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Isotopenpraxis Isotopes in Environmental and Health
Studies
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Effective Radium Content in Egyptian Soil by CR-39
and LR-115 Plastic Nuclear Track Detectors
A. F. Hafez , B. M. Moharram , A. M. El-Khatib & A. Abdel-Naby
a
Alexandria University, Faculty of Science, Physics Department , Alexandria, Egypt
b
Cairo University, Faculty of Science, Khartoum Branch , P.O. Box 10 55, Khartoum,
Sudan
c
Alexandria University, Faculty of Education, Physic. Chemistry Department ,
Alexandria, Egypt
Published online: 27 Aug 2008.
To cite this article: A. F. Hafez , B. M. Moharram , A. M. El-Khatib & A. Abdel-Naby (1991) Effective Radium Content in
Egyptian Soil by CR-39 and LR-115 Plastic Nuclear Track Detectors, Isotopenpraxis Isotopes in Environmental and Health
Studies, 27:4, 185-188, DOI: 10.1080/10256019108622504
To link to this article: http://dx.doi.org/10.1080/10256019108622504
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Hafez et al.: Effective Radium Content in Egyptian Soil
Isotopcnpraxis 27 (1991) 4, pp. 185-188
Effective Radium Content in Egyptian Soil by CR-39
and LR-115 Plastic Nuclear Track Detectors
A. F. Hafez1), B. M. Moharram1), A. M. El-Khatibl), A. Abdel-Naby3)
') Alexandria University, Faculty of Science, Physics Department, Alexandria, Egypt
) Cairo University, Faculty of Science, Khartoum Branch, P.O. Box 10 55, Khartoum, Sudan
3
) Alexandria University, Faculty of Education, Physic. Chemistry Department, Alexandria, Egypt
Downloaded by [Columbia University] at 03:46 07 October 2014
2
CR-39 and LR-115 plastic detectors were used in air volume of several sealed cylinderical plastic tubes (can-technique) for time
integrated long term measurement of radon activity concentrations. Dried soil samples, collected from the Egyptian Nile delta, were
used. The tubes, 3.8 cm inner diameter and 20 — 170 cm in heights, were filled at the bottom with the dry soil samples up to different
thicknesses (10 —160 cm). The registeration sensitivities of CR-39 and LR-115 nuclear plastic detectors in the sealed tube, taking
into consideration the plated-out activity on the walls, arc discussed. The porosity of the soil samples, the diffusion length, the effective
and real radium-226 contents were estimated. The results showed that the sealed tube technique could be used as a useful tool for the
measurement of the radium concentration with reasonable accuracy.
CR-39 und LR-115 Plast-Kernspurdetektoren wurden in einem Luftvolumen von mehreren abgedichteten zylindrischen Plastrohren
(Behdller-Technik) fur die zeitintegrierte Langzeitmessung der Radonaktivitciten verwendet. Getrocknete Bodenproben, die vom
Nildella gesammelt wurden, siml zur Messung gelangt. Die Rohren, 3,8 cm Innendurclimesser und 20 ... 770 cm Hohe, wurden vom
Boden her mit den trockenen Bodenproben bis zu unterschiedlichen Hb'hen (10 ... 160 cm) gefullt. Die Nachweisempfindlichkeiten
von CR-39- und LR-115-PIastedetektoren in den abgedichteten Rohren wurden diskutiert, wobei die Abscheidungsaktivitat an den
IVa'nden mit in Betracht gezogen wurde. Die Porositdt der Bodenproben, die Diffusionslangen, die effekliven und die echlen Radium-226
Gehalte wurden bestimmt. Die Ergebnisse zeigten, dafi die Mefitechnik mit den abgedichteten Rohren zur Bestimmung des
Radiumgehaltes mit einer vertretbaren Genauigkeit als wertvolles Hilfsmittel durchaus eingesetzt werden kann.
Keywords
dielectric track detectors; diffusion; diffusion length; etching; particle tracks; porosity; radioactivity; radon 226; soils
1.
Introduction
Air borne charged particles e.g. alpha-particles emitted from radon
and its daughters may be detected conveniently using plastic track
detectors [1—3]. The usual aim of the application of track techniques
is to measure uranium concentrations in dating or related geological
poroblems [4] and to carry out time integrated radon exposures
under different environmental conditions e.g. in soils and natural
waters for uranium prospection and radon transport studies [5] also
in dwellings, mines as well as in waste and construction materials
for estimation of health hazards [6, 7].
The indirect method of radium measurements via the a-decay of
its first daughter element is shortly called "radon-a-method of
radium analysis". Practically, an effective radioactive equilibrium
(about 98%) for the radium-radon members of the decay series is
reached in about 3 weeks.
The aim of the present work is to estimate the radium content
in Egyptian soil samples, from the Nile delta, using the "radon-amethod of radium analysis" using sealed tubes each equipped at
the top by two adjacent detectors of different sensitivities (CR-39
and LR-115) and to measure some of radon diffusion parameters
in the soil e.g. porosity and diffusion length.
2.
through the soil occurs by two mechanisms; diffusion under the
influence of the radon concentration gradient between the pore air
and the external air, and convective transport (i.e. the movement
of radon along with the pore air) occasioned by the existance of a
pressure difference, which can be ignored in this assessment. The
diffusion of radon in the soil may thus be represented by the
one-dimensional diffusion eqn.; [6, 8]
d2C(x)
dC(.x)
df
= Ds — —2
d.v
Isotopenpraxis 27 (1991) 4
(1)
/.C(x) + a = 0 ,
where,
dC(.v)
df
= 0
in the steady state condition,
is the concentration in the direction of
depth,
C(x)
—
the diffusion coefficient of radon in the soil,
the diffusion coefficient of radon in air,
the porosity of the soil,
the decay constant of radon, and
is the production rate of radon.
A.
p
Diffusion of radon in soil;
porosity and diffusion length determination
Assume a soil sample of thickness L enclosed in a scaled tube of
cross-sectional area A and Volume V = Ah, where h is the air height
above the sample. The sealed tube is equipped at its top by a piece
of solid state nuclear track detector (SSNTD). Transport of radon
'
The flux is given by Fick' first law:
9(x) = - D .
dC(.x)
dx
(2)
185
Hafez et al.: Effective Radium Content in Egyptian Soil
where <p{x) is the activity flux per unit time per unit area of the
soil. If the bottom of the sealed tube is taken as the origin, then
the radon flux is equal to zero at X = 0, i.e.
(3)
d.v
For diffusion occuring from a soil sample of thickness L into a
sealed tube of air volume V= Ah at the steady state, the flux into
the tube compensates the volume decay rate of radon in the tube i.e.
dC(.x)
-AD,
= Ah C(L).
d.x
(4)
The steady state solution of eqn. (1) under the boundary conditions shown in eqns. 3, 4 has the form;
c o s h (X/Xd)
C(X) = !L( i
/. V
cosh (Ljxd) + P(xjh) sinh (L/xd)
Downloaded by [Columbia University] at 03:46 07 October 2014
cosh (.V/.Y,,)
cosh (L/xd) (1 + P(xjh) tanh (L/xd),
(5)
where xd = yDJ). is the diffusion length of radon and the factor
G = (1 + P(xjh) tanh (L/xd) is the back diffusion geometrical
correction factor of the used tube.
Porosity: If the thickness L of the soil sample in the sealed tube is
L < xd i.e. tanh (L/xd) = Ljxd\ hence,
Hungary) and LR-115 pieces type II (Kodak Pathe, France). In this case,
s-particles emitted directly from the upper layers of the soil sample, could
manage to reach the detectors at the top simultanously. The detectors were
left undisturbed for exposure time period of 24 days. At the end of the
experiment, the detectors were taken out for normal chemical etching as
follows:
CR-39 sheets were etched in 20% NaOH solution at 70 CC for 6 hrs, and
LR-115 films were etched in 10% NaOH solution at 60 CC for 2.5 hrs
(l'B = 3 um/h). After etching, all the detectors were washed in distilled water,
then CR-39 detectors were dipped for few minutes in 3% acetic acid solution,
then washed again with distilled water, while LR-115filmswere treated with
the solution B (50 cm3 distilled water + 5 cm3 ethyl alcohol). After that, the
detectors were mounted on a glass slide and counted by optical microscope
(Leitz-Germany) at magnification of 500 x , with a total number of 500 fields.
To minimize the background in CR-39 nuclear track detectors, the detectors
were treated, pre-used, with PEW 40 solution (15 gm KOH + 40gm
C 2 H 5 • OH + 45 gm H2O) at 70 CC [9].
4.
Results and discussion
4.1.
Porosity measurements
The linear variation of 1/g and VJVm ratio, eqn. 6 for CR-39 and
LR-115 plastic nuclear track detectors, are shown in Fig. 2.
Applying the least squares method on the data, theregression lines
were found to obey the relations; Y= (1.63 x 10" 5 + 4.25 x 10"6)
+ (4.63 x l O " 5 ± 8.67 x 1(T6) x,Y= (1.11 x 1(T4 ±O.19xlO" 4 )
+ (2.38 x 10" 4 + 0.38 x 10"4) x for CR-39 and LR-115 respectively. From the slopes and intercepts of the two lines the average
value for porosity is P = 0.41 ± 0.06. Another method was followed
to measure the porosity, in which a known mass in of the soil of
volume Kwas saturated with a known volume of water and the
new volume Vs was determined; applying the eqn. P = 1
1 _ 1
Q
Qo
1 Ah(Fa)
(6)
PQo AL(K r o )
,
(>n IK)
we found P = 0.35 + 0.03 which is in good agreement with that
obtained from the track method and those in literature [10].
where (VJVm) is the ratio of the air volume of the tube to that of
the soil, Q/Q0 = C]Ca, Co = al"/., g and Q0 are the track density and
the maximum track density respectively.
Diffusion length: By using eqns. 3,5 when:x = L, it will be possible
to get:
<pl<Po =
1 +P.v d //i)tanh(L/.v d )
7-
(7)
1 + Pxjh tanh (L/xd)
where (p0 is the maximum flux density.
By obtaining Q, and Q0 experimentally and using eqns. 6 and 7
the porosity P and the diffusion length xd, can be obtained.
3.
Experimental procedure
In this experimental investigation several sealed cjlinderical plastic tubes of
different lengths, 20 — 170 cms, and each of the same inner diameter, 3.8 cm,
were used. The soil samples were collected from the middle of Egyptian delta.
The tubes were filled up to different thicknesses of dried soil samples (from
10 to 160 cms) while the upper 10 cm part of the tubes serves asfixedmeasuring
air volume V^ as shown in Fig. 1.
To measure the effective radium content of the soil sample i.e. radium in
radioactive equilibrium with the measured radon released from the soil, each
tube was equipped on its top by CR-39 nuclear detector ( M A - N D / J , 1000 um.
'
' '
i' 1' 11 p i ' i ] i|i|liL ^
~JHJ 1 I I ! 11 | T , I I ' Ijii
L = (soil thickness of mass m)
Fig. 1. CR-39 & LR-115 detectors in the sealed tube
186
h = 70 cm
(air volume)
r
•-
0
0,5
7,0
Ratio of air volume Va , to soil sample volume Vs
7,5
Fig. 2. Variation of the reciprocal value of the measured track density against
the ratio of the air volume Va, to the soil volume Vm in the sealed
plastic tubes
4.2.
T[xe diffusion length of radon in soil
Fig. 3. shows the radon profiles obtained for CR-39 and LR-115
trach detectors. To get the diffusion length .vd the radon concentration profiles are fitted by theoretically expected shape of relation 7,
using the average value of the porosity (~0.41), the average value
of xd was found to be 1.50 ± 0.14.
Isotopenpraxis 27 (1991) 4
Hafez et al.: Effective Radium Content in Egyptian Soil
4.3.1. Calculation of the registeration sensitivity
CR-39
Assuming that Rn-222 is present only in the air volume of the tube,
and Po-218, Po-214 are plated-out (deposited) on the tube's wall.
For CR-39 track detector fixed at the top of the tube, of inner radius
a and air gap height /i, the sensitivity of Rn is given by;
S.
= - •
where
0c is the critical angle of etching and is given by,
50
100
Soil thickness [L/cm]
0c = Sin" 1 —, V is the response function [11].
150
Downloaded by [Columbia University] at 03:46 07 October 2014
Fig. 3. Radon-concentralion profile of soil measured by the sealed tube _
technique
For the plated-out daughters;
where i = 2, 3, Kis the air gap volume, A is the total surface area
of the wall of the tube [12].
In the case of LR-115 foils, the sensitivity S for radon in the air
volume is given by;
i2 - - - - ) •
S, = — a Cos 0c
4
for
«! ^ a ^ a0 .
For plated-out activity;
2
50
WO
Soil thickness [L/cm]
150
~ ~2~A~'
2 /I
Fig. 4. Variation of the back diffusion geometrical correlation factor G, with
the soil sample thickness L
where
Fig. 4., represents the variation of the calculated G factor with
the thickness L of the sample soil, in the calculations we used the
experimentally obtained values for P and xd. This curve is valid
only when Vz ^ Vm.
4.3.
Effective radium content
The effective radium content, CRa _ e(Te. (Bq/kg), of the soil i.e. radium
in radioactive equilibrium with the measured radon released from
the soil sample, can be obtained from the formula; [6]
_ /
C R a eff:
QG
\ hA
(8)
1
= K-R niM (4.2MeV),
= Rl Cos0 c ,
0
=
= Ro Cos 0C,
R~Rmin(l.9Me\),
and R is the range of alpha-particles in air, and 0c = 40° + 5° for
the optimum residual thickness 5 — 6 um, for track hole observation
[12].
Making use of formula (8) in the results represented in Fig. 3., for
both CR-39 and LR-115, the effective radium content obtained is
CBa eff = 3.00 + 0.53 (Bq/kg).
4.4.
Estimation of real Ra-226 content
The real Ra content of the soil may be given by the relation;
where
s= Ii s,
i=r
Isotopenpraxis27(1991)4
is the registeration sensitivity of the detector in the used tube, i denotes Rn-222,
Po-218 and Po-214, assuming that these
isotopes are in radioactive equilibrium
is the effective exposure time, = T— (///.)
x (( — e~ ir ), is the exposure time and A is
the decay constant of radon and
is the mass of the soil in the tube
where £ is the total emanation coefficient 8. In this work, we used
the average value of e = 0.12 ± 0.034. Therefore, the real radium
content obtained is;
C Ra r
±
0.12 ±0.034
which is in quite agreement with the published data [13 — 15].
187
I
Hafez et al.: Field Measurements of Radon Exhalation
The mass of Ra-226 (real) corresponding to the activity per unit
mass of the soil can be calculated from the relation; mRa(kg Ra/kg
sample) = C R I /W"A>-R»T ' = 1.7x l<r 1 7 C R a r 1 / 2 where
/l Ra
nA
/ Ra
is the mass number of radium isotope,
is Avogadro's number and
is the decay constant of radium.
Assuming a radioactive equilibrium of the U-series i.e. the activity
of CRa = Cu gives the value of uranium content of ~ 2 ppm.
Received April 20, 1990
Accepted ion revised form July 22, 1990
References
[1] G. Somogyi, G. Nemeih, J. Palfalvi, 1. Gerzson, Subsurface radondistribution measurements with LR-115, Cr-39 TL-detectors. Proc. 1 lth
Int. Conf. on SSNTDs, Bristol 1981, Pergamon Press, (1982) 525
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[2] L. B. Cohen, J. Rakowski, R. Nason, Health Phys. 50 (1986) 133
[3] C. F. Wong, N. Espic, L. A. Meara, Nucl. Instrum. Methods 206 (1983)
569
[4] R. L. Fleischer, P. B. Price, R. M. Walker, University of California
Press, Ltd. London (1975)
[5] //. II'. Alter, P. B. Price, Terradex Corporation, U.S. Patent, 3, 665,
194 (1972)
[6] G. Somogyi, A. F. Hafez, L. Hunyadi, M. Tolh-Szilagyi, Nucl. Tracks,
12(1986)701
[7] Bureau of Radiological Health; Radiological Health Handbook, Rockville, Maryland 20 852 Jan. (1970)
[8] R. L. Fleischer, A. Magro-Cambero, J. Geophys. Res. 83 (1978) 3539
[9] G. Somogyi, I, Hunyadi, Etching properties of the CR-39 polymeric
nuclear track detector. In: Proc. 10th Int. Conf. Solid State Nucl. Track
Detectors, Lyon, and Suppl. 2, Nucl. Tracks. Pergamon Oxford (1980)
443
[10] J. Ktasnicka, Nucl. Instrum. Methods 174 (1980) 599
[11] G. Somogyi, M. Tolh-Szilagyi, L. Hunyadi, A. F. Hafez, Nucl. Tracks,
12 (1980) 97
[12] G. Somogyi, B. Paripas, Zs. Varga, Nucl. Tracks and Rad. Meas. 8
(1984) 423
[13] M. Gallyas, I. Torok, Rad. Prot. Dosim. 7 (1984) 69
[14] /. Siotos, A. D. Wrixon, Rad. Prot. Dosim. 7 (1984) 101
[15] A'. Brown, P. J. Dimbylou; P. Wilkinson, Rad. Prot. Dosim. 7 (1984) 91
Isotopenpraxis 27 (1991) 4, pp. 188-190
Field Measurements of Radon Exhalation and Ra-226 Content
in Soil using the Can-Technique
A. F. Hafez, A. M. El-Khatib
Alexandria University, Faculty of Science, Physics Department, Alexandria, Egypt
B. M. Moharram
Cairo University, Faculty of Science, Khartoum Branch, P.O. Box 1055, Khartoum, Sudan
At. A. Kotb
Alexandria University, Medical Research Institute, Biophysics Department, Alexandria, Egypt
A. Abdel-Naby
Alexandria University, Faculty of Education, Physics-Chemistry Department, Alexandria, Egypt
CR-39 and LR-115 plastic nuclear track detectors in the can-technique have been employed in the field measurements of radon
exhalation, Ra-226 and U-238 content in dry-soil air at numerous regions in Sudan (the Blue and White Nile and Mogran regions).
Measurements gave an average radon exhalation from the soil to the atmosphere and Ra-226 content of (23.4 ± 2.60) kBq • m~2
and (123 ± 13.65) Bq • kg~' respectively. A polyethylene permeable membrane cover was used to eliminate the contribution ofthoron
activity inside the can. Assuming a radioactive equilibrium between the U-series, the average U-238 content in the soil was found to
be (9.92 ± 1.01) ppm. This survey may be used for uranium prospection in soil.
Die CR-39- und LR-115-Plast-Kernspurdetektoren wurden verwendet, um die Radon-Exhalation, den Ra-226- und den U-238-Gehalt
in zahlreichen Regionen des Sudans (Blauer und Weifier Nil und Mogran-Region) zu ermitteln. Dabei wurdefiir diese Feldmessungen
die Biichsentechnik angewendet. Die Messungen ergeben eine miltlerc Radon-Exhalation vom Boden in die Atmosphare
(23,4 + 2,60) kBq • m~2 und einen mittleren Ra-226-Gehalt von (123 ± 13,65) Bq • kg'1. Um die Tlwronaktivitat zu verhindern,
wurde innerhalb des Behdlters eine permeable Polyethylenmembran zur Abdeckung verwendet. Unter der Annahme eines radioaktiven
Gleichgewichts zwischen den Uran-Zerfallsreihen wird ein mittlerer U-238-Gehalt iin Boden von (9,92 + 1,01) ppm gefwiden. Diese
Erkundung kannfur eine Uranprospektion im Boden eingesetzt werden.
Keywords
dielectric track detectors; exhalation; geochemical surveys; prospecting; quantity ratio; radioactivity; radon isotopes; radium 226;
soils; sudan; uranium 238
1.
Indroduction
Radon, which is naturally alpha-radioactive noble gas, is every
where present in our natural environment. It diffuses readly through
most soils and porous rocks [1, 2].
188
The real concentration of radon in the soils is influenced by a
relatively large number of factors which results in a significantly
oscillation of radon concentrations in the soils. The most important
arc e.g. the humidity of the soil, atmospheric pressure changes, low
winds over the surface of the soil, rains and temperature of the soil
Isotopenpraxis 27 (1991) 4