1. Evaluation of the characteristics of TLD LiF:Mg.Ti-100 Powder: A Measure of Consistency Between Multiple Batches of Powder Paola Alvarez,Jose Francisco Aguirre, Susan Smith, David Followill Dept of Radiation Physics, UT MD Anderson Cancer Center, Houston, TX Methods Reproducibility of the system is defined as the analysis of the response from reading many capsules irradiated to the same dose under same geometry. On the order of 50 capsule randomly selected from the batch are used for this test. Irradiations are done in a 60 Co beam at a dose level of 300 cGy. Fig 3: Linearity correction for eight different TLD batches Results (continued) More than 15 batches of TLD powder were commissioned to determine correction factors for the calculation of dose. The reproducibility of the TLD has always been the first characteristic determined for each new batch of TLD. This test is also performed during commissioning of a new reader. Results show a uniformity in the TLD response with a standard deviation of  1.2%. Example of data for one batch is shown on Figure 1. Results (continued) LiF:Mg.Ti-100 powder shows a predictable behavior in terms of the characteristics of fading, linearity and energy/block correction. The system sensitivity or calibration factor for each TLD batch, although variable, follows a pattern that allows the determination of maintenance actions when changes are observed. Each batch of LiF-100 TLD powder showed minimal variability in their powder characteristics, except for system sensitivity. However, an accurate calculation of dose using TLD with a minimal uncertainty, requires a new commissioning for each new batch. Introduction Materials The Radiological Physics Center (RPC) and the Radiations Dosimetry Services (RDS) have used LiF:Mg.Ti-100 encapsulated powder for many years to measure the dose delivered from photon or electron beams as a part of their audit services. The determination of dose based on this thermoluminescent dosimeter (TLD) has characteristics that influence the calculation of dose through the signal emitted by the powder when it is thermally stimulated. The reproducibility of the signal, fading of signal after irradiation, lack of linear response and energy/block dependence was analyzed for each batch of powder used going back to 1988. The gathered data on these parameters from batch to batch has shown a remarkable reproducibility that has been determined though the analysis of the commissioning data for the past 14 years. The variations in these factors have been studied and are presented in this poster. The RPC and RDS have used TLD LiF-100 encapsulated powder to verify the output for 60 Co to 23 MV photon beams and 6 to 23 MeV electron beams, for the past 14 years. The capsules are 1.5 cm in length and 0.4 cm diameter and filled with 20 to 22 mg of powder. The reading of the dosimeter is defined as the ratio between the signal from the reading process and the mass of powder. The mass is measured with highly sensitive digital balances. The make and model of TLD readers used are Harshaw 3500 for RPC and Rexon UL-320 for RDS. Commissioning of each batch of powder encompassed determining the linearity response vs. dose level, energy and fading characteristics of each batch of powder to determine the correction factors for the calculation of dose. These characteristics are reader independent and batch dependent. During each reading session the system sensitivity, dose response, is defined. This parameter works like a calibration factor for the reader in use. The linearity correction is based on irradiations in a 60 Co beam. The dose range is between 25 and 600cGy. The normalization point is 300cGy. The correction is defined as a way to characterize the lack of linearity based on dose level delivered to the TLD. Fig 1: Reproducibility study for one TLD batch The system sensitivity is determined every reading session. This parameter is the signal/mg per unit known dose of 60 Co. The sensitivity is based on 60 Co irradiations to an accurate dose of 300 cGy. This correction is used as a calibration factor for the TLD reading during a reading session. Conclusion The correction for fading, defined between 1 to 180 days, is a known characteristic of the LiF crystal. This correction factor varied by  1% between the multiple batches. See Figure 2 The correction for fading takes into account the loss of signal between the irradiation and read date. TLD capsules are irradiated at same dose in a 60 Co beam. Irradiations are done between 1 to 180 days before the read date. All readings are done within a single session. Fig 2: Fading correction for two TLD batches The linearity correction, showed a maximum variation of ±3% between batches. See Figure 3 The energy correction factor is defined as the change in TLD response because of the energy of the beam. The RPC/RDS TLD systems are based on the use of acrylic miniphantoms under specific irradiation geometry and 60 Co is the reference energy. The energy correction factors, as defined for the RPC/RDS TLD audit systems varied within ±1.7% (one std dev.) between batches. The correction factors are defined for various electron and photon energies. Values for these corrections are shown on Table 1. The system sensitivity is highly dependent on each TLD batch, specific TLD reader and reading session conditions. The sensitivity shows variability that depends on the specific powder batch and changes in the reflecting characteristics of the heating planchet. The system sensitivity, while accounting for the changes in readers and reading sessions, varied by as much as 20% between batches. See Figure 4. The changes in sensitivity from session to session follow a pattern that can be used as an indicator of the time to either clean or change the planchet. Results Energy6 MeV9MeV12MeV20MeV6MV10MV15MV18MV Average Correction 1.0381.0341.0281.0491.0131.0401.0461.061 %Standard deviation 0.7%1.4%1.1% 0.8%1.7%0.9%1.1% Table 1: Energy correction factors. Work supported by PHS CA010953 awarded by NCI, DHHS Fig 4: System sensitivity for one specific reader