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In situ density measurement in aqueous solutions by the gamma-ray backscattering method
Nuclear Instruments and Methods 192 (1982) 619-621 North-Holland Publishing Company
619
Letter to the Editor
IN SlTU DENSITY MEASUREMENT IN AQUEOUS SOLUTIONS BY THE
GAMMA-RAY
BACKSCATTERING
METHOD
A. GAYER, S. BUKSHPAN and D. KEDEM Soreq Nuclear Research Centre, Yavne, lsrael
Received 16 March 1981 and in revised form 10 August 1981
The possible application of the "r-ray backscattering method for in situ measurement of the density prof'de in aqueous solutions was studied experimentally, following Monte Carlo calculations.
Gamma-ray backscattering techniques are widely used today in various instruments for density measurement. This application is possible because the Compton scattering cross section is proportional to the material electron density which, to a good approximation, is proportional to the material density [1,2]. One instrument, already used commercially, is the soil surface density gauge. Much experimental and computational work has been done to improve its response and to eliminate various sources of error [3-6]. In this work we present a similar possible application of the gamma-ray backscattering method, i.e. in situ measurement of the density profile in aqueous solution. The proposed method can be used, for example, in oceanography and solar-pond research, where the density depth profile is an important parameter. The Monte Carlo method of calculation was employed to obtain a realistic picture of the behavior of the backscattered gamma-ray flux as a function of the density and chemical composition of aqueous solutions. The dependence of the backscattered flux on the y-ray source energy and the geometrical configuration was also studied so that an optimal system could be devised. The Monte Carlo program used has been described previously [7]. The calculations were performed for two y-ray sources, 241Am which has a principal line at 59.5 keV and 192 Ir which has several lines in the range 200-600 keV. The geometric configuration used in the mathematical simulation is shown schematically in fig. 1. A thin layer of an aqueous solution of a given chemical composition was irradiated by a y-ray source with 0029-554X/82/0000-0000/$0 ° 75 © 1982 North-Holland
a 27r geometry, positioned at a height h above the solution surface. The layer width of 10 mm was chosen in accordance with the desired depth resolution. The detector, a 1" long, 1.75" diameter NaI(T1) scintillator, was also positioned at a height h above the solution surface. The detector efficiency was taken into consideration in the calculations, and photons were allowed to enter the detector only through its bottom surface. Calculations were performed for aqueous solutions of NaCI and KBr. Since these two salts have very different average atomic numbers, we were able to study the dependence of the backscattered y-ray flux on the chemical composition. The density range considered was 1.00-1.22 g/cm 3 , as limited by the solubility of the salts. The curves for the number N.r(p) of the scattered photons detected, for the 241Am source and geometrical configurations in which d = 5.0 cm and h = 0 or 2.0 cm, exhibit a decreasing linear semi-log dependence on the solution density. The calculations for
I I I I"
Detector ]
hi
~
d
I I I I
I
.....
Fig. 1. Schematic representation .~f the geometric configuration used in the calculations.
620
A. (layer et al. / 1,7 situ density measurements
AI cao
Source lOmm Glass vessel
Fig. 2. Schematic representation of the experimental arrangement.
the 241Am source revealed a strong dependence of the system response on the salt type. For example when h = 0 , the slopes are - 1 . 6 cma/g and -16.1 cm a/g for NaC1 and KBr solutions, respectively. The practical consequences of this behavior will be discussed later. The corresponding results of NvCo ) for the 192Ir source were obtained for various geometric configurations with d = 5.0 or 6.6 cm and h = 0, 2.0 or 2.5 cm. All the curves exhibit an increasing dependence on density, this dependence being almost linear on a semi-logarithmic scale. In addition, the salt-type dependence is very small and is observed only in the high density region. After obtaining the results of the Monte Carlo calculations, we tested the feasibility of the technique experimentally. The experiments were done with the same 3,-ray sources and aqueous solutions used in the calculations. For each 3'-ray source, we used the optimal geometric configuration derived by the calculations. A schematic representation of the geometric arrangement of the system is shown in fig. 2. The Yray source antt the 1 " × 1.75" NaI(T1) detector were positioned in a lead brick, machined according to the form presented in fig. 2. In order to maintain a c o n stant gap of height h between the source-detector plane and the measured solution layer, for a given experiment, a 0.5 mm thick cylindrical aluminum cap was mounted on the cylindrical lead protrusion. The width of the measured solution layer was kept at 10 mm by a 10 mm thick round lead plate, mounted with four pins on the main system housing. The whole system, except for the upper part of the lead brick, was immersed in the measured aqueous solution, in a 150 mm diameter glass vessel so that the surface of the solution was 20 mm above the
detector-source plane. The influence on the backscattered 3'-ray flux of the glass vessel, its stand and the solution outside t h e measured layer were examined and found to be negligible. Hence the measured solution can be regarded as an infinite medium, corresponding to the realistic situation to which the technique would be applied in practice. A standard spectroscopic system, including a single-channel analyzer and a scaler, was used in the measurements. The counting was performed in an energy window whose limits were set to include the backscattered photons and to reject as much as possible of the photons coming directly from the source. The background in the measured ")'-ray flux was caused by photons backscattered from the lead plate and direct photons from the source. In the case of the 192Ir source, most of the background was due to direct radiation leaking through insufficient detector shielding and to energy overlap between the direct and backscattered spectra. A constant background contribution, determined by performing a measurement without solution, was subtracted from the raw data. In the case of the 241Anlsource, most of the contribution to the background was due to scattering from lead plate. It was density-dependent, because of the attenuation of the primary and scattered radiation in the solution, especially for the high Z solution. We found empirically that a good approximation to the net results for tile 242 A m source can be obtained by subtracting a constant background which is measured with the lead plate adjacent to the detector-source plane. In fig. 3 we present the experimental results of N-r(,o ) for the 241 Am source, obtained with a 30 mCi point source in a geometric configuration in which d = 5 cm and h = 0. The results show a decreasing linear dependence on solution density, in agreement with the calculations. The corresponding experimental results for the 192Ir source, shown in fig. 4, were obtained with a 1 mCi point source in a geometric configuration where d = 6.6 cm and h = 2.5 cm. In this case, also, the experimental results are in agreement with the calculations. The salt-type effect which is also observed in the experimental results, amounts to a maximum of 3% for the two salts used in the measurements. The experimental results for the two "/-ray sources were found to have logarithmic linear correlation coefficients high than 0.999 for the two salts. Similar behavior was also observed for aqueous solutions of
A. Gayer et aL / In situ density measurements
°51
I
I 241Am
× bo"
Z
iO3
IDO
[
I.I0 Density (g / cm3)
i
1.20
Fig. 3. The experimental backscattered "r-ray flux N,r after background subtraction for the 241Am sourcel as a function of the solution density p for NaC1 (o) and KBr (~). KC1, which have an intermediate average atomic number. Hence, the logarithmic linear response, which is an empirical consequence o f the present work, is a I
I
621
property of the density region and the experimental arrangement studied here, and not of the chemical composition of the solution. From the results obtained so far, the application of the ")'-ray backscattering method for in situ measurement of the density profile in aqueous salt solutions is feasible with some limitations. The use of a low energy 3,-ray source, such as 241 Am, results in a strong salt-type effect and hence cannot be used in solutions of unknown or changing salt composition. However, for constant salt composition, the technique using a low energy 7-ray source is highly sensitive and is easily calibrated because of its linear response on a logarithmic scale. In this sense the backscattering technique is superior to the 3'-ray transmission method, where the response is nonlinear, because the mass attenuation coefficients of aqueous salt solutions are density dependent. By using a high energy 3'-ray source in the region of the ~92Ir spectrum the salt type effect is almost eliminated and amounts to a maximum of 3% for the extremes in average atomic number of the measured salts. Hence, it can be expected that the salt type effect will be less than 1% for measurements in a solar pond or the ocean where the salt composition does not change radically.
1921r
References
Z
I 1.00
I I.IO
[ 1.20
Density (O/cm3) Fig. 4. The experimental backscattered "r-ray flux N 3, after background subtraction for the 192ir source, as a function of the solution density p for NaC1 (o) and KBr (z~).
[1] E.S. Gamett, T.J. Kennett, D.B. Kenyon and C.E. Webber, Radiology 106 (1973) 209. [2] R.P. Gardner and D.R. Whitaker, Nucl. Appl. 3 (1967) 298. [3] D. Taylor and M. Kansara, Nucl. Instr. and Meth. 59 (1968) 305. [4] P.C. Curtayne, Nucl. Technol. 11 (1971) 61. [5] E.R. Christensen, Nucl. Eng. Des. 22 (1972) 342. [6] W.C. Hopkins and R.P. Gardner, Nucl. Eng. Des. 24 (1973) 332. [7] A. Gayer, S. Bukshpan and E. Nardi, Nucl. Instr. and Meth. 180 (1981) 589.
619
Letter to the Editor
IN SlTU DENSITY MEASUREMENT IN AQUEOUS SOLUTIONS BY THE
GAMMA-RAY
BACKSCATTERING
METHOD
A. GAYER, S. BUKSHPAN and D. KEDEM Soreq Nuclear Research Centre, Yavne, lsrael
Received 16 March 1981 and in revised form 10 August 1981
The possible application of the "r-ray backscattering method for in situ measurement of the density prof'de in aqueous solutions was studied experimentally, following Monte Carlo calculations.
Gamma-ray backscattering techniques are widely used today in various instruments for density measurement. This application is possible because the Compton scattering cross section is proportional to the material electron density which, to a good approximation, is proportional to the material density [1,2]. One instrument, already used commercially, is the soil surface density gauge. Much experimental and computational work has been done to improve its response and to eliminate various sources of error [3-6]. In this work we present a similar possible application of the gamma-ray backscattering method, i.e. in situ measurement of the density profile in aqueous solution. The proposed method can be used, for example, in oceanography and solar-pond research, where the density depth profile is an important parameter. The Monte Carlo method of calculation was employed to obtain a realistic picture of the behavior of the backscattered gamma-ray flux as a function of the density and chemical composition of aqueous solutions. The dependence of the backscattered flux on the y-ray source energy and the geometrical configuration was also studied so that an optimal system could be devised. The Monte Carlo program used has been described previously [7]. The calculations were performed for two y-ray sources, 241Am which has a principal line at 59.5 keV and 192 Ir which has several lines in the range 200-600 keV. The geometric configuration used in the mathematical simulation is shown schematically in fig. 1. A thin layer of an aqueous solution of a given chemical composition was irradiated by a y-ray source with 0029-554X/82/0000-0000/$0 ° 75 © 1982 North-Holland
a 27r geometry, positioned at a height h above the solution surface. The layer width of 10 mm was chosen in accordance with the desired depth resolution. The detector, a 1" long, 1.75" diameter NaI(T1) scintillator, was also positioned at a height h above the solution surface. The detector efficiency was taken into consideration in the calculations, and photons were allowed to enter the detector only through its bottom surface. Calculations were performed for aqueous solutions of NaCI and KBr. Since these two salts have very different average atomic numbers, we were able to study the dependence of the backscattered y-ray flux on the chemical composition. The density range considered was 1.00-1.22 g/cm 3 , as limited by the solubility of the salts. The curves for the number N.r(p) of the scattered photons detected, for the 241Am source and geometrical configurations in which d = 5.0 cm and h = 0 or 2.0 cm, exhibit a decreasing linear semi-log dependence on the solution density. The calculations for
I I I I"
Detector ]
hi
~
d
I I I I
I
.....
Fig. 1. Schematic representation .~f the geometric configuration used in the calculations.
620
A. (layer et al. / 1,7 situ density measurements
AI cao
Source lOmm Glass vessel
Fig. 2. Schematic representation of the experimental arrangement.
the 241Am source revealed a strong dependence of the system response on the salt type. For example when h = 0 , the slopes are - 1 . 6 cma/g and -16.1 cm a/g for NaC1 and KBr solutions, respectively. The practical consequences of this behavior will be discussed later. The corresponding results of NvCo ) for the 192Ir source were obtained for various geometric configurations with d = 5.0 or 6.6 cm and h = 0, 2.0 or 2.5 cm. All the curves exhibit an increasing dependence on density, this dependence being almost linear on a semi-logarithmic scale. In addition, the salt-type dependence is very small and is observed only in the high density region. After obtaining the results of the Monte Carlo calculations, we tested the feasibility of the technique experimentally. The experiments were done with the same 3,-ray sources and aqueous solutions used in the calculations. For each 3'-ray source, we used the optimal geometric configuration derived by the calculations. A schematic representation of the geometric arrangement of the system is shown in fig. 2. The Yray source antt the 1 " × 1.75" NaI(T1) detector were positioned in a lead brick, machined according to the form presented in fig. 2. In order to maintain a c o n stant gap of height h between the source-detector plane and the measured solution layer, for a given experiment, a 0.5 mm thick cylindrical aluminum cap was mounted on the cylindrical lead protrusion. The width of the measured solution layer was kept at 10 mm by a 10 mm thick round lead plate, mounted with four pins on the main system housing. The whole system, except for the upper part of the lead brick, was immersed in the measured aqueous solution, in a 150 mm diameter glass vessel so that the surface of the solution was 20 mm above the
detector-source plane. The influence on the backscattered 3'-ray flux of the glass vessel, its stand and the solution outside t h e measured layer were examined and found to be negligible. Hence the measured solution can be regarded as an infinite medium, corresponding to the realistic situation to which the technique would be applied in practice. A standard spectroscopic system, including a single-channel analyzer and a scaler, was used in the measurements. The counting was performed in an energy window whose limits were set to include the backscattered photons and to reject as much as possible of the photons coming directly from the source. The background in the measured ")'-ray flux was caused by photons backscattered from the lead plate and direct photons from the source. In the case of the 192Ir source, most of the background was due to direct radiation leaking through insufficient detector shielding and to energy overlap between the direct and backscattered spectra. A constant background contribution, determined by performing a measurement without solution, was subtracted from the raw data. In the case of the 241Anlsource, most of the contribution to the background was due to scattering from lead plate. It was density-dependent, because of the attenuation of the primary and scattered radiation in the solution, especially for the high Z solution. We found empirically that a good approximation to the net results for tile 242 A m source can be obtained by subtracting a constant background which is measured with the lead plate adjacent to the detector-source plane. In fig. 3 we present the experimental results of N-r(,o ) for the 241 Am source, obtained with a 30 mCi point source in a geometric configuration in which d = 5 cm and h = 0. The results show a decreasing linear dependence on solution density, in agreement with the calculations. The corresponding experimental results for the 192Ir source, shown in fig. 4, were obtained with a 1 mCi point source in a geometric configuration where d = 6.6 cm and h = 2.5 cm. In this case, also, the experimental results are in agreement with the calculations. The salt-type effect which is also observed in the experimental results, amounts to a maximum of 3% for the two salts used in the measurements. The experimental results for the two "/-ray sources were found to have logarithmic linear correlation coefficients high than 0.999 for the two salts. Similar behavior was also observed for aqueous solutions of
A. Gayer et aL / In situ density measurements
°51
I
I 241Am
× bo"
Z
iO3
IDO
[
I.I0 Density (g / cm3)
i
1.20
Fig. 3. The experimental backscattered "r-ray flux N,r after background subtraction for the 241Am sourcel as a function of the solution density p for NaC1 (o) and KBr (~). KC1, which have an intermediate average atomic number. Hence, the logarithmic linear response, which is an empirical consequence o f the present work, is a I
I
621
property of the density region and the experimental arrangement studied here, and not of the chemical composition of the solution. From the results obtained so far, the application of the ")'-ray backscattering method for in situ measurement of the density profile in aqueous salt solutions is feasible with some limitations. The use of a low energy 3,-ray source, such as 241 Am, results in a strong salt-type effect and hence cannot be used in solutions of unknown or changing salt composition. However, for constant salt composition, the technique using a low energy 7-ray source is highly sensitive and is easily calibrated because of its linear response on a logarithmic scale. In this sense the backscattering technique is superior to the 3'-ray transmission method, where the response is nonlinear, because the mass attenuation coefficients of aqueous salt solutions are density dependent. By using a high energy 3'-ray source in the region of the ~92Ir spectrum the salt type effect is almost eliminated and amounts to a maximum of 3% for the extremes in average atomic number of the measured salts. Hence, it can be expected that the salt type effect will be less than 1% for measurements in a solar pond or the ocean where the salt composition does not change radically.
1921r
References
Z
I 1.00
I I.IO
[ 1.20
Density (O/cm3) Fig. 4. The experimental backscattered "r-ray flux N 3, after background subtraction for the 192ir source, as a function of the solution density p for NaC1 (o) and KBr (z~).
[1] E.S. Gamett, T.J. Kennett, D.B. Kenyon and C.E. Webber, Radiology 106 (1973) 209. [2] R.P. Gardner and D.R. Whitaker, Nucl. Appl. 3 (1967) 298. [3] D. Taylor and M. Kansara, Nucl. Instr. and Meth. 59 (1968) 305. [4] P.C. Curtayne, Nucl. Technol. 11 (1971) 61. [5] E.R. Christensen, Nucl. Eng. Des. 22 (1972) 342. [6] W.C. Hopkins and R.P. Gardner, Nucl. Eng. Des. 24 (1973) 332. [7] A. Gayer, S. Bukshpan and E. Nardi, Nucl. Instr. and Meth. 180 (1981) 589.