Document 4229

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IC/93/101
INTERNAL REPORT
(Limited Distribution)
International Atomic Energy Agency
\
and
United Nations Educational Scientific and Cultural Organization
INTERNATIONAL CENTRE FOR THEORETICAL PHYSICS
ANALYSIS OF ATMOSPHERIC CONCENTRATIONS
OF RADON AND THORON USING BETA COUNTING TECHNIQUE
G.S. Islam*
International Centre for Theoretical Physics, Trieste, Italy,
A.K.F. Haque
and
S.M. Basunia
Department of Physics, University of Rajshahi, Rajshahi-6205, Bangladesh.
ABSTRACT
This paper presents a detailed theory and experimental procedure for measurement and analysis nf mixed radon and thoron in the environment. The technique has been
successfully applied to the study of seasonal variations of radon and thoron in Rajshahi
atmosphere during the years 1989-1991. The maximum radon concentration in outdoor
air was observed in the winter period of December to January while the indoor radon
concentration was found to be maximum during the monsoon months of July and August.
The implication of results is briefly discussed in the paper.
INTRODUCTION
The Earth's crust and most of the common building materials contain trace
amounts of 23iU and n2Th which decay to radon (222Rn) and thoron (2WRn), respectively. The Rn gas molecules diffuse out of the ground through pore spaces in rocks
and soils and mix with the atmosphere. Inhalation of Rn and its daughters can cause
a significant health hazard when they are present in enhanced levels in enclosed indoor
environments like human dwelling if it is poorly ventilated and if the radon input from
the soil or the building materials is high. It has been suggested that the indoor radon
concentration in the US is responsible for about 10% of the total risk of lung cancer (1) .
During the past decade numerous devices for assaying the radon content of specific
air sample have been developed. But there is not a single technique that can meet all
the requirements of the different types of radon measurements. The choice of the most
appropriate technique depends on the particular information needed, the type of radon
survey, the cost of the apparatus, etc.
However, when considering the vise of a specific technique for radon monitoring,
one of the most important parameter to be considered is its accuracy. Still there is some
confusion between accuracy and precision, so that often the roproducibility of replicate
measurements (i.e. precision}, is erroneously taken as an indication of the goodness of a
result.
Although radon measurements with active sampling of air on filters associated
with electronics to count alpha or beta radiations emitted by radon daughters is a standard
technique, we have made an attempt to analyze radon and thoron in air employing beta
counting method in measuring the filter activity with an adequate theoretical framework.
THEORY
The build up of radon and thoron progeny atoms on the filter during the sampling
period i, is described by the following differential equations
dNA(t,
dt, = vnA dNB(t,) = vn + X N (t.) B
A A
dt,
dNc(t.) = vn
c
dt.
MIRAMARE - TRIESTE
May 1993
(a 3J
' :
(1)
(2)
(3)
and
(4)
dt,
Permanent Address; Department of Physics, University of Rajshahi, Rajshahi-6205, Bangladesh.
dt,
=
vnThc
(5)
where NA(t.), NB(t,),Nc(t,).NThB(t,),NThc(t,)
are the numbers of atoms of RaA(*'BPo),
2M
2M
212
212
RaB( Pb), RaC( Bi), ThB( Ph) and ThCf ^) on the filter; nA, nB, nc, nTkB,
nThc are the numbers of atoms of RaA, RaB, RaC, ThB and ThC per unit volume of the
sampled air in atoms/m 3 ; v is the volume of air passing through the filter per unit time
(m 3 / min) and XA, XB, Ac, ^ThB and Xjkc are the decay constants (min" 1 ) of the radon
and thoron daughters respectively.
These equations are solved to get the number of radon and thoron progeny atoms
which were collected on the fiiter during the sampling period.
= TTl 1 - e "^ 1 ']
(6)
Similarly, equations (7) and (8) yield the ratio of the activity of RaC to the
activity of RaB:
(12)
The variations of Ri(t,) and Ri{t,) with sampling time t, are shown in figures 1
and 2.
Once the flow of air through the filter stops, the equations governing the decay
of radon and thoron progeny atoms on the filter are
=
(7)
-XANA{t)
dNB(t)
dt
(13)
(14)
(15)
XAnA -
dt
XAnB)
(16)
( A H -
dt
C
-A,,)
(8)
T
ThC
(IT)
~dT
0)
The solutions of these equations are as follows,
(18)
(10)
{10)
NB(t) =
k
NA{-^Y-)[e- "'-
Although RaA, being an alpha emitter, is not counted directly, its continuous
arrival and rapid decay during the sampling period increase the amount of RaB present.
However, once the filtering has stopped, the small number of RaA atoms present has little
effect, and the subsequent counting rate ran be described primarily in terms of RaB and
RaC.
(20)
Equations (C) and (7) yield the ratio of the activity of RaB to the activity of
RaA,
NThB(t)
= JV^
(21)
(22)
after the filtering process has gone for a time f,:
(11)
where it has been assumed that the activities in the atmosphere are in secular equilibrium
with radon, i.e. that \AnA — A B 7i s = i c n f ,
where, NA = NA(t,), N°B = NB{t,), N£ = NcV.h N<j-hB = NThB(t,) and AT«hc =
NThc(t.) are the number of atoms of RaA, RaB, RaC, ThB and ThC respectively on the
filter at the end of sampling period and JV^(t), NB(t), Nc(t), AV*a(*)- a n d ^ThcU) are
the corresponding numbers on the filter paper at any time t after the end of sampling.
Using the relationships A°B/AA = R^t,) and Aac/AaB = R2(t,), one obtains for
the beta activities AB = XBNB(t) of RaB and Ac = XcNc(t) of RaC the expressio
pressions
Using the conditon of secular equilibrium as before, equation (29) reduces to:
.n
,,
An.,
\ _ i ,
Ap
, _\ „ t
_i.J.,
(23)
(30)
From equation (30) ng is calculated and finally the radon (222Rn) concentration is obtained
from the equation
Cfln = A fln n fln = XBnB
(31)
*,) ( A c -
-xA
(24)
where < is measured from the end of the sampling period.
If the small effect of RaA is neglected, equations (23) and (24) will be reduced to
simple forms
AB = A%e-X
For the decay products of thoron (2WRn), the sum ThB + ThC was measured.
Because of the relatively small counting rate due to these activities, it is sufficient to
approximate this contribution by a simple exponential of half-life 10.64 hr, corresponding
to the decay of ThB( 212 P6). Equation (9) gives the activity of ThB on the filter at the
instant of terminating the filtration:
(25)
(26)
Xc-Xt
The beta activities of ThB + ThC at a time t after the end of filtration can be
written as
The observed variation of counting rate with time, or the total beta activity
on
the filter paper at a time t after the end of filtration should be given primarily by the
sum
of the activities of RaB and RaC:
-ft
-
A
ThBe
Hence, the concentration of ll7Pb in atmospheric air is obtained from
(27)
= XfhBnThf)
Substituting the values of AR and Ac from equations (23) and (24) into equation (27),
h
V[\ -
(32)
The relative magnitude of this contribution depends on a number of external
circumstances; it can be estimated experimentally as described in the analysis of results.
As-A/
-A*)
—
EXPERIMENTAL
RA*,
AflAc
ni(t,)(Xc-XB)[Xc-XA)
AC-ABV
>
(28)
Knowing D, experimentally. A% is calculated using equation (28).
Since A°B — \nN% therefore, from equation (7),
A a — \A
(29)
Beta counting technique was employed in measuring the filter activity. The apparatus used here consists of an air sampler capable of drawing air at 0.5 m3/ min through
a millipore filter paper and a shielded beta-sensitive end-window GM tube (type CENCO
71210, window diameter 3.5cm) connected to a model 575 solid state scaler/timer/counter
of Nucleus Inc., U.S.A. The air sampling equipment was calibtrated using appropriate instrumentation. The collection efficiency of the filter paper was found to be almost 100%.
This was determined by fixing a second filter at the outlet of the vacuum pump and measuring its activity after the end of sampling. We used a metal grid which serves as a
mechanical support for the filter. The active filter area was ~ 8.5ct7i2. Since the filter
• * -
area was slightly larger than the window of the GM tube, a minor size correction was duly
taken into account.
In performing the experiment, air was drawn through a filter paper for 15 to 90
minutes. After the sampler was turned off, the filter paper was removed and was placed
on the sliding tray of the source holder mounted below the GM tube. The background
counting rate was recorded during the sampling period. The activity of the sample was
then followed through a series of 5 minute counts. The outdoor and indoor radon daughter
levels were measured in different seasons in the University Campus of Rajshahi. The periods of measurement included the following months: winter = December-January; spring =
February-March; summer = April-June; monsoon = July-August; autumn = SeptemberNovember. At least three measurements were made every month and seasonal concentrations of radon and thoron were obtained from the average of monthly data. Nuclear
Physics Laboratory was chosen as the reference room for indoor radon measurements. The
ventilation condition of the room was kept identical throughout the period of investigation.
Relative humidity and room temperature were recorded during each measurement.
In order to obtain the overall beta counting efficiency of the GM tube a standard
Sr + 90 1' thin disc shaped beta source (0.1/iCi), whose beta-radiation is similar to the
beta-radiation of RaB( 214 P6) and RaC( 2l4 i?!), was used for calibration. The maximum
energies of the beta-radiaiton of mSr + 9 ° 1' are 0.5(50%), 2.2(50%) MeV; for 3 l 4 P6 : 0.6
MrV; 2UB) : 3.1(20%), 1.6(80%) MeV.
m
In order to verify the accuracy and reliability of the measurement technique,
reprodncibility measurements were conducted in a special room without any windows.
The room was contaminated with some weak radium sources to further increase the radon
activity. The room was kept closed for at least 10 days before each measurement was
made. This time period was sufficient for the building up of radon and thoron in the room
to their initial levels. Seven measurements were perfomed successively using 30 minute
sampling time. Inside this room, it was observed that there is excessive deposition and
overlapping of dust particles on the filter bod when the sampling time exceeded ~ 45 min.
However, for other indoor and outdoor measurements, longer sampling periods were used.
The observed count rate was corrected for background and for the dead time of the GM
tube.
ANALYSIS OF RESULTS
Figures 3 and 4 show the results for 15 and 30 minutes sampling times respectively.
The curves dearly reveal the effects of the thoron decay products. An examination of the
decay processes involved shows that after about 90 min the counting rate due to the thoron
decay products should fall off exponentially with a half-life of 10.64 hr. The straight line
fitted to the tail of these decay curves corresponds to this half-life. When the ordinates
of this line are subtracted from the observed counting rates, the resulting rates should be
due soleley to RaB(214Pfc) and RaC{2UBi). The function AB + Ac is shown normalized
to these points. The calculated counting rates due to RaB and RaC alone are shown on
graph 4 only (broken lines). Good fitting between the observed data and calculated count
rates is observed. It was quite surprising that the activity on the reverse side of the filter
paper was found to be almost the same as on the front surface. Hence, in the estimation
of activity concentrations of radon and thoron, the observed activity was multiplied by a
factor of 2 along with other necessary corrections.
The result of reproducibility measurements is provided in Table 1 which shows
reasonably good accuracy for the estimation of the concentrations of radon and thorou.
Table 2 gives the seasonal mean concentrations of radon and thoron in the surface air
(both outdoor and indoor) and are displayed in figures 5 and 6. The outdoor radon
concentration was found to be maximum during winter season while the maximum indoor
radon concentration was observed during the monsoon months of July and August, The
increased indoor radon activity during monsoon was due to humid weather condition.
This was confirmed from an investigaiton of the relation between relative humidity and
the concentations of radon in indoor and outdoor air (Table 3). The humidity variation is
displayed in figure 7.
Heavy rainfalls in the monsoon months of July and August contributed to the
reduction of radon concentration in outdoor air. This may be due to the fact that during
or following a heavy rainfall, considerable amount of dust particles settle on the ground
thus leaving the atmosphere relatively more cleaner. On the other hand, the rainfall did
not affect the radon activity inside the Laboratory in the above manner, rather the radon
activity was found to increase due to increased relative humidity. Similarly, during spring
and summer at Rajshahi the weather turns windy and plenty of dust particles (mainly
sands) float in the surface air. Considerable reduction in radon concentrations during
these seasons is probably due to dilutions of fine radioactive aerosols by the non-radioactive
sands in the surface air.
The results of measurements of radon and thoron concentrations in several office
rooms and laboratories in the physics building yielded are average radon and thoron concentrations of 31.78 ± 1.12 Bqm'3 and 2.27±0.145?-n"' respectively. These values are
far below the current upper limit of 150 Bq-m~3 recommended by the U.S. Environmental
Protection Agency (EPA) (4) for indoor radon-in-air concentation. The mean indoor concentration was normally found to be about three to six times more than that of outdoor
radon activity.
CONCLUSIONS
Radon and thoron concentrations in air can be analyzed with reasonably good
statistical accuracy by utilizing a simple and inexpensive instrumentation. In general it
can be considered that radon is mostly in equilibrium with its daughters. Radioactive
disequilibrium in a particular plare merely means that all of the daugheter activities are
not present in the location being studied. From this point of view, the assumption of
secular equilibrium in the theoretical model, in particular for closed spaces, describes very
nearly the real physical condition.
The measurements also reveal that the proportion of noRn/222Rn
in environmental air is quite low and the net effect is that the thoron daughters may be neglected in
most Rn exposure studies.
REFERENCES
1. Nero, A.V. Jr., Estimated Risk of Lung Cancer from Expoturr. to Radon Decay
Products in US Homes: A Brief Review. Atmos. Environ. 22 (10), 2205-2211
(1988).
2. Zhang C.X. and Lno D., "Measurement of mixed radon and, thoron daughters in
«>", Nucl. Inst. and Methods. 215, 481 (19S3).
3. Evans R.D., The Atomic Nucleus, McGraw-Hill Book Company Inc. New York
(1955).
Based on the present results further measurement on a large scale in the country
;is a whole are planned.
4. Science News, Vol. 134, No. 13, Sept. 24 (1988).
Acknowledgments
One of the authors (G.S.I.) would like to thank Professor Abdus SaJam, the
International Atomic Energy Agency and UNESCO for hospitality at the International
Centre for Theoretical Physics, Trieste. He would also like to thank the Swedish Agency
for Research Cooperation with Developing Countries (SAREC) for financial support during
lii^ visit at the ICTP under the Associateship scheme.
10
Figure Captions
Table 1
Reproducibility of Radon and Thoron Measurements
Fig.l. Dependence of the initial RaB-RaA ratio on sampling time.
Fig.2. Dependence of the initial RaC-RaB ratio on sampling time.
Fig.3. The variation of counting rate with time following a 15-minute sampling period.
Assembled unit
Radon concentration
(Bq-m-3)
1
95,20 ± 1.02 *
2
96.03 ± 1.08
3
95.81 ± 1.01
4
97,01 ± 1.10
5
96.12 ± 1.08
6
94.35 ± 1.02
7
98.41 ± 1.21
Mean
96.13 ±1.08
* statistical counting error
Fig,4. The variation of counting rate with time following a 30-minute sampling period.
Fig.5. Seasonal variations of the concentrations of 222Rn and 312Pb in open atmosphere.
Fig.6. Seasonal variations of the concentrations of 222Rn and 212Pb inside the Laboratory.
Fig.7. Variation of Radon Concentration with humidity.
Thoron concentration
[Bq-m-1)
5.14 ±0.15
5.61 ±0.16
6.21 ±0.16
5.12 ±0.15
5,90 ± 0.16
5.S2±0.16
6.24 ±0.17
5.72 ±0.16
Table 2
Seasonal variations of the concentrations
of 212Rn and 212Pb in the surface air
Outdoor
activity
Indoor
activity
Season
Winter
Spring
Summer
Monsoon
Autumn
Mean
11
concentration
of 2nRn
(Bq-m-3)
7.83 ± 0.38
3.36 ± 0.28
2.86 ±0.21
1.94 ±0.18
3.37 ±0.26
3.87 ± 0.26
concentration
oJ2r2Pb
(Bq-m-3)
0.94 ±0.15
0.48 ± 0.08
0.61 ±0.09
0.34 ± 0.07
0.55 ± 0.08
0.58 ± 0.09
12
concentration
of 122Rn
{Bq-m-1)
16.60 ±0.40
10.86 ±0.31
15.01 ± 0,45
29.00 ± 1.01
13.77 ±0.36
17.19 ±0.51
concentration
of 7l2Pb
(Bq-irr*)
2.19 ±0.17
1.66 ±0.14
3.25 ±0.21
5.38 ± 0.29
3.20 ±0.19
3.14 ±0.20
Table 3
Variation of Radon concentration
with relative humidity
Relative humidity
Range (percent)
20-40
Outdoor Radon concentration
(Bq-m-3)
2.26 ±0.18
40-60
4.28 ±0.26
3.04 ±0.20
3.68 ±0.24
GO-SO
SO-100
13
Indoor Radon concentration
(Bq-m-3)
Data not available
(room temperature was
always above 40%)
9.98 ± 0.32
13.74 ±0.34
19.08 ±0.52
10
100
SAMPLING TIMEtt, ) MtN
Fig.2
COUNTS
PER MINUTE
1000
ioo,oool-
O OBSERVED RATES
•
OBSERVED RATES WITH Z '*Pb +
2l2
B i SUBTRACTEO
10
10,000
a.
iii
a.
SAM PL INS TIMEa JO MIN
z
o
a
K
I-
h«
Ul
u
4
O
o
o
o
2
X
!-0
l,000h
Winter
Spring
Summar
SEASON
Fig.5
100
300
400
300
600
700
TIME (MIN)
Fig.4
17
18
Momoon
Autumn
-12
-10
m INDOOR
O 24
z
[~] OUT DOOR
g
2 20
z
S 16
LU
o
o
o
§12
o
L-0
Winter
Spring
Summer
SEASON
Monsoon
Autumn
Z
O
8
S 4
DC
Fig. 6
0
20
40
60
80
RELATIVE HUMIDITY (PERCENT)
Fig. 7
19
20
100
`