- Email: [email protected]
Natural radioactivity of granite rocks in Wadi Qena
Radrar.
Php.
Chem. Vol. 44. No.
I/2, pp. 95-98.
1994
Coovrieht Xi” 1994 Elsevier Science Ltd
Pergamon
Printed’ib &ealBritain. All rights reserved 0969-806X/94 $6.00 + 0.00
NATURAL
RADIOACTIVITY IN WAD1
OF GRANITE QENA
M. H. SAIED, A. ABBADY, A. H. EL-KAMEL Physics
Department,
Faculty
ROCKS
of Science, Assiut
and A. EL-ARABI
University,
Qena,
Egypt
Abstract-Compressed powdered
granite samples brought from G. El Missikat and G. El Garra with different ages were analyzed by low-level y spectrometry. The contents of U, Th and K were determined and the dependence on age was tested. U/Th and U/K ratios for young and old granite samples were evaluated. Results were discussed and compared with other experiments.
energy spectra of the samples were recorded 900min measurement time for every sample.
INTRODUCTION
The aim of the present radioactivity in some Wadi Qena, Upper method. That region natural background
work is to measure the natural geological samples, located in Egypt, by a y spectroscopic was found to have a higher than other regions in Upper
RESULTS AND DISCUSSION
The present measurements include y spectroscopic analyses of compressed powdered granite samples of different ages brought from G. El-Missikat and ElGarra. Figure 1 represents the energy spectra of two granite samples: (a) young; and (b) old.
Egypt. A y spectroscopic method is preferred with respect to the various possible methods of measuring U and Th concentrations. The main y spectrometry techniques are by NaI(TI) scintillation and by semiconductor systems. For low-level counting, reduction of background and increased precision is required (Adams and Lowder, 1964).
Determination of the activity of ‘j8Li, “=Th and “OKin granite samples by y Estimation of 238U and 232Th radioactivity spectroscopy is possible when radioactive equilibrium exists throughout the 238U and 232Th series while 40K is a direct y-ray emitter. For calculations, ;he daughters activities in Bq/g are determined from the measured photopeaks. For the samples under investigation, the activity of each daughter A(x) was calculated from the formula (Boyle, 1982):
EXPERIMENTAL Sixty-five
granite
samples
with
different
ages
were
rocks of the G. El-Missikat and El-Garra areas. These granitic plutons represent a part of the basement rocks forming the red sea hills of the Eastern desert of Egypt. They are located at distance of about 3 km to the south of El-Farokuya station which is located midway between Qena and Safaga on the highway, 85 km from Qena. The rock samples were crushed to small pieces and dried at 105°C. Then the dried samples were ground to a grain-size of about 50 mesh. Each powdered sample was shaken well to a homogeneous state, and then shaped in the form of a circular disc of 55 mm dia. and 13 mm thick. Each sample was fixed in a plastic ring whose inner diameter was equal to the diameter of the detector in face-to-face geometry. Finally, each sample was stored for 4 wk to reach the equilibrium state. The measuring system consisted of a complete y spectrometer with 1024 channels and a NaI(TI) detector. Before measuring the activities of the samples under investigation, the spectrometer was adjusted, calibrated and the background was measured (Abbady et al., 1993 (this RPC issue)). Then the collected
from
after
the granitic
A(x)=-
Net area RTq
’
where R is the emission probability of the y-ray, T is the counting time (s) and 1 is the full energy peak efficiency under the given experimental conditions. In the present experiment the activity of “‘U was determined from its daughter ‘14Bi while for 232Th its activity was measured from its daughters ‘=*Ac and 208Tl. The 4oK activity was measured directly since it is a direct y-ray emitter. The calculated values of activity of the samples under investigation fluctuate between 0.02 and 0.05 Bq/g for 238U while it varies from 0.03 to 0.22 Bq/g and from 0.14 to 2.00 Bq/g for 40K.
The contents of U, Th and K in granite samples The concentration of U, Th and K in the granite samples under investigation was determined from the measured activities. The average amount of U in the young sample was found to be 21.3 f 6.4 ppm while the corresponding values for old granite samples is 8.2 k 3.7 ppm. Generally, the values found for all 95
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Natural
radioactivity
of granite
2.0
1.0
1.X F
L
1.6 1.4
0
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97
rocks in Wadi Qena
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U ppm Fig. 3. The relation between U content and U/Th ratio for young granite samples.
decrease
with
age
in accordance
with
the present
results. The variation of U and Th content with age of the granite rocks can be due to their chemical and geological properties. Many studies have been concerned with the distribution of U and Th in igneous rocks (John et al., 1961) in particular the variation of U and Th in a number of igneous differentiation sequences. Most studies of igneous rocks, whether from abroad or from individual local sampling, show that both U and Th increase in abundance toward later crystallizing and more acidic rock types. Different studies disagree, however, concerning the behaviour of the U to Th ratio. Granitic rocks rich in U are referred to as “hot granite”. These
7o r 60
t 50 “0
3
k
between U content and U/Th ratio for old granite samples.
granites normally contain 15 ppm and may sometimes as great as 120ppm of U (Heier and Rogers, 1963). For Th in igneous rocks, it is usually present in accessory minerals such as monazite, thorite, zircon and allanite. The content of Th generally increases with SiO, content and closely follows U during differentiation or partial melting. The increase of U with both SiO, and alkali content is usually more marked than the increase of Th. Figures 2 and 3 describe the relation between U concentration and both U/Th and U/K ratios for young samples, while Figs 4 and 5 for old ones. The relations show that there is no obvious relation because the presence of both U and Th in these samples is largely determined by the oxidization and leaching of U during weathering, with the immobile Th left behind and fixed. The U/Th ratio is less than unity in most cases for old and young granites. Therefore it can be concluded that the radioactive occurrence in this case is related to Th rather than to U which is in good agreement with many published results (Thornton, 1983).
40REFERENCES
: 2
U wm Fig. 5. The relation
3o 20 -
10
0
0 0
0
0
0,
0
10
5
15
U mm
Fig. 4. The relation
between U content old granite samples.
and U/K ratio for
Abbady A., Saied M., El Kamel A. and El-Arrabi A. (1994) Radiar. Phys. Chem. 44, 225-228. Adams J. and Lowder W. (1964) The Nafural Radiation Environment. University of Chicago Press. Bovle R. (1982) Geochemical Prospecting for Uranium and I _ ?hori&. Elskvier, Amsterdam. _ El-Assaly M. (1981) IAEA symposium in Methods of‘ Lot+ Level Counting and Specrrometry, Berlin. p. 41. Heier K. and Rogers W. (1963) Geochim. Cosmochim. 27, 131. Hok A., EL-Kamel A., Sansoni B. and Abbady A. (1990) Bulletin Faculty of Science, Assiut University, 19(2-A), p. 39.
M. H.
98
John
J.. Rogers
W. and Paul C. (1961) Geochim.
Cosmo-
er al
Richter
P. and Stettner
G. (1979) Grochcwti.str,~ und Petrol of Fichtelgehirge Granites, p. 78. Thornton I. (1983) Applied En~ironntcwtul Geochmttstr~~ ruphy
chim. 25, 99.
Larsen E. S. and 151.
SAIED
Gottfied
D. (1960)
Amer.
J. Sci. 258,
Academic
Press, New York.
Php.
Chem. Vol. 44. No.
I/2, pp. 95-98.
1994
Coovrieht Xi” 1994 Elsevier Science Ltd
Pergamon
Printed’ib &ealBritain. All rights reserved 0969-806X/94 $6.00 + 0.00
NATURAL
RADIOACTIVITY IN WAD1
OF GRANITE QENA
M. H. SAIED, A. ABBADY, A. H. EL-KAMEL Physics
Department,
Faculty
ROCKS
of Science, Assiut
and A. EL-ARABI
University,
Qena,
Egypt
Abstract-Compressed powdered
granite samples brought from G. El Missikat and G. El Garra with different ages were analyzed by low-level y spectrometry. The contents of U, Th and K were determined and the dependence on age was tested. U/Th and U/K ratios for young and old granite samples were evaluated. Results were discussed and compared with other experiments.
energy spectra of the samples were recorded 900min measurement time for every sample.
INTRODUCTION
The aim of the present radioactivity in some Wadi Qena, Upper method. That region natural background
work is to measure the natural geological samples, located in Egypt, by a y spectroscopic was found to have a higher than other regions in Upper
RESULTS AND DISCUSSION
The present measurements include y spectroscopic analyses of compressed powdered granite samples of different ages brought from G. El-Missikat and ElGarra. Figure 1 represents the energy spectra of two granite samples: (a) young; and (b) old.
Egypt. A y spectroscopic method is preferred with respect to the various possible methods of measuring U and Th concentrations. The main y spectrometry techniques are by NaI(TI) scintillation and by semiconductor systems. For low-level counting, reduction of background and increased precision is required (Adams and Lowder, 1964).
Determination of the activity of ‘j8Li, “=Th and “OKin granite samples by y Estimation of 238U and 232Th radioactivity spectroscopy is possible when radioactive equilibrium exists throughout the 238U and 232Th series while 40K is a direct y-ray emitter. For calculations, ;he daughters activities in Bq/g are determined from the measured photopeaks. For the samples under investigation, the activity of each daughter A(x) was calculated from the formula (Boyle, 1982):
EXPERIMENTAL Sixty-five
granite
samples
with
different
ages
were
rocks of the G. El-Missikat and El-Garra areas. These granitic plutons represent a part of the basement rocks forming the red sea hills of the Eastern desert of Egypt. They are located at distance of about 3 km to the south of El-Farokuya station which is located midway between Qena and Safaga on the highway, 85 km from Qena. The rock samples were crushed to small pieces and dried at 105°C. Then the dried samples were ground to a grain-size of about 50 mesh. Each powdered sample was shaken well to a homogeneous state, and then shaped in the form of a circular disc of 55 mm dia. and 13 mm thick. Each sample was fixed in a plastic ring whose inner diameter was equal to the diameter of the detector in face-to-face geometry. Finally, each sample was stored for 4 wk to reach the equilibrium state. The measuring system consisted of a complete y spectrometer with 1024 channels and a NaI(TI) detector. Before measuring the activities of the samples under investigation, the spectrometer was adjusted, calibrated and the background was measured (Abbady et al., 1993 (this RPC issue)). Then the collected
from
after
the granitic
A(x)=-
Net area RTq
’
where R is the emission probability of the y-ray, T is the counting time (s) and 1 is the full energy peak efficiency under the given experimental conditions. In the present experiment the activity of “‘U was determined from its daughter ‘14Bi while for 232Th its activity was measured from its daughters ‘=*Ac and 208Tl. The 4oK activity was measured directly since it is a direct y-ray emitter. The calculated values of activity of the samples under investigation fluctuate between 0.02 and 0.05 Bq/g for 238U while it varies from 0.03 to 0.22 Bq/g and from 0.14 to 2.00 Bq/g for 40K.
The contents of U, Th and K in granite samples The concentration of U, Th and K in the granite samples under investigation was determined from the measured activities. The average amount of U in the young sample was found to be 21.3 f 6.4 ppm while the corresponding values for old granite samples is 8.2 k 3.7 ppm. Generally, the values found for all 95
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.H ‘w.
96
Natural
radioactivity
of granite
2.0
1.0
1.X F
L
1.6 1.4
0
0
97
rocks in Wadi Qena
r
0.8 1
0
0
0
1.2 s , 3
0
1
0
1.0 0.8
0.6
t
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o.
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0
0.2
t oc5
0
10
5
1.5
25
20
30
35
40
U ppm Fig. 3. The relation between U content and U/Th ratio for young granite samples.
decrease
with
age
in accordance
with
the present
results. The variation of U and Th content with age of the granite rocks can be due to their chemical and geological properties. Many studies have been concerned with the distribution of U and Th in igneous rocks (John et al., 1961) in particular the variation of U and Th in a number of igneous differentiation sequences. Most studies of igneous rocks, whether from abroad or from individual local sampling, show that both U and Th increase in abundance toward later crystallizing and more acidic rock types. Different studies disagree, however, concerning the behaviour of the U to Th ratio. Granitic rocks rich in U are referred to as “hot granite”. These
7o r 60
t 50 “0
3
k
between U content and U/Th ratio for old granite samples.
granites normally contain 15 ppm and may sometimes as great as 120ppm of U (Heier and Rogers, 1963). For Th in igneous rocks, it is usually present in accessory minerals such as monazite, thorite, zircon and allanite. The content of Th generally increases with SiO, content and closely follows U during differentiation or partial melting. The increase of U with both SiO, and alkali content is usually more marked than the increase of Th. Figures 2 and 3 describe the relation between U concentration and both U/Th and U/K ratios for young samples, while Figs 4 and 5 for old ones. The relations show that there is no obvious relation because the presence of both U and Th in these samples is largely determined by the oxidization and leaching of U during weathering, with the immobile Th left behind and fixed. The U/Th ratio is less than unity in most cases for old and young granites. Therefore it can be concluded that the radioactive occurrence in this case is related to Th rather than to U which is in good agreement with many published results (Thornton, 1983).
40REFERENCES
: 2
U wm Fig. 5. The relation
3o 20 -
10
0
0 0
0
0
0,
0
10
5
15
U mm
Fig. 4. The relation
between U content old granite samples.
and U/K ratio for
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