Mass attenuation coefficients of Li2O- B2O3 glass ...

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 (13) . In our previous paper (1), it is showed that Li2O-B2O3 glass system has good TL properties 121


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 (crosssections) 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): Zeff = b a1Z 1b + a2 Z 2b + ......

(1)

with ai =

ni ( Z i ) , ∑ i ni (Zi )

ni = N A Z i

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 n ⎛μ⎞ ⎛μ⎞ = wi ⎜ ⎟ (2) ∑ ⎜ ⎟ i ⎝ ρ ⎠m ⎝ ρ ⎠i (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 crosssections 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.

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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. %) 95 90 85 80 75 70 65 60

LiB5 LiB10 LiB15 LiB20 LiB25 LiB30 LiB35 LiB40

5 10 15 20 25 30 35 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. 3.1 Photoelectric mass absorption cross section ( μmph ) Based on our calculations, it was found that the photoelectric cross section ( μmph ) 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 M eV 1.25 M eV

1.10

y=-3E -08x + 1E -05 R² = 1

1.00

-5

2

Photoelectric interaction x 10 (cm /g)

1.20

0.90 0.80 0.30 0.25 0.20

y = -5E -09x + 2E -06 R ² = 0.99

0.15 0.10 5

10

15

20

25

30

35

40

% LiB 5 -4 0

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 %). 123


It is clear from Fig. 1 that the values of μmph at low photon energy (0.662 MeV) are higher than those at high energy (1.25 MeV). In addition, the slope of the straight line between μmph and x at 0.662 MeV is higher than that at 1.25 MeV. Therefore, the effect of increasing Li2O, and accordingly decreasing the B2O3 concentrations, showed a decrease in the μmph values of the studied glass systems at both energies. 3.2 Compton and coherent scattering Compton scattering mass attenuation coefficients ( μmcs ) of the studied glass system were calculated using WinXcom software. Results are given in Fig. 2 where μmcs values show an observed decrease with increasing Li2O concentration. Also, values of μmcs at 0.662 MeV are

-2

2

Compton Interaction x 10 (cm /g)

higher than those at 1.25 MeV. Therefore, both μmph and μmcs values are more effective at low energy value, i.e at 0.662 MeV. 7.90 7.88 7.86 7.84 7.82 7.80 7.78 7.76 7.74 7.72 7.70

0.662 MeV 1.25 MeV

y = -3.3E-05x + 0.07865 2 R =0.997

5.65 5.60 5.55 5.50

y = -2.3E-5x + 0.05555 2 R =0.994

5.45 5.40 5.35 5

10

15

20

25

30

35

40

% 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 %). The Coherent scattering mass attenuation coefficients ( μmch ) of the studied glass system were also calculated and results are represented in Fig. 3. μmch shows (see Fig.3) a decrease with increasing Li2O concentration. The rate of increase in μmch is obvious at energy of 0.662 MeV where values of μmch are higher than those at 1.25 MeV.

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0.160 0.155

0.662 MeV 1.25 MeV

-3

2

Coherent interaction x 10 (cm /g)

0.150

y = -3.6E-7 x + 1.435E-4 R² = 0.999

0.145 0.140 0.135 0.130 0.055 0.050

y = -8E-08x + 3E-05 R² = 0.999

0.045 0.040 0.035 0.030 0.025 5

10

15

20

25

30

35

40

% of LiB5-40

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 LiB5 LiB10 LiB15 LiB20 LiB25 LiB30 LiB35 LiB40

Energy 0.60 1.25 0.60 1.25 0.60 1.25 0.60 1.25 0.60 1.25 0.60 1.25 0.60 1.25 0.60 1.25

Coherent 2 1.42×10-4 3.27×10-5 1.40×10-4 3.23×10-5 1.38×10-4 3.19×10-5 1.36×10-4 3.15×10-5 1.35×10-4 3.11×10-5 1.33×10-4 3.06×10-5 1.31×10-4 3.02×10-5 1.29×10-4 2.98×10-5

Compton (cm2/g) 7.85×10-2 5.54×10-2 7.83×10-2 5.53×10-2 7.82×10-2 5.52×10-2 7.80×10-2 5.51×10-2 7.78×10-2 5.50×10-2 7.77×10-2 5.49×10-2 7.75×10-2 5.48×10-2 7.73×10-2 5.46×10-2

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Photoelectri 9.56×10-6 1.98×10-6 9.43×10-6 1.87×10-6 9.30×10-6 1.93×10-6 9.17×10-6 1.82×10-6 9.04×10-6 1.88×10-6 8.91×10-6 1.77×10-6 8.78×10-6 1.83×10-6 8.65×10-6 1.72×10-6

Sum 2 7.86×10-2 5.55×10-2 7.84×10-2 5.53×10-2 7.83×10-2 5.52×10-2 7.81×10-2 5.51×10-2 7.80×10-2 5.50×10-2 7.78×10-2 5.49×10-2 7.76×10-2 5.48×10-2 7.74×10-2 5.46×10-2


From table (2), one can notice that the total absorption coefficient closes to the Compton interaction values. To confirm the obtained values of the attenuation coefficient of the studied glass, the effective atomic number (Zeff ) should be calculated and compared the obtained data by the attenuation coefficient of the element with Z has the same or near Zeff . 4. ZEFF -CALCULATION Zeff was calculated for the studied glass samples [LiB(5-40)] and given in Table 3 using Eq. (2). Column 1 gives the glass composition, element under study is given under column 2, the atomic weight, A and atomic number Z are given under columns 3 and 4, respectively. Weight fraction, w, and the total number of electrons per gm, NAZw/A are displayed under columns 5 and 6 respectively. The fraction number of electrons due to each element in the composition is then calculated and given under column 7 and denoted by a. The values of aZb were then calculated and given under the last two columns where x takes the values 2.94 and 3.5. Finally, values of Zeff were calculated and given in the last row of each glass composition. For example, Zeff values of LiB5 glass composition were found to be 7.33 and 7.39 with a percentage difference of about 0.8% and of an average of 7.36.

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Table (3). Zeff calculation for our studied glass samples [LiB(5-40)]. LiB5

LiB10

LiB15

LiB20

LiB25

LiB30

LiB35

LiB40

Element B O Li Sum Zeff B O Li Sum Zeff B O Li Sum Zeff B O Si Sum Zeff B O Li Sum Zeff Element B O Li Sum Zeff B O Li Sum Zeff B O Li Sum Zeff

A 10.81 16 6.941

Z 5 8 3

W 0.295 0.684 0.021 1

NAZ W/A 8.19×1022 2.06×1023 5.47×1021 2.93×1023

a 0.279 0.702 0.019 1

10.81 16 6.941

5 8 3

0.27781 0.67318 0.049 1

7.738×1022 2.027×1023 1.275×1022 2.928×1022

0.264 0.692 0.043 1

10.81 16 6.941

5 8 3

0.265 0.674 0.061 1

7.35×1022 2.03×1023 1.59×1022 2.92×1023

0.252 0.694 0.054 1

10.81 16 6.941

5 8 3

0.245 0.65714 0.09742 1

6.824×1022 1.979×1023 2.536×1022 2.915×1022

0.234 0.679 0.087 1

10.81 16 6.941

5 8 3

0.235 0.664 0.101 1

6.52×1022 2.00×1023 2.63×1022 2.91×1023

0.224 0.686 0.090 1

A 10.81 16 6.941

Z 5 8 3

W 0.21347 0.64128 0.1452 1

NAZ W/A 5.946×1022 1.931×1023 3.779×1022 2.903×1023

a 0.205 0.665 0.130 1

10.81 16 6.941

5 8 3

0.205 0.654 0.141 1

5.68×1022 1.97×1023 3.67×1022 2.90×1022

0.196 0.678 0.126 1

10.81 16 6.941

5 8 3

0.18194 0.62565 0.19242 1

5.068×1022 1.884×1023 5.008×1022 2.891×1023

0.175 0.652 0.173 1

127

a Z 2.94 31.719 317.203 0.472 349.394 7.329 29.991 312.833 1.101 343.925 7.290 28.576 313.582 1.374 343.532 7.287 26.573 306.809 2.199 335.582 7.230 25.412 309.937 2.283 337.632 7.245 a Z 2.94 23.243 300.562 3.290 327.095 7.167 22.227 306.268 3.197 331.692 7.201 19.892 294.451 4.378 318.721 7.104

a Z3.5 78.118 1016.407 0.872 1095.397 7.387 73.861 1002.404 2.037 1078.302 7.354 70.376 1004.804 2.542 1077.722 7.352 65.442 983.105 4.068 1052.616 7.303 62.584 993.125 4.223 1059.931 7.318 a Z3.5 57.241 963.084 6.0873 1026.412 7.251 54.740 981.368 5.915 1042.023 7.282 48.989 943.504 8.100 1000.593 7.198


The variation of the mean values of Zeff of the present glass system with Li2O is given in Fig. 4. It is clear that (see Fig. 4), Zeff decreases with increasing the Li2O concentration and their values range from 7.15 to 7.36. This variation is represented by an excellent linearity form with R2=0.99.

Fig(4) The effective atomic number of (100‐x) B2O3‐ xLi2O glass system (where x=5, 10, 15, 20, 25, 30, 35 and 40 mole %).

From the Zeff calculations, one can notice that, the Zeff for the studied glass are decreased from 7.35 to 7.15. So, the attenuation coefficient can be compared by an element closes to this range as Nitrogen atom (9) which has an attenuation coefficient value of 0.081 and 0.057 at gamma energies of 0.66 and 1.25 MeV respectively which is in a good agreement with our data (see table 2). Zeff of LiB glass system have the same as soft tissue (Zeff = 7.30 ± 1.25%) (10) , which is recommended for dose measurements using insertion of LiB TL-detectors in various phantoms [1]. 5. CONCLUSION The total mass attenuation coefficients and partial interactions at photon energies 0.662 and 1.25 MeV (100-x) B2O3-xLi2O glass system (where x=5, 10, 15, 20, 25, 30, 35 and 40 mole %) have been investigated using the WinXCom software. Results showed that total mass attenuation coefficients decreased with increasing Li2O concentration, due to a decrease in Compton scattering of glass samples which contribute dominantly to the total interaction. Although, the coherent scattering showed a decrease with increasing Li2O concentrations but it has lower values than Compton scattering. The photoelectric interaction has a slight influence in these glass samples on comparison to the effects of other two interactions. In addition, the Zeff values of our studied glasses showed a decrease with increasing Li2O concentration and found to have values from 7.15 to 7.35. Results recommend Li2O-B2O3 128


glass system as a TL γ-dosimeter especially at low Li2O content. In addition the low Li2O content in the glass system makes it very close to tissue equivalent material. Therefore, it is recommended to use low Li2O content in Li2O-B2O3 glass system as good gamma detectors at energy of 0.66 MeV.

6. REFERENCE (1) (2)

(3)

(4) (5) (6) (7) (8) (9) (10)

A. El-Adawy, N. E. Khaled, A. R. El-Sersy, A. Hussein, and H. Donya, Appl. Radiat. Isot., 68 – 6 (2010) 1132. N.E.Khaled, A.R.El-Sersy, H. M. El-samman, A.Hussein, A. El-Adawy and H. Donya, World Acad. of Sci. Eng. and Techn., 76 (2011) 894. H. Donya, H.M. El-Samman, A. El-Adawy A. Hussein, A.R. El-Sersy and. N.E. Khaled , Tenth Radiation Physics & Protection Conference, EG1100464, 42(33) (2011) 135 www.iaea.org/inis/collection/NCLCollectionStore/Public/42/076/42076638.pdf A.R. El-Sersy, A. Hussein, H.M. El-Samman, N.E. Khaled, , A. El-Adawy and H. Donya, J. of Anal. and Nucl. Chem., 288 (2010) 65. P.R. González, , C. Furetta, B. E. Calvo, M. I. Gaso, E. Cruz-Zaragoza , Nucl. Inst. & Meth. Phys. Res. B 260 (2007) 685. C. Furetta, B.E. Calvo, M.I. Gaso, E. Cruz-Zaragoza, Mod. Phys. Lett. B, 22(2008) 1997. D.F. Jackson, D.J. Hawkes, Phys. Rep. 70 (1981) 169. L. Gerward, N. Guilbert, K.B. Jensen, H. Levring, Radiat. Phys. Chem. 60 (2001) 23. http://physics.nist.gov/PhysRefData/XrayMassCoef/tab3.html (2013). Faiz M. Khan, “The Physics of Radiation Therapy” Lippincott Williams & Wilkins , 3rd ed., 2003.

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