- Email: [email protected]
Determination of soil parameters by gamma-ray transmission
Radiation Measurements 35 (2002) 17–21
www.elsevier.com/locate/radmeas
Determination of soil parameters by gamma-ray transmission A. Filiz Bayta(sa; ∗ , Sevgi Akbalb a Istanbul
Technical University, Institute for Nuclear Energy, 80626 Maslak, Istanbul, Turkey Nuclear Research and Training Center, PK1 Havaalani 34831, Istanbul, Turkey
bC ekmece
Received 23 August 2000; received in revised form 10 April 2001; accepted 12 May 2001
Abstract Gamma-ray transmission methods have been used accurately for the study of the properties of a porous medium such as soil. In this study, di0erent soil parameters are determined by using gamma-ray transmission method. To this end, the soil samples were collected from various regions of Turkey and a NaI (Tl) detector measured the attenuation of strongly collimated monoenergetic gamma beam through soil samples. The mass attenuation coe3cients of dry soil samples were calculated from the transmission measurements for di0erent photon energies. Furthermore, the soil samples were irrigated by adding known quantities of water and the soil–water properties were examined. The local water saturation and porosity were estimated from c 2002 Elsevier Science Ltd. All rights reserved. the transmission measurements for each soil sample. Keywords: Gamma ray transmission; Soil; Attenuation coe3cients; Porosity; Saturation
1. Introduction The gamma-ray transmission method is often used for the study of the soil–water properties. This is done by adding a known amount of water to the system and accounting for this by the attenuation of gamma radiation. To obtain non-destructive measurements of bulk density and water content, soil scientists have extensively used single and dual source gamma attenuation measurements (Phogat et al., 1991). For example, a dual gamma-ray apparatus was used to determine water content variations in soil samples during in:ltration (Barataud et al., 1999). The photon attenuation coe3cient is an important parameter characterising the penetration and di0usion of gamma-rays in composite materials such as alloys, organic and inorganic compounds, soils and biological materials (Singh and Mudahar, 1992). The e0ects of di0erent parameters on the attenuation coe3cients of soils have been discussed in several studies by using the gamma transmission method. ∗ Corresponding author. Tel.: +90-212-285-3945; fax: +90-212285-3884. E-mail address: [email protected] (A. Filiz Bayta(s).
Soils have a chemical composition characterised by the presence of SiO2 ; Al2 O3 ; CaO; MgO; K2 O and others in variable concentrations. A large number of trace elements are also present in soils, but the presence of trace elements has generally no inCuence on the soil attenuation coe3cient (Cesareo et al., 1994). The mass attenuation coe3cients depend on the chemical composition of the absorbing material and the energy of the gamma rays. However, for the total photon interaction, the variation of total mass attenuation coe3cients with soil composition is much below 50 keV and it is negligible above 300 keV up to 3 MeV (Mudahar et al., 1991). The size distribution of particles is important for physical characterisation of soil. The gamma-ray attenuation method is also used to determine the soil particle size as a new procedure (Oliveira et al., 1997; Vaz et al., 1999). To support the use of soil as a radiation protection material, the e0ects of soil grain size and pressure on gamma-ray attenuation have been tested in the di0erent energy regions (Mudahar and Sahota, 1988). The purpose of this paper is to determine soil parameters such as photon attenuation coe3cient, porosity, local water saturation and :eld capacity of soils by using the gamma-ray transmission technique. The soils under consideration were collected from di0erent
c 2002 Elsevier Science Ltd. All rights reserved. 1350-4487/02/$ - see front matter PII: S 1 3 5 0 - 4 4 8 7 ( 0 1 ) 0 0 2 5 3 - 0
18
A. Filiz Baytas , S. Akbal / Radiation Measurements 35 (2002) 17–21
regions of Turkey. The gamma attenuation measurements have been carried out at intermediate energy region. The effects of :eld capacity of soils and density of soils on the porosity and the local water saturation are studied by another set of experiments. 2. Background The attenuation of gamma-rays in a medium is expressed by I = I0 exp(−x);
(1)
where I0 is the initial intensity of gamma rays and I is the intensity of gamma-rays after attenuation through a media of length x; is the linear attenuation coe3cient of he material. is a macroscopic cross-section for gamma ray interaction like macroscopic neutron cross-section and it has units cm−1 (Lamarsh, 1977). It can be described as = (=);
(2)
where (=) is the mass attenuation coe3cient and is the physical density. If we assume that Ids represents the beam intensity after having passed through the column :lled with dry soil and Iss represents the intensity after having passed through fully saturated soil, then Eq. (1) can be rewritten as follows: Ids = I0 exp(−s xs );
(3a)
Iss = I0 exp(−s xs − w xw );
(3b)
where the subscripts s and w denote soil and water, respectively, and xw is the equivalent thickness of water which is spread in the soil system of thickness xs . Combining Eqs. (3a) and (3b) gives Ids w ; (4) xwsat = ln Iss
measurements as xwp Sp = ; xwsat where xwp is given by Ids xwp = ln w Ip
(8)
(9)
and Ip represents the beam intensity after passing through a p depth of the column. By using Eq. (8), Eq. (9) can be rearranged as follows: ln(Ids =Ip ) Sp = : (10) ln(Ids =Iss ) Thus, one can estimate soil porosity and local water saturation characteristics for each soil by performing gamma-ray transmission measurements, and using Eqs. (7) and (10). 3. Experimental set-up and measurements The experimental set-up consists of a NaI(Tl) detector and a gamma source both encased in lead as shown in Fig. 1. Since the narrow beam was obtained by using portable parts of columnator in this system, the diameter of the columnator for the source and the detector can be selected di0erent values in the experimental set-up. For this study, the diameter of columnator was adjusted to 5 mm. The source and the detector were mounted on a base plate. A narrow beam of gamma rays is absorbed by the detector after passing through the test column. A multichannel analyzer was used to count the signal magnitude of the transmitted gamma-rays. A soil sample was used in a 5 × 5 cm cross-sectional and 25 cm length perspex column that could be moved downward and upward. The bottom of the column is conical in shape with the provision of an outlet hole that could be closed. The soil samples vary widely due to di0erences in composition, topography and climate (Akbal, 1999). The chemical composition of the soil samples considered is given
where xwsat represents the thickness of the sample column :lled water and soil at saturation (Moseley and Dhir, 1992). The porosity () of a medium can be expressed as =
void volume : total volume
(5)
Also, the porosity of the saturated soil medium can be rewritten by using transmission measurements as xwsat : (6) = xwsat + xs By using Eq. (4), Eq. (6) can be rearranged as follows: −1 x s w = 1 + : (7) ln(Ids =Iss ) Also, the local water saturation (Sp ) at a p depth of the column axis can be written by using transmission
Fig. 1. The schematic diagram of gamma transmission system; 1 is the gamma-ray source, 2 Pb shields, 3 Portable Pb collimators, 4 NaI(Tl) detector, 5 HV, 6 ampli:er, 7 MCA, 8 soil column, p1, p2 and p3 measuring points.
A. Filiz Baytas , S. Akbal / Radiation Measurements 35 (2002) 17–21
19
Table 1 The chemical composition of soils Soil locations
Ankara Konya Fatsa Bergama Batman
Chemical components (%) SiO2
Al2 O3
Fe2 O3
CaO
MgO
SO3
K2 O
Na2 O
PF
34.25 38.39 52.78 56.98 58.12
6.83 7.62 19.06 16.1 13.16
24.1 3.23 6.59 4.73 5.73
12.2 22.4 3.76 5.01 5.46
1 1.62 0.94 1.89 3.74
0.68 0.31 0.16 0.28 0.23
1.32 1.5 4.47 2.62 2.17
1.13 0.28 2.67 2.24 1.48
16.7 23.3 7.84 7.58 9.64
in Table 1. The densities of soils vary from 1:54 to 1:9 g=cm3 . Dry soil was passed through a 2 mm sieve and packed in the perspex column. In order to :nd the compaction of the soil column, the gamma-ray transmission measurements were taken at di0erent depths of the column. For dry soil samples, three di0erent photon energies (468 keV from 192 Ir; 662 keV from 137 Cs and 1173 keV from 60 Co) were used. After the gamma-ray transmission measurements were taken for dry soil, the bottom of the perspex column was closed and a known volume of water was poured on the surface of the soil column. Irrigation was applied to the surface up to the saturation. When water was ponding on the surface, the saturation occurred. After each irrigation and also after saturation, the transmission measurements were repeated at di0erent depths of the column. Then after saturation, the bottom of the perspex column was opened and the system was drained of water. The system was left in this condition for several hours. The soil column retains a certain amount of water according to its :eld capacity. The gamma-ray transmission measurements at the same depth of soil column were also repeated in this condition. Local water saturation and porosity were calculated from the transmission measurements for each soil sample. Also, the water attenuation coe3cient was determined experimentally by measuring the gamma attenuation through a known thickness of water. 4. Results and discussion The mass attenuation coe3cients of soils were calculated from Eqs. (1) and (2) for known physical densities by using gamma transmission measurements for dry soil samples. The e0ect of chemical composition of soil on the mass attenuation coe3cient is shown by means of the SiO2 concentration in soil, since SiO2 is most abundant chemical components of soil (Fishman et al., 1981). Fig. 2 presents a plot of the mass attenuation coe3cients (=)ds for di0erent soils as a function of the SiO2 concentration. From Fig. 2, it is seen that the mass attenuation coe3cient for the di0erent soil samples changes in a narrow range in the intermediate energy region (468; 662 and 1173 keV). After irrigation, the gamma transmission measurements were achieved by using 137 Cs (662 keV) gamma source and the total linear
Fig. 2. Mass attenuation coe3cients of soils for di0erent photon energies.
attenuation coe3cients are calculated by using transmission measurements. The linear attenuation coe3cient increases linearly with the water content (Phogat et al., 1991). The amount of water added for each irrigation is 10% of the mass of dry soil in this investigation. As shown in Fig. 3, ds =wet ratio decreases signi:cantly as the water content increases since the total linear attenuation coe3cient of wet soil increases where, wet is the linear attenuation coe3cient of wet soil. The porosity of the considered soils was determined by using the transmission measurements and also Eq. (7) as presented in Fig. 4. From the graph, it can be seen that as the porosity of soil increases, ln(Ids =Iss ) increases since the gamma transmission dramatically decreases in the saturated soil column. It is also shown the relationship between the :eld capacity factor, R , and porosity in Fig. 4. The :eld capacity factor explains the change of the linear attenuation coe3cient of wet soil after drainage according to that of dry soil and we propose for it here the following
20
A. Filiz Baytas , S. Akbal / Radiation Measurements 35 (2002) 17–21
Fig. 3. The water content e0ect on linear attenuation coe3cient of soil for 662 keV.
Fig. 4. The porosities of the studied soils vs ln(Ids =Iss ) and the :eld capacity factor.
expression: ds ; R = drain
(11)
where drain is linear attenuation coe3cient of soil which is measured after drainage. As can be clearly seen from Fig. 4, as the porosity of soil increases the :eld capacity factor of soil decreases (drain ds ). Following each irrigation and also after saturation, the transmission measurements were performed with sources at di0erent depths of the soil column. The measuring points are p1, p2 and p3 as shown in Fig. 1. The water saturation vs soil density is shown for different water contents and for di0erent measuring locations (p1; p2; p3) in Fig. 5. As shown in Fig. 5, the saturation occurs more rapidly as soil density increases.
Fig. 5. The behaviour of water saturation vs soil density for measuring locations, (a) p1, (b) p2, and (c) p3.
A. Filiz Baytas , S. Akbal / Radiation Measurements 35 (2002) 17–21
5. Conclusions The gamma-ray transmission system is used for the determination of soil parameters such as the photon attenuation coe3cient, the local water saturation and porosity. To this end, soils of di0erent densities from 1:54 to 1:9 g=cm3 were collected from di0erent locations of Turkey. The mass attenuation coe3cients of soils were determined for di0erent energy levels and they change in a narrow range as expected from theoretical considerations. Moreover, the e0ect of water content on the linear attenuation coe3cient of soil was examined. Furthermore, the results show that the :eld capacity factor of soil decreases as the porosity of soil increases, and the soil is saturated at low water content as soil density increases. Acknowledgements This work was supported by the Istanbul Technical University Research Fund through grant 1116. The gamma sources were provided by the courtesty of C(ekmece Nuclear Research and Training Centre, Istanbul. References Akbal, S., 1999. Measurement of photon attenuation coe3cient in soil samples. M.Sc. Thesis, ITU, Istanbul.
21
Barataud, F., Moyne, C., Stemmelen, D., 1999. Measurement of soil water di0usitivity of an undisturbed forest soil using dual-energy gamma radiation technique. Soil Sci. 164, 493–502. Cesareo, R., Teixeira de Assis, J., Crestana, S., 1994. Attenuation coe3cients and tomographic measurements for soil in the energy range 10–300 keV. Appl. Radiat. Isot. 45, 613–620. Fishman, A., Notea, A., Segal, Y., 1981. Gamma transmission gauge for assay of integral water content in soil. Nucl. Instrum. Methods 184, 571–576. Lamarsh, J.R., 1977. Introduction to Nuclear Engineering. Addison-Wesley, Reading, MA. Moseley, W.A., Dhir, V.K., 1992. Capillarity in porous media and the e0ect of wettability. Heat and Mass Transfer in Porous Media ASME HTD-216, 1–7. Mudahar, G.S., Modi, S., Singh, M., 1991. Total and partial mass attenuation coe3cients of soil as a function of chemical composition. Appl. Radiat. Isot. 42, 13–18. Mudahar, G.S., Sahota, H.S., 1988. Soil: a radiation shielding material. Appl. Radiat. Isot. 39, 21–24. Oliveira, J.C.M., Vaz, C.M.P., Reichardt, K., Swartzendruber, D., 1997. Improved soil particle-size analysis by gamma-ray attenuation. Soil Sci. Soc. Am. J. 61, 23–26. Phogat, V.K., Aylmore, L.A.G., Schuller, R.D., 1991. Simultaneous measurement of the spatial distribution of soil water content and bulk density. Soil Sci. Soc. Am. J. 55, 908–915. Singh, M., Mudahar, S., 1992. Energy dependence of total photon attenuation coe3cients of composite materials. Appl. Radiat. Isot. 43, 907–911. Vaz, C.M.P., Naime, J.M., Macedo, A., 1999. Soil particle size fractions determined by gamma-ray attenuation. Soil Sci. 164, 403–410.
www.elsevier.com/locate/radmeas
Determination of soil parameters by gamma-ray transmission A. Filiz Bayta(sa; ∗ , Sevgi Akbalb a Istanbul
Technical University, Institute for Nuclear Energy, 80626 Maslak, Istanbul, Turkey Nuclear Research and Training Center, PK1 Havaalani 34831, Istanbul, Turkey
bC ekmece
Received 23 August 2000; received in revised form 10 April 2001; accepted 12 May 2001
Abstract Gamma-ray transmission methods have been used accurately for the study of the properties of a porous medium such as soil. In this study, di0erent soil parameters are determined by using gamma-ray transmission method. To this end, the soil samples were collected from various regions of Turkey and a NaI (Tl) detector measured the attenuation of strongly collimated monoenergetic gamma beam through soil samples. The mass attenuation coe3cients of dry soil samples were calculated from the transmission measurements for di0erent photon energies. Furthermore, the soil samples were irrigated by adding known quantities of water and the soil–water properties were examined. The local water saturation and porosity were estimated from c 2002 Elsevier Science Ltd. All rights reserved. the transmission measurements for each soil sample. Keywords: Gamma ray transmission; Soil; Attenuation coe3cients; Porosity; Saturation
1. Introduction The gamma-ray transmission method is often used for the study of the soil–water properties. This is done by adding a known amount of water to the system and accounting for this by the attenuation of gamma radiation. To obtain non-destructive measurements of bulk density and water content, soil scientists have extensively used single and dual source gamma attenuation measurements (Phogat et al., 1991). For example, a dual gamma-ray apparatus was used to determine water content variations in soil samples during in:ltration (Barataud et al., 1999). The photon attenuation coe3cient is an important parameter characterising the penetration and di0usion of gamma-rays in composite materials such as alloys, organic and inorganic compounds, soils and biological materials (Singh and Mudahar, 1992). The e0ects of di0erent parameters on the attenuation coe3cients of soils have been discussed in several studies by using the gamma transmission method. ∗ Corresponding author. Tel.: +90-212-285-3945; fax: +90-212285-3884. E-mail address: [email protected] (A. Filiz Bayta(s).
Soils have a chemical composition characterised by the presence of SiO2 ; Al2 O3 ; CaO; MgO; K2 O and others in variable concentrations. A large number of trace elements are also present in soils, but the presence of trace elements has generally no inCuence on the soil attenuation coe3cient (Cesareo et al., 1994). The mass attenuation coe3cients depend on the chemical composition of the absorbing material and the energy of the gamma rays. However, for the total photon interaction, the variation of total mass attenuation coe3cients with soil composition is much below 50 keV and it is negligible above 300 keV up to 3 MeV (Mudahar et al., 1991). The size distribution of particles is important for physical characterisation of soil. The gamma-ray attenuation method is also used to determine the soil particle size as a new procedure (Oliveira et al., 1997; Vaz et al., 1999). To support the use of soil as a radiation protection material, the e0ects of soil grain size and pressure on gamma-ray attenuation have been tested in the di0erent energy regions (Mudahar and Sahota, 1988). The purpose of this paper is to determine soil parameters such as photon attenuation coe3cient, porosity, local water saturation and :eld capacity of soils by using the gamma-ray transmission technique. The soils under consideration were collected from di0erent
c 2002 Elsevier Science Ltd. All rights reserved. 1350-4487/02/$ - see front matter PII: S 1 3 5 0 - 4 4 8 7 ( 0 1 ) 0 0 2 5 3 - 0
18
A. Filiz Baytas , S. Akbal / Radiation Measurements 35 (2002) 17–21
regions of Turkey. The gamma attenuation measurements have been carried out at intermediate energy region. The effects of :eld capacity of soils and density of soils on the porosity and the local water saturation are studied by another set of experiments. 2. Background The attenuation of gamma-rays in a medium is expressed by I = I0 exp(−x);
(1)
where I0 is the initial intensity of gamma rays and I is the intensity of gamma-rays after attenuation through a media of length x; is the linear attenuation coe3cient of he material. is a macroscopic cross-section for gamma ray interaction like macroscopic neutron cross-section and it has units cm−1 (Lamarsh, 1977). It can be described as = (=);
(2)
where (=) is the mass attenuation coe3cient and is the physical density. If we assume that Ids represents the beam intensity after having passed through the column :lled with dry soil and Iss represents the intensity after having passed through fully saturated soil, then Eq. (1) can be rewritten as follows: Ids = I0 exp(−s xs );
(3a)
Iss = I0 exp(−s xs − w xw );
(3b)
where the subscripts s and w denote soil and water, respectively, and xw is the equivalent thickness of water which is spread in the soil system of thickness xs . Combining Eqs. (3a) and (3b) gives Ids w ; (4) xwsat = ln Iss
measurements as xwp Sp = ; xwsat where xwp is given by Ids xwp = ln w Ip
(8)
(9)
and Ip represents the beam intensity after passing through a p depth of the column. By using Eq. (8), Eq. (9) can be rearranged as follows: ln(Ids =Ip ) Sp = : (10) ln(Ids =Iss ) Thus, one can estimate soil porosity and local water saturation characteristics for each soil by performing gamma-ray transmission measurements, and using Eqs. (7) and (10). 3. Experimental set-up and measurements The experimental set-up consists of a NaI(Tl) detector and a gamma source both encased in lead as shown in Fig. 1. Since the narrow beam was obtained by using portable parts of columnator in this system, the diameter of the columnator for the source and the detector can be selected di0erent values in the experimental set-up. For this study, the diameter of columnator was adjusted to 5 mm. The source and the detector were mounted on a base plate. A narrow beam of gamma rays is absorbed by the detector after passing through the test column. A multichannel analyzer was used to count the signal magnitude of the transmitted gamma-rays. A soil sample was used in a 5 × 5 cm cross-sectional and 25 cm length perspex column that could be moved downward and upward. The bottom of the column is conical in shape with the provision of an outlet hole that could be closed. The soil samples vary widely due to di0erences in composition, topography and climate (Akbal, 1999). The chemical composition of the soil samples considered is given
where xwsat represents the thickness of the sample column :lled water and soil at saturation (Moseley and Dhir, 1992). The porosity () of a medium can be expressed as =
void volume : total volume
(5)
Also, the porosity of the saturated soil medium can be rewritten by using transmission measurements as xwsat : (6) = xwsat + xs By using Eq. (4), Eq. (6) can be rearranged as follows: −1 x s w = 1 + : (7) ln(Ids =Iss ) Also, the local water saturation (Sp ) at a p depth of the column axis can be written by using transmission
Fig. 1. The schematic diagram of gamma transmission system; 1 is the gamma-ray source, 2 Pb shields, 3 Portable Pb collimators, 4 NaI(Tl) detector, 5 HV, 6 ampli:er, 7 MCA, 8 soil column, p1, p2 and p3 measuring points.
A. Filiz Baytas , S. Akbal / Radiation Measurements 35 (2002) 17–21
19
Table 1 The chemical composition of soils Soil locations
Ankara Konya Fatsa Bergama Batman
Chemical components (%) SiO2
Al2 O3
Fe2 O3
CaO
MgO
SO3
K2 O
Na2 O
PF
34.25 38.39 52.78 56.98 58.12
6.83 7.62 19.06 16.1 13.16
24.1 3.23 6.59 4.73 5.73
12.2 22.4 3.76 5.01 5.46
1 1.62 0.94 1.89 3.74
0.68 0.31 0.16 0.28 0.23
1.32 1.5 4.47 2.62 2.17
1.13 0.28 2.67 2.24 1.48
16.7 23.3 7.84 7.58 9.64
in Table 1. The densities of soils vary from 1:54 to 1:9 g=cm3 . Dry soil was passed through a 2 mm sieve and packed in the perspex column. In order to :nd the compaction of the soil column, the gamma-ray transmission measurements were taken at di0erent depths of the column. For dry soil samples, three di0erent photon energies (468 keV from 192 Ir; 662 keV from 137 Cs and 1173 keV from 60 Co) were used. After the gamma-ray transmission measurements were taken for dry soil, the bottom of the perspex column was closed and a known volume of water was poured on the surface of the soil column. Irrigation was applied to the surface up to the saturation. When water was ponding on the surface, the saturation occurred. After each irrigation and also after saturation, the transmission measurements were repeated at di0erent depths of the column. Then after saturation, the bottom of the perspex column was opened and the system was drained of water. The system was left in this condition for several hours. The soil column retains a certain amount of water according to its :eld capacity. The gamma-ray transmission measurements at the same depth of soil column were also repeated in this condition. Local water saturation and porosity were calculated from the transmission measurements for each soil sample. Also, the water attenuation coe3cient was determined experimentally by measuring the gamma attenuation through a known thickness of water. 4. Results and discussion The mass attenuation coe3cients of soils were calculated from Eqs. (1) and (2) for known physical densities by using gamma transmission measurements for dry soil samples. The e0ect of chemical composition of soil on the mass attenuation coe3cient is shown by means of the SiO2 concentration in soil, since SiO2 is most abundant chemical components of soil (Fishman et al., 1981). Fig. 2 presents a plot of the mass attenuation coe3cients (=)ds for di0erent soils as a function of the SiO2 concentration. From Fig. 2, it is seen that the mass attenuation coe3cient for the di0erent soil samples changes in a narrow range in the intermediate energy region (468; 662 and 1173 keV). After irrigation, the gamma transmission measurements were achieved by using 137 Cs (662 keV) gamma source and the total linear
Fig. 2. Mass attenuation coe3cients of soils for di0erent photon energies.
attenuation coe3cients are calculated by using transmission measurements. The linear attenuation coe3cient increases linearly with the water content (Phogat et al., 1991). The amount of water added for each irrigation is 10% of the mass of dry soil in this investigation. As shown in Fig. 3, ds =wet ratio decreases signi:cantly as the water content increases since the total linear attenuation coe3cient of wet soil increases where, wet is the linear attenuation coe3cient of wet soil. The porosity of the considered soils was determined by using the transmission measurements and also Eq. (7) as presented in Fig. 4. From the graph, it can be seen that as the porosity of soil increases, ln(Ids =Iss ) increases since the gamma transmission dramatically decreases in the saturated soil column. It is also shown the relationship between the :eld capacity factor, R , and porosity in Fig. 4. The :eld capacity factor explains the change of the linear attenuation coe3cient of wet soil after drainage according to that of dry soil and we propose for it here the following
20
A. Filiz Baytas , S. Akbal / Radiation Measurements 35 (2002) 17–21
Fig. 3. The water content e0ect on linear attenuation coe3cient of soil for 662 keV.
Fig. 4. The porosities of the studied soils vs ln(Ids =Iss ) and the :eld capacity factor.
expression: ds ; R = drain
(11)
where drain is linear attenuation coe3cient of soil which is measured after drainage. As can be clearly seen from Fig. 4, as the porosity of soil increases the :eld capacity factor of soil decreases (drain ds ). Following each irrigation and also after saturation, the transmission measurements were performed with sources at di0erent depths of the soil column. The measuring points are p1, p2 and p3 as shown in Fig. 1. The water saturation vs soil density is shown for different water contents and for di0erent measuring locations (p1; p2; p3) in Fig. 5. As shown in Fig. 5, the saturation occurs more rapidly as soil density increases.
Fig. 5. The behaviour of water saturation vs soil density for measuring locations, (a) p1, (b) p2, and (c) p3.
A. Filiz Baytas , S. Akbal / Radiation Measurements 35 (2002) 17–21
5. Conclusions The gamma-ray transmission system is used for the determination of soil parameters such as the photon attenuation coe3cient, the local water saturation and porosity. To this end, soils of di0erent densities from 1:54 to 1:9 g=cm3 were collected from di0erent locations of Turkey. The mass attenuation coe3cients of soils were determined for di0erent energy levels and they change in a narrow range as expected from theoretical considerations. Moreover, the e0ect of water content on the linear attenuation coe3cient of soil was examined. Furthermore, the results show that the :eld capacity factor of soil decreases as the porosity of soil increases, and the soil is saturated at low water content as soil density increases. Acknowledgements This work was supported by the Istanbul Technical University Research Fund through grant 1116. The gamma sources were provided by the courtesty of C(ekmece Nuclear Research and Training Centre, Istanbul. References Akbal, S., 1999. Measurement of photon attenuation coe3cient in soil samples. M.Sc. Thesis, ITU, Istanbul.
21
Barataud, F., Moyne, C., Stemmelen, D., 1999. Measurement of soil water di0usitivity of an undisturbed forest soil using dual-energy gamma radiation technique. Soil Sci. 164, 493–502. Cesareo, R., Teixeira de Assis, J., Crestana, S., 1994. Attenuation coe3cients and tomographic measurements for soil in the energy range 10–300 keV. Appl. Radiat. Isot. 45, 613–620. Fishman, A., Notea, A., Segal, Y., 1981. Gamma transmission gauge for assay of integral water content in soil. Nucl. Instrum. Methods 184, 571–576. Lamarsh, J.R., 1977. Introduction to Nuclear Engineering. Addison-Wesley, Reading, MA. Moseley, W.A., Dhir, V.K., 1992. Capillarity in porous media and the e0ect of wettability. Heat and Mass Transfer in Porous Media ASME HTD-216, 1–7. Mudahar, G.S., Modi, S., Singh, M., 1991. Total and partial mass attenuation coe3cients of soil as a function of chemical composition. Appl. Radiat. Isot. 42, 13–18. Mudahar, G.S., Sahota, H.S., 1988. Soil: a radiation shielding material. Appl. Radiat. Isot. 39, 21–24. Oliveira, J.C.M., Vaz, C.M.P., Reichardt, K., Swartzendruber, D., 1997. Improved soil particle-size analysis by gamma-ray attenuation. Soil Sci. Soc. Am. J. 61, 23–26. Phogat, V.K., Aylmore, L.A.G., Schuller, R.D., 1991. Simultaneous measurement of the spatial distribution of soil water content and bulk density. Soil Sci. Soc. Am. J. 55, 908–915. Singh, M., Mudahar, S., 1992. Energy dependence of total photon attenuation coe3cients of composite materials. Appl. Radiat. Isot. 43, 907–911. Vaz, C.M.P., Naime, J.M., Macedo, A., 1999. Soil particle size fractions determined by gamma-ray attenuation. Soil Sci. 164, 403–410.