Influence of readout parameters on TL response, re-usability and residual signal in LiF:Mg,Cu,P

AN IMPROVED METHOD FOR THE COMPUTERISED ANALYSIS OF GR-200A LiF:Mg,Cu,P TL SIGNALS

The profile of the residual signals of LiF:Mg,Cu,P within the region of dosimetric interest was approximated by a cubic curve. The conventional analysis, the GCA developed in CIEMAT and the GCA developed in this laboratory were compared. For GCA developed in this laboratory, the 5 μGy test dose could be well measured even after measurement at 10 mGy and obviously at lower doses. The 20 μGy test dose could be well measured even after measurement at 50 mGy. The residual signals caused by previous irradiations in the case of using only reader anneal can be more effectively eliminated by GCA developed in this laboratory than GCA developed in CIEMAT. The program developed in this laboratory, effectively estimating the contribution of the residual signals caused by previous irradiations, permits work to be done using only reader anneal in a dose range of more than three orders of magnitude.

Developing an Optimum Protocol for Thermoluminescence Dosimetry with GR-200 Chips using Taguchi Method

Thermoluminescence dosimetry (TLD) is a powerful technique with wide applications in personal, environmental and clinical dosimetry. The optimum annealing, storage and reading protocols are very effective in accuracy of TLD response. The purpose of this study is to obtain an optimum protocol for GR-200; LiF: Mg, Cu, P, by optimizing the effective parameters, to increase the reliability of the TLD response using Taguchi method. Taguchi method has been used in this study for optimization of annealing, storage and reading protocols of the TLDs. A number of 108 GR-200 chips were divided into 27 groups, each containing four chips. The TLDs were exposed to three different doses, and stored, annealed and read out by different procedures as suggested by Taguchi Method. By comparing the signal-to-noise ratios the optimum dosimetry procedure was obtained. According to the results, the optimum values for annealing temperature (°C), Annealing Time (s), Annealing to Exposure time (d), Exposure to Readout time (d), Pre-heat Temperature (°C), Pre-heat Time (s), Heating Rate (°C/s), Maximum Temperature of Readout (°C), readout time (s) and Storage Temperature (°C) are 240, 90, 1, 2, 50, 0, 15, 240, 13 and -20, respectively. Using the optimum protocol, an efficient glow curve with low residual signals can be achieved. Using optimum protocol obtained by Taguchi method, the dosimetry can be effectively performed with great accuracy.

Long-term fade study of the DT-702 LiF: Mg,Cu,P TLD

LiF thermoluminescent dosemeters (TLDs) are used by the US Navy to record radiation exposure of personnel. The Model DT-648 LiF:Mg,Ti TLD has been replaced by a new Model DT-702 LiF:Mg,Cu,P TLD. The DT-648 was used for many years and has undergone extensive testing to identify its pre- and post-irradiation fade operating characteristics. Studies have shown that the addition of copper increases the thermoluminesence sensitivity of the TLD for improved low-level radiation monitoring. This study evaluates various fading characteristics of the new copper-doped dosemeter using current equipment for processing of TLDs and calibrating to a National Institute of Standards and Technology standard source. The 57-week study took place at the Naval Dosimetry Center, Bethesda, MD, USA. TLDs were stored for various lengths of time before and after being exposed to a National Institute of Standards and Technology calibrated radiation sources. TLDs were then processed using current US Navy instructions and the resulting dose compared with the calibrated exposure. Both loss of signal and loss of sensitivity were evaluated. The results of this study have shown that the DT-702 TLD has no statistically significant change in sensitivity or change in signal with up to 57 weeks of pre- or post-irradiation time. The results of this study will increase the accuracy of exposure record keeping for the Navy and will allow longer issue periods. This will increase flexibility with international and domestic shipping procedures, as well as reduce workload requirements for dosimetry processing.

Sensitivity loss of Lif:Mg,Cu,P thermoluminescence dosemeters caused by oven annealing

LiF:Mg,Cu,P is a very sensitive thermoluminescence material that can be used for personal dosimetry in radiology. But if this material is heated too much during annealing or reading it quickly loses its sensitivity. This study shows that an annealing oven in wide use can cause thermal damage to the dosemeters owing to inhomogeneous temperature distribution in the annealing tray. At annealing temperatures>240 degrees C, differences of only 1 degrees C lead to significant losses of sensitivity. Therefore, it is necessary to measure the distribution of temperature in the annealing tray for correct placement of the dosemeters in the tray.