Sintered LiF:Mg,Cu,Na,Si thermoluminescence (TL) pellets have been developed for application in radiation dosimetry. LiF:Mg,Cu,Na,Si TL pellets were made from TL powders using a sintering process, that is, pressing and heat treatment. These pellets have a diameter of 4.5 mm, and a thickness of 0.8 mm are blue in colour and have a mass of 28 mg each. After 400 pellets had been produced they were irradiated with 137Cs gamma radiation and samples having a sensitivity within a 5% standard deviation were selected for experimental use. In the present study, the physical and dosimetric properties of LiF:Mg,Cu,Na,Si TL pellets were investigated for their emission spectrum, dose response, energy response and fading characteristics. Photon irradiation for the experiments was carried out using X ray beams and a 137Cs gamma source at the Korea Atomic Energy Research Institute (KAERI). The average energies and the dose were in the range of 20–662 keV and 106 102 Gy respectively. The glow curves were measured with a manual type thermoluminescence dosimetry reader (system 310, Teledyne) at a constant nitrogen flux and a linear heating rate. For a constant heating rate of 5°C.s1 , the main dosimetric peak of the glow curve appeared at 234°C, its activation energy was 2.34 eV and the frequency factor was 1.001023. The TL emission spectrum appeared at the blue region centred at 410 nm. A linearity of photon dose response was maintained up to 100 Gy. The photon energy responses relative to the 137Cs response were within 20% in the overall photon energy region. No fading of the TL sensitivity of the pellets stored at room temperature was found over the course of a year. Therefore LiF:Mg,Cu,Na,Si TL pellets can be used for personal dosimetry, but more research is needed to improve the characteristics for repeated use.
ICRP recommend that a personal dose should be maintained as low as reasonably achievable (ALARA) to protect against the stochastic effect predicted by the linear non-threshold model in their report(1). Precise dose evaluation is needed in the low-dose ranges of 104 –101 mGy to maintain this ALARA principle, and it can be achieved by using TL materials which are more sensitive than existing ones.
LiF:Mg,Ti TL material was first sold commercially as TLD-100(2) and it is still used for personal monitoring even though it has a low sensitivity. In 1978, Nakajima et al(3) developed LiF:Mg,Cu,P TL material which has a sensitivity 20 to 40 times greater than the widely used TLD-100, and much research is now being done on this material in China (GR-200), Poland (MCP-N) and the USA (TLD-100H)(4).
A new TL material, LiF:Mg,Cu,Na,Si phosphors, with a higher sensitivity than LiF:Mg,Cu,P and good fading characteristics, has been developed by Doh et al(5) in Korea, and sintered LiF:Mg,Cu,Na,Si pellets have recently been developed at the Korea Atomic Energy Research Institute (KAERI)(6). Because TL pellets have a totally different material structure and dosimetric characteristics from TL phosphors due to the high-temperature sintering process, research on the physical and dosimetric characteristics of these pellets is needed.
In the present work the physical and dosimetric characteristics of LiF:Mg,Cu,Na,Si TL pellets were investigated. The glow curves were separated by a peak analysis program, and the activation energy and frequency factor calculated for the main peak. The threedimensional emission spectrum was measured with the heating temperature and frequency. The energy response in the range of 20–662 keV, the dose response from 106 to 102 Gy and the fading rate for a storage period of 1 year were also investigated for application to a personal dosimetry system.
MATERIALS AND METHODS
LiF:Mg,Cu,Na,Si TL pellets were made from TL phosphors by pressing at room temperature followed by a sintering process. These pellets have a diameter of 4.5 mm, are 0.8 mm thick with a blue colour and have a mass of 28 mg each. After 400 pellets had been produced they were irradiated with 137Cs gamma radiation and samples having a sensitivity within a 5% standard deviation were selected for experimental use.
The TL glow curves and intensities of the LiF:Mg,Cu,Na,Si TL pellets were measured using a commercial TLD reader (system 310 TLD reader: Teledyne Brown Engineering) controlled by a personal computer. Measurements were carried out with a linear heating rate of 5°C.s1 in a nitrogen flow of 70 kPa.
These pellets were irradiated with 137Cs gamma and X ray radiation on the 10 10 2 mm PMMA phantom at KAERI to investigate the physical structure, energy response, dose response, reusability and fading characteristics. The irradiating energies were in the range of 20 to 662 keV.
RESULTS AND DISCUSSION
Glow curve and TL sensitivity
Figure 1 shows the glow peak separated from the glow curve of LiF:Mg,Cu,Na,Si TL pellets using a TL analysis program. The heating rate was 5°C.s1 and the reading temperature was up to 320°C. The glow curve can be separated into five peaks at 104, 145, 189, 234 and 268°C. Of these peaks, the main peak which can be used for dose evaluation is the fourth peak (the 234°C peak), and the activation energy and the frequency factor of the main peak analysed by the TL analysis program are 2.34 eV and 1.00 1023 s1 respectively.