Commercial Ge-doped SiO2 optical fibers; Cat(MM-50/125): optical properties: losses @850 nm ≤10 dB/ km @ 900 nm & losses @1300 nm ≤10 dB/km, {typical core diameter 50 µm//cladding diameter 125.0 ±0.1 µm that provided by CorActive, Canada, were used in this study. To investigate the enhancement achieved by the high atomic number of the gold coating for the dose measured by the fiber optics, the following practical methodology was used:
1. Preparation of fiber optics
A. Remove of the acrylate coating: The outer acrylate coating is carefully removed using a fiber mechanical stripper. The remaining fiber is then cleaned using a pad of cotton moistened with a small amount of methanol to remove any residual polymer coating [12].
B. Cutting the fibers: small pieces of fiber optics were cut and divided into two groups: one group was coated with a gold layer, and the other was the uncoated control group. A total of 140 pieces of fiber optic tissue, each with a length of 1 ± 0.1 cm, and another group of fibers of the same number, cut into lengths of 0.5 ± 0.1 cm, were prepared by a CT 30 cleaver (Fujikura, Japan), the results of which are presented in Figure 1A. To reduce the uncertainty in TL yield, the gross TL yield was normalized to the unit mass of each irradiated fiber, and an electronic balance was used to ensure that the mean mass of the fiber and standard deviation were 4.5 ± 0.02 mg.
C. Storage and handling of the fiber pieces: Vacuum tweezers, which are shown in Figure 1B, were used to handle the fiber optic pieces [19]. To ensure unimpeded reception of luminescence yield by the photomultiplier tube arrangement within the TLD reader, it is essential that TLD materials do not become contaminated, particularly with grease [19]. Moreover, the TL sensitivity, stability, reusability, precision and minimum detectable dose for optical fibers may be affected by other physical and environmental factors [20]. Then, the optical fibers were placed into gelatin capsules to avoid exposure to elevated humidity and corrosive agents because these environmental parameters have been shown to reduce sensitivity by enhancing fading [21]. In addition, the capsule temperature decreases when exposed to high ambient light levels, the optical fibers were kept in a darkened environment prior to being irradiated or read out [22]. Figure (1C) shows the fiber optic encapsulation and storage methods.
D. Annealing procedures: The fibers were first annealed in a furnace (Carbolite, UK) before any irradiation or subsequent TL measurements were taken. For annealing purposes, the optical fibers were placed in a ceramic boat and covered with aluminum foil before being placed in the middle of the furnace. The temperature in the furnace was gradually increased from room temperature to 420°C, after which the mixture remained at that temperature for a period of 1 hour. To minimize thermal stress, the samples were then left in the oven for 18 hours to finally equilibrate at room temperature. This annealing step of TL material is carried out when the fibers are to be used for the first time or to be reused for three reasons: to find a good combination of annealing temperature and time for erasing any effect of previous irradiation, to produce the lowest intrinsic background and the highest sensitivity and to obtain reproducibility for both TL and background signals [21].
E. Screening process: Prior to the use of TL dosimeters in clinical practice, they should be screened by irradiating them with a known dose from a calibrated radiation source, which is related to the conditions for which the dosimeters will be used [22]. This process, which is shown in Figure 1D, enables the selection of samples that show good beam uniformity. Figure (1E) shows the instrument used to measure and choose a suitable fiber optic yield. Any TL sample outside the specified tolerance limits was rejected (see Figure 2 A). According to ICRU specifications, this was chosen to be within a limit of ±5% of the group mean [3], which is illustrated in Figure 2B.
F. The Glow Curve: The common method of presenting TL data is to plot light intensity against temperature or time, which is called the glow curve (represented in Figure 3). With increasing temperature, the light intensity increases as more electrons are released from the trap and allowed to excite. A decrease in light intensity occurs as the trap is gradually depleted. The number of peaks in a glow curve corresponds to the number of different types of traps existing in the TL material. The temperature at which the maximum of each peak occurs is correlated with its energy depth, E. The area under a glow curve is related to the number of electrons trapped and, in turn, corresponds to the quantity of absorbed radiation [23]. One of the important experimental problems in TL is the presence of several overlapping peaks within the TL glow curve. Very few experimental methods exist that allow decomposition of the TL glow curve into its individual components [24].
G. Gold coating process: First, 1 cm long pieces of fiber optics were prepared for coating by adhering them to small discs of hard card paper. These discs were centrally located on a miniature low inertia solar fan motor, the latter being protected by an aluminum foil cover that also covered the battery. Figure (1F) shows the low inertia solar fan motor and fiber optic samples attached to the centrally located card paper disc.
The five discs with ten fibers per disc were coated with gold in five steps as follows:
- 20 nm thickness - 40 nm thickness - 60 nm thickness - 80 nm thickness - 100 nm thickness. Figure (1G) shows the coating instrument used (K675X Turbo Large Chromium Coater 8; Quorum Technologies Ltd., Kent, UK). After the coating, the optical fibers were ready for irradiation, together with their corresponding uncoated (control) fibers.
H. Gold etching:
The TL yield from the fiber optics is strongly affected by the presence of gold during reading, so the gold should be removed. The gold-etching methodology uses aqua regia;
A mixture of concentrated nitric acid and concentrated hydrochloric acid at a ratio of 1:3 was used for gold removal from the fiber optics [25].
2. X-ray machines used in the experiment: An X-ray machine with potentials ranging from 150 to 500 kV was used. The operating voltage ranged between 200 and 300 kV, and the current was between 10 and 20 mA. Filters of different designs are used to achieve half-value layers between 1 and 4 mm Cu.
2.1. Irradiation of the fibers: The fibers were put in gelatin capsules and irradiated at Royal Surrey County Hospital, Guildford, UK, using an orthovoltage X-ray unit (GULMAY MEDICAL, Surrey UK), which has a 1 mm lead window. The half-value layer HVL is 2.7 mm Cu, the FSD is 50 cm, the added filtration in mm is 1.5 mm Al + 0.25 mm Cu + 0.5 mm Sn, the tube current is 12 mA, and the tube potential is 250 kVp.
The optical fibers were exposed to a 3 Gy single dose.
2.2. Reading the fibers for screening: The two requirements for determining the dose given to the fiber optics are a reliable form of heating and a method of measuring the light output [19]. To maintain control of thermal fading, the readout was performed after a set period post irradiation of 12 hours. The TLD reader used was a TOLEDO system (Pitman Instruments, Weybridge, UK), which is presented in Figure 1E.
The readout was carried out in the presence of nitrogen gas to prevent the influence of triboluminescence. In addition, this reduces the amount of oxidation that would otherwise occur on the surface of the dosimeter. Triboluminescence is caused by the mechanical disturbance of the surface of the fiber when the fiber is cut into small pieces [20].
The parameters that provided an optimal glow curve and were used during the readout process were as follows: preheating temperature of 160°C for 10 seconds and readout temperature of 300°C for 25 seconds with a ramp rate of 25°C/sec. An annealing temperature of 300°C for 10 seconds was subsequently used to eliminate any residual signal; its reproducibility was (±1.5), and a low residual signal was obtained for a readout temperature of 300°C and negligible fading. After reading and performing the statistical analysis for the screening process, the steps for coating and subsequent irradiation were carried out.
3. The irradiation setup of the fiber optics inserted in the phantom simulated the RT dosimeter:
The optical fibers were distributed in the phantom in steps according to their gold coating thickness. The phantom, a model for the synovial membrane, is formed of a Perspex cube with a length of 7 cm [26]. The phantom was designed at the University of Surrey. Figure (1 H) illustrates the shape of this phantom and its different steps. The irradiation setup was the same as that for the screening process, with the same monitoring units, time and dose rate.