Appl. Radiat. ht. Vol. 42, No. I, pp. 13-18, 1991 ht. J. Radiat. Appl. Instrum. Part A Printed in Great Britain. All rights reserved

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Total and Partial Mass Attenuation

Coefficients of Soil as a Function of

Chemical Composition

GURMEL S. MUDAHAR,* SANJAY MOD1 and MAKHAN SINGH

Department of Physics, Punjabi University, Patiala 147002, India

(Received for publication I I June 1990)

The mass attenuation coefficient (p,) of soil for total and partial photon interaction processes has been calculated for five different elemental compositions of soils, in the energy range from 10 keV to 100 GeV, to study the influence of chemical composition. For the total photon interaction coefficient, appreciable variations in aS due to chemical composition (soil type) are seen only below 300 keV and above 3 MeV photon energy. For partial interaction processes significant variations in pE are seen for photoeffect, coherent scattering and pair production in the nuclear field at all energies. No significant variations were noted in pS due to soil chemical composition for incoherent scattering and pair production in the electron field. We present graphs of the variations in pr for total and partial photon interaction processes in soil due to the Z-dependences of the interaction processes.

Introduction

In comparison with other, more-conventional methods of soil-water measurement, a non destruc- tive y-ray transmission method has many advantages, and makes use of the fact that scattering and absorp- tion of y-rays are related to the density and effective atomic number (chemical composition) of the matter. Different attempts (Gurr, 1962; Fritton, 1969; Corey et al., 1971; Saksena et al., 1974; Mudahar and Sahota 1985, 1986, 1988a,b) have been made to improve the accuracy of this radiometric method and to make it more convenient. A knowledge of the mass attenuation cofficient (p(,) of soil is of prime importance for these measurements. Hence we pre- sent a study of the mass attenuation coefficients of soils of different chemical compositions for different photon interaction processes covering a large energy region.

Reginato and Van Bavel (1964) calculated the theoretical pI of nine representative oven dry soil samples from the U.S. for 662 keV y-rays, and reported variations in ph of the order of I%, whereas Fritton (1969) found 4% variations in p, values for four different column lengths of soil (5.04, 10.13, 15.18 and 20.26 cm) for the same 662 keV y-rays.

Zavel’skii (1964) studying p, of different soils in different energy regions, concluded that for the aver- age photon energy (E = 0.5-1.25 MeV) the change

*Author for correspondence.

in ps in many sandy and clayey soil samples is less than 1%.

From the results of Corey et al. (1971) in connec- tion with their simultaneous measurements of soil density and moisture content, it is seen that the variation of p, in two different soils was found to be 1% for 662 keV and about 20% for 60 keV photons.

Saksena et al. (1974) in demonstrating the useful- ness of the y-ray transmission method for soil-water measurements, concluded that for the integrated pho- tons coming from ‘34Cs above 511 keV, p(5 is nearly constant for three different textured soils (medium sand, fine sand, and clay). Fishman et al. (1981) reported that p(5 of different soil units changes within 4.5% for 13’Cs and 6oCo isotopes.

In our previous work (Mudahar and Sahota, 1985) pI values were measured for two different soils at three-y energies from the decay of *03Hg, 13’Cs and “Co sources. Our results indicated that for a particu- lar photon energy, p(, is independent of the type of soil. We have also conducted y-ray transmission studies in different soils (Mudahar and Sahota, 1986, 1988a,b), and other workers (Christensen, 1974; Gardener and Calissendorf, 1967; King, 1967; Zvolsky and Vovk, 1971; Kirkham and Corey, 1973; Fahad, 1989) have similarly used the transmission method in different soil systems using different photon energies.

Our present investigations of the mass attenuation coefficients pcs of different soils for both total and partial photon interaction processes (e.g. photo-

I3

14 GURMEL S. MUDAHAR et al.

electric absorption, coherent scattering, incoherent scattering and pair production) should be useful to soil scientists and workers in related fields, filling a gap in the available information.

In this paper, detailed calculations have been made to study the effect of chemical composition on p(5 for total and partial photon interaction processes in five different soils covering the extensive energy region from 10 keV to 100 GeV.

Calculation Work

In the present studies, five types of soils (Coppola and Reiniger, 1974) of different chemical composition were chosen, which are given in Table I. The vari- ation in elemental abundance in these soils is signifi- cant and is suitable for the present investigations e.g. the percentage of Al, Si, Ca and Fe etc.

The calculations were made with the help of a state-of-the-art convenient computer program and data base prepared by Berger and Hubbell (1987) named “XCOM: photon cross sections on a personal computer”. Our calculations were made on a per- sonal computer (PC/XT) in the Department of Physics, Punjabi University, Patiala (India) for all five type of soils. Taking advantage of the capability of the XCOM program, p, values were computed in cm’/g over a wide energy range from IO keV to 100 GeV.

Results and Discussion

The results of the present investigation are shown graphically in Figs I to 7, where p5 is given as a function of photon energy in all photon interaction processes.

In Fig. I, the p5 values of Cecil sandy loam (CSL) soil for total and partial photon interactions have been plotted against the photon energy. The plots of p5 for all the photon interactions are similar to those of elements of low atomic number such as Al, Si, P, S etc. because of the fact that effective Z values of the soils studied lie in this range. As the present work is

to check the effect of soil chemical composition on pY, the plots of pI vs energy for different interactions are discussed in the following paragraphs.

For the total photon interaction, the variation of p5 (total) with soil composition is large below 50 keV and negligible above 300 keV up to 3 meV, beyond which there is again significant variation in p, up to 100 GeV. Present results are consistent with the find- ings of Coppola and Reiniger (1974) who have also studied the influence of chemical composition on p, from IO keV to 3 MeV for the total photon inter- action process, and who concluded that the signi- ficant variations occur only up to 300 keV photon energy, beyond which there is no significant change in per due to soil chemical composition up to 3 MeV. Above 3 MeV, a region not previously studied to our knowledge, significant variations are seen in p5 due to chemical composition up to 100 GeV (Fig. 2). Along with the total, the partial photon interaction pro- cesses have also been considered in the same energy range.

It is also clear in Fig. 2 that the soil (such as Nipe clay), which contains a higher percentage of heavy elements such as iron, has higher values of p5 in the energy regions where variation is signi- ficant. This result was also noted by Zavel’skii (1964) who has proposed a direct relationship of p(5 with heavy metals in rocksalt for low photon energies.

The above interpretation of Fig. 2 can also be extended to the other figures (3 to 7) in which the effect of soil chemical composition on p, is investi- gated for partial photon interactions (atomic photo- effect (photo), coherent (cob) and incoherent (incoh) scattering, pair production in the electron field, and pair production in the nuclear field) in the same five soils.

Figure 3 shows the most significant variation in p\ (photo) due to soil chemical composition. In com- parison to the photoelectric absorption, the variation in p(5 (cob) due to soil chemical composition is less but still significant (Fig. 4). The results of p, (photo) and pI (cob) clearly explain the variation of p(, (total) with

Element

Table 1. Percentage (by weight) of elements in different soils

Average earth C~ICWSXIS Norfolk Cecil Cr”St Clay sandy loam sandy loam

WC) KC) (NW (CSL)

Nipe ClkiY

(NC)

H c N 0

NLi

W Al SI P

a K

CL+ TI

Mn Fe

46.46 2 75 2.07 8 07

2161 0.12 0.06 0.05 2.58 3.64 0.62 0.09 5.06

0.02

4.78 -

49.00 0.36 I 35’ 6.18,

19.45

- I .46

I2 48 0.47

2.85

- - I.13 I .02 0.24 0.27 0.07 0.03 0.03

52.09 47.55 40.33 0.08 0.1 I 0.02 0.05 0.05 0.16 I.10 14.60 7.78

44.18 24.52 3.14 0.03 0.07 0.02 0.03 0 03 0.10 -

0.08 0.2x 0.43 0.01 0.58

0.51 0.31 0.29 0.6X 0.44 0.02 0.84

II.35 44.76

Attenuation coefficients of soil 15

s- IU

E

Y ib3 =i I

Photon energy(MeV)

Fig. I. Mass attenuation coefficients of Cecil sandy loam (CSL) soil vs photon energy for different photon interaction processes.

soil type below 300 keV in Fig. 2, because in this energy region the photoelectric and coherent scatter- ing are predominant. These variations in pr are further related to Z-dependence of the interaction processes, where it is Z4m5 for photoeffect and Z2~3 for coherent scattering. Perumallu el al. (1985) have also suggested the Z-dependence of atomic photoeffect and coherent scattering in multi-element materials of

biological importance, of the order of 2”’ for photo- effect and Z*-’ for coherent scattering.

For incoherent (Compton) scattering, no signifi- cant variation is seen in pr (incoh) due to the soil type (Fig. 5). This may be because Z dependence of incoherent scattering is linear. As this process is dominant in the medium energy region, the middle portion of Fig. 2 from 300 keV to 3 MeV can be

I I I ,

IO2 Id lo” IO5 - Photon energy(MeV) --+

Fig. 2. Total mass attenuation coefficients of soils as a function of photon energy. AEC, average earth crust: CC. calcareous clay; NSL. Norfold sandy loam; CSL, Cecil sandy loam: NC, Nipe clay.

GURMELSMLJDAHAR~~ al. 16

- Photon erlergy (MeV)--t

Fig. 3. Mass attenuation coefficients of soils vs photon energy energy for photoelectric absorption.

related to the results of pS (incoh). Because of the linear Z-dependence of this process, there is no significant variation in pS (total) due to soil chemical composition, and in this region pair production is not yet dominant.

The results of pL, for pair production in nuclear

and electron fields are shown in Figs 6 and 7 respect-

Photon energy ( MeV) IO'

Fig. 4. Mass attenuation coefficients of soils vs photon energy for coherent scattering.

ively. Significant variations are seen in the mass attenuation coefficient of soil for pair production

Fig. 5. Mass attenuation coeficienrs of soils vs photon energy for incoherent scattering

Attenuation coefficients of soil 17

10' / NC

EC

- Photon energy(MeV) -

Fig. 6. Mass attenuation coefficients of soils vs photon energy for pair production in the nuclear field.

in the nuclear field (Fig. 6) but no variation is seen for pair production in the electron field (Fig. 7)

References

due to the soil chemical composition. It may be because pair production in the nuclear field is Z* dependent, whereas the Z dependence of pair pro- duction in the electron field is linear. In the high energy region, the variation in total p5 (Fig. 2) is due to Z?-dependence of the pair production in the nuclear field.

Acknowledgements-The authors thank Dr H. S. Sahota, Professor of Physics, Punjabi University, Patiala, for fruitful discussions and one of us (G.S.M.) is grateful to the University Grants Commission, Government of India, for

Berger M. J. and Hubbell J. H. (1987) XCOM: Photon cross sections on a personal computer. NBSIR 87-3597.

Christensen E. R. (1974) Use of the gamma density gauge in combination with the neutron moisture probe. In Isotope and Radiation Techniques in Soil Physics and Irrigation Studies, 1973. IAEA Report SM-176/l, pp. 2744.

Coppola M. and Reiniger P. (1974) Influence of the chemi- cal composition on the gamma-ray attenuation by soils. Soil Sri. 117, 331.

Corey J. C., Peterson S. F. and Wakat M. A. (1971) Measurement of “‘Cs and 14’Am gamma rays for soil density and water content determination. Soil Sci. Sot.

the award of a research grant to carry out this work. Am. j. 35, 215

k---- NC+ CSL+ AEC+CC+NSL

I I I I

Id lo2 3

10 lo’ 1; 1 - Photon energy (MeV) -

Fig. 7. Mass attenuation coefficients of soils vs photon energy for pair production in the electron field.

18 GURMEL S. MUDAHAR et al.

Fahad A. A. (1989) A computer-controlled dual gamma scanner for measurement of soil water content and bulk density. Appi. Radiat. Isot. 40, 340.

Fishman A., Notea A. and Segal Y. (1981) Gamma trans- mission gauge for assay of integral water content in soil. Nucl. Instrum. methods 184, 571.

Fritton D. D. (1969) Resolving time, mass attenuation coefficient and water content with gamma ray attenu- ation. Soil Sci. Sot. Am. Proc. 33, 651,

Gardener W. H. and Calissendorff C. (1967) Gamma ray and neutron attenuation in the measurement of soil bulk density and water content. In Symposium on the Use of Isotopes and Radiation Techniques, Istanbul, 1967. IAEA, Vienna.

Gurr C. G. (1962) Use of gamma ray measuring water content and permeability in unsaturated columns of soil. Soil Sci. 94, 224.

King L. G. (1967) Gamma ray attenuation for soil-water content measurement using *“‘Am. In Symp. on the use of Isotopes and Radiation Techniques in Soil Physics and Irrigation Studies, Istanbul, 1967. IAEA Report SM-94120 pp. 17-29. IAEA, Vienna.

Kirkham D. and Corey J. C. (1973) Recent progress in the design of radiation equipment and its practical impli- cation. In Soil-moisture and Irrigation Studies II Vienna, 1973; pp. 17-39. IAEA, Vienna.

Mudahar G. S. and Sahota H. S. (1985) Optimal thickness of soil between source and detector for different gamma- ray energies. J. Hydrol. 80, 265.

Mudahar G. S. and Sahota H. S. (1986) A new method for simultaneous measurement of soil bulk density and water content. Int. J. Appl Radiat. Isot. 31, 563.

Mudahar G. S. and Sahota H. S. (1988a) Soil: A radiation shielding material. Appl. Radial. Isot. 39, 21.

Mudahar G. S. and Sahota H. S. (1988b) Effective atomic number studies in different soils for total photon inter- action in the energy region 10-5000 keV. Appl. Radiat. Isot. 39, 1251.

Perumallu A., Nageswara Rao A. S. and Krishna Rao G. (1985) Z-dependence of photon intractions in multi- elements materials. Physica 132C, 388.

Reginato R. J. and Van Bavel C. H. M. (1964) Soil water measurement with gamma attenuation. Soil Sci. Sot. Am. Proc. 28, 721.

Saksena R. S., Chandra S. and Singh B. P. (1974) A gamma-ray-transmission method for the determination of moisture content in soils. J. Hydrol. 23, 341.

Zavel’skii F. S. (1964) Mass absorption coefficients of y-radiation in soils and errors in measurements made by the y method. At. Energy 16, 266.

Zvolsky S. T. and Vovk P. K. (1971) Gamma-ray properties of dispersive grounds. Bull. Ukrainian Acad. Sci. No. 2. 133.