^^^^^^^^^^^^ КРАТКИЕ СООБЩЕНИЯ ^^^^^^^^^^^^
ENERGY RESPONSE OF Ge-DOPED FIBER DOSIMETER SUBJECTED
TO PHOTON IRRADIATION
© 2012 г. I. Hossain*, H. W&giran, H. Asni, N. H. Yaakob
*Department of Physics, Rabigh Colleage, King Abdulaziz University, Post Box 344, 21911 Rabigh, Saudi Arabia
Department of Physics, Universiti Teknologi Malaysia 81310 Johor Bahru, Johor, Malaysia
E-mail: firstname.lastname@example.org Поступила в редакцию 03.02.2012 г. В окончательном виде 28.02.2012 г.
This paper focuses on the effect of dopant concentration of Ge-doped optical fiber subjected to photon irradiation with energy 1.25 MeV by Monte Carlo Simulation. Optical fibers are made of glass with different refractive indices in the inner core and the outer cladding regions. The effect of different dopant concentrations on the energy responses were analysed the range of 0—0.8% mol and the result from simulation shows that effect of energy response depends on different doping concentration of Ge-doped optical fiber and response are increased slowly with increased in concentrations. The simulation results are compared with experimental results.
ХИМИЯ ВЫСОКИХ ЭНЕРГИЙ, 2012, том 46, № 6, с. 500-502
The investigation of thermoluminescence (TL) of phosphor materials have been studied enormously, become commercially available and applied to many areas of ionizing radiation dosimetry including personal, clinical, environmental, charged particle and neutron dosimetry. The optical fiber has several characteristics such as respond monotonically to gamma photon radiation and in some dose regions observed linearly, carries a low residual TL signal, can be reused several times and low degree of fading. As claimed by Espinosa et al.  optical fiber could be very attractive for used in variety of radiation dosimetry applications due to its small size, flexibility, low cost and commercially available.
TL performance of an irradiated fiber depends on the type of fiber and by the radiation parameters . Different impurities in the fiber could give different performance as TL material. Abdulla et al.  has carried out a TL study on commercially available Ge-doped silica based optical fiber in dose range of 1— 1230 Gy. Germanium doped optical fiber was found to have linear dose response up to 4 Gy for 6 MV photons, and up to 3.5 Gy for 6-, 9- and 12 MeV electrons irradiation. Besides photon and electron irradiations, a linear dose response was also observed for 2.5 MeV protons irradiations. Germanium doped fibre was also prove to have strong TL response to fast neutron irradiation, whereas for aluminium doped fibers the TL response was practically negligible [2, 4].
Nowadays, other than carried out by experiment, the response studies also could be done by Monte Carlo simulation is one of the most important tools to study particle transport and interaction with matter as well as radiation protection and dosimetry. MCNP5 is a general purpose Monte Carlo n-particle transport
code that is continuous-energy, generalized-geometry, time-dependent code, and can be used for single or coupled neutron/photon/electron transport.
There are several studies showing the useful of optical fiber as TL material [1, 2, 5, 6] in terms of TL sensitivity, fading and dose response. Previous studies in thermo luminescence were carried out mainly by experiment. However, this research tries alternative approach which is Monte Carlo simulation, specifically MCNP5 . It is a common but not widely used method in thermoluminescence dosimetry area.
These studies will applied Monte Carlo N Particle code version 5 (MCNP5). MCNP was chosen because of the simplicity and ease when using the code compared to the other codes. Many research group use EGS Monte Carlo system as a tool to investigate their problems in the area of thermoluminescence dosimetry. Miles  studied the photon energy response for filtered CaSO4 dosimeter using EGS4 Monte Carlo system. Since the response of CaSO4 is not air-equivalent, it needs filtration or any other energy correlation method if dosimeters are to be exposed at lower energy photon. Miles compared response of the dosimeter from simulation with response from experiment and also with those calculated based on simple linear attenuation. His finding shows EGS4 calculated results were in close agreement with experimental data than the calculation based on attenuation. Miles reported discrepancies in the EGS4 calculation are believed to be due to energy dependent thermo luminescence efficiency of the phosphor. Davis studied the potential of TLD 100 H as a detector for environmental dosimeter using extended code EGS4 and found Monte Carlo calculations were good agreement with measured results .
ENERGY RESPONSE OF GE-DOPED FIBER DOSIMETER SUBJECTED
1.15 r Photon Energy Response at 1.25 MeV
J 1.10 -
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Dopant Concentretion (mol %)
Fig. 1. TL response in (nC/mg) vs dopant concentration for Ge-doped optical fibre (mol %).
The lack ofpublished literature referencing MCNP shows that there is a need for more research to be conduct with MCNP. Recently, we have thermo luminescence energy response of TLD 100 and Ge-doped optical fiber subject to photon irradiation using Monte Carlo N-particle code simulation [10—13]. This research utilized MCNP5 simulation in order to study the energy response as a function of doping concentration with the aim to promote the usage of computer simulation as well as provide basic knowledge in MCNP5 application. This research will contribute to the knowledge that can create new experts in thermoluminescence area as well as Monte Carlo area.
MATERIALS AND METHODS
Doped silica is a key material in several technological fields. Ge is the dopant that has been most frequently reported in diffusion studies on optical fibers. In optical fiber, doping with germanium atoms permits the creation of the radial profile of the refractive index, which is essential for an optical guide. The Ge-doped optical fiber consist of element Silica 53.6%, Oxygen 46.1% and Germinium 0.3%. The fiber initial length is 10 m; it was cut into 50 tiny pieces. Each pieces had 5 mm long. The fractions of elements in each fiber were measured by SEM technique as measured by Yaakob et al. [14, 15].
RESULTS AND DISCUSSION
The optical fiber used in this research was cut into 5 mm long from the 10 meter of optical fiber. Studying the effect of dopant concentration arise from the fact that the concentration of dopant along this 10 meter of optical fiber may be varies in the ranges zero to 0.8 mol %. Thus, 30 input file were simulated using tally F6 and photon energy of 1.25 MeV, in order to study the behaviour of different concentration of germanium in the optical fiber. Tally F6 is the track heating tallies modified to tally reaction rate convolved with an energy-dependent heating function instead of flux. The units of this tally are MeV/g.
Fig. 1 depicts the responses of the optical fiber with different germanium concentration. Optical fiber is fabricated so that there is a difference in the refractive index between the fiber core and the outer cladding region. This refractive index difference is achieved normally by adding a dopant to the inner core region. It is obviously shows that photon energy response of optical fiber had affected slightly linear even though the concentration of the germanium is different along the fibre for dopant concentrations of zero percent — 0.8 percent mol. As was expected, the TL output increases with dopant concentration since the number of electron traps and recombination centres increase with dopant concentration. The results of this work indicate that TL response increases slightly linear with increase of dopant concentration.
It is an important to note that comparison of thermoluminescence energy response subject to photon irradiation of optical fiber by simulation and experiments was in good agreement from energy range 200 keV to 10 MeV . The simulation response was in agreement with existing data of experiment energy response and overlap to each other at higher energies larger than 1000 keV. Energy response of Ge-doped optical fiber as a function of dopant concentration for 1.25 MeV photon irradiation using simulation and experiments are obviously verified. Thermo luminescence properties of Ge-doped optical fibers were experimentally studied at different percentage concentration of germanium subjected to 6 MV photon irradiation [16, 17]. The concentrations of Ge-doped fibers were in the range of 0.02—0.71 mol %. These
о X 1.2 и Й Й Й
^ £ H Й 0.8
' 0 0.2 0.4 0.6 0.8
Dopany concentration for Ge-doped optical fiber (mol %)
Fig. 2. Comparison of TL response between experiments and simulation as a function of dopant concentration (% mol) of germanium doped SiO2 optical fiber using photon energy 1.25 MeV
ХИМИЯ ВЫСОКИХ ЭНЕРГИИ том 46 № 6 2012
HOSSAIN и др.
ranges were determined by using SEM analysis at Ibnu sina Institute, UTM, Malaysia. Fig. 2 shows comparison of average simulation and experimental values of TL energy response as a function of Ge-doped concentrations (mol. %). The experimental values were multiplied by a factor after considering normalization for simulation and experimental values at concentration 0.35% mole. The results show that MCNP5 and experimental values are in good agreement.
Beside time saving, there are several advantages that can be offered when using simulation, specifically MCNP5, in investigating photon energy response or any TL research. MCNP5 provides interactive software; called MCNP vised, which can assist the user to create the input file, as well as gave the user visual of the geometry created. Other than that, MCNP also can be cost saving. Simulation can provide preliminary result of photon energy respons
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