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Mass Attenuation Coefficients of Li2o- B2O3 Glass System at 0.662 and 1.25 Mev Gamma Energies

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Mass Attenuation Coefficients of Li2o- B2O3 Glass System at 0.662 and 1.25 Mev Gamma Energies XI Radiation Physics & Protection Conference, 25-28 November 2012, Nasr City - Cairo, Egypt Mass attenuation coefficients of Li2O- B2O3 glass system at 0.662 and 1.25 MeV gamma energies H.E. Donya* Faculty of Science, Physics Department, Menoufia University, Egypt. *Hossam Elsayed Donya ([email protected]), Tel: 00201004164148 ABSTRACT Borate glasses are very promising materials for the radiation dosimetry applications in view of the fact that their effective atomic numbers (Zeff) are very close to that of human tissue and having a high ability of hosting activators. The total mass attenuation coefficients, partial interactions and Zeff of glass system (100-x)B2O3-xLi2O (where x=5, 10, 15, 20, 25, 30, 35 and 40 mole %) have been calculated at photon energies 0.662 and 1.25 MeV using WinXCom software on the basis of mixture rule. Results indicated that the total mass attenuation coefficients showed a decrease with increasing the Li2O content, due to a decrease in Compton scattering probability, which gave a dominant contribution to the total mass attenuation coefficients for the studied glass samples at both energies. However, the photoelectric absorption and coherent scattering showed an increase with increasing the Li2O, concentrations at same energies. For a comparison, the total mass attenuation coefficients of the glass system had lower values at the energy 1.25 MeV than that at 0.662 MeV. Zeff was found to increase linearly with the increase of Li2O concentrations. It was concluded that low Li2O concentrations in glass system, under study, have Zeff closed to that of biological tissue (Zeff=7.42) and have higher total absorption coefficients at energy of 0.662 MeV than that at 1.25 MeV. These results are very useful in designing gamma radiation detectors using thermoluminescence technique. Therefore, it is recommended to use low Li2O content in Li2O-B2O3 glass system which makes it suitable for radiation detection purposes in medical applications. Keywords: Borate glasses/Effective atomic number/Mass attenuation coefficients /Thermoluminescence. 1. INTRODUCTION New thermoluminescent (TL) materials are suitable for radiation detection in the last several years that have been produced and studied. Special attention was given to different glass systems by our group because of their high TL sensitivity and their negligible fading (1- 3) (1) . In our previous paper , it is showed that Li2O-B2O3 glass system has good TL properties 191 XI Radiation Physics & Protection Conference, 25-28 November 2012, Nasr City - Cairo, Egypt to be considered as a new added candidate to the member of the currently being used dosimetric tree. The response of a thermoluminescent material to γ- ray detection depends on the atomic number of its constituents; then it is important to know first the effective atomic number of such material, Zeff, for obtaining its expected TL response at different energies. This was carried out by calculating the strength of different interaction probabilities (cross- sections) between gamma rays and the studied materials (detectors) (4). 2. METHOD OF CALCULATION The effective atomic number, Zeff of a thermoluminescent material (a composite) can be calculated according to the following equation (5, 6): b bb Zeff=++aZ 1 1 aZ 2 2 ...... (1) with nZ() anNZ==ii , iiAinZ() ∑i ii where a1, a2,… are the fractional contents of electrons belonging to different elements of atomic number Z1, Z2, ….etc in the composite, ni is the number of electrons, in one mole, belonging to each element Zi and NA is the Avogadro's number. The values of b are in the range from 2.94 to 3.5. The total mass attenuation coefficient of a mixture or compound (µ/ρ)m has been calculated by WinXCom, based on the mixture rule (7), where ⎛⎞μ n ⎛⎞μ ⎜⎟= ∑w i ⎜⎟ (2) ⎝⎠ρ mii ⎝⎠ρ (µ/ρ)i is the mass attenuation coefficient for the individual element in each component and wi is the fractional weight of the element in each component. This equation is valid when the effects of molecular binding, chemical and crystalline environment are negligible. Berger and Hubbell developed XCOM for calculating the total mass absorption coefficients or photon interaction cross- sections for any element, compounds or mixtures in a wide range of photon energies (from 1keV to 100 GeV). Recently, XCOM was transformed to the Windows platform by Gerward et al. (8), called WinXCom and our calculations were extracted using this software. 3. RESULTS AND DISCUSSIONS In this study, the (100-x)B2O3-xLi2O glass system (where x=5, 10, 15, 20, 25, 30, 35 and 40 mole %) was converted to weight fraction and given in table (1). For an example LiB5 sample refers to the composition 95B2O3-5Li2O. 192 XI Radiation Physics & Protection Conference, 25-28 November 2012, Nasr City - Cairo, Egypt Table (1). Chemical composition of samples and compound mole fraction of each in the mixture of the studied glass systems Sample B2O3 (Mol. %) Li2O (Mol. %) LiB5 95 5 LiB10 90 10 LiB15 85 15 LiB20 80 20 LiB25 75 25 LiB30 70 30 LiB35 65 35 LiB40 60 40 The total mass attenuation coefficients, of the studied glass systems were calculated at two photon energies 0.662 (137Cs-source) and 1.25 MeV (average energy of 60Co-source) using the WinXCom software on the basis of mixture rule. ph 3.1 Photoelectric mass absorption cross section ( μm ) ph Based on our calculations, it was found that the photoelectric cross section ( μm ) of the studied glass system at 0.662 and 1.25 MeV showed a decrease with increasing the Li2O concentrations (see Fig. 1). 1.30 0.662 MeV 1.20 1.25 MeV /g) 1.10 2 1.00 y=-3E-08x + 1E-05 (cm -5 R² = 1 0.90 0.80 0.30 0.25 0.20 0.15 y = -5E-09x + 2E-06 Photoelectric interaction x 10 interaction Photoelectric R² = 0.99 0.10 5 10152025303540 % LiB 5-40 Fig. (1). The photoelectric mass absorption coefficient of (100-x) B2O3-xLi2O glass system (where x=5, 10, 15, 20, 25, 30, 35 and 40 mole %). 193 XI Radiation Physics & Protection Conference, 25-28 November 2012, Nasr City - Cairo, Egypt ph It is clear from Fig. 1 that the values of μm at low photon energy (0.662 MeV) are higher ph than those at high energy (1.25 MeV). In addition, the slope of the straight line between μm and x at 0.662 MeV is higher than that at 1.25 MeV. Therefore, the effect of increasing Li2O, ph and accordingly decreasing the B2O3 concentrations, showed a decrease in the μm values of the studied glass systems at both energies. 3.2 Compton and coherent scattering cs Compton scattering mass attenuation coefficients ( μm ) of the studied glass system were cs calculated using WinXcom software. Results are given in Fig. 2 where μm values show an cs observed decrease with increasing Li2O concentration. Also, values of μm at 0.662 MeV are ph cs higher than those at 1.25 MeV. Therefore, both μm and μm values are more effective at low energy value, i.e at 0.662 MeV. 7.90 7.88 0.662 MeV 7.86 7.84 1.25 MeV 7.82 /g) 7.80 2 7.78 (cm 7.76 y = -3.3E-05x + 0.07865 -2 7.74 R2=0.997 7.72 7.70 5.65 5.60 5.55 5.50 y = -2.3E-5x + 0.05555 5.45 Compton Interaction x 10 R2=0.994 5.40 5.35 5 10152025303540 % LiB 5-40 Fig. (2) The Compton scattering, interaction of (100-x) B2O3-xLi2O glass system (where x=5, 10, 15, 20, 25, 30, 35 and 40 mole %). ch The Coherent scattering mass attenuation coefficients ( μm ) of the studied glass system ch were also calculated and results are represented in Fig. 3. μm shows (see Fig.3) a decrease ch with increasing Li2O concentration. The rate of increase in μm is obvious at energy of 0.662 ch MeV where values of μm are higher than those at 1.25 MeV. 194 XI Radiation Physics & Protection Conference, 25-28 November 2012, Nasr City - Cairo, Egypt 0.160 0.155 0.662 MeV 0.150 1.25 MeV 0.145 y = -3.6E-7 x + 1.435E-4 /g) 2 R² = 0.999 0.140 (cm 0.135 -3 0.130 0.055 0.050 0.045 y = -8E-08x + 3E-05 0.040 R² = 0.999 0.035 0.030 Coherent interaction interaction x 10 Coherent 0.025 5 10152025303540 % of LiB 5-40 Fig. (3) The coherent scattering mass attenuation coefficient of (100-x) B2O3-xLi2O glass system (where x=5, 10, 15, 20, 25, 30, 35 and 40 Mol. %). From the above calculations of photoelectric, Compton and coherent interaction, the total mass absorption coefficient were calculated and illustrated in Table 2. Table (2). Total absorption coefficient values of the studied glass system. Sample Energy Coherent Compton (cm2/g) Photoelectri Sum 2 -4 -2 -6 2 -2 LiB5 0.60 1.42×10 7.85×10 9.56×10 7.86×10 1.25 3.27×10-5 5.54×10-2 1.98×10-6 5.55×10-2 -4 -2 -6 -2 LiB10 0.60 1.40×10 7.83×10 9.43×10 7.84×10 1.25 3.23×10-5 5.53×10-2 1.87×10-6 5.53×10-2 -4 -2 -6 -2 LiB15 0.60 1.38×10 7.82×10 9.30×10 7.83×10 1.25 3.19×10-5 5.52×10-2 1.93×10-6 5.52×10-2 -4 -2 -6 -2 LiB20 0.60 1.36×10 7.80×10 9.17×10 7.81×10 1.25 3.15×10-5 5.51×10-2 1.82×10-6 5.51×10-2 -4 -2 -6 -2 LiB25 0.60 1.35×10 7.78×10 9.04×10 7.80×10 1.25 3.11×10-5 5.50×10-2 1.88×10-6 5.50×10-2 -4 -2 -6 -2 LiB30 0.60 1.33×10 7.77×10 8.91×10 7.78×10 1.25 3.06×10-5 5.49×10-2 1.77×10-6 5.49×10-2 -4 -2 -6 -2 LiB35 0.60 1.31×10 7.75×10 8.78×10 7.76×10 1.25 3.02×10-5 5.48×10-2 1.83×10-6 5.48×10-2 -4 -2 -6 -2 LiB40 0.60 1.29×10 7.73×10 8.65×10 7.74×10 1.25 2.98×10-5 5.46×10-2 1.72×10-6 5.46×10-2 195 XI Radiation Physics & Protection Conference, 25-28 November 2012, Nasr City - Cairo, Egypt From table (2), one can notice that the total absorption coefficient closes to the Compton interaction values.
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