An investigation of major factors affecting LiF:Mg;Ti TLD response and accuracy was carried out. The factors that investigated were dose response, field size response, dose rate dependence, wedge angle and percentage depth dose (PDD) response for a Perspex phantom. The reference conditions for these TLD experiments were 100 cGy of dose, 6 MV X-ray energy spectrums, and zero days of fading. The absorbed doses were measured at the surface as well as at various depths and the results were verified using ionization chamber used in radiation therapy. The mean reproducibility of TLD readings was within 3% for one standard deviation. Doses from TLDs were compared to the calculated doses from the Treatment Planning System (TPS).
Lithium fluoride is one of the most popular types of thermoluminescent dosimeter (TLD) because of its good sensitivity (light emitted per dose), tissue equivalency (effective Z# of LiF = 8.2, effective Z# = 7.64 for human muscle tissues) (Furetta. and Weng, 1998.; Horowitz, 1984b.; McKeever et al, 1995.; McKinlay, 1981.; Kron, 1994.; Duggan, 2002) and stability (Furetta. and Weng, 1998). As LiF was purified to a higher level it was found that its sensitivity actually decreased (Furetta. and Weng, 1998.; Horowitz, 1984b). Other study (Moscovitch.M, 1991) was conducted to investigate the dosimetric properties of LiF:Mg,Cu,P, emphasizing the characteristics that are particularly relevant to medical dosimetry, focusing on the sensitivity , the dose response and the photon energy dependence.
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Duggan (2002) studied the dose, energy and fading response of LiF TLD. However, he investigated LiF made by China and Poland, while this work investigated LiF made by the St. Gobain company (Solon, OH), and uses different equipment and procedures. It is not surprising more clinics are undertaking such studies since it is important to determine TLD characteristics to ensure the most accurate measurements of patient dose are made. Reproducibility plays a major role in the accuracy of TLD measurements, so high reproducibility is desirable (Furetta C. and Weng P, 1998.; McKeever S et al, 1995.; McKinlay A, 1981.; Cameron J et al, 1968).
2. TLD Usage
TLDs have three major uses; as personal dosimeters for people who work with or around radiation, environmental monitoring around nuclear installations, and for verifying the radiation doses given to radiation patients (Furetta. and Weng, 1998.; Horowitz, 1984a.; McKeever et al, 1995.; McKinlay, 1981.; Ginjaume et al, 1999.; Cameron et al, 1968.; Kron, 1994.; Saez-Vergara and Delgado, 1995.;Saez-Vergara et al, 1993.; Yuen, 1995.; Nakajima et al, 1978.; Kron,1995.; Hutton, 1984.; Aukett ,1991). Personal dosimeters must be sensitive to small amounts of radiation, accurately record a wide range of doses and different types of radiation, and be stable for wearing periods of one to three months.
2.4 Pre-readout Anneal
When using TLDs the lower traps are often deliberately eliminated (emptied). One easy way to accomplish that is to wait 24 hours or more after the radiation exposure is given before reading the TLD. That allows time for the lower energy thermoluminescence to decay away. The disadvantage is the delay in getting results.
For LiF:Mg,Ti a thermal treatment of 20 hours at 80°C before irradiation results in a transformation of lower energy traps into higher level ones. The process of transformation is not well known, but it is thought to involve the conversion of molecular complexes of magnesium dimers to trimers (McKeever et al, 1995). A dimer is composed of two Mg dipoles, while a trimer is composed of three Mg dipoles. The low-temperature heating may allow the rearrangement of the low energy Van der Waals bonds, which are on the order of 0.1 evlatom (Callister, 2000), enabling conversion from the dimer to trimer form. This third alternative allows immediate readout of the TLDs after irradiation has occurred, but a longer preparation time is required.
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1. Labeling TLDs
Forty pieces of LiF:Mg;Ti TLDs were labeled, from 1 - 40 in one series, with a graphite pencil to aid in identification and analysis of reader orientation dependence
2. Calibration of TLDs
The TLDs used in this study were selected from a batch of LiF:Mg;Ti square chip having dimensions of 3 x 3 x 0.9 mm3 as obtained from the manufacturers (Bicron NE, USA). A Siemens Mevatron MD2 linac (Siemens Inc. USA) was the radiation sources used in this investigation. A Nabertherm oven (Nabertherm, Germany) is used to anneal the TLDs and a Harshaw model 3500 (Harshaw, USA) is used as the TLD reader. The TLDs were selected after a careful initialization procedure (Furetta C and Weng P, 1998). The LiF:Mg;Ti TLDs were annealed at a temperature of 400 °C for 1 hour (high temperature anneal) followed by 80 °C for 20 hours (low temperature anneal) (Horowitz Y, 1990). The TLDs with reproducibility within ± 3 % (I SD) and sensitivity ranging not more than ± 5 % were selected for calibration and subsequent measurements.
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The TLDs were then calibrated in a Perspex phantom at 1.5 cm depth for 6 MV beams. The ionization chamber used for the TLD calibration was the FC65-G (Welhofer Germany) for the Mevatron linear accelerator. The FC65-G chamber was calibrated at the Welhofer Calibration Laboratory, Germany.
The field size of 10 cm x 10 cm and 100 cm source to surface distance (SSD) was employed for the linear accelerator. For calibration purposes, a dose of 100 cGy at measurement point is delivered from 6 MV photon beam.
A number of 4 mm thick slabs of perspex phantom are fabricated with holes within the surface having diameter of 4.5 mm. The TLDs were placed in the holes for calibration and dose measurement purposes. The holes are about 1cm apart in order to avoid any influence on dose because of the slightly higher density of TLDs (2.64 gm/cm3) compared to the phantom slabs (l gm/cm3).
Each TLD was initially calibrated as many times as needed until response stability is achieved. The criterion for this is a relative response within 3 % of the mean of the relative response of previous calibrations. All TLDs were calibrated immediately before and after being used for a measurement run and checked for stability after each calibration. A few TLDs were separated into sub groups, which are used as control. Although common and individual calibration is established for all dosimeters, these controls are irradiated to a known dose at the same time as a measurement run and read out along with the measurement TLDs to rule out any uncertainties in TLD calibration.
In this study the dose was derived using the common calibration factor (CF common )after applying elemental correction coefficients (ECC) for individual readings is the mean dose in TLD (100cGY), TL Ave is the average reading of TLD after exposed, Dose TLD is the dose in TLD after exposed, and TL Signal is the TLD reading.
3. Response Behavior of LiF:Mg;Ti TLD
TLDs were calibrated at 6 MV photon beam for reference conditions to the dose at dmax using calibrated ion chamber, calibration setup is presented in Figure1. The reference conditions for TLD calibration were 100 cm SSD, a field size of 10 x 10 cm2 at 1.5 cm depth in solid water phantom with fullback scatter. The dose from 1 cGy to 400 cGy from a linear accelerator beam was measured to find out any variation in linearity of dose for those beams. Non-reference conditions include variations in field size and SSD were studied. The effect of standard wedges (30°, 45° and 60°) on the TLD signal was also determined. A percentage depth dose (PDD) curve was determined as well.
Perspex Phantom Phantom
Figure1:TLD calibration setup.
RESULTS AND DISCUSSION
Calibration of TLDs
The individual calibration factors were determined at six irradiated times, and then the average result was used. The standard deviation of each individual calibration factor gives a measure of the reproducibility of the results. Reproducibility plays a major role in the accuracy of TLD measurements, so high reproducibility is desirable. The individual backgrounds for each TLD were not subtracted from the gross readings since the background is so low compared with the TL of 100 cGy (less than 0.01% for LiF:Mg,Ti).
2. Linearity Response
for different dose values using 6 MV X-rays. As shown in the figure 2, the response TLD curve as a function of dose ranging between 25 to 400 cGy was linear. A fixed SSD (100 cm), 10 cm thickness of perspex phantom for back scattered, and 10 x 10 cm2 field sizes were fixed during linearity response study.
Figure 2: Linearity of thermoluminescence intensity (TLD) against absorbed dose.
3. Field Size Response
Field size dependence was performed by exposing the TLDs and ion chamber for different field size, reference setting was used for all measurment, only the field size changed from 5 x 5cm2 to 20 x 20 cm2. The relationship between the field size and dose is presented in Figure 3. It shows an increase of the dose by increasing fild size. The radiation scatter is increasing with field size, so with it will add up some dose.
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Figure 4 shows the output factors for both TLDs and ion chamber for various field sizes. The output factor was determined as ratio of measured dose at a given field size to the calculated dose at reference field size (10 x 10 cm2). By refering to Figure 4, output factor increases with increasing ï¬eld size for both TLDs and ion chamber. For the field size ranging from 5 to 20 cm2, the maximum discrepancies were 6 %. The discrepancies between output factor measurements using TLDs and ionization chambers appeared because TLDs and ion chamber have different energy responses.
Figure 3: Field size versus dose.
Figure 4: Output factor for TLD and ion chamber.
Effect of Wedge
The wedge correction factor (CFWedge) is defined as the ratio between the wedge transmission factor for a 10 x 10 cm2 field size, measured with the ionization chamber placed at dmax, and the wedge transmission factor for the same field size, measured with the TLDs placed at the centre of the field at dmax in the phantom.
Figure 6 shows that there is a downward trend in the plot of ionization chamber to TLDs correction factors with increasing wedge angle up to 45Ëš. For 60Ëš wedge this ratio slightly increase. It is because the thickness of 45Ëš wedge compare to 60Ëš wedge is slightly greater. This difference is also observed in TLD and ionization chamber reading seperately. Variations in wedge factors arise from changes due to beam transmission through the wedge. It would be expected that the TLD reading should increase due to wedge beam hardening because of greater beam penetration.
Table 5.3: Transmission Factor and Wedge Factor for both TLD and ion chamber Reading (Mount Miriam Hospital).
Ion Chamber Reading
Figure 5: Wedge angle verses wedge factor for TLD and Ion Chamber.
Figure 6: Wedge correction factor for TLD.
Dose Rate Dependence
the SSD value was changed from 80 to 130 cm. TLDs data obtained from measurements are normalized to the measurement value for 100 cm SSD as shown in Figure 7. The correction factor is within 3 %. Dose rate was indicated through SSD values using inverse square law. Figure 8 shows a linear fit curve between dose and dose rate. By decreasing SSD, the number of contaminating electrons and head scatter low energy photons able to reach the sensitive part of the detector is larger and the ratio of ionization chamber and TLD reading decreases. This effect is reverse for increases of SSD to more than 100 cm.
Figure 7: SSD dependence of TLD.
Figure 8: Dose rate dependence for TLD using various SSD.
Percentage Depth Dose Curve
PDD curve was performed by exposing the TLDs and ion chamber at different depths in the phantom, reference setting was used for all measurment using both perspex and solid water phantoms, only the depth value was changed from 0 to 20 cm, 10 cm thickness as full backscatter was used for both phantoms.
Three PDD curves are presented in Figure 9, blue and green color curves refer to the ion chamber dose reading using solid water and perspex phantom, respectevily. While the red color PDD curve resulted from TLDs dose reading by solid water phantom. For the depth ranging from 1.5 to 19 cm using a solid water phantom, the maximum discrepancies between ion chamber and TLDs reading were 5 %, while the discrepancy was 37 % at the surface. On the other hand, the maximum discrepancy for ion chamber reading was 3 % using two different phantom materials. The discrepancies between the measurements using TLDs and ionization chambers appeared because TLDs and ion chamber have different energy responses.
Figure 9: PDD curve in water phantom and Pespex phantom for a 10 x 10 cm² field at an SSD of 100 cm for 6 MV photon beam using LiF 100 TLDs and ion chamber.
The study of the TLD dose response showed that the response TLD curve as function of dose was linear . Affixed SSD (100cm), 10 cm thickness of solid water phantom and 10 x10 cm2 field size were fixed during linearity response study. Increasing dose increased TLD reading.
The finding showed that increasing field size from 5 to 20 cm2 increased the dose because of scattered radiation and this increasing some dose. Output factor increased with increasing field size for both TLD and ion chamber, for the field size ranging from 5 to 20 cm2.
Also there is a downward trend in the plot of ionization chamber to TLDs correction factors with increasing wedge angle from 30° to 45° and slightly increased from 45° to 60°, because the thickness of 45° wedge compare to 60° wedge is slightly greater.
In the case of dose rate dependence, increasing source surface distance decreases the TLD reading and dose in TLDs. Dose rate increases by increasing dose. This occurred when SSD from 80 to 130 cm.
The study percentage depth dose curve is done by exposing TLDs and ion chamber at different depths. In the Perspex phantom and solid water phantom, the PDD decreased with depth ranged from 1.5 to 19 cm. However, the PDD increased for depth range from surface to 1.5 cm. The PDD reach the maximum value at 1.5 cm by fixed 10x10 cm2 field size at 100 cm SSD and 6MV photon beam, using LIF 100 TLDs and ion chamber.
The maximum obtainable accuracy of 3 % occurs when no adjustments for dose, field size, and wedge responses are required. In other words, when the calibration TLDs are irradiated to the same dose, with the same energy, and on the same day as the TLDs of interest. The reference conditions for these TLD experiments were 100 cGy of dose, 6 MV X-ray energy spectrums, and zero days of fading. Dose response, field size response, and wedge response have all been determined for LiF:Mg ,Ti TLDs
6.2 Future Work
The study recommends following works for further exploration of the current findings; to use a phantom of particle board (Rhizophara spp) wood and a Perspex phantom to study the parameters of the LiF:Mg;Ti TLDs and compare it with the current results of solid water phantom. On the other hand, a comparison between LiF:Mg;Ti and LiF:Mg;Cu;P TLDs can be investigated.