Minimum Detectable Dose:

Here, the Minimum Detectable Dose (MDD), D0, refers to the lowest dose that the dosimeter is capable of measuring, over and above that of background, where:

𝐷₀ = ( 𝐵_{𝑚𝑒𝑎𝑛}+ 2σ)F (1)

𝐹 = 1 𝑚⁄ (2)

The MDDs are calculated as discussed by Furreta et al. (2001) using equation 1 (in present work units of mGy having renormalized to the particular dosimeter mass), with Bmean representing the mean TL signal obtained from the annealed but unirradiated samples and σ the standard deviation of the mean background. F (in equation 2) is determined from the slope m of the calibrated TL dose response for each of the TL materials (the uncertainty of the slope being of the order of ± 5%), expressed in mGy/TL herein. In present study, the MDD values were found to be 0.1 µGy and 1.0 µGy for the Ge-B-

than ± 8%, obtained in quadrature (for independent contributions) taking into account that for the slope, use of dosimeters screened to give variability uncertainties to within ± 4%, uncertainties in machine output (estimated to be up to ± 3%) and TLD reader drift (expected to be within ± 1%). The results show the samples reporting values of dose down to the least delivered dose of 5 µGy. The MDD for TLD-100, discussed by others for high kVp chest radiography, and thus not directly applicable herein, quote a value of 10 µGy under such circumstance (Burke and Sutton, 2014). The fibres fabricated herein provide excellent dose detection, from low doses in the lower range of medical diagnostic X-ray exposures though to the relatively higher values that can be obtained from a mammographic machine.

5.18 Conclusion:

Present work has sought to investigate the applicability of Ge-B-doped collapsed (FF) and disc shaped Ge-doped silica in respect of one low dose and one more elevated dose diagnostic-radiology situation, respective use being made of dental and mammography medical irradiating equipment. We have investigated the capability of these tailor made doped-silica glass thermoluminescence (TL) in medical diagnostic imaging dosimetry with the aim of developing a dosimeter of sensitivity greater than that of the commonly used LiF (Mg,Ti) phosphor. In so doing, we have examined the ability of such doped glass media to detect the typically low levels of radiation in diagnostic applications (from fractions of a mGy through to several mGy or more), including, chest, mammography and dental radiology, Excellent sensitivity within these two diagnostic dose regimes have been demonstrated, also low fading, representing a water impervious, robust dosimetric system. In addition, these silica-based fibres have shown good linearity over a wide dynamic range of dose and dose-rate. The results show that novel fabrications of doped- silica glass produce thermoluminescence (TL) yields that make them attractive as dosimeters in the diagnosis range of doses, offering high sensitivity for x ray doses and MDDs down to the µGy level.

5.19 References:

Abdulla,Y.A., Amin,Y.M. and Bradley,D.A., 2001. The thermoluminescence response of Ge doped optical fibre subjected to photon irradiation. Radiat.Phys.Chem. 61(3-6), pp.409- 410. Alyahyawi, A., Siti Rozaila, Z., Siti Shafiqah, A.S., Sabtu, S.N., Alsubaie, A., Alanazi, A., Daar, E., Abdul Sani, S.F., Bradley, D.A., 2016. Investigation of silica-based TL media for diagnostic x-ray dosimetry. Radiat. Phys. Chem. doi:10.1016/j.radphyschem.2016.12.018 Bradley, D.A., Hugtenburg, R.P., Nisbet, A., Abdul Rahman, A.T., Issa, F., Mohd Noor, N. and

Alalawi, A., 2012. Review of doped silica glass optical fibre: their TL properties and potential applications in radiation therapy dosimetry. Appl. Radiat. Isot. 71,pp.2–11. Burke, K., and Sutton, D., 2014. Optimization and deconvolution of lithium fluoride TLD-100 in

diagnostic radiology. Brit. J. Radiol. doi.org/10.1259/bjr.70.831.9166051.

Dambul, K.D., Mahdiraji, G.A., Amirkhan, F., Chow, D.M., Gan, G.K., Wong, W.R., Hassan, M.A., Tee, D.C., Ismail, S., Ibrahim, S.A., Tamchek, N. and Adikan, F.M., 2012. Fabrication and development of Flat Fibers.Proc.PGC.pp.1-3.

Entezam, A., Khandaker, M.U., Amin, Y.M., Ung, N.M., Maah, J. and Bradley, D.A., 2016. Thermoluminescence response of Ge-doped SiO2 fibres to electrons, X- and y-radiation.

Radiat.Phys.Chem. 121, pp.115–121.

Espinosa, G.,Golzarri, J.I., Bogard, J., and García-Macedo, J., 2006. Commercial optical fibre as TLD material. Radiat.Phys.Chem.119(1-4), pp.197–200.

Furetta, C., Prokic, M., Salamon, R., Prokic, V., and Kitis, G., 2001. Dosimetric characteristics of tissue equivalent thermoluminescent solid TL detectors based on lithium borate. Nucl.

Instrum. Methods Phys. Res. Sect. A Accel. Spectrometers, Detect. Assoc. Equip. 456, 411–

417. doi:10.1016/S0168-9002(00)00585-4

Horowitz, Y.S.and Yossian, D., 1995. Computerized glow curve deconvolution: application to thermoluminescence dosimetry. Radiat. Prot. Dosim. 60, 1-114.

Issa, F., Latip, N.A.A., Bradley, D.A., and Nisbet, A., 2011. Ge-doped optical fibres as thermoluminescence dosimeters for kilovoltage X-ray therapy irradiations. Nucl. Instrum.

Methods Phys. Res. Sect. A Accel. Spectrometers, Detect. Assoc. Equip. 652(1),834–837.

Jones D.E.A.,1978. The calculation of effective atomic number, Z. Brit. J. Radiol. https://doi.org/10.1259/0007-1285-52-616-330-c.

Mahdiraji, G.A., Dermosesian, E., Safari, M.J., Adikan, F.R.M., and Bradley, D.A., 2015a. Collapsed-hole Ge-doped photonic crystal fiber as a diagnostic radiation dosimeter. J. Light. Technol. 33, 3439–3445. doi:10.1109/JLT.2015.2406775

Molina, P., Sommer, M., Marcazzó, J., Santiago, M., Henniger, J. & Caselli, E., 2014. Thermoluminescent kinetics for negligible retrapping : Its application to the analysis of the glow curve of Y 2 O 3 : Eu þ 3. Radiation Physics and Chemistry, 97, pp.81–84. Available at: http://dx.doi.org/10.1016/j.radphyschem.2013.11.002.

Rahman, A.A., Bradley, D.A., Doran, S.J., Thierry, B., Bräuer-Krisch, E. and Bravin, A., 2010. The thermoluminescence response of Ge-doped silica fibres for synchrotron microbeam radiation therapy dosimetry. Nucl. Instrum.MethodsPhys. Res., Sect. A. 619(1),167–170. Rozaila, Z.S., Alyahyawi, A., Khandaker, M.U., Amin, Y.M., Bradley, D.A. and Maah, M.J.,

2016. Ge and B doped collapsed photonic crystal optical fibre, a potential TLD material for low dose measurements. Radiat.Phys.Chem. 126, pp.9-13.

Chapter 6

6 Enhancement of TL yield using gold-coated

fibres and variation in fibre fabrication:

6.1 Origin of TL yield enhancement in coated fibres:

As previously mentioned, dose is defined as the radiation absorbed per unit mass of an absorbing medium, measured in Gy (where 1 Gy is equivalent to 1 J of energy absorbed per unit mass of the particular medium). Focusing on photons, the radiations of primary interest herein, as a result of interactions between the photons and electrons in the absorbing medium, energy transfers take place between the photons and the absorber electrons. Subsequently, the energetic electrons lose their excess energy by collisions with other electrons within the medium, creating a large number of secondary electrons of various energy. The energy of the secondary electrons, clearly less than primary values, lose their excess energy over a highly limited distance (of the order of a few microns to tens of microns in substantial matter). For low energy x-rays (those generated at limited kVp values, taken herein to be <150 kVp), the various interactions with matter are dominated by the photoelectric process, at the particular energies being highly dependent on the atomic number (Z) of the absorber material. The greater the atomic number of material, the greater the number of secondary electrons released (favouring, as an instance, the use of gold and platinum, Z= 79 and 78 respectively, as photoelectron and Auger electron conversion media). Dose enhancement is produced as a result of the deposition of energy generated from such secondary electrons. In the photoelectric effect, the energy of the incident photon energy must be equal to or greater than the shell binding energy (Khan, 2010), K-shell photoelectrons being the particular interest. A typical x-ray tube spectrum dominated by the bremsstrahlung continuum is shown in figure (6.1), the peaks corresponding to characteristic x-rays at the K and L-shell binding energies.

Figure 6-1 A typical example x-ray tube spectrum showing the bremsstrahlung continuum as well as peaks corresponding to characteristic x-rays (Ahmed., 2007).