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Studies on mass attenuation coefficient, effective atomic number and electron density of some amino acids in the energy range 0.122–1.330 MeV
Radiation Physics and Chemistry 92 (2013) 22–27
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Studies on mass attenuation coefficient, effective atomic number and electron density of some amino acids in the energy range 0.122–1.330 MeV Pravina P. Pawar n, Govind K. Bichile Department of Physics, Dr.Babasaheb Ambedkar Marathwada University, Aurangabad 431004, India
H I G H L I G H T S
Compute the Mass Attenuation Coefficient, effective atomic number and electron density of some amino acids. Gamma ray attenuation studies on biologically important molecules have been carried out using narrow beam good geometry set up. The values of Mass Attenuation Coefficient, effective atomic number and electron density of some amino acids are in agreement with the XCOM programme. The measured mass attenuation coefficient for some amino acids are useful in medical field. The data is useful in radiation dosimetry and other fields.
art ic l e i nf o
a b s t r a c t
Article history: Received 15 February 2013 Accepted 4 July 2013 Available online 17 July 2013
The total mass attenuation coefficients of some amino acids, such as Glycine (C2H5NO2), DL-Alanine (C3H7NO2), Proline (C5H9NO2), L-Leucine (C6H13NO2 ), L-Arginine (C6H14N4O2) and L-Arginine Monohydrochloride (C6H15ClN4O2), were measured at 122, 356, 511, 662, 1170, 1275 and 1330 keV photon energies using a well-collimated narrow beam good geometry set-up. The gamma rays were detected using NaI (Tl) scintillation detection system with a resolution of 10.2% at 662 keV. The attenuation coefficient data were then used to obtain the effective atomic numbers (Zeff) and effective electron densities (Neff) of amino acids. It was observed that the effective atomic number (Zeff) and effective electron densities (Neff) tend to be almost constant as a function of gamma-ray energy. The results show that, the experimental values of mass attenuation coefficients, effective atomic numbers and effective electron densities are in good agreement with the theoretical values with less than 1% error. Published by Elsevier Ltd.
Keywords: Mass attenuation coefficients Atomic cross-section Electronic cross-section Effective atomic number Effective electron density.
1. Introduction The study of photon interactions with matter is important. The data on the transmission and absorption of X-rays and gammarays in biological shielding and dosimetric materials assumed great significance by virtue of their diverse applications in the field of medical physics and medical biology (Kaewkhao et al., 2008). Gamma radiations in the energy region above 200 keV up to about 1500 keV interact with matter predominantly by photoelectric effect, Compton effect, and pair production processes. The photon interaction cross-section can be expressed as a function of the photon energy and the atomic number Z. At a given photon energy, the interaction cross section is proportional to Zn where n
n
Corresponding author. Tel.: +91 0240 2486706. E-mail address: [email protected] (P.P. Pawar).
0969-806X/$ - see front matter Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.radphyschem.2013.07.004
is expected to be between 4 and 5 for the photoelectric effect, 1 for the Compton effect, and 2 for pair production (Murthy, 1965). It is often found convenient to represent the gamma-ray interaction properties of a composite material consisting of a number of different elements in varying proportions by an effective atomic number Zeff. This number depends on the incident energy as well as the atomic number of the constituent elements. It indicates on average, the number of electrons of the material that actively participate in the photon–atom interaction. Hence, the Zeff is used frequently in calculations of mass energy absorption coefficients and Kerma in radiation dosimetry (Manjunathaguru and Umesh, 2009). This parameter is also used in the calculation of Compton profiles of complex materials and hence may yield valuable information about the chemical environment that surrounds the atom in a quantitative manner. Hine (1952) pointed out that there is a different atomic number for each interaction process in a complex material.
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In order to make use of the fact that scattering and absorption of gamma-radiations are related to the density and effective atomic numbers of the material, a knowledge of the mass
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attenuation coefficients is of prime importance. From the mass attenuation coefficient, a number of related parameters can be derived, such as the mass energy-absorption coefficient, the total interaction cross-section, the molar extinction coefficient, the effective atomic number, and the effective electron density. The mass attenuation coefficient, μ/ρ, is a measure of the average number of interactions between incident photons and matter that occur in a given mass per unit area thickness of the substance encountered. A precise knowledge of the effective atomic number of the composite material is necessary for various purposes. However, the theoretical expressions for effective atomic number of composite materials are not very accurate at all energies. Early calculations of effective atomic numbers were based on parameterization of the photon interaction cross-section by fitting data over limited ranges of photon energy and atomic number (Jackson and Hawkes, 1981). Today, accurate databases of photon interaction cross-sections and interpolation programs (Gerward et al., 2001,2004); (Berger and Hubbell, 1999) have made it possible to calculate effective atomic numbers with much improved accuracy and information content over wide ranges of photon energy and elemental composition. A simple and widely used method of obtaining Zeff of a composite material consisting of different elements in definite proportions is based on the determination of total attenuation coefficients for gamma-ray interactions by the transmission method. In the present work, the method of deriving effective atomic numbers in composite materials was followed by using the experimental results of mass attenuation coefficients. Experimental results were compared with theoretical values. A variety of physiological functions inside living systems are performed by complex molecules such as carbohydrates, proteins, fats and oils composed of H, C, N and O elements. Sugars and amino acids are the building blocks of carbohydrates and proteins. Photons of energy from 1500 keV down to about 5 keV are widely used in medical and biological applications (Hubbell, 1999) especially during diagnosis and therapy. A thorough knowledge of the
0.2 Fig. 1. (a) The schematic view of the experimental set up. (b) Nuclear electronic system of a scintillation spectrometer.
Molar mass (g/ Chemical mol) formula
Mean atomic number, Z
Glycine DL-Alanine Proline L-Leucine L-Arginine LR L-Arginine Monohydrochloride LR
75.10 89.10 115.13 131.17 174.20 210.66
4.00 3.69 3.65 3.27 3.62 4.00
C2H5NO2 C3H7NO2 C5H9NO2 C6H13NO2 C6H14N4O2 C6H15ClN4O2
2/g) m(cm
Table 1 The mean atomic numbers calculated from the chemical formula for amino acids Amino acids
Exp.
0.16
Theo.
0.12 0.08 0.04 0
0
500 1000 Energy (keV)
1500
Fig. 2. The typical plot of μm versus energy E for Glycine (C2H5NO2).
Table 2 Mass attenuation coefficient μm (cm2/g) of amino acids. Sr. no.
1. 2. 3. 4. 5. 6.
Amino acids
Glycine DL-Alanine Proline L-Leucine L-Arginine LR L-Arginine Monohydro chloride LR
122 keV
356 keV
511 keV
662 keV
1170 keV
1275 keV
1330 keV
Exp.
Theo.
Exp.
Theo.
Exp.
Theo.
Exp.
Theo.
Exp.
Theo.
Exp.
Theo.
Exp.
Theo
0.154 0.155 0.155 0.160 0.156 0.162
0.155 0.156 0.156 0.157 0.155 0.160
0.104 0.107 0.107 0.107 0.108 0.107
0.106 0.108 0.108 0.110 0.108 0.106
0.094 0.093 0.093 0.095 0.095 0.091
0.092 0.093 0.092 0.094 0.094 0.090
0.084 0.084 0.083 0.083 0.083 0.083
0.083 0.083 0.082 0.084 0.084 0.082
0.062 0.065 0.065 0.065 0.063 0.064
0.0621 0.064 0.064 0.064 0.064 0.063
0.061 0.060 0.061 0.063 0.061 0.059
0.061 0.060 0.060 0.062 0.060 0.058
0.059 0.059 0.059 0.061 0.058 0.058
0.059 0.059 0.058 0.060 0.059 0.057
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P.P. Pawar, G.K. Bichile / Radiation Physics and Chemistry 92 (2013) 22–27
Table 3 Atomic cross-sections, st (barn/molecule) of amino acids. Sr. no.
1. 2. 3. 4. 5. 6.
Amino acids
122 keV
Glycine DL-Alanine Proline L-Leucine L-ArginineLR L-Arginine Monohydro Chloride LR
356 keV
511 keV
662 keV
1170 keV
1275 keV
1330 keV
Exp.
Theo.
Exp.
Theo.
Exp.
Theo.
Exp.
Theo.
Exp.
Theo.
Exp.
Theo.
Exp.
Theo
19.20 22.92 29.62 34.83 45.11 56.64
19.32 23.07 29.81 34.18 44.82 55.94
12.96 15.82 20.45 23.30 31.23 37.41
13.21 15.97 20.64 23.95 31.23 37.06
11.72 13.75 17.77 20.68 27.47 31.81
11.47 13.75 17.58 20.47 27.18 31.46
10.47 12.42 15.86 18.07 23.99 29.02
10.35 12.27 15.67 18.29 24.29 28.67
7.73 9.61 12.42 14.15 18.22 22.37
7.74 9.46 12.23 13.93 18.50 22.03
7.60 8.87 11.66 13.72 17.64 20.62
7.60 8.87 11.47 13.50 17.35 20.28
7.35 8.72 11.27 13.28 16.77 20.28
7.34 8.72 11.08 13.06 16.06 19.93
Table 4 Electronic cross-sections, se (barn/molecule) of amino acids. Sr. no.
1. 2. 3. 4. 5. 6.
Amino acids
122 keV
Glycine DL-Alanine Proline L-Leucine L-Arginine LR L-Arginine Monohydro Chloride LR
356 keV
511 keV
1275 keV
1330 keV
Theo.
Exp.
Theo.
Exp.
Theo.
Exp.
Theo.
Exp.
Theo.
Exp.
Theo.
Exp.
Theo
4.54 6.30 8.25 11.15 12.71 14.05
4.57 6.34 8.30 10.94 12.63 13.87
3.23 4.34 5.68 7.40 8.78 9.3
3.29 4.38 5.74 7.61 8.78 9.22
2.93 3.77 4.94 6.55 7.71 7.92
2.86 3.77 4.88 6.49 7.63 7.83
2.62 3.41 4.40 5.72 6.73 7.23
2.58 3.36 4.35 5.79 6.82 7.14
1.93 2.63 3.44 4.45 5.11 5.58
1.94 2.59 3.39 4.39 5.18 5.50
1.90 2.43 3.23 4.32 4.94 5.14
1.90 2.43 3.18 4.25 4.86 5.06
1.84 2.38 3.12 4.18 4.70 5.06
1.83 2.38 3.07 4.11 4.50 4.97
6 Exp. Theo.
15 10
e exp
5 e (b/atom)
20 t(b/atom)
1170 keV
Exp.
25
e theo
4 3 2 1
5
0 0
662 keV
0
500
1000 Energy (keV)
0
1500
500 1000 Energy (keV)
1500
Fig. 4. The typical plots of se versus E are displayed for Glycine (C2H5NO2).
Fig. 3. The typical plots of st versus E for Glycine (C2H5NO2).
nature of interaction of these biologically important complex molecules such as amino acids is desirable over this energy region. Hence, in recent years, this has motivated many investigators over the years to determine the total attenuation cross-sections as well as composition dependent quantities such as effective atomic numbers (Zeff), and effective electron densities (Neff) of such complex molecules of biological interest in this energy region by employing different methods (Kirby et al., 2003; Midgley, 2004, 2005; Shivaramu, 2001; Shivaramu et al., 2001; Sandhu et al., 2002; Gowda et al., 2004, 2005; Manjunathaguru and Umesh, 2006; Manohara and Hanagodimath, 2007; El-Kateb and Abdul-hamid, 1991). Theoretical values for the mass attenuation coefficients can be found in the tabulation by Hubbell and Seltzer (1995). A convenient alternative to manual calculations, using tabulated data, is to generate attenuation data, as needed using a computer. For this purpose Berger and Hubbell (1999) developed XCOM software for calculating mass attenuation coefficients or photon interaction cross-sections for any element, compound for a wide range of energies. There have been a great number of experimental and theoretical investigations to determine mass attenuation coefficients for
complex biological molecules such as carbohydrates, proteins, fats and oils composed of H, C, N and O elements in varying proportions. Sandhu et al. 2002 have investigated fatty acids in the energy region 81–1330 keV. Gowda et al. (2004, 2005) have reported total attenuation cross-sections for sugar and amino acids. Recently Manjunathguru and Umesh (2006), Manohara and Hanagodimath (2007), have determined the effective atomic numbers of several biomolecules. Measurements on the sample containing H, C, and O in the energy range 54–1333 keV have been reported by El-Kateb and Abdul-hamid (1991). In this work, we have measured the mass attenuation coefficients, the atomic cross-sections, the effective atomic numbers, and the effective electron densities for H, C, N and O based amino acids in the energy range 122–1330 keV and then compared these experimentally evaluated parameters with theory using XCOM program.
2. Theory In this section we summarize some theoretical relations that have been used for the determination of mass attenuation coefficients in the present work. When a monoenergetic beam of gamma photons is incident on a target, some photons are removed
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Table 5 Effective atomic number, Zeff of amino acids. Sr. No.
Amino acids
1. 2. 3. 4. 5. 6.
122 keV
Glycine DL-Alanine Proline L-Leucine L-Arginine LR L-Arginine Monohydro Chloride LR
356 keV
511 keV
1330 keV
Exp.
Theo.
Exp.
Theo.
Exp.
Theo.
Exp.
Theo.
Exp.
Theo.
Exp.
Theo
4.23 3.64 3.59 3.12 3.55 4.03
4.23 3.64 3.59 3.12 3.55 4.03
4.01 3.65 3.60 3.15 3.56 4.02
4.02 3.65 3.60 3.15 3.56 4.01
4.00 3.65 3.60 3.16 3.56 4.01
4.01 3.65 3.60 3.15 3.56 4.01
3.99 3.64 3.60 3.16 3.56 4.01
4.01 3.65 3.60 3.16 3.56 4.01
4.01 3.65 3.61 3.18 3.57 4.01
3.99 3.65 3.61 3.17 3.57 4.01
4.00 3.65 3.61 3.18 3.57 4.01
4.00 3.65 3.61 3.18 3.57 4.01
3.99 3.66 3.61 3.18 3.57 4.01
4.01 3.66 3.61 3.18 3.57 4.01
chemical compound the fraction by weight (wi) is given by;
Exp.
4.015
Zeff
1275 keV
Theo.
4.02
wi ¼
Theo.
ni Ai ∑j nj Aj
ð5Þ
where Ai is the atomic weight of the ith element and ni is the number of formula units. The values of the mass attenuation coefficients were then used to determine the total molecular cross-section (st,m) by the following relation:
4.01 4.005 4
ðst;m Þ ¼ μm ðM=N A Þ;
3.995 0
500 1000 Energy (keV)
1500
Fig. 5. The typical plots of Zeff versus E for Glycine (C2H5NO2).
from the beam due to the dominant interaction processes and, therefore, the transmitted beam is attenuated. The extent of attenuation depends for a given elemental target on the photon energy. This attenuation of the beam is described by the following equation: I ¼ I 0 eμt
ð1Þ
where I0 and I are the unattenuated and attenuated photon intensities, respectively, and μ (cm 1) is the linear attenuation coefficient of the material. Linear attenuation coefficient more accurately characterizing a given material is density-independent. Rearrangement of Eq. (1) yields the following equation for the linear attenuation coefficient: μ¼
1170 keV
Exp.
4.025
3.99
662 keV
1 lnðI 0 =IÞ t
ð2Þ
In Eq. (2), t (cm) is the sample thickness. The mass attenuation coefficients μ/ρ(cm2 gm 1) for the samples were obtained from Eq. (3) by using the density of the corresponding samples: 2
1
μm ¼ μ=ρðcm gm
1 Þ ¼ lnðI 0 =IÞ ρt
ð3Þ
where ρ (cm2/gm) is a measured density of the corresponding sample. If the materials are composed of different elements in varying proportions, then it is assumed that the contribution of each element of the compound to the total interaction is additive, yielding the well known mixture rule of Hubbell and Seltzer (1995) which represent the total mass attenuation coefficient (μ/ρ)T of any compound as the sum of the appropriately weighted proportions of the individual atoms. The mass attenuation coefficient for a compound or mixture is given by μm ¼ ∑i wi ðμm Þi
ð4Þ
where wi and (μm)i are the weight fraction and mass attenuation coefficient of the ith constituent element, respectively. For a
ð6Þ
where M¼∑iniAi is the molecular weight of the compound, NA is the Avogadro′s number, ni is the total number of atoms (with respect to mass number ) in the molecule and Ai is the atomic weight of the ith element in a molecule. The total atomic cross-sections (st,a) can be easily determined from the following equation: st;a ¼
1 ∑ f A ðμ Þ NA i i i m i
ð7Þ
Similarly, effective electronic cross-section (st,el) for the individual element is given by: st;el ¼
1 st;a ∑ f A ðsm Þi ¼ NA Z A i i i Z eff
ð8Þ
where fi ¼ni/∑jnj and Zi are the fractional abundance and atomic number of constituent element I, respectively, ni is the total number of atoms of the constituent element i and ∑jnj is the total number of atoms present in the molecular formula. Now, the effective atomic number (Zeff) can be given as st;a Z eff ¼ ð9Þ st;el
3. Experimental set up and measurements The five radioactive sources 57Co, 133Ba, 137Cs, 60Co and 22Na were used in the present investigation. Gamma rays of energy 122, 360, 511, 662, 1170, 1275 and 1330 keV emitted by the above radioactive sources (procured from Bhaba Atomic Research Centre, Mumbai) were collimated and detected by NaI (Tl) detector having energy resolution of 10.2% at 662 keV. The signals from the detector were amplified and analyzed with 8 K multichannel analyzer. The amino acids (Glycine (C2H5NO2), DL-Alanine (C3H7NO2), Proline (C5H9NO2), L-Leucine (C6H13NO2 ), L-Arginine (C6H14N4O2) and L-Arginine Monohydrochloride (C6H15ClN4O2) under investigation were pellet shaped and confined in a cylindrical plastic container having the same diameter as that of sample pellet. The diameters of the samples were determined with the help of a traveling microscope. It was observed that the attenuation of photons of the empty containers were negligible. Each sample pellet was weighed in a sensitive digital balance having an accuracy 0.001 mg. The weighing was repeated several times to obtain consistent value of the mass. The mean of this set of values was taken
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P.P. Pawar, G.K. Bichile / Radiation Physics and Chemistry 92 (2013) 22–27
Table 6 Effective electron densities, Neff (1024) of amino acids. Sr. No. Amino acids
1. 2. 3. 4. 5.
122 keV
Glycine DL-Alanine Proline L-Leucine L-Arginine LR 6L-Arginine Monohydro Chloride LR
356 keV
511 keV Theo.
Exp.
Theo.
Exp.
Theo.
Exp.
Theo.
Exp.
Theo.
Exp.
Theo
0.3395 0.3199 0.3193 0.3152 0.3192 0.3220
0.3395 0.3199 0.3193 0.3152 0.3192 0.3220
0.3218 0.3208 0.3202 0.3182 0.3201 0.3210
0.3226 0.3208 0.3202 0.3182 0.3201 0.3210
0.3210 0.3208 0.3202 0.3193 0.3201 0.3210
0.3218 0.3208 0.3202 0.3182 0.3201 0.3210
0.3202 0.3199 0.3202 0.3193 0.3201 0.3210
0.3218 0.3208 0.3202 0.3193 0.3201 0.3210
0.3218 0.3208 0.3211 0.3213 0.3210 0.3200
0.3202 0.3208 0.3211 0.3203 0.3210 0.3200
0.3210 0.3208 0.3211 0.3213 0.3210 0.3210
0.3210 0.3208 0.3211 0.3213 0.3210 0.3200
0.3202 0.3217 0.3211 0.3213 0.3210 0.3200
0.3218 0.3217 0.3211 0.3213 0.3210
Neff
0.322
Theo.
0.3215 0.321 0.3205 500 1000 Energy (keV)
1500
Fig. 6. Shows the typical variation of effective electron density versus photon energy for Glycine (C2H5NO2) sample.
0.323 0.3225
Neff
0.322 Neff
0.3215 0.321 0.3205 4
4.01 Zeff
1330 keV
Exp.
Exp.
0.32 3.99
1275 keV
Theo.
0.3225
0
1170 keV
Exp.
0.323
0.32
662 keV
4.02
4.03
Fig. 7. Plot of effective electron number (Neff) as a function of effective atomic number (Zeff) for Glycine (C2H5NO2) sample.
to be the mass of the sample. By using the diameter of the pellet and mean value of the mass of the pellet, the mass per unit area was determined in each case. The sample thicknesses were selected in order to satisfy the following ideal condition as far as possible (Creagh, 1987): 2 o lnðI 0 =IÞ ≤4 For measurement of incident and transmitted photon energies a narrow beam good geometry set up was used. The schematic view of the experimental set up is displayed in Fig. 1(a) and the nuclear electronic system of the scintillation spectrometer is shown in Fig. 1(b). From the measured values of unattenuated photon intensity I0 (with empty plastic container) and attenuated photon intensity I (with sample), the mass attenuation coefficients (μ/ρ) for all the samples of amino acids were calculated using Eq. (3). The values of mass attenuation coefficients were also obtained using XCOM program at all photon energies of current interest. Apart from multiple scattering and counting statistics, the other possible
sources of error due to the small angle scattering contribution, sample impurity, nonuniformity of the sample, photo built-up effects, dead time of the counting instrument and pulse pile effect were evaluated and taken care. By proper adjustment of the distance between detector and source (30 cm≤d≤50 cm), the maximum angle of scattering was below 30 min. According to Hubbell and Berger (1968) the contribution of coherent as well as incoherent scattering at such angles in the measured crosssections at intermediate energies is negligible. Hence no small angle scattering corrections were applied to the measured data. The error due to the sample impurities can be high only when large percentage of high Z impurities is present in the sample. All the amino acid samples used in the present study were of high purity (99.9%) and no content of high Z impurities was present hence, sample impurity corrections were not applied to the measured data. The non-uniformity of the sample material introduces a fraction of error of about half the root mean square deviation in mass per unit area. In the present work, the uncertainty in the mass per unit area and the error due to the nonuniformity of the sample is ≤0.05% for all energies of the interest. The photon built-up effect was kept minimum by choosing optimum count rate and the counting time. The photon builtup depends on the atomic number and the sample thickness, and also on the incident photon energy. It is also a consequence of the multiple scattering inside the sample. In the present work the multiple scattering effects have been corrected and detector of good resolution was used with optimum value of count rate and counting time, it is expected that effects of photon built-up were negligible. In the multichannel analyzer used in the present study, there was a built-in provision for dead time correction. The pulse pile of effects were kept minimum by selecting an optimum count rate and counting time.
4. Results and discussion The values of mean atomic number calculated from the chemical formulae of amino acids studied in the present are displayed in Table 1. The experimentally measured Mass attenuation coefficient μm (cm2/g) for the six amino acids Glycine (C2H5NO2), DL-Alanine (C3H7NO2), Proline (C5H9NO2), L-Leucine (C6H13NO2), L-Arginine (C6H14N4O2) and L-Arginine Monohydrochloride (C6H15ClN4O2) at 122, 360, 511, 662, 1170, 1275 and 1330 keV photon energies have been tabulated in Table 2 and the typical plot of ln μ/ρ versus energy E for Glycine (C2H5NO2 is displayed in Fig. 2. Fig. 2 also includes the variation of theoretically determined μ/ρ values versus energy. It is clearly seen that the mass attenuation coefficient (μm) depends on photon energy and decreases with increasing photon energy. As can be seen in Table 2 and the typical plot displayed in Fig. 2, the experimental (μm) values agree with theoretical values calculated using the XCOM program based on the mixture rule. The total experimental uncertainty of the (μm) values depends on
P.P. Pawar, G.K. Bichile / Radiation Physics and Chemistry 92 (2013) 22–27
the uncertainties of I0 (without attenuation) and I (after attenuation), mass thickness measurements and counting statistics. Typical total uncertainty in the measured experimental (μm) values is estimated to be 2–3%. Measured total atomic and electronic crosssections, st and se values for the presently studied amino acids have been displayed in Table 3 and Table 4, respectively. The typical plots of st versus E and se versus E are displayed in Figs. 3 and 4 respectively. The behavior of st and se with photon energy shows almost similar behavior to (μm) plots. Effective atomic numbers (Zeff) values were determined from Eqs. (7)–(9) by using (μm) values and the same are given in Table 5. The variation of (Zeff) values versus photon energy is displayed graphically for Glycine (C2H5NO2) in Fig. 5. It is seen from Table 5 and Fig. 5 that (Zeff ) values for present samples vary and decrease with photon energy. In the composite materials, like the amino acids studied in the present case, the interaction of gamma rays with these materials is related to the obtained (Zeff) values and photon energies. Effective electron numbers or electron densities (Neff) for present samples were determined using (μm) and (se) values and the same are given in Table 6. Figs. 6 and 7 show the typical variation of effective electron density versus photon energy for Glycine (C2H5NO2) sample. From Figs. 5 and 6, it is clearly observed that (Zeff) and (Neff) show linear behavior with photon energy. Fig. 6 gives the plot of (Neff) versus (Zeff) for photon energy and shows that (Neff) increases linearly with increase of (Zeff). Further, it is worth noting that in the energy range of present interest (Zeff) values of the H, C, N and O based samples (amino acids) used in the present work are related to their respective effective atomic weight by the relation (Zeff)¼ 0.533Aeff where the quantity Aeff is defined as the ratio of molecular weight of a sample to the total number of atoms of all types in it. The variation of effective atomic number was systematically studied with respect to effective atomic weight and the above relation has been satisfied. Similar results were also observed by Manjunathaguru and Umesh (2006) for H, C, N and O based samples. 5. Conclusion The present experimental study has been undertaken to get information on mass attenuation coefficient μm values and related parameters (Zeff, Neff, st and se) for six amino acid samples. It has been found that the μm is useful and sensitive physical quantity to determine the Zeff and Neff for H, C, N and O based biological compounds. In the interaction of photon with matter, μm values are dependent on the physical and chemical environments of the samples. The mass attenuation coefficient (μm) values were found to decrease with increasing photon energies. The variation of st and se is identical to μm. The Neff is closely related to the Zeff and the energy dependence of Zeff and Neff is the same. Results of the study help to understand how μm values change with variation of the Zeff and Neff values in the case of the H, C, N and O based biological compounds like amino acids. It is also worth noting that in the energy region of interest that the Zeff values of the amino acid samples used in the present work are related to their respective effective atomic weight.
27
Acknowledgment The authors are thankful to UGC for giving financial support for Major Research Project on gamma ray attenuation coefficient studies of biologically important compounds.
References Berger,M,J.,Hubbell,J.H.,XCOM: photon cross sections database, web version 1.2, 1999. Available from: 〈http://physics.nist.gov/xcom〉, Originally published as NBSIR 87-3597: XCOM: Photon Cross Sections on a Personal Computer, Washington, DC, 1987. Creagh, D.C., 1987. Nucl. Instrum. Methods A 255, 1–16. El-Kateb, A.H., Abdul-Hamid, A.S., 1991. Photon attenuation coefficients study of some materials containing hydrogen,carbon and oxygen. Appl. Radiat. Isot. 42, 303–307. Gerward, L.,Guilbert, N., Jensen, K.B. Levring,H. Radiat. Phys. Chem. 60 (2001) 23–24. Gerward, L., Guilbert, N., Jensen, K.B., Levring, H., 2004. Radiat. Phys. Chem. 71, 653–654. Gowda, S., Krishnaveni, S., Yashoda, T., Umesh, T.K., Gowda, R., 2004. Photon mass attenuation coefficients,effective atomic numbers and electron densities of some thermoluminescent dosimetric compounds. Pramana-J. Phys. 63, 529–541. Gowda, S., Krishnaveni, S., Gowda, R., 2005. Studies on effective atomic numbers and electron densities in amino acids and sugars in the energy range 30–1333 keV. Nucl. Instrum. Methods Phys. Res. B 239, 361–369. Hine, G.J., 1952. The effective atomic number of materials for various gamma ray process. Phys. Rev. 85, 725. Hubbell, J.H., 1999. Review of photon interaction cross section data in the medical and biological context. Phys. Med. Biol. 44, R1–R22. Hubbell J.H. Seltzer S.M.. Tables of X-ray mass attenuation coefficients from 1 keV to 20 Mev for elements Z¼ 1–92. National Institute of Standards and Technology (IR) Report No. 5632. 1995. Hubbell, J.H., Berger, M.J, 1968. Photon attenuation. In: Jaerger, R.G. (Ed.), Engineering Compendium on Radiation Shielding, 1. 9IAEA, Vienna, Austria, p. 198. (4). Jackson, D.F., Hawkes, D.J., 1981. Phys. Rep. 70, 169–233. Kaewkhao, J..Laopaiboon J.,W. Chewpraditkul,2008.Determination of effective atomic numbers and effective electron densities for Cu/Zn alloy.J. Quant. Spectrosc. Radiat. Transfer 109(2008) 1260–1265. Kirby, B.J., Davis, J.R., Grant, J.A., Morgan, M.J., 2003. Extracting material parameters from X-ray attenuation: a CT feasibility study using kilo voltage synchrotron x-rays incident upon low atomic number absorbers. Phys. Med. Biol. 48, 3389–3409. Murthy, R.C., 1965. Effective atomic number of heterogeneous materials. Nature 207, 398–399. Manjunathaguru, V., Umesh, T.K., 2009. Simple parameterization of photon mass energy absorption coefficients of H-,C-,N-,and O-based samples of biological interest in the energy range 200–1500 keV. Pramana-J. Phys. 72 (2), 375–387. Midgley, S.M., 2004. A parameterization scheme for the x-ray linear attenuation coefficient and energy absorption coefficient. Phys. Med. Biol. 49, 307–325. Midgley, S.M., 2005. Materials analysis using x-ray linear attenuation coefficient measurements at four photon energies. Phys. Med. Biol. 50, 4139–4157. Manjunathaguru, V., Umesh, T.K., 2006. Effective atomic numbers and electron densities of some biologically important compounds containing H,C,N and O in the energy range 145–1330 keV. J. Phys. B: At. Mol. Opt. Phys., 39; , pp. 3969–3981. Manohara, S.R., Hanagodimath, S.M., 2007. Studies on effective atomic numbers and electron densities of essential amino acids in the energy range 1 keV– 100 GeV. Nucl. Instrum. Methods Phys. Res. B 258, 321–328. Shivaramu, Vijay Kumar, R., Rjasekaran, L., Ramamurthy, N., 2001. Effective atomic numbers for photon energy absorption of some low –Z substances of dosimetric interest. Radiat. Phys. Chem. 62, 371–377. Shivaramu, 2001. Effective atomic numbers for photon energy absorption and photon attenuation of tissues from human organs. Med. Dosimetry 27, 1–9. Sandhu, G.K., Singh, Kulwant, Lark, B.S., Gerward, L., 2002. Molar extinction coefficients of some fatty acids. Radiat. Phys. Chem. 65, 211–215.
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Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem
Studies on mass attenuation coefficient, effective atomic number and electron density of some amino acids in the energy range 0.122–1.330 MeV Pravina P. Pawar n, Govind K. Bichile Department of Physics, Dr.Babasaheb Ambedkar Marathwada University, Aurangabad 431004, India
H I G H L I G H T S
Compute the Mass Attenuation Coefficient, effective atomic number and electron density of some amino acids. Gamma ray attenuation studies on biologically important molecules have been carried out using narrow beam good geometry set up. The values of Mass Attenuation Coefficient, effective atomic number and electron density of some amino acids are in agreement with the XCOM programme. The measured mass attenuation coefficient for some amino acids are useful in medical field. The data is useful in radiation dosimetry and other fields.
art ic l e i nf o
a b s t r a c t
Article history: Received 15 February 2013 Accepted 4 July 2013 Available online 17 July 2013
The total mass attenuation coefficients of some amino acids, such as Glycine (C2H5NO2), DL-Alanine (C3H7NO2), Proline (C5H9NO2), L-Leucine (C6H13NO2 ), L-Arginine (C6H14N4O2) and L-Arginine Monohydrochloride (C6H15ClN4O2), were measured at 122, 356, 511, 662, 1170, 1275 and 1330 keV photon energies using a well-collimated narrow beam good geometry set-up. The gamma rays were detected using NaI (Tl) scintillation detection system with a resolution of 10.2% at 662 keV. The attenuation coefficient data were then used to obtain the effective atomic numbers (Zeff) and effective electron densities (Neff) of amino acids. It was observed that the effective atomic number (Zeff) and effective electron densities (Neff) tend to be almost constant as a function of gamma-ray energy. The results show that, the experimental values of mass attenuation coefficients, effective atomic numbers and effective electron densities are in good agreement with the theoretical values with less than 1% error. Published by Elsevier Ltd.
Keywords: Mass attenuation coefficients Atomic cross-section Electronic cross-section Effective atomic number Effective electron density.
1. Introduction The study of photon interactions with matter is important. The data on the transmission and absorption of X-rays and gammarays in biological shielding and dosimetric materials assumed great significance by virtue of their diverse applications in the field of medical physics and medical biology (Kaewkhao et al., 2008). Gamma radiations in the energy region above 200 keV up to about 1500 keV interact with matter predominantly by photoelectric effect, Compton effect, and pair production processes. The photon interaction cross-section can be expressed as a function of the photon energy and the atomic number Z. At a given photon energy, the interaction cross section is proportional to Zn where n
n
Corresponding author. Tel.: +91 0240 2486706. E-mail address: [email protected] (P.P. Pawar).
0969-806X/$ - see front matter Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.radphyschem.2013.07.004
is expected to be between 4 and 5 for the photoelectric effect, 1 for the Compton effect, and 2 for pair production (Murthy, 1965). It is often found convenient to represent the gamma-ray interaction properties of a composite material consisting of a number of different elements in varying proportions by an effective atomic number Zeff. This number depends on the incident energy as well as the atomic number of the constituent elements. It indicates on average, the number of electrons of the material that actively participate in the photon–atom interaction. Hence, the Zeff is used frequently in calculations of mass energy absorption coefficients and Kerma in radiation dosimetry (Manjunathaguru and Umesh, 2009). This parameter is also used in the calculation of Compton profiles of complex materials and hence may yield valuable information about the chemical environment that surrounds the atom in a quantitative manner. Hine (1952) pointed out that there is a different atomic number for each interaction process in a complex material.
P.P. Pawar, G.K. Bichile / Radiation Physics and Chemistry 92 (2013) 22–27
In order to make use of the fact that scattering and absorption of gamma-radiations are related to the density and effective atomic numbers of the material, a knowledge of the mass
23
attenuation coefficients is of prime importance. From the mass attenuation coefficient, a number of related parameters can be derived, such as the mass energy-absorption coefficient, the total interaction cross-section, the molar extinction coefficient, the effective atomic number, and the effective electron density. The mass attenuation coefficient, μ/ρ, is a measure of the average number of interactions between incident photons and matter that occur in a given mass per unit area thickness of the substance encountered. A precise knowledge of the effective atomic number of the composite material is necessary for various purposes. However, the theoretical expressions for effective atomic number of composite materials are not very accurate at all energies. Early calculations of effective atomic numbers were based on parameterization of the photon interaction cross-section by fitting data over limited ranges of photon energy and atomic number (Jackson and Hawkes, 1981). Today, accurate databases of photon interaction cross-sections and interpolation programs (Gerward et al., 2001,2004); (Berger and Hubbell, 1999) have made it possible to calculate effective atomic numbers with much improved accuracy and information content over wide ranges of photon energy and elemental composition. A simple and widely used method of obtaining Zeff of a composite material consisting of different elements in definite proportions is based on the determination of total attenuation coefficients for gamma-ray interactions by the transmission method. In the present work, the method of deriving effective atomic numbers in composite materials was followed by using the experimental results of mass attenuation coefficients. Experimental results were compared with theoretical values. A variety of physiological functions inside living systems are performed by complex molecules such as carbohydrates, proteins, fats and oils composed of H, C, N and O elements. Sugars and amino acids are the building blocks of carbohydrates and proteins. Photons of energy from 1500 keV down to about 5 keV are widely used in medical and biological applications (Hubbell, 1999) especially during diagnosis and therapy. A thorough knowledge of the
0.2 Fig. 1. (a) The schematic view of the experimental set up. (b) Nuclear electronic system of a scintillation spectrometer.
Molar mass (g/ Chemical mol) formula
Mean atomic number, Z
Glycine DL-Alanine Proline L-Leucine L-Arginine LR L-Arginine Monohydrochloride LR
75.10 89.10 115.13 131.17 174.20 210.66
4.00 3.69 3.65 3.27 3.62 4.00
C2H5NO2 C3H7NO2 C5H9NO2 C6H13NO2 C6H14N4O2 C6H15ClN4O2
2/g) m(cm
Table 1 The mean atomic numbers calculated from the chemical formula for amino acids Amino acids
Exp.
0.16
Theo.
0.12 0.08 0.04 0
0
500 1000 Energy (keV)
1500
Fig. 2. The typical plot of μm versus energy E for Glycine (C2H5NO2).
Table 2 Mass attenuation coefficient μm (cm2/g) of amino acids. Sr. no.
1. 2. 3. 4. 5. 6.
Amino acids
Glycine DL-Alanine Proline L-Leucine L-Arginine LR L-Arginine Monohydro chloride LR
122 keV
356 keV
511 keV
662 keV
1170 keV
1275 keV
1330 keV
Exp.
Theo.
Exp.
Theo.
Exp.
Theo.
Exp.
Theo.
Exp.
Theo.
Exp.
Theo.
Exp.
Theo
0.154 0.155 0.155 0.160 0.156 0.162
0.155 0.156 0.156 0.157 0.155 0.160
0.104 0.107 0.107 0.107 0.108 0.107
0.106 0.108 0.108 0.110 0.108 0.106
0.094 0.093 0.093 0.095 0.095 0.091
0.092 0.093 0.092 0.094 0.094 0.090
0.084 0.084 0.083 0.083 0.083 0.083
0.083 0.083 0.082 0.084 0.084 0.082
0.062 0.065 0.065 0.065 0.063 0.064
0.0621 0.064 0.064 0.064 0.064 0.063
0.061 0.060 0.061 0.063 0.061 0.059
0.061 0.060 0.060 0.062 0.060 0.058
0.059 0.059 0.059 0.061 0.058 0.058
0.059 0.059 0.058 0.060 0.059 0.057
24
P.P. Pawar, G.K. Bichile / Radiation Physics and Chemistry 92 (2013) 22–27
Table 3 Atomic cross-sections, st (barn/molecule) of amino acids. Sr. no.
1. 2. 3. 4. 5. 6.
Amino acids
122 keV
Glycine DL-Alanine Proline L-Leucine L-ArginineLR L-Arginine Monohydro Chloride LR
356 keV
511 keV
662 keV
1170 keV
1275 keV
1330 keV
Exp.
Theo.
Exp.
Theo.
Exp.
Theo.
Exp.
Theo.
Exp.
Theo.
Exp.
Theo.
Exp.
Theo
19.20 22.92 29.62 34.83 45.11 56.64
19.32 23.07 29.81 34.18 44.82 55.94
12.96 15.82 20.45 23.30 31.23 37.41
13.21 15.97 20.64 23.95 31.23 37.06
11.72 13.75 17.77 20.68 27.47 31.81
11.47 13.75 17.58 20.47 27.18 31.46
10.47 12.42 15.86 18.07 23.99 29.02
10.35 12.27 15.67 18.29 24.29 28.67
7.73 9.61 12.42 14.15 18.22 22.37
7.74 9.46 12.23 13.93 18.50 22.03
7.60 8.87 11.66 13.72 17.64 20.62
7.60 8.87 11.47 13.50 17.35 20.28
7.35 8.72 11.27 13.28 16.77 20.28
7.34 8.72 11.08 13.06 16.06 19.93
Table 4 Electronic cross-sections, se (barn/molecule) of amino acids. Sr. no.
1. 2. 3. 4. 5. 6.
Amino acids
122 keV
Glycine DL-Alanine Proline L-Leucine L-Arginine LR L-Arginine Monohydro Chloride LR
356 keV
511 keV
1275 keV
1330 keV
Theo.
Exp.
Theo.
Exp.
Theo.
Exp.
Theo.
Exp.
Theo.
Exp.
Theo.
Exp.
Theo
4.54 6.30 8.25 11.15 12.71 14.05
4.57 6.34 8.30 10.94 12.63 13.87
3.23 4.34 5.68 7.40 8.78 9.3
3.29 4.38 5.74 7.61 8.78 9.22
2.93 3.77 4.94 6.55 7.71 7.92
2.86 3.77 4.88 6.49 7.63 7.83
2.62 3.41 4.40 5.72 6.73 7.23
2.58 3.36 4.35 5.79 6.82 7.14
1.93 2.63 3.44 4.45 5.11 5.58
1.94 2.59 3.39 4.39 5.18 5.50
1.90 2.43 3.23 4.32 4.94 5.14
1.90 2.43 3.18 4.25 4.86 5.06
1.84 2.38 3.12 4.18 4.70 5.06
1.83 2.38 3.07 4.11 4.50 4.97
6 Exp. Theo.
15 10
e exp
5 e (b/atom)
20 t(b/atom)
1170 keV
Exp.
25
e theo
4 3 2 1
5
0 0
662 keV
0
500
1000 Energy (keV)
0
1500
500 1000 Energy (keV)
1500
Fig. 4. The typical plots of se versus E are displayed for Glycine (C2H5NO2).
Fig. 3. The typical plots of st versus E for Glycine (C2H5NO2).
nature of interaction of these biologically important complex molecules such as amino acids is desirable over this energy region. Hence, in recent years, this has motivated many investigators over the years to determine the total attenuation cross-sections as well as composition dependent quantities such as effective atomic numbers (Zeff), and effective electron densities (Neff) of such complex molecules of biological interest in this energy region by employing different methods (Kirby et al., 2003; Midgley, 2004, 2005; Shivaramu, 2001; Shivaramu et al., 2001; Sandhu et al., 2002; Gowda et al., 2004, 2005; Manjunathaguru and Umesh, 2006; Manohara and Hanagodimath, 2007; El-Kateb and Abdul-hamid, 1991). Theoretical values for the mass attenuation coefficients can be found in the tabulation by Hubbell and Seltzer (1995). A convenient alternative to manual calculations, using tabulated data, is to generate attenuation data, as needed using a computer. For this purpose Berger and Hubbell (1999) developed XCOM software for calculating mass attenuation coefficients or photon interaction cross-sections for any element, compound for a wide range of energies. There have been a great number of experimental and theoretical investigations to determine mass attenuation coefficients for
complex biological molecules such as carbohydrates, proteins, fats and oils composed of H, C, N and O elements in varying proportions. Sandhu et al. 2002 have investigated fatty acids in the energy region 81–1330 keV. Gowda et al. (2004, 2005) have reported total attenuation cross-sections for sugar and amino acids. Recently Manjunathguru and Umesh (2006), Manohara and Hanagodimath (2007), have determined the effective atomic numbers of several biomolecules. Measurements on the sample containing H, C, and O in the energy range 54–1333 keV have been reported by El-Kateb and Abdul-hamid (1991). In this work, we have measured the mass attenuation coefficients, the atomic cross-sections, the effective atomic numbers, and the effective electron densities for H, C, N and O based amino acids in the energy range 122–1330 keV and then compared these experimentally evaluated parameters with theory using XCOM program.
2. Theory In this section we summarize some theoretical relations that have been used for the determination of mass attenuation coefficients in the present work. When a monoenergetic beam of gamma photons is incident on a target, some photons are removed
P.P. Pawar, G.K. Bichile / Radiation Physics and Chemistry 92 (2013) 22–27
25
Table 5 Effective atomic number, Zeff of amino acids. Sr. No.
Amino acids
1. 2. 3. 4. 5. 6.
122 keV
Glycine DL-Alanine Proline L-Leucine L-Arginine LR L-Arginine Monohydro Chloride LR
356 keV
511 keV
1330 keV
Exp.
Theo.
Exp.
Theo.
Exp.
Theo.
Exp.
Theo.
Exp.
Theo.
Exp.
Theo
4.23 3.64 3.59 3.12 3.55 4.03
4.23 3.64 3.59 3.12 3.55 4.03
4.01 3.65 3.60 3.15 3.56 4.02
4.02 3.65 3.60 3.15 3.56 4.01
4.00 3.65 3.60 3.16 3.56 4.01
4.01 3.65 3.60 3.15 3.56 4.01
3.99 3.64 3.60 3.16 3.56 4.01
4.01 3.65 3.60 3.16 3.56 4.01
4.01 3.65 3.61 3.18 3.57 4.01
3.99 3.65 3.61 3.17 3.57 4.01
4.00 3.65 3.61 3.18 3.57 4.01
4.00 3.65 3.61 3.18 3.57 4.01
3.99 3.66 3.61 3.18 3.57 4.01
4.01 3.66 3.61 3.18 3.57 4.01
chemical compound the fraction by weight (wi) is given by;
Exp.
4.015
Zeff
1275 keV
Theo.
4.02
wi ¼
Theo.
ni Ai ∑j nj Aj
ð5Þ
where Ai is the atomic weight of the ith element and ni is the number of formula units. The values of the mass attenuation coefficients were then used to determine the total molecular cross-section (st,m) by the following relation:
4.01 4.005 4
ðst;m Þ ¼ μm ðM=N A Þ;
3.995 0
500 1000 Energy (keV)
1500
Fig. 5. The typical plots of Zeff versus E for Glycine (C2H5NO2).
from the beam due to the dominant interaction processes and, therefore, the transmitted beam is attenuated. The extent of attenuation depends for a given elemental target on the photon energy. This attenuation of the beam is described by the following equation: I ¼ I 0 eμt
ð1Þ
where I0 and I are the unattenuated and attenuated photon intensities, respectively, and μ (cm 1) is the linear attenuation coefficient of the material. Linear attenuation coefficient more accurately characterizing a given material is density-independent. Rearrangement of Eq. (1) yields the following equation for the linear attenuation coefficient: μ¼
1170 keV
Exp.
4.025
3.99
662 keV
1 lnðI 0 =IÞ t
ð2Þ
In Eq. (2), t (cm) is the sample thickness. The mass attenuation coefficients μ/ρ(cm2 gm 1) for the samples were obtained from Eq. (3) by using the density of the corresponding samples: 2
1
μm ¼ μ=ρðcm gm
1 Þ ¼ lnðI 0 =IÞ ρt
ð3Þ
where ρ (cm2/gm) is a measured density of the corresponding sample. If the materials are composed of different elements in varying proportions, then it is assumed that the contribution of each element of the compound to the total interaction is additive, yielding the well known mixture rule of Hubbell and Seltzer (1995) which represent the total mass attenuation coefficient (μ/ρ)T of any compound as the sum of the appropriately weighted proportions of the individual atoms. The mass attenuation coefficient for a compound or mixture is given by μm ¼ ∑i wi ðμm Þi
ð4Þ
where wi and (μm)i are the weight fraction and mass attenuation coefficient of the ith constituent element, respectively. For a
ð6Þ
where M¼∑iniAi is the molecular weight of the compound, NA is the Avogadro′s number, ni is the total number of atoms (with respect to mass number ) in the molecule and Ai is the atomic weight of the ith element in a molecule. The total atomic cross-sections (st,a) can be easily determined from the following equation: st;a ¼
1 ∑ f A ðμ Þ NA i i i m i
ð7Þ
Similarly, effective electronic cross-section (st,el) for the individual element is given by: st;el ¼
1 st;a ∑ f A ðsm Þi ¼ NA Z A i i i Z eff
ð8Þ
where fi ¼ni/∑jnj and Zi are the fractional abundance and atomic number of constituent element I, respectively, ni is the total number of atoms of the constituent element i and ∑jnj is the total number of atoms present in the molecular formula. Now, the effective atomic number (Zeff) can be given as st;a Z eff ¼ ð9Þ st;el
3. Experimental set up and measurements The five radioactive sources 57Co, 133Ba, 137Cs, 60Co and 22Na were used in the present investigation. Gamma rays of energy 122, 360, 511, 662, 1170, 1275 and 1330 keV emitted by the above radioactive sources (procured from Bhaba Atomic Research Centre, Mumbai) were collimated and detected by NaI (Tl) detector having energy resolution of 10.2% at 662 keV. The signals from the detector were amplified and analyzed with 8 K multichannel analyzer. The amino acids (Glycine (C2H5NO2), DL-Alanine (C3H7NO2), Proline (C5H9NO2), L-Leucine (C6H13NO2 ), L-Arginine (C6H14N4O2) and L-Arginine Monohydrochloride (C6H15ClN4O2) under investigation were pellet shaped and confined in a cylindrical plastic container having the same diameter as that of sample pellet. The diameters of the samples were determined with the help of a traveling microscope. It was observed that the attenuation of photons of the empty containers were negligible. Each sample pellet was weighed in a sensitive digital balance having an accuracy 0.001 mg. The weighing was repeated several times to obtain consistent value of the mass. The mean of this set of values was taken
26
P.P. Pawar, G.K. Bichile / Radiation Physics and Chemistry 92 (2013) 22–27
Table 6 Effective electron densities, Neff (1024) of amino acids. Sr. No. Amino acids
1. 2. 3. 4. 5.
122 keV
Glycine DL-Alanine Proline L-Leucine L-Arginine LR 6L-Arginine Monohydro Chloride LR
356 keV
511 keV Theo.
Exp.
Theo.
Exp.
Theo.
Exp.
Theo.
Exp.
Theo.
Exp.
Theo
0.3395 0.3199 0.3193 0.3152 0.3192 0.3220
0.3395 0.3199 0.3193 0.3152 0.3192 0.3220
0.3218 0.3208 0.3202 0.3182 0.3201 0.3210
0.3226 0.3208 0.3202 0.3182 0.3201 0.3210
0.3210 0.3208 0.3202 0.3193 0.3201 0.3210
0.3218 0.3208 0.3202 0.3182 0.3201 0.3210
0.3202 0.3199 0.3202 0.3193 0.3201 0.3210
0.3218 0.3208 0.3202 0.3193 0.3201 0.3210
0.3218 0.3208 0.3211 0.3213 0.3210 0.3200
0.3202 0.3208 0.3211 0.3203 0.3210 0.3200
0.3210 0.3208 0.3211 0.3213 0.3210 0.3210
0.3210 0.3208 0.3211 0.3213 0.3210 0.3200
0.3202 0.3217 0.3211 0.3213 0.3210 0.3200
0.3218 0.3217 0.3211 0.3213 0.3210
Neff
0.322
Theo.
0.3215 0.321 0.3205 500 1000 Energy (keV)
1500
Fig. 6. Shows the typical variation of effective electron density versus photon energy for Glycine (C2H5NO2) sample.
0.323 0.3225
Neff
0.322 Neff
0.3215 0.321 0.3205 4
4.01 Zeff
1330 keV
Exp.
Exp.
0.32 3.99
1275 keV
Theo.
0.3225
0
1170 keV
Exp.
0.323
0.32
662 keV
4.02
4.03
Fig. 7. Plot of effective electron number (Neff) as a function of effective atomic number (Zeff) for Glycine (C2H5NO2) sample.
to be the mass of the sample. By using the diameter of the pellet and mean value of the mass of the pellet, the mass per unit area was determined in each case. The sample thicknesses were selected in order to satisfy the following ideal condition as far as possible (Creagh, 1987): 2 o lnðI 0 =IÞ ≤4 For measurement of incident and transmitted photon energies a narrow beam good geometry set up was used. The schematic view of the experimental set up is displayed in Fig. 1(a) and the nuclear electronic system of the scintillation spectrometer is shown in Fig. 1(b). From the measured values of unattenuated photon intensity I0 (with empty plastic container) and attenuated photon intensity I (with sample), the mass attenuation coefficients (μ/ρ) for all the samples of amino acids were calculated using Eq. (3). The values of mass attenuation coefficients were also obtained using XCOM program at all photon energies of current interest. Apart from multiple scattering and counting statistics, the other possible
sources of error due to the small angle scattering contribution, sample impurity, nonuniformity of the sample, photo built-up effects, dead time of the counting instrument and pulse pile effect were evaluated and taken care. By proper adjustment of the distance between detector and source (30 cm≤d≤50 cm), the maximum angle of scattering was below 30 min. According to Hubbell and Berger (1968) the contribution of coherent as well as incoherent scattering at such angles in the measured crosssections at intermediate energies is negligible. Hence no small angle scattering corrections were applied to the measured data. The error due to the sample impurities can be high only when large percentage of high Z impurities is present in the sample. All the amino acid samples used in the present study were of high purity (99.9%) and no content of high Z impurities was present hence, sample impurity corrections were not applied to the measured data. The non-uniformity of the sample material introduces a fraction of error of about half the root mean square deviation in mass per unit area. In the present work, the uncertainty in the mass per unit area and the error due to the nonuniformity of the sample is ≤0.05% for all energies of the interest. The photon built-up effect was kept minimum by choosing optimum count rate and the counting time. The photon builtup depends on the atomic number and the sample thickness, and also on the incident photon energy. It is also a consequence of the multiple scattering inside the sample. In the present work the multiple scattering effects have been corrected and detector of good resolution was used with optimum value of count rate and counting time, it is expected that effects of photon built-up were negligible. In the multichannel analyzer used in the present study, there was a built-in provision for dead time correction. The pulse pile of effects were kept minimum by selecting an optimum count rate and counting time.
4. Results and discussion The values of mean atomic number calculated from the chemical formulae of amino acids studied in the present are displayed in Table 1. The experimentally measured Mass attenuation coefficient μm (cm2/g) for the six amino acids Glycine (C2H5NO2), DL-Alanine (C3H7NO2), Proline (C5H9NO2), L-Leucine (C6H13NO2), L-Arginine (C6H14N4O2) and L-Arginine Monohydrochloride (C6H15ClN4O2) at 122, 360, 511, 662, 1170, 1275 and 1330 keV photon energies have been tabulated in Table 2 and the typical plot of ln μ/ρ versus energy E for Glycine (C2H5NO2 is displayed in Fig. 2. Fig. 2 also includes the variation of theoretically determined μ/ρ values versus energy. It is clearly seen that the mass attenuation coefficient (μm) depends on photon energy and decreases with increasing photon energy. As can be seen in Table 2 and the typical plot displayed in Fig. 2, the experimental (μm) values agree with theoretical values calculated using the XCOM program based on the mixture rule. The total experimental uncertainty of the (μm) values depends on
P.P. Pawar, G.K. Bichile / Radiation Physics and Chemistry 92 (2013) 22–27
the uncertainties of I0 (without attenuation) and I (after attenuation), mass thickness measurements and counting statistics. Typical total uncertainty in the measured experimental (μm) values is estimated to be 2–3%. Measured total atomic and electronic crosssections, st and se values for the presently studied amino acids have been displayed in Table 3 and Table 4, respectively. The typical plots of st versus E and se versus E are displayed in Figs. 3 and 4 respectively. The behavior of st and se with photon energy shows almost similar behavior to (μm) plots. Effective atomic numbers (Zeff) values were determined from Eqs. (7)–(9) by using (μm) values and the same are given in Table 5. The variation of (Zeff) values versus photon energy is displayed graphically for Glycine (C2H5NO2) in Fig. 5. It is seen from Table 5 and Fig. 5 that (Zeff ) values for present samples vary and decrease with photon energy. In the composite materials, like the amino acids studied in the present case, the interaction of gamma rays with these materials is related to the obtained (Zeff) values and photon energies. Effective electron numbers or electron densities (Neff) for present samples were determined using (μm) and (se) values and the same are given in Table 6. Figs. 6 and 7 show the typical variation of effective electron density versus photon energy for Glycine (C2H5NO2) sample. From Figs. 5 and 6, it is clearly observed that (Zeff) and (Neff) show linear behavior with photon energy. Fig. 6 gives the plot of (Neff) versus (Zeff) for photon energy and shows that (Neff) increases linearly with increase of (Zeff). Further, it is worth noting that in the energy range of present interest (Zeff) values of the H, C, N and O based samples (amino acids) used in the present work are related to their respective effective atomic weight by the relation (Zeff)¼ 0.533Aeff where the quantity Aeff is defined as the ratio of molecular weight of a sample to the total number of atoms of all types in it. The variation of effective atomic number was systematically studied with respect to effective atomic weight and the above relation has been satisfied. Similar results were also observed by Manjunathaguru and Umesh (2006) for H, C, N and O based samples. 5. Conclusion The present experimental study has been undertaken to get information on mass attenuation coefficient μm values and related parameters (Zeff, Neff, st and se) for six amino acid samples. It has been found that the μm is useful and sensitive physical quantity to determine the Zeff and Neff for H, C, N and O based biological compounds. In the interaction of photon with matter, μm values are dependent on the physical and chemical environments of the samples. The mass attenuation coefficient (μm) values were found to decrease with increasing photon energies. The variation of st and se is identical to μm. The Neff is closely related to the Zeff and the energy dependence of Zeff and Neff is the same. Results of the study help to understand how μm values change with variation of the Zeff and Neff values in the case of the H, C, N and O based biological compounds like amino acids. It is also worth noting that in the energy region of interest that the Zeff values of the amino acid samples used in the present work are related to their respective effective atomic weight.
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Acknowledgment The authors are thankful to UGC for giving financial support for Major Research Project on gamma ray attenuation coefficient studies of biologically important compounds.
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