uranium Ore with emanation rate of 6.8 ± 5.6 KBq.m -z from the sub-floor soil but because of the presence of sub-floor ventilation almost all emanation was eliminated before entering the house s .
532
F.A. ABU-JARAD
measurements concluded that the concentration of the radon daughters decreases with increased distance from the ground level zs, but the ventilation rate was not measure in that study. In another study, forty weasurewents were made in identical rooms using WL N o n i t o r l s . The ventilation rate was measured this time. The rooms had been closed and unoccupied for at least two weeks. The rooms chosen , were identical (i.e. with the same volume and on the same side of the building). The summary of the study was that the radon concentration was independent of the level of the room but only depended on the ventilation rate of that r o o m . A l s o i t was n o t i c e d that the geometrical mean of the results was 1 . 0 wWL , w h i c h i s h a l f o f t h e q u o t e d m e a n ( 2 . 0 mWL) f o r 65 m e a s u r e m e n t s in typical houses in the same city z°. 4-Passive radon dosimeter This is a closed chamber into which radon diffuses. The cross section of the diffusion chamber is shown in Figure 9. One CR-39 d e t e c t o r is fixed to the bottom. In the cover there is a hole sealed w i t h a 5 ms t h i c k n e s s of foam plastic. The design of the chamber ensures that the aerosol particles and the radon products are deposited on the foam from outside and that only radon, among other gases, diffuses through it to the sensitive volume of the chamber z. During disintegration of the Rn-222 in the chamber into its products, three ~- particles will be emitted. Some of these alpha particles will hit the CR-39 detector if it falls within their range. The d e t e c t o r will thus accumulate a number of tracks proportional to the concentration of radon gas in the room. 7.6
Ca
Fig.
9. a cross section of environmental dosimeter radon concentration in
~ - ~ det~tor
the passive for measuring houses.
i ..... 5.8
cm
The s y s t e m has been calibrated participation in the second and dosimeters for radon and decay international intercomparieon dosimeters. The system has been Sadui Arabia 3. The p a s s i v e technique was applied
at the NRBP i n U n i t e d Kingdom. Also third CEC i n t e r c o m p e r i s o n of passive products in 1984 and 1987 as part of showed good agreement with other used for radon survey in dwellings in
in
different
A. ~ _ e ~ _ s . ~ _ . Y _ ~ _ r _ ~ _ ~ _ ~ _ _ r _ @ ~ _ _ ~ _ Y ~ _ ~ 9 ~ : used to study the seasonal variation of radon The d o s i m e t e r s were distributed i n t h e Same four-month periods throughout a full yearZl. as shown in the following T a b l e 7: Table
7.
Summary of
radon
Seasons
# of
Spring Summer Winter
101 83
11.5 8.9
80
]5.5
houses
concentration Radon
in
applications
as
follow:
The passive dosimeter was concentration in dwellings. houses for three different The s u m m a r y o f t h e r e s u l t s
houses
concentration
for
different
seasons.
Bq.m -3
± 0.7 ± 0.4 ± 0.?
The above results show that the concentration of radon in houses measured over a full year varied with the seasons of the year, with the highest concentration in winter and the lowest in summer. B. R a d o n _ l n _ _ S a u d i _ _ h o _ u s e B . P a s s i v e dosimeters were used in a recent survey in 372 houses in Saudi Arabia 3. A total o f 637 p a s s i v e dosimeters were used in this survey. The radon concentration was found to vary from 5 to 36 B q . m a w i t h a mean of 16 B q . m s . The unoccupied houses showed a
RADON RELATED
MEASUREMENTS
533
c o n c e n t r a t i o n of 29 ± 7 B q . m z d o u b l e t h a t o f t h e o c c u p i e d h o u s e s 14 ± 1 B q . m s in t h e s a m e a r e a a n d m a d e o f t h e s a m e b u i l d i n g materials. This s u r v e y is t h e first in S a u d i A r a b i a which can be considered as a hot climate country. Comparing the results of this survey with another s u r v e y p e r f o r m e d w i t h a s i m i l a r t e c h n i q u e in a c o l d c l i m a t e c o u n t r y zz, in which, 367 houses were surveyed in United Kingdom (U.K.), the equilibrium equivalent radon concentration (EER) in U.K. a n d in S a u d i A r a b i a w a s 12 a n d I0 B q . m -~ r e s p e c t i v e l y . Although, the f i r s t is a c o l d c l i m a t e a n d the s e c o n d is h o t c l i m a t e c o u n t r y , t h e E E R is not far f r o m e a c h other. T h i s m i g h t b e e x p l a i n e d as f o l l o w : The t e m p e r a t u r e in S.A. is h i g h in s u m m e r ( 3 5 - 4 5 o C), t h e r e f o r e a l m o s t all h o u s e s in S.A. a r e c o o l e d b y a i r - c o n d i t i o n i n g and they are usually k e p t c l o s e d m o s t of the time. In w i n t e r also, being cold especially at night, f o r c e d a i r h e a t i n g is c o m m o n , particularly in C e n t r a l Provence. For m o s t o f the time , t h e n S a u d i h o u s e s a r e c l o s e d and air ventilated. T h u s the c o n d i t i o n s in S.A. a r e t h e r e f o r e l i t t l e d i f f e r e n t f r o m t h o s e in U.K. w h e r e the h o u s e s a r e u s u a l l y c l o s e d f o r o p p o s i t e r e a s o n (i.e to s a v e e n e r g y ) . It m i g h t therefore be concluded that the radon concentration in h o u s e s d o e s not d e p e n d on t h e l o c a t i o n a n d c l i m a t e of the c o u n t r y .
If a group of three track detectors placed at different positions in particular room, the number of ~-tracks on them do not always show a consistent pattern. These variations attributed in part to the inhomogeniety of radon and its daughters in the air of the room z~. This may be caused by convention currents inside the room. In addition,to continuous inlet air from around closed windows which will always reduce the concentration close to them. The variation of the concentration of radon and its daughters in the same room is supported by results of another experiment in which concentrations of two tracer gases(Freon and Halothene) in a room were measured by a gas chromatographic technique at different positions in the same room, readings w e r e a s i n T a b l e 8: T a b l e 8: T h e r e a d i n g o f gas c h r o m a t o g r a p h at d i f f e r e n t s h o w i n g d i f f e r e n t c o n c e n t r a t i o n s o f t h e t r a c e r gas. Freon -12
peak
heights *Site
T i m e min. 1.5 6.5 11.5 16.5 21.5 26.5 31.5 36.5 41.5 46.5 51.5 56.5 61.5 .
.
1
4
(cm) No. 6
Halothane *Site 2
105 93 87 30 49.5 54 42.5 17.6 23 23.6 28.8 10.8 15
.
* Sites No. 1 a n d 6 n e a r t h e f l o o r . in the middle of the room. Site No.
T i m e min. 4 9 14 19 24 29 34 39 44 49 54 59 64
1
sites
peak No. 4
in
heights
6
aroom
(cm)
2
17.6 15.8 15.8 5.9 9 9.9 10.4 4.1 5.6 5.6 6.7 2.6 3.6 .
.
.
Sites No. 4 , 1 m e t r e 2, lm a b o v e t h e f l o o r
above the floor near the window
I t w a s n o t i c e d z t h a t w h e n the " C a n t e c h n i q u e " a p p l i e d to b r i c k s c o v e r e d with wallpaper it s h o w e d m o r e r a d o n e m a n a t i o n than bricks covered with liquid paints. T h i s l e d to a n o t h e r experiment, w h i c h w a s to f i n d the uranium content o f the wallpaper. This was carried out b y placing d i f f e r e n t s a m p l e s of w a l l p a p e r s a d j a c e n t to L e x a n a n d i r r a d i a t e d w i t h a f l u e n c e o f I0 Is t h e r m a l n e u t r o n s p e r c m z. A p i e c e of r e f e r e n c e glass containing lppm of uranium concentration also irradiated at t h e s a m e time. In s o m e w a l l p a p e r the c o n c e n t r a t i o n w a s 6 . 3 , 6 a n d 0.31, 0 . 2 9 p p m for the f r o n t a n d b a c k f a c e s , r e s p e c t i v e l y , s e e f i g u r e 10. T h e f r o n t f a c e s of t h e s e p a p e r s w e r e d e c o r a t e d w i t h c o l o r e d p a i n t . T h i s i n d i c a t e s that the uranium content on the front faces is mainly from the paint itself. Thus, some wallpapers may be another source of radon inside the house.
534
F.A. ABU-JARAD
(115 x 6.3 magnification)
Fig.
I0.
(12.5 x 16 magnification)
Uranium d i s t r i b u t i o n in a w a l l p a p e r s h o w n by track after i r r a d i a t i o n by thermal n e u t r o n s .
fission-fragment
ACKNOWLEDGEMENTS The author would like to thank Research Committee of King Fahd U n i v e r s i t y of P e t r o l e u m and M i n e r a l s for its s u p p o r t in part of the above mentioned research and to the R e s e a r c h Institute for its support to present this paper in this conference.
REFERENCES I. F. Ahu-Jarsd, J H Frezlin and R. Bull. Phys. Ned. Biol. 25, 683(1980). 2. F. Abu-Jarad and J.H. Fremlin. Health Physics, 44, 243- (1983). 3. F. A b u - J a r a d a n d M . I . Al-Jarallah. Radiation Protection Dosimetry, !4, 243- (1986). 4. U.Mcl. Michie. trans. Instr. Mio. Metal]. ( Section B. A p p l . E a r t h . Sci. 8!, 53- (1978). 5 . F. A b u - J a r a d and J.H. Fremlin. Proceeding llth ICSSNTD, 565- (1981), Pergamon Press. 6. International C o m m i s s i o n on R a d i o l o g i c a l protection ( I C R P ) 32 ( 1 9 8 1 ) . 7 . M . E . W r e n n , M. E i s e n b u d , C. C o s t a - R i b e i r t o , A.J. Razle and R.D. Siek. Health Physics, 17,405(1969). 8. R.F. Houlb, R.F. Droullard, W.Bo, P.K. Hopke,R. Parsley and J.J. Stuckle. Health Physics, 36, 497- (1979). 9. J . B i g u . gealth Physics, 44, 266- (1983). 10. S . N . R u d n i c k , W.C. H i n d s , E . F . M a h e r , J . M . P r i c e and M.T. First. presented at I n t . M e e t i n g on r a d o n a n d r a d o n p r o g e n y m e a s u r e m e n t s , M o n t g o m e r y , A l a b a m a , 2 7 - 2 8 Aug. ( 1 9 8 1 ) 1 1 . F. A b u - J a r a d and J. Fremlin. Health Physics, 42, 82- (1982). 1 2 . B. K a h n , Z . W a n g a n d E. S e n s i n t a f f a r . Nucl. Inst. Meth. 2!9, 419(1984) 13. F. kbu--Jarad and R. Sextro. 14th ICSSNTD'S, Lahore~ 2-6 April 1988. 14. F. Abu-Jarad, C.K. W i l s o n and J.H. Fremlin. N u c l e a r Track and R a d i a t i o n M e a s u r e m e n t s , 5, 285- (1981). 15. F . A b u - J a r a d and J.H. Fremlin. R a d i a t i o n P r o t e c t i o n Dosimetry, !, 221(1981). 16. K.D. Cliff. Phys. Bed. Biol. 23, 55- (1978). 17. J . C H. R i l e s and J.Sinnaeve. Report EUR 10403 EN, ( 1 9 8 6 ) . 18. F. A b u - J a r a d and J.H.Fremlin. Health Physics, 44, 479- (1983). 1 9 . A. T o t h . H e a l t h P h y s i c s , 23, 281- (1972). 2 0 . F. A b u - J a r a d Environmental International, 8, 37-(1982). 2 1 . F. A b u - J a r a d and J.R. Fremlin. Health Physics, 46, 1126- (1984). 2 2 . L. B r o w n , B. M. R. G r e e n , J . C. R. M i l e s a n d A. D. W r i x o n . In proc. C o n f . on I n d o o r A i r , 2 0 - 2 4 A u g u s t 1 9 8 4 , S t o c k h o l m , Sweden. 23. F.Abu-Jarad. Radiation Protection Dosimetry, 3, 227-(1982).
Nucl. Tracks Radiat. Meas., Vol. 15, Sos. 1-4, pp. 525--534, 1988 Int. J. Radiat. Appl. lnstrum., Part D
Printed in Great Britain
APPLICATION OF NUCLEAR TRACK DETECTORS F O R R A D O N R E L A T E D MEASUREMENTS FALAH
A. A B U - J A R A D
ENERGY RESOURCES D I V I S I O N / R E S E A R C H I N S T I T U T E KING FAHD U N I V E R S I T Y OF P E T R O L I U M AND M I N E R A L S P.O.Box 2020, D H A H R A N 31261, SAUDI ARABIA.
ABSTRACT Personal experience in the application of nuclear track detectors for radon related measurements are discussed. The paper summarizes previously published w o r k . The " C a n T e c h n i q u e " used for measuring radon emanation from building materials, walls and soil.nWorking Level Monitor" used for measuring Working levels of radon daughters in houses in a short period. "Passive radon dosimeters" used to measure radon levels in houses for long term (few months) periods. Application of nuclear track detectors for measuring the radon daughters plate-out on s u r f a c e s of mixing fan blades and walls are discussed. Uranium content of some wall papers were f o u n d t o b e 6 ppm. V a r i a t i o n of radon progeny concentration in same room is measured and supported by another study through Gas Chromatograh measurements. Independent of radon concentration on r o o m , s level in highrise buildings is concluded. Effect of Sub-floor radon emanation on radon concentration in houses is depend on wether there is sub-floor ventilation or not.
Applications, Can dosimeter, Plate-out,
KEY WORDS Technique, Working Level Monitor, Fan, paint, surfaces, floors.
Passive
Radon
INTRODUCTION Nuclear track detectors have been applied in the last few years by many authors for measuring radon and progeny activities in houses. Reviewing of all their published work in different conferences and journals is worthwhile to be done. Since too much time is needed to perform this type of work. I thought it might be useful to summarize previously published work related to radon and progeny measurements in which I have participated. For more details and method of calculations it would be worthwhile to refer to the original papers.
Different the need
TECHNIQUES, APPLICATIONS AND RESULTS techniques were established and applied in different arose. A summary of these follow:
ways
as
For long-term measurements of radon exhalation from building materials or walls a "Can technique" was developed 1. In this a track detector CR-39 was placed in u small, impervious vessel and sealed to a typical part of the wall or individual bricks, Figure 1. The d e t e c t o r was placed at 2 in from the surface of the wall or brick, hidden by a diaphragm (250 polystyrene) but freely exposed to the emergent radon. It records the decay of radon in the outer part of the can plus the decay of Po-218 and Po-214 |rlck or wall deposited on the inner walls of the can. This would reach equilibrium concentration after a week or so; hence knowing the geometry of the Fig. l. The Can Technique system and time of exposure ~1, 1 5 ~ 1 / 4 - z i
525
526
F.A. ABU-JARAD
(3 m o n t h s obtained.
or more), t h e To m e a s u r e t h e C V d/ A
equilibrium activity of emergent radon could be exhalation r a t e t h e f o l l o w i n g e q u a t i o n w a s used:
E =
T + (I/d) Where
e -dT-
(I/d)
E: C:
is the r a d o n e x h a l a t i o n p e r u n i t a r e a a n d t i m e (Bq. m -2 . h -1 ) is the integrated radon exposure as measured by the CR-39 detector ( B q .m - 3 . h ) V: i s t h e v o l u m e o f t h e c a n (m z ) d: i s t h e d e c a y c o n s t a n t of radon (h -1 ) A: i s t h e a r e a c o v e r e d b y t h e c a n ( m2 ) By s t i c k i n g three or more "Cans" to different walls of each room, a good estimation of the exhalation rate from the walls to the inside air of the room can be made. This technique applied in three different cases as follow:
The can technique was applied to compare the emanation rate from different building materials as shown in Figure 2. It shows that the hidden detectors give more tracks than the detector facing the bricks, while in the clay bricks the opposite happened. This mean that the emanation from the granite bricks escaped from the depth of the brick, while in the clay brick it emanates from the surface only. This does not correlate with the porosity measurements which indicate that the clay brick absorbed more water than the granite ones. This measurement gave the volume of the pores while it is the internal surface area which determines the amount of radon can escape. To c l a r i f y this, bricks from the two types were cut into slices with different thickness and, applying the "Can technique" for each slice. These results are shown in Figures 3 and 4 which show clearly that increasing the thickness of clay bricks does not increase the count as it does from the crushed granite.
KIO~
_7 2 ~=sooo
I
li.D"h 2~m
iE
Brick
~" .c /,O00
I
2
[]f C32
~3ooQ e~
c
e 2000
-
._c Iooo
f 3
< ~g
time
brick
Fig.
2.
~icks mode Gronfte
from crdshed Mock stone or from granite Aberdeen
Oifforent clny txlcks
CoolorlcKSl(~s
12 ~i ~0 ~SO 220 Thicknesses of brick (ram)
Fig. 3. Radon emanation for various thicknesses of crushed graint bricks. Hat~hed and plain areas as figure 2.
Radon emanation from different building materials. Case hatched areas, CR-39 detectors facing the brick surface; plain areas, CR-39 facing away from the surface.
1, case
2,
RADON RELATED MEASUREMENTS
|r~],,,o.t.~'o'
--12
527
~'
I~p~inted brick
£
200o i.I
t
O~
lsoo
o
O5
2
1
4
8
•
Fig. 5. Two Cans on the same b r i c k c o v e r the same area in two d i f f e r e n t c o n d i t i o n s : a: with u n p a i n t e d b r i c k and b: w i t h p a r t i a l l y p a i n t e d brick.
32
Thicknesll#$ @ brick (cm)
Fig. B.
4.
Radon emanation for various thicknesses Hatched and plain a r e a s as f i g u r e 2.
Effect
of
internal
wall
cover
on
radon
of
coal
emanation
slag inside
brick. houses:
The
the radon emanation z. Eleven new, bare bricks were selected from crushed granite and two cans were sealed on t h e s u r f a c e of each brick. After 79 days, the CR-39 detectors were collected and stored. Then every brick was coated with one primer coat and 4 coats of different types of paints on a l l s i d e s of the brick except the areas where the cans were sealed previously. Then, the experiments were repeated again f o r 97 d a y s ( i e . a fresh CN-39 d e t e c t o r was sealed inside each can, covering the unpainted areas). The representative diagram is shown in Figure 5. All the detectors stored and the new ones, were etched together in the same beaker. The r e s u l t s of these are shown in Table l, which shows that the number of alpha tracks on CN-39 detectors used with the partially painted bricks was more than with the bare bricks by factors ranging f r o m 3 t o 14 with an average of 7. This tends to indicate that the c o n c e n t r a t i o n of radon increases inside the p a i n t e d brick itself leading to an increase in the e m a n a t i o n from the u n p a i n t e d area u n d e r the can. The effect of paint, in general, is to reduce the radon e m a n a t i o n from the brick surface, which also lead to increase the radon concentration inside the brick. Paint (No. 9) i s e p o x y p a i n t which shows that the emanation increased under the can by a factor o f 12 w h e n i t partially painted. This means that this paint is efficient in reducing the emanation from the painted area. This was proved to be true in another experiment. Table I. Number of ~-trecks cm - z p e r 79 d a y s o n b o t h CR-39 detectors within two Cans sealed to the same unpainted areas on every brick in two different conditions:(a) with bare brick, (b) w i t h p a r t i a l l y painted b r i c k (see Fig. 5) paint
No.
1 2 3 4 5 6 7 8 9 10
(a) bare b r i c k (2.1, (3.4, (3.6, (5.9, (2.7, (6.2, (2.9, (2.3, (2.0, (2.9,
(b) p a r t i a l l y
2.1)xlO s 2.7)x103 8.4)x103 2.5)xlOS 2.1)xl0S 2.9)x10 s 4.7)x10 s 3.1)xlO s 2.4)x10 s 4.7)x103
(18.6, (20.8, (48.9, (12.1, (15,2, (23.0, (15.5, (19.6, (24.8, (18.3,
18.9)x10 s 18.O)xlO s 50.5)x10 s 9.6)x10 s 15.5)x103 23.3)x10 s 16.3)x10 s 24.9)x10 s 27.9)x10 s 23.9)x10 s
av.
3xlO 3
Av. r a d o n * concent. Bq.m -3
167
llO0
Av. e x h a l a t i o n rate Bq . h - Z . m - ~
0.06
0.40
Calculated
according
to
22x
equation
6 in
painted
lO s
reference
b/a 9, 6, 14, 2, 6, 4, 5, 9, 12, 6, 7
No 1 .
9 7 6 4 7 8 4 8 12 fi
528
F.A. ABU-JARAD
C. E m a n a t i o n f r o m e x i s t i n ~ . w a l l R i n _ d w e _ l l S n g s : T w o or three nCans" were sealed to different walls o f 30 m e a s u r e d rooms for a period of 3 months or Bore to estiaate the exhalation rate of radon from the already existing walls. The s u m m a r y o f t h e r e s u l t s as shown in Table 2. Table
2.
City
Summary o f No. o f Houses
results No. o f to the
of
wall
exhalation
Cans sealed walls
measurementsa.
Activity under C a n s ( B q . m- a )
Exhalation (Bq.m-i-h -1)
Dhahran Riyadh Jeddah
21 6 10
55 12 28
1.0 3.3 2.0
± 0.1 ± 1.2 ± 0.5
0.35 1.2 0.7
± 0.05 ± 0.5 ± 0.2
Total
37
95
1.6
± 0.3
0.6
± 0.1
Radon emanation from uranium ore: Some h o u s e s built on uranium ore on Orkeny Island/U.K., where the uranium concentration varied between 500 to 1 2 0 0 ppm 4 . T h e ~Can t e c h n i q u e N w a s a p p l i e d to find the level of radon emanation from sub-floor soil and what the effect of this is on the radon level inside houses. The radon daughters concentration inside the houses were measured by using the Working Level Monitor technique ( to be described latter in this paper). The cans were buried a t a d e p t h o f 30 cm below the soil surface in the garden of each measured house s . The radon concentration inside the Cans was found to vary from 0.26-22 KBq.m - 3 w i t h an average of 8.5 ± 7.0 KBq.m - 3 , see Table 3. The average equivalent exhalation rate was found to be 6.8 ± 5.6 KBq.m-Z.h -1If the average exhalation rate is used to estimate the total amount of radon emanated from the sub-floor soil of a house covering an area o£ soil equal t o 10 x 10 mz , t h e r a d o n e m a n a t e d from the soil under the house should b e 680 KBq . h - 1 . A s s t m i n g t h e v o l u m e o f t h e h o u s e i s 3 0 0 m31 t h e n t h e level of radon.m -3 should be around 2.5 KBq.m - 3 , taking into consideration the derived air concentration of the equilibrium-equivalent Rn-222 concentration = 1 . 5 KBq.m -3 ( I C R P ) e . Thus, the calculated value is equal to nearly double the derived limit, while the measured level of radon daughters in these h o u s e s w a s 2 . 5 mWL ( = 9 B q . m - 3 ) w h i c h i s e q u i v a l e n t to ordinary houses built in ordinary soil. The emanation from the sub-floor soil was not able to enter the houses because there was a space (about 0.5 m) u n d e r the houses and because strong winds blow throughout the year. Figure 6 shows the expected fate of emanated radon from the subsoil.
D.
Table 3. The radon concentration inside cans buried in the garden's soil of houses built the Orkeny uranium ore and their exhalation Rouse No. 1 2 3 4 5 6 7 8 9 10 11 Average
Radon conc. inside the can gBq.m -3 3.7 20.7 4.2 10.2 9.2 0.3 4.0 6.9 6.9 5.1 22.5 8 . 5 +- 7
Exhalation rate K B q . m - Z . h -1 2.8 16.4 3.3 8.1 7.4 0.2 3.0 5.5 5.5 4.1 17.9 6.75 + 5.6
on rate ~
'. i / I Fig. 6. The emanated radon from sub-soil flow with the sub-floor ventilation
RADON RELATED MEASUREMENTS
529
2. P~ ~ = . 9 _ ~ k _ ~ . . ~ ~ @ ~ The plate-out follows:
of
radon
daughters
were
studied
for
different
surfaces
as
A~_.On__m~xinK_fan__blades_~andwall__surfeces~The nuclear track detectors also used to measure the e f f e c t of a i r t u r b u l e n c e i n s i d e the r o o m s on the radon progeny. A running air m i x i n g fan was f o u n d to r e d u c e the W o r k i n g Level (WL) in air of the r o o m v-13. The r e d u c t i o n in radon d e c a y p r o d u c t s was m a i n l y due to p l a t e - o u t on i n t e r n a l s u r f a c e s in the r o o m (for m o r e details see another p a p e r in this p r o c e e d i n g ) z3. To m e a s u r e the r a t i o of p l a t e - o u t activity on the w a l l s u r f a c e s and on the fan b l a d e s CR-39 detectors were used. S e v e r a l T r a c k d e t e c t o r s w e r e s e a l e d onto the front and back face of every b l a d e of the fan u s i n g d o u b l e - s i d e d c e l l o t a p e . At the same time and for the s a m e p e r i o d o v e r w h i c h the fan was used, C R - 3 9 d e t e c t o r s w e r e exposed in d i f f e r e n t p o s i t i o n s on the walls of the room. The results of this s t u d y s h o w e d that the n u m b e r of radon d a u g h t e r s w h i c h are p l a t e d - o u t per unit a r e a on the two sides of the b l a d e s of the fan are more than those p l a t e d - o u t per u n i t a r e a on the s u r f a c e of the w a l l s by a f a c t o r of 10 on the average. B e c a u s e the s u r f a c e area of the fan blades is n e g l i g i b l e in c o m p a r i s o n of the s u r f a c e a r e a of the walls, the main d e c r e a s e in a i r b o r n e r a d o n d a u g h t e r s is due to p l a t e - o u t on the surfaces of the walls of the room. T h e p l a t e - o u t ratio d e p e n d s on the n u m b e r of aerosol p a r t i c l e s in the air of the room. It i n c r e a s e s w i t h by the r e d u c t i o n of the n u m b e r of a e r o s o l p a r t i c l e s . B. The d e ~ c ~ a t o r experiment. A n o t h e r e x p e r i m e n t was d e s i g n e d to s t u d y the p l a t e - o u t of radon d a u g h t e r s on t r a c k d e t e c t o r s s u r f a c e s T M . F i g u r e 7 shows the d e s i g n of the e x p e r i m e n t . The Track detectors were placed inside the chamber in t h r e e p o s i t i o n s ( f a c i n g upwards, downwards and s i d e w a y s a w a y from the wall). By i n j e c t i n g same a m o u n t of radon in the c h a m b e r and for the same p e r i o d , t h e o n l y v a r i a b l e was the type and n u m b e r of a e r o s o l p a r t i c l e s .
v.,._ , = - - Tj
T,
.J Warn
Fig.
nec:Jl
7. The a r r a n g e m e n t for i n t r o d u c i n g the radon into the d e s i c c a t o r w h i c h show the p o s i t i o n of the p l a s t i c as follows: a- f a c i n g upwards, b- f a c i n g d o w n w a r d s , c - f a c i n g sidways.
The experiment was repeated three times and the results are shown in Table 4. The conclusion of those experiments showed that response of detectors facing downwards and sideways are less than those facing u p w a r d s in the p r e s e n c e of s m o k e p a r t i c l e s , w h i l e all are equal in the absence of the smoke. This is explained because of less plate out in the presence of smoke particles registered by sideways and downwards d e t e c t o r s . The i n c r e a s e d r e s p o n s e on the u p w a r d s d e t e c t o r s is due to the settling of smoke particles on the surface of the detectors. The equal response of all detectors in absence of smoke is because of equal plateout on all of them without regard to the position of the detectors.
530
F.A. ABU-JARAD
Table 4. Comparison of track d e n s i t i e s o b t a i n e d under varying experimental conditions. No of Exp.
experimental conditions
I.
with smoke from KMnO4
i ii
2.
with smoke from one cigarette
3.
CR-39 cond.
Track
densities
Upwards
without smoke
on C R - 3 9
xl0 ~
detectors
(cm -z)
Downwards
Sideways
(4.8 !0.2) (0.9 ± 0 . 0 4 )
(2.5 ± 0 . 1 ) (0.2 ± 0 . 0 1 )
(2.7 ±0.1) (0.2 ±0.01)
i ii
(4.7 ± 0 , 1 ) (0.9 ±0.03)
(3.7 ± 0 . I ) (0.3 ± 0 . 0 l )
(3.6 ±0.I) (0.3 ±0.I)
i ii
(3.4 ±0.I) (0.4 ±0.02)
(3.7 ± 0 . 1 ) (0.5 ± 0 . 0 2 )
(3.7 ±0.I) (0.5 ±0.02)
(i) CR-39 detectors p l a c e d in d i f f e r e n t sides t h r o u g h the e x p e r i m e n t . (ii) new CR-39 placed on the s u r f a c e of (i) d e t e c t o r s i m m e d i a t e l y after f i n i s h i n R their e x p o s u r e to r e c o r d the p l a t e d - o u t radon p r o g e n y on its surfaces.
3.~....W.o.~.n,~...le.~.~_L_~i.to~ N u c l e a r track detectors w e r e used to m o n i t o r radon levels inside houses as a s e m i - a c t i v e technique. The d e s i g n of the w o r k i n g level m o n i t o r is shown in Figure 8. Track d e t e c t o r s LR-115 are used w i t h an a i r - s a m p l e r and glass fiber filter zs. A Known v o l u m e of air is d r a w n t h r o u g h a glass microfiber filter at a rate of 65 liter per minute. Parallel to and d i r e c t l y front of this is a disc of LR-115. This is s e p a r a t e d from the filter by a spacer ring 2 mm in t h i c k n e s s w i t h 56 r a d i a l l y d r i l l e d holes of I mm diameter. The outer d i a m e t e r of the ring is 32 mm and the inner d i a m e t e r is 25 mm. The latter b e i n g the e f f e c t i v e d i a m e t e r of both the filter and plastic. I
I II -...,~...,w . . . . . ~
I
(a)
(d)
Fig. 8:(a) C r o s s s e c t i o n a l view of W L - M o n i t o r head. (b) The a s s e m b l e d r e p l a c e a b l e unit (c) The a s s e m b l e d WL-Honitor (d) The alpha track holes in L R - 1 1 5 s h o w i n g the s e p a r a t i o n b e t w e e n e n e r g i e s 6 Mev (left) and 7.7 May (right)
RADON RELATED MEASUREMENTS
531
The filter paper, LR-115 plastic and the spacer are clamped together as a single unit. This unit can be exchanged with another unit to be used in the head of the air sampler as required, without removing the filter and the plastic from the first unit, while the activity on the filter surface decays. The assembled WL m o n i t o r and the replaceable unit are shown in Figure 8.b and 8.c. Because the plastic is not sensitive to alpha particles with energies above 4.3 Mev, for high detection-efficiency an absorber is needed in front of the detector to reduce the energy of alpha particles ( Po-218 Mev a n d P o - 2 1 4 7.68 Mev) to the registrar range. Aluainium foil thicknesses of 12 a n d 2 4 um a r e used to separate and register the above two energies respectively as shown in Figure 8.d. T h e WL i s d e t e r m i n e d according to the following equation: WL= ( 4 . 6 2 N 1 / ~ + 5 . 9 2 N z / k z ) / IO s V By c o u n t i n g Nx a n d Nz on the two halves of the plastic and knowing the total volume of air passing through the filter paper( V), and the efficiency ( k l a n d k2 ) o f the plastic for the registration of the two energies , the activity of radondaughters i n WL c a n b e c a l c u l a t e d . This technique was c o m p a r e d with the Radon Daughter Monitor (RDM) u s e d by the National Radiological Protection Board xs and also in the second CEC r a d o n a c t i v e intercomparison xv a n d s h o w e d g o o d c o r r e l a t i o n . T h e WL m o n i t o r
was applied
in
different
applications
as
follow:
A. T - ~ - ~ - m ~ a s . ~ t ~ - ~ h ~ - c ~ c ~ n . t ~ r a . t ~ i . ~ 9 ~ f - ~ d ~ - ~ a u g h t ~ r s ` - . ~ . n s . i d ~ - ~ h ~ u s e s ~ of measurements were performed inside houses in different areas t h e WL M o n i t o r . The results of these are shown in Table 5. Table
5.
Summary of
Area
measurement
with
concentration (mwl)
Birmingham/U.K Abardeen Orkney Dhahran/S.A
xs
z
# of houses
2 7 2.5
65 I0 II
7
17
house s~To study this effect, radon daughters and ventilation rooms. There was no sub-floor of this study is shown in the Table radon
6. The effect of daughters activity
H o u s e No Construction of the Floor construction
house
WL-monitor
in
different
by
Croup using
areas.
conditions
walls are normal clay bricks. walls are granite. houses built on uranium ore with sub-floor ventilation. ground floor and unoccupied.
two floor houses were chosen. T h e WL o f rate was found in ground and first floors ventilation under the houses. The results following T a b l e 6:
the subsoil under the inside the ground floor l wood conc.
2 brick cone.
house on rooms 18.
increasing
5 brick cone.
the
3 brick cone.
4 brick cone.
6 stone cone.
10.1 0.7 7.8
3.4 1.4 4.7
5.0 0.3 1.5
12.2 -
6.7 0.2 1.4
5.2 0.3 1.6
1.7 0.2 0.3
2.5 0.1 0.3
4.2 -
1.8 0.3 0.5
a.r.o.u.Dd...,..t.lo, o.r. C o n e . o f Rn d a u g h ' s . ( m W L ) Ventilation rate h -l mWL. h - 1
13.7 0.1 1.4
C o n e . o f Rn d a u g h ' s . ( m W L ) Ventilation rate h -1 nWL. h - 1
-
The ground floor results showed more concentration than the first floor. This is due to the sub-floor emanation passing directly through the solid floor to the air of the room, while in Orkney houses although the houses built on Uranium Ore with emanation rate of 6.8 ± 5.6 KBq.m -z from the sub-floor soil but because of the presence of sub-floor ventilation almost all emanation was eliminated before entering the house s .
532
F.A. ABU-JARAD
measurements concluded that the concentration of the radon daughters decreases with increased distance from the ground level zs, but the ventilation rate was not measure in that study. In another study, forty weasurewents were made in identical rooms using WL N o n i t o r l s . The ventilation rate was measured this time. The rooms had been closed and unoccupied for at least two weeks. The rooms chosen , were identical (i.e. with the same volume and on the same side of the building). The summary of the study was that the radon concentration was independent of the level of the room but only depended on the ventilation rate of that r o o m . A l s o i t was n o t i c e d that the geometrical mean of the results was 1 . 0 wWL , w h i c h i s h a l f o f t h e q u o t e d m e a n ( 2 . 0 mWL) f o r 65 m e a s u r e m e n t s in typical houses in the same city z°. 4-Passive radon dosimeter This is a closed chamber into which radon diffuses. The cross section of the diffusion chamber is shown in Figure 9. One CR-39 d e t e c t o r is fixed to the bottom. In the cover there is a hole sealed w i t h a 5 ms t h i c k n e s s of foam plastic. The design of the chamber ensures that the aerosol particles and the radon products are deposited on the foam from outside and that only radon, among other gases, diffuses through it to the sensitive volume of the chamber z. During disintegration of the Rn-222 in the chamber into its products, three ~- particles will be emitted. Some of these alpha particles will hit the CR-39 detector if it falls within their range. The d e t e c t o r will thus accumulate a number of tracks proportional to the concentration of radon gas in the room. 7.6
Ca
Fig.
9. a cross section of environmental dosimeter radon concentration in
~ - ~ det~tor
the passive for measuring houses.
i ..... 5.8
cm
The s y s t e m has been calibrated participation in the second and dosimeters for radon and decay international intercomparieon dosimeters. The system has been Sadui Arabia 3. The p a s s i v e technique was applied
at the NRBP i n U n i t e d Kingdom. Also third CEC i n t e r c o m p e r i s o n of passive products in 1984 and 1987 as part of showed good agreement with other used for radon survey in dwellings in
in
different
A. ~ _ e ~ _ s . ~ _ . Y _ ~ _ r _ ~ _ ~ _ ~ _ _ r _ @ ~ _ _ ~ _ Y ~ _ ~ 9 ~ : used to study the seasonal variation of radon The d o s i m e t e r s were distributed i n t h e Same four-month periods throughout a full yearZl. as shown in the following T a b l e 7: Table
7.
Summary of
radon
Seasons
# of
Spring Summer Winter
101 83
11.5 8.9
80
]5.5
houses
concentration Radon
in
applications
as
follow:
The passive dosimeter was concentration in dwellings. houses for three different The s u m m a r y o f t h e r e s u l t s
houses
concentration
for
different
seasons.
Bq.m -3
± 0.7 ± 0.4 ± 0.?
The above results show that the concentration of radon in houses measured over a full year varied with the seasons of the year, with the highest concentration in winter and the lowest in summer. B. R a d o n _ l n _ _ S a u d i _ _ h o _ u s e B . P a s s i v e dosimeters were used in a recent survey in 372 houses in Saudi Arabia 3. A total o f 637 p a s s i v e dosimeters were used in this survey. The radon concentration was found to vary from 5 to 36 B q . m a w i t h a mean of 16 B q . m s . The unoccupied houses showed a
RADON RELATED
MEASUREMENTS
533
c o n c e n t r a t i o n of 29 ± 7 B q . m z d o u b l e t h a t o f t h e o c c u p i e d h o u s e s 14 ± 1 B q . m s in t h e s a m e a r e a a n d m a d e o f t h e s a m e b u i l d i n g materials. This s u r v e y is t h e first in S a u d i A r a b i a which can be considered as a hot climate country. Comparing the results of this survey with another s u r v e y p e r f o r m e d w i t h a s i m i l a r t e c h n i q u e in a c o l d c l i m a t e c o u n t r y zz, in which, 367 houses were surveyed in United Kingdom (U.K.), the equilibrium equivalent radon concentration (EER) in U.K. a n d in S a u d i A r a b i a w a s 12 a n d I0 B q . m -~ r e s p e c t i v e l y . Although, the f i r s t is a c o l d c l i m a t e a n d the s e c o n d is h o t c l i m a t e c o u n t r y , t h e E E R is not far f r o m e a c h other. T h i s m i g h t b e e x p l a i n e d as f o l l o w : The t e m p e r a t u r e in S.A. is h i g h in s u m m e r ( 3 5 - 4 5 o C), t h e r e f o r e a l m o s t all h o u s e s in S.A. a r e c o o l e d b y a i r - c o n d i t i o n i n g and they are usually k e p t c l o s e d m o s t of the time. In w i n t e r also, being cold especially at night, f o r c e d a i r h e a t i n g is c o m m o n , particularly in C e n t r a l Provence. For m o s t o f the time , t h e n S a u d i h o u s e s a r e c l o s e d and air ventilated. T h u s the c o n d i t i o n s in S.A. a r e t h e r e f o r e l i t t l e d i f f e r e n t f r o m t h o s e in U.K. w h e r e the h o u s e s a r e u s u a l l y c l o s e d f o r o p p o s i t e r e a s o n (i.e to s a v e e n e r g y ) . It m i g h t therefore be concluded that the radon concentration in h o u s e s d o e s not d e p e n d on t h e l o c a t i o n a n d c l i m a t e of the c o u n t r y .
If a group of three track detectors placed at different positions in particular room, the number of ~-tracks on them do not always show a consistent pattern. These variations attributed in part to the inhomogeniety of radon and its daughters in the air of the room z~. This may be caused by convention currents inside the room. In addition,to continuous inlet air from around closed windows which will always reduce the concentration close to them. The variation of the concentration of radon and its daughters in the same room is supported by results of another experiment in which concentrations of two tracer gases(Freon and Halothene) in a room were measured by a gas chromatographic technique at different positions in the same room, readings w e r e a s i n T a b l e 8: T a b l e 8: T h e r e a d i n g o f gas c h r o m a t o g r a p h at d i f f e r e n t s h o w i n g d i f f e r e n t c o n c e n t r a t i o n s o f t h e t r a c e r gas. Freon -12
peak
heights *Site
T i m e min. 1.5 6.5 11.5 16.5 21.5 26.5 31.5 36.5 41.5 46.5 51.5 56.5 61.5 .
.
1
4
(cm) No. 6
Halothane *Site 2
105 93 87 30 49.5 54 42.5 17.6 23 23.6 28.8 10.8 15
.
* Sites No. 1 a n d 6 n e a r t h e f l o o r . in the middle of the room. Site No.
T i m e min. 4 9 14 19 24 29 34 39 44 49 54 59 64
1
sites
peak No. 4
in
heights
6
aroom
(cm)
2
17.6 15.8 15.8 5.9 9 9.9 10.4 4.1 5.6 5.6 6.7 2.6 3.6 .
.
.
Sites No. 4 , 1 m e t r e 2, lm a b o v e t h e f l o o r
above the floor near the window
I t w a s n o t i c e d z t h a t w h e n the " C a n t e c h n i q u e " a p p l i e d to b r i c k s c o v e r e d with wallpaper it s h o w e d m o r e r a d o n e m a n a t i o n than bricks covered with liquid paints. T h i s l e d to a n o t h e r experiment, w h i c h w a s to f i n d the uranium content o f the wallpaper. This was carried out b y placing d i f f e r e n t s a m p l e s of w a l l p a p e r s a d j a c e n t to L e x a n a n d i r r a d i a t e d w i t h a f l u e n c e o f I0 Is t h e r m a l n e u t r o n s p e r c m z. A p i e c e of r e f e r e n c e glass containing lppm of uranium concentration also irradiated at t h e s a m e time. In s o m e w a l l p a p e r the c o n c e n t r a t i o n w a s 6 . 3 , 6 a n d 0.31, 0 . 2 9 p p m for the f r o n t a n d b a c k f a c e s , r e s p e c t i v e l y , s e e f i g u r e 10. T h e f r o n t f a c e s of t h e s e p a p e r s w e r e d e c o r a t e d w i t h c o l o r e d p a i n t . T h i s i n d i c a t e s that the uranium content on the front faces is mainly from the paint itself. Thus, some wallpapers may be another source of radon inside the house.
534
F.A. ABU-JARAD
(115 x 6.3 magnification)
Fig.
I0.
(12.5 x 16 magnification)
Uranium d i s t r i b u t i o n in a w a l l p a p e r s h o w n by track after i r r a d i a t i o n by thermal n e u t r o n s .
fission-fragment
ACKNOWLEDGEMENTS The author would like to thank Research Committee of King Fahd U n i v e r s i t y of P e t r o l e u m and M i n e r a l s for its s u p p o r t in part of the above mentioned research and to the R e s e a r c h Institute for its support to present this paper in this conference.
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