A theoretical evaluation on radiation shielding features of Van-Ercis and Rize-İkizdere Türkiye obsi

Abstract

Keywords: Radiation Attenuation Parameters; Obsidian; Radiation Shielding; Phy-X/PSD

Introduction

Radiation protection and hence, radiation shielding materials have great interest due to the increase of radiation applications in people’s daily lives. Many studies were carried out for determining shielding properties of different materials before [1-6]. Obsidian is a natural glassy structure associated to volcanic rocks. These glassy volcanic rocks are commonly preferred in engineering, medical and nuclear applications. It is important to learn if these kind of widely used natural materials have radiation shielding features or not. There

are many volcanic regions in the world and therefore many obsidian types that vary according to the regional differences. Erciş and Ikızdere obsidians are the two of them in Türkiye. Erciş-Van obsidians are located in 20 km Northern of Van in Northeast of Anatolian plate [7]. Ikizdere-Rize obsidians covers around 10 km2 north-northeast of Ikizdere region. The obsidians were formed during Upper Pliocene Pleistocene period in the last phase of volcanism [8].

*Corresponding author. *E-mail address: zeynep.yarbasi@gmail.com, zaygun@beu.edu.tr This paper was recommended for publication in revised form by Regional Editor Hayriye Sundu Published by Yıldız Technical University Press, İstanbul, Turkey Copyright 2021, Yıldız Technical University. This is an open access article under the CC BY-NC license (http://creativecommons.org/licenses/by-nc/4.0/).

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There are some softwares in the literature such as XCOM [9], GEANT4 [10], WinXCOM [11,12], and XMuDat [13] used for calculation of MAC values for elements, compounds and mixtures. We preferred recently reported and widely used Phy-X/PSD code which can calculate quickly and accurately all specified shielding parameters for different materials in the continuous energy range [14]. In this study, we aimed to calculate radiation attenuation parameters which give significant knowledge about the radiation shielding abilities of the materials. For this purpose, we determined the mass attenuation coefficient (MAC), linear attenuation coefficient (LAC), effective atomic number (Zeff), half-value layer (HVL), tenth-value layer (TVL), total atomic cross section (ACS), total electronic cross section (ECS), effective conductivity (Ceff) and effective electron number (Neff) in a wide photon energy range (4keV-100GeV) by Phy-X/PSD code. It is also aimed to compare the MAC and LAC values of the obsidians with those of ordinary concrete which is a commonly used shielding material.

Theory

In this study, Erciş obsidians and Ikizdere obsidians were obtained from Van-Erciş Ulupamir Village and RizeIkizdere Büyükyayla region, respectively. Plaques were cut from the rocks and thin sections were prepared for the analysis of chemical compositions. Chemical compositions of the samples were determined by X-ray fluorescence technique and the used compositions were taken from literature [7,15]. Phy-X/PSD software, which is recently developed to determine the radiation shielding parameters of different materials, was used in the calculation. Calculation process is started by defining the chemical composition and the density of the material in the program. In the software, the material composition can be entered as mole fraction or weight fraction. The parameters are determined in a wide energy range by selecting the energy sources (22Na, 55Fe, 60 Co, 109Cd, 131I, 133Ba, 137Cs, 152Eu, 241Am and the K-shell energies of Cu, Rb, Mo, Ag, Ba and Tb elements) [14]. Lastly, the desired shielding parameters are selected for the purpose and the parameters used in calculation process are given with their formulas below. The MAC is a quantity that defines the interaction possibility between gamma photons and the mass per unit area for a particular medium and can be calculated by the Beer– Lambert formulated as: I = I0e–μt μm =

where I0 and I are incident and attenuated photon intensities, ρ (g/cm3) is the density of material, μm(cm2/g) and

μ(cm−1) are mass and linear attenuation coefficients, tm (g/ cm2) and t (cm) are sample mass thickness (the mass per unit area) and the thickness, respectively. If the sample has various elements, we can write the total mass attenuation coefficient for any compound as follows [16]; μ ⁄ ρ = ∑iwi (μ ⁄ ρ)i

where wi and (μ ⁄ρ)i are the weight fraction and the mass attenuation coefficient of the ith constituent element, respectively. The total atomic cross-section (σa) for any sample can be calculated using the equation formulated as; ACS = σa =

where NA and N respectively are the Avogadro’s number and the atomic mass of materials. The total electronic cross-section (σe) is formulated the following equation [17]; ECS = σe =

By using the Equations (4) and (5), we can find the effective atomic number, Zeff, of the material as follows; Zeff =

We can calculate the effective electron number, Neff, as follows [18], Neff =

HVL and TVL are the thicknesses parameters that are the used to reduce the radiation intensities by one half and one tenth, respectively. MFP is the average distance at which a photon travels through the material between two interactions. The μ is used to obtain the parameters given by HVL =

Effective conductivity (Ceff) of materials can be given by the following equation [19]:

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Figure 1. The changes of MAC (a) and LAC (b) as a function of incident photon energy.

where me (kg) and e (C) are mass and charge of electron, respectively.

Results And Discussion

The chemical compositions of Ikizdere obsidian (IO) and Ercis obsidian (EO) are obtained from literature and are given in Table 1 [7,15]. The theoretically calculated radiation attenuation parameters of the EO and IO are listed in Table 2-3, respectively. Variations of the calculated MAC values of the obsidians versus photon energies (4keV-100GeV) are shown in Fig. 1(a). In the low energy region (1-100keV) where the photoelectric process is predominant, MAC values decreased sharply with increasing energy. In the intermediate energy region (100keV–5MeV) where the Compton scattering is dominant, MAC values slightly changed. Above 5MeV, the Pair production process starts and an increase in MAC values was observed with increasing energy [1,20]. LAC is an important parameter for defining the photon-matter interaction, but it is not sufficient. The value of LAC depends on both MAC and density of compound. Dependence of the calculated LAC values versus photon

energies (4keV-100GeV) is shown in Fig. 1(b). Differences of LAC values are greater than those of MAC values depending on the density effect. It was obtained that the MAC and LAC values of both obsidians were very near to each other for the given energies. In low energies, MAC values of IO are slightly bigger than those of EO, it can be said that IO has more absorption feature than EO. It was determined that the shielding potentials of the both EO and IO are more than the shielding potential of ordinary concrete (OC) [21], when the obtained MAC and LAC values of the obsidians are compared with those of OC (Table 4). The interaction possibility of per atom and per electron in a unit volume of any material is given by ACS and ESC, respectively. Changing of ACS and ECS values as a function of incident photon energies are given in Fig. 2(a-b). The obsidian with higher ACS and ECS values can be defined as better shielding obsidian. According to the obtained results for ACS and ECS parameters of the samples, the shielding potential of the IO is slightly higher than the EO. The HVL and TVL parameters give the information about the penetration ability of the radiations in materials. HVL, TVL and MFP parameters changing as a function of incident photon energies are given in Fig 3(a-c). In the mid-energy region where Compton scattering is dominant, most photons are more likely to be scattered. Therefore, their absorption probabilities are lower and hence thicker

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Table 2. Photon attenuation parameters of EO between the energies of 4keV and 100GeV. Energy

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Figure 2. The variations of ACS (a) and ECS (b) as a function of incident photon energy.

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Table 3. Photon attenuation parameters of IO between the energies of 4keV and 100GeV. Energy

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Figure 3. Dependence of HVL (a) TVL (b) and MFP (c) versus incident photon energy.

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Figure 4. The changes of Zeff (a) Neff (b) and Ceff (c) as a function of incident photon energy. Table 4. MAC and LAC values of EO, IO and OC between the energies of 10keV-1GeV Energy

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materials are required and longer MFP would be. It is preferred to have low HVL, TVL and MFP values in the high energy regions for better shielding property. Although, HVL, TVL and MFP values of the obsidians are very close to each other, lower HVL, TVL and MFP values were obtained for EO at high energies. The energy dependence of Zeff, Neff and Ceff are given in Fig. 4(a-c). In the low energy region due to the photoelectric effect, maximum Zeff values were obtained. By increasing energy, these values decreased sharply. Then the values gradually increased and remained constant in high energies. Due to the higher Zeff values of IO than those of EO, it can be said that IO shows higher shielding potential. Neff is one of the most important parameter that represents the effective conductivity of the compound depending on the excitatory photon energy [20]. As shown in Fig. 4, the variation of the Neff values on the incident photon energies is similar with the variation of Zeff values. The interactions between photons and material with photoelectric effect, Compton scattering, and pair production interaction processes cause changes in the number of free electrons in the material. Changing of Ceff values versus photon energies showed that EO has higher Ceff values than those of IO.

Conclusion

In the present study, radiation-matter interaction parameters of IO and EO were obtained to determine the radiation shielding capabilities. For this purpose, the MAC, LAC, HVL, TVL, MFP, ACS, ECS, Zeff, Neff and Ceff parameters of the present samples were calculated by Phy-X / PSD code in the range of 4keV-100GeV. According to the obtained results, although, the parameters of the studied obsidians have near values to each other, it was concluded that IO has higher shielding potential compared to EO. It was also obtained that both IO and EO have more shielding ability than that of OC.

Authorship Contributions

There are no ethical issues with the publication of this manuscript.

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