Measurement of Attenuation Cross-Sections of Some Fatty Acids in the Energy Range 122–1330 Kev

Pramana – J. Phys. (2016) 87: 12 c Indian Academy of Sciences DOI 10.1007/s12043-016-1213-y Measurement of attenuation cross-sections of some fatty acids in the energy range 122–1330 keV D K GAIKWAD1,∗,PPPAWAR1 and T P SELVAM2 1Department of Physics, Dr Babasaheb Ambedkar Marathwada University, Aurangabad 431 004, India 2Radiation Physics and Advisory Division, BARC, Anushaktinagar, Mumbai 400 085, India ∗Corresponding author. E-mail: [email protected] MS received 14 June 2015; revised 7 August 2015; accepted 7 September 2015; published online 18 June 2016 Abstract. The mass attenuation coefficients (μm) have been measured for undecylic acid (C11H22O2), lau- ric acid (C12H24O2), tridecylic acid (C13H26O2), myristic acid (C14H28O2), pentadecylic acid (C15H30O2)and 57 133 137 60 22 palmitic acid (C16H32O2)using Co, Ba, Cs, Co and Na emitted γ radiation with energies 122, 356, 511, 662, 1170, 1275 and 1330 keV, respectively. The accurate values of the effective atomic number (Zeff), atomic cross-section (σt,), electronic cross-section (σe) and the effective electron density (Neff) have great signifi- cance in radiation protection and dosimetry. These quantities were obtained by utilizing experimentally measured values of mass attenuation coefficients (μm). A NaI(Tl) scintillation detector with 8.2% (at 662 keV) resolution was used for detecting of attenuated γ -photons. The variation in Zeff and Neff of fatty acids with energy is discussed. The experimental and theoretical results are in good agreement within 2% deviation. Keywords. Fatty acids; NaI(Tl) scintillation detector; Zeff and Neff. PACS Nos 32.80.−t; 32.90.+a; 32.80.Fb; 20; 33.80.−b 1. Introduction coefficient (μen) is a measure of imparted interacting photon energy to kinetic energy of charge particles, Radioactive isotopes are widely used in different fields and can be calculated directly by employing sphere such as radiation protection, radiation therapy, agri- transmission method [3]. The mass energy absorption culture, medicine, industry and research. Data on the coefficient can be utilized for calculating the absorbed absorption and scattering of X-rays and γ -photons in dose, which is useful in radiotherapy [4]. Hubbell and the biological system have been used in medical phy- Seltzer [5] have given tables of μm and μen for some sics and radiation biology [1]. Biologically important elements and materials of dosimetric interest as these systems such as carbohydrates, amino acids, fatty acids are the basic parameters for calculating the penetration and proteins are the building blocks of human body and of γ -ray photons in an absorber. The values of mass other living systems. These complex biomolecules are attenuation coefficient (μm) derived from X-rays and necessary for the body to function and are responsi- γ -photons are valuable in radiation biology, dosimetry, ble for various physiological functions inside the living etc. [2]. Similarly, scattering cross-section data for var- system. Hubbell [2] has carried out comprehensive ious interaction processes have potential applications, theoretical studies on the interaction of γ -rays, and including the area of radiation shielding, industrial has reported that the energy range 5–1500 keV has irradiation and monitoring, crystallography and X-ray tremendous application in diagnostic radiology. spectroscopy, besides medical and biological appli- The mass attenuation coefficient (μm) is the basic cations [6]. Hine [7] showed that a constant value of ato- parameter for understanding the absorption and scat- mic number is not suitable for photon interaction across tering of X-ray and γ -ray photons in matter, and can the entire energy region. The parameter denoting the ato- be defined as a measure of the probability of inter- mic number of a sample with different chemical con- action between incident photons and sample of mass stituents is the effective atomic number and it can per unit area. Similarly, the mass energy absorption be obtained by different ways such as logarithmic 1 12 Page 2 of 7 Pramana – J. Phys. (2016) 87: 12 interpolation, power law, auto-Zeff and direct method of γ -ray is collimated on a sample, photons are atten- [8–11]. Zeff is not constant and it varies with energy uated by the sample due to absorption and scattering. depending on dominant interaction processes. In low- The transmitted beam of photons is represented by energy region (E<0.05 MeV) photoelectric absorp- −μt I = I e , (1) tion is dominant and proportional to Z4 of an absorber. 0 Dominance of Compton scattering and pair production where I0 and I are respectively the initial and final inten- − in intermediate (0.05 MeV <E<1.02 MeV) and sities of the interacting photons. μ (cm 1) is the linear high-energy region (E>1.02 MeV) is proportional to attenuation coefficient of the sample that decreases Z and Z2 respectively. exponentially and t (cm) is the thickness of the mate- Several researchers have investigated γ -photon inter- rial. The linear attenuation coefficient (μ)ofthemate- action on different types of materials. The electron rials from eq. (1) is expressed as N Z density ( eff) and effective atomic number ( eff)were 1 I found to be higher at low energy for carbohydrates μ = 0 , t ln I (2) and fatty acids as studied by Manohara et al [12]. Manjunathaguru and Umesh [13] computed the electron where ln(I /I) is the absorbance which depends on the N 0 density ( eff) of amino acids using the new matrix me- density, Z and thickness of the sample. By introducing thod. Recently, mass attenuation coefficients for a few the density of the material in eq. (2), the mass attenua- medicinal plants were measured by Morabad and Kerur tion coefficients for a sample is given by [14]. Gamma-ray transmission studies on C-, H-, N- and O-based biological molecules have been carried out μ − 1 I0 μ = (cm2 g 1) = ln , (3) with a narrow-beam good geometry set-up, and it is re- m ρ ρt I ported that the mass attenuation coefficient decreases with increase in energy [15–18]. The attenuations of where the density (ρ) is expressed in g/cm3. high-energy photons by biologically important sys- The fatty acids are composed of the elements C, H tems, tissues and materials of dosimetric interest have and O in different proportions, and their constituent been carried out by a few investigators [19–28]. elements participate in the interaction process and is A fatty acid is an acid consisting of a saturated or an un- additive. The mass attenuation coefficient (μm)ofsuch saturated hydrocarbon chain with a terminal carboxyl a compound is represented by the sum of the weighted group. Fatty acids do not exist in a free state, and are al- fractions of the individual atoms of the element accord- ways found combined with triglycerides. Fatty acids are ing to the mixture rule [5]. Then, the mass attenuation necessary for the growth and functioning of living cells coefficient (μm) for the selected compound can be and their deficiency may cause critical disease or abnor- written as mality. No experimental data on atomic and electronic μ = W (μ ) , scattering cross-sections are reported in the literature for m i m i (4) the present samples, which are most convenient for the i computation of Z and other parameters. This promo- eff where Wi and (μm)i are the weight fraction and mass ted a systematic and comprehensive investigation of attenuation coefficient for the ith constituent element, fatty acids in the energy range 122–1330 keV. In the respectively. The weight fraction (Wi) for the com- μ present work, the mass attenuation coefficient ( m), pound is the ratio of atomic weight of the individual Z effective atomic number ( eff), effective electron den- elements and total atomic weight of the compound and N σ sity ( eff), atomic cross-section ( t) and electronic is represented by cross-section (σe) of fatty acids have been determined experimentally in the energy range 122–1330 keV. The niAi Wi = , (5) experimental results were compared with the theoreti- j niAj cal values. where Ai and ni are the atomic weight of the ith constituent element and number of formula units respec- 2. Theory tively. The total photon interaction cross-section (σ) contributed by various partial interaction processes is The theoretical formulation for the calculation of mass expressed as attenuation coefficient (μm) and other related parameters are reviewed below. When a monochromatic beam σ = σpho + σincoh + σcoh + σpair + σtri + σph.n., (6) Pramana – J. Phys. (2016) 87: 12 Page 3 of 7 12 where σpho is the atomic photoelectric absorption acid (C14H28O2), pentadecylic acid (C15H30O2)and cross-section that occurred in the energy region palmitic acid (C16H32O2) were weighed using a sensi- below 0.01 MeV and σincoh is the Compton scattering tive digital balance to the third decimal place. Radioac- (incoherent) dominant at intermediate energy region, tive sources such as 57Co (250 kBq), 133Ba (86 kBq), 0.05 MeV <E<5 MeV. The scattering pro- 137Cs (97 kBq), 60Co (134 kBq) and 22Na (73kBq) cess which occurred in the high-energy region, above used for transmission experiments were obtained from 1.02 MeV, such as σpair and σtri, are pair produc- Bhabha Atomic Research Centre, Mumbai. The trans- tion in the field of nucleus and electron. σph.n is the mission experiment was carried out using a 3 × 3 photonuclear scattering cross-section. The molecu- NaI(Tl) scintillation detector having 8.2% resolution at lar scattering cross-section (σt,m) determined using the 662 keV. The detector and γ -ray source were shielded mass attenuation coefficient (μm)isgivenby well using lead. A plastic container was used as a sample holder, and attenuation of the photon by the unfilled σt,m = μm(M/NA), (7) container was found negligible.

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