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ORIGINAL ARTICLE
Year : 2016  |  Volume : 39  |  Issue : 2  |  Page : 62-67  

Effective atomic numbers for photon energy absorption and energy dependence of some thermoluminescent dosimetric materials


Department of Nuclear Science and Technology, University of Petroleum and Energy Studies, Dehradun, Uttarakhand, India

Date of Web Publication13-Sep-2016

Correspondence Address:
Balajiranganathan Anupreethi
Department of Nuclear Science and Technology, University of Petroleum and Energy Studies, Dehradun, Uttarakhand
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/0972-0464.190389

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  Abstract 

Effective atomic number for photon energy absorption (Zpeaeff), the relative effective atomic number of the thermoluminescent dosimetric (TLD) materials with respect to air (Zreff), and energy dependence (ED) of the TLD materials such as beryllium oxide (BeO), lithium tetraborate (Li2B4O7: Cu, Ag, P), aluminum oxide (Al2O3), calcium fluoride (CaF2), and titanium oxide (TiO2) in the energy range of 1 keV to 20 MeV have been studied. The values of ZPEAeff and ZReff show a broad peak and a maximum value around 20 keV for BeO, around 60 keV for CaF2 and TiO2, and around 40 keV for Al2O3 and Li2B4O7: Cu, Ag, P. Overall, the effective atomic number (Zeff) varies linearly with ED for all the materials in the photon energy range of 1 keV to 20 MeV except TiO2 and thereby confirming the validity of a more commonly employed method of using the Zeff to calculate the measure of the ED in TLD. The ED values show a broad peak around 30 keV for CaF2 and TiO2 and around 20 keV for Al2O3 and remain almost constant for the other two materials.

Keywords: Dosimetry, effective atomic number, energy dependence, mass energy absorption coefficient, thermoluminescent dosimetric materials


How to cite this article:
Anupreethi B, Shivaramu. Effective atomic numbers for photon energy absorption and energy dependence of some thermoluminescent dosimetric materials. Radiat Prot Environ 2016;39:62-7

How to cite this URL:
Anupreethi B, Shivaramu. Effective atomic numbers for photon energy absorption and energy dependence of some thermoluminescent dosimetric materials. Radiat Prot Environ [serial online] 2016 [cited 2022 Jan 19];39:62-7. Available from: https://www.rpe.org.in/text.asp?2016/39/2/62/190389


  Introduction Top


Thermoluminescent dosimetry (TLD) method is an important measurement technique of absorbed dose which is a measure of ionizing radiation effect in the irradiated material, and the TLDs have wide commercial applications in environmental monitoring and personal dosimetry. To select the most suitable TLD detector for a given dose measurement, the dosimetric characteristics of the various materials, in particular, their energy dependence (ED) and ZPEAeff must be known. When the ionizing radiation deposits energy in the TL material, electrons in the atoms get excited to higher energy state where they get trapped in deliberately introduced impurities in the crystal. The exposed TL material is subjected to heating which causes the electrons to drop back to the ground state with the emission of photon. The photon intensity can be measured, and it corresponds to the amount of energy absorbed from the exposure. In these kinds of composite materials, the atomic number cannot be represented by a single number (as in the case of elements) for the entire energy spectrum. The equivalent atomic number for the composite material is known as effective atomic number, Zeff, and it is found to vary with energy. Further, this Zeff was subdivided into two kinds, the effective atomic number for photon interactions ZPIeff, and effective atomic number for photon energy absorption ZPEAeff.[1] The ZPIeff is obtained from mass attenuation coefficient, µ/ρ which is defined as an average number of interactions between incident photon and the matter in a given mass per area thickness of the material. The ZPEAeff is found from mass energy absorption coefficient, µen/ρ which is defined as the measure of the average fractional amount of incident photon energy transferred to kinetic energy of the charged particles as a result of interactions (including bremsstrahlung and escaping secondary photons). The net kinetic energy of imparted charged particle is approximately equal to the amount of photon energy which caused chemical, biological, and other effects related to exposure of ionizing radiation. Since ZPEAeff represents the absorbed dose, it is preferable to use ZPEeff instead of commonly used ZPIeff in TLD and for the calculation of absorbed dose in radiation therapy. The ED is by far the most important and the first selection criteria of a detector for a TLD. The ED and ZPEAeff values further shed light into estimation of sensitivity of TLD materials for various photon energies.

The present work aims at estimating the ZPEAeff, ZReff, and ED of some TLD materials such as beryllium oxide (BeO), lithium tetraborate (Li2B4O7: Cu [0.03 weight %], Ag [0.03 weight %], P [0.8 weight %]), aluminum oxide (Al2O3), calcium fluoride (CaF2), and titanium oxide (TiO2) in the energy range of 1 keV to 20 MeV and study their variation as a function of energy. Among the materials studied, BeO and lithium tetraborate are tissue equivalent materials (i.e., Zeff is equivalent to that of human tissue 7.3) and other three materials are non-tissue equivalent.


  Materials and Methods Top


The ZPEAeff is obtained by interpolating the corresponding energy absorption cross-section (σen) of the TLD material in the plot of σen versus atomic number of elements following the method reported earlier.[1] The µen/ρ of each TLD material and the corresponding σen are obtained from individual µen/ρ of the constituent elements and their fractional weights (w) by employing the Equations 1 and 2.[1],[2],[3]





Where NA is the Avogadro number and Ai is the atomic weight of the ith constituent element of TLD material. The ED and Zreff of the TLD materials are calculated using the Equations 3 and 4, respectively, and are given below:






  Results and Discussion Top


The values of ZPEAeff, ED, and ZReff are calculated for BeO, Li2B4O7: Cu, Ag, P (Cu [0.03 weight %], Ag [0.03 weight %], P [0.8 weight %]), Al2O3, CaF2, and TiO2 in the energy range of 1 keV to 20 MeV, and the results have been tabulated and are given in [Table 1],[Table 2],[Table 3]. The composition of these materials is taken from the published literature.[4],[5] The values of ZPEAeff are plotted as a function of energy for various TLD materials studied in the present work and are shown in [Figure 1],[Figure 2],[Figure 3],[Figure 4],[Figure 5]. As can be seen from these figures, the ZPEAeff values are found to steadily increase up to 50 keV for BeO, Al2O3, and Li2B4O7: Cu, Ag, P and up to 60 keV for CaF2 and TiO2. The ZPEAeff values show a broad peak and a maximum value at 20 keV for BeO, 60 keV for CaF2 and TiO2, and 40 keV for Al2O3 and Li2B4O7: Cu, Ag, P. The variation in ZPEAeff with energy is due to the ED of various partial interaction processes (photoelectric effect, Compton scattering, and pair production) in the present energy region studied.
Table 1: ZPEAeff, Zreff, and ED values of beryllium oxide and lithium tetraborate for photon energies from 1 keV to 20 MeV

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Table 2: ZPEAeff, ZReff, and ED values of aluminum oxide and calcium fluoride for photon energies from 1 keV to 20 MeV

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Table 3: ZPEAeff, ZReff, and ED values of titanium oxide and ZPEAeff of air for photon energies from 1 keV to 20 MeV

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Figure 1: Variation of ZPEAeffand Zrefffor BeO with photon energy from 1 keV to 20 MeV

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Figure 2: Variation of ZPEAeffand ZRefffor Li2B4O7: Cu, Ag, P with photon energy from 1 keV to 20 MeV

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Figure 3: Variation of ZPEAeffand ZRefffor Al2O3with photon energy from 1 keV to 20 MeV

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Figure 4: Variation of ZPEAeffand ZRefffor CaF2with photon energy from 1 keV to 20 MeV

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Figure 5: Variation of ZPEAeffand ZRefffor TiO2with photon energy from 1 keV to 20 MeV

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At energies above 5 MeV, the contribution from pair production becomes significant and becomes very significant for TLD materials with high ZPEAeff. The same trend in the variation of ZReff as a function of energy is seen. The values of ED are plotted as a function of energy for various TLD materials studied in the present work and are shown in [Figure 6]. The ED graph has a broad peak near 30 keV for CaF2 and TiO2 and 20 keV for Al2O3 and remains almost constant for the other two materials. As can be seen from [Table 2] and [Table 3], there is a sudden increase in ED value from 15 to 20 keV for Al2O3 and similarly at 40–50 keV for CaF2 and TiO2. This rapid variation in ZPEAeff and hence in ED is due to K-edge. The K-edge energies of aluminum, calcium, and titanium are 15.60, 40.38, and 49.66 keV. This can be correlated to the sudden change in ED values at the specified energies.
Figure 6: Variation of ED as a function of photon energy from 1 keV to 20 MeV for various TLD materials

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The values of ZPEAeff are plotted as a function of ED for various TLD materials studied in the present work at selected photon energies and are shown in [Figure 7],[Figure 8],[Figure 9],[Figure 10],[Figure 11]. Overall, the ZPEAeff and ZReff vary linearly with ED in the photon energy range of 1 keV to 20 MeV with the exception of CaF2 and TiO2 for photon energies from 0.5 to 20 MeV. If the ED values of CaF2 and TiO2 are closely observed for 1 and 2 MeV photon energies, there is not much variation and that the values of TiO2 are less compared to CaF2. This leads to the vertical rise as can be seen in Figures 8-11. At low energies, the sensitivity of all the materials is good and as the energy increases, TiO2 reaches the minimum sensitivity region, especially for the energies >0.5 MeV, and hence, it is not suitable for use as dosimeter material in those energy ranges.
Figure 7: Variation of ZPEAeffwith ED at photon energies 10, 50, and 100 keV for various TLD materials

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Figure 8: Variation of ZPEAeffwith ED at photon energies 0.5, 5, and 10 MeV for various TLD materials

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Figure 9: Variation of ZPEAeffwith ED at photon energies 1 MeV for various TLD materials

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Figure 10: Variation of ZPEAeffwith ED at photon energies 2 MeV for various TLD materials

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Figure 11: Variation of ZPEAeffwith ED at photon energies 15 and 20 MeV for various TLD materials

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  Conclusions Top


New data on ZPEAeff, ED, and ZReff on some TLD materials have been reported for photon energies from 1 keV to 20 MeV. Variation in ZPEAeff with energy is due to the ED of various partial interaction processes (photoelectric effect, Compton scattering, and pair production) in the present energy region studied. Validity of calculating the measure of ED from Zeff has also been verified, and it is found to be almost linear for all the materials for the photon energies from 1 keV to 20 MeV except for TiO2. Since ZPEAeff represents the absorbed dose, it is preferable to use ZPEAeff instead of commonly used ZPIeff in TLD and for the calculation of absorbed dose in radiation therapy. To the best of our knowledge, these calculations have been done and are reported for the above TLD materials for the first time.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

 
  References Top

1.
Shivaramu, Ramprasath V. Effective atomic numbers for photon energy absorption and energy dependence of some thermoluminescent dosimetric compounds. Nucl Instrum Methods Phys Res B 2000;168:294-304.  Back to cited text no. 1
    
2.
Manohara SR, Hanagodimath SM, Gerward L. Energy dependence of effective atomic numbers for photon energy absorption and photon interaction: Studies of some biological molecules in the energy range 1 keV-20 MeV. Med Phys 2008;35:388-402.  Back to cited text no. 2
    
3.
Hubbell JH, Seltzer SM. Tables of X-ray Mass Attenuation Coefficients and Mass Energy-absorption Coefficients 1 keV to 20 MeV for Elements Z=1 to 92 and 48 Additional Substances of Dosimetric Interest. NISTIR 5632; 1995.  Back to cited text no. 3
    
4.
Azorin J. Preparation methods of thermoluminescent materials for dosimetric applications: An overview. Appl Radiat Isot 2014;83(Pt C):187-91.  Back to cited text no. 4
    
5.
Prokic M. Dosimetric characteristics of Li2B4O7: Cu, Ag, P solid TL detectors. Radiat Prot Dosimetry 2002;100:265-8.  Back to cited text no. 5
    


    Figures

  [Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11]
 
 
    Tables

  [Table 1], [Table 2], [Table 3]


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