Analysis of the glow curves obtained from LiF:Mg,Cu,Na,Si TL material using the general order kinetics model

Three-dimensional thermoluminescence (TL) spectra based on temperature, wavelength and intensity for newly developed LiF:Mg,Cu,Na,Si TL material at the Korea Atomic Energy Research Institute (KAERI) were measured and analysed. The glow curves were obtained by integration of luminescence intensity over all wavelengths at each temperature, and various trapping parameters related to the traps were determined by analysing these curves. A computerised glow curve deconvolution (CGCD) method which was based on the general order kinetics (GOK) model was used for the glow curve analysis. The glow curves of LiF:Mg,Cu,Na,Si TL material were deconvoluted to six isolated glow curves which have peak temperatures at 333, 374, 426, 466, 483 and 516 K. The main glow peak of peak temperature at 466 K had activation energy of 2.06 eV and a kinetic order of 1.05. This TL material was also found to have three recombination centres, 1.80 eV, 2.88 eV and 3.27 eV by analysis of the TL spectra.     

Developments in the synthesis of LiF:Mg,Cu,Na,Si TL material

A procedure for synthesis of the highly sensitive pellet-type LiF:Mg,Cu,Na,Si thermoluminescent (TL) detector has been newly developed. It was found that the optimum concentrations of dopants for a pellet-type LiF:Mg,Cu,Na,Si TL detector were found to be Mg: 0.2 mol %, Cu: 0.05 mol %, Na: 0.9 mol%, and Si: 0.9 mol%. The TL sensitivity of this new detector was about 30 times higher than that of the TLD-100 by light integration measurements. Reusability study of the detector was carried out for 10 cycles. The results show that the coefficients of variation for each detector separately did not exceed 0.016, and that for all 10 detectors collectively was 0.0054.     

Thermoluminescence emission spectra for the LiF:Mg,Cu,Na,Si thermoluminescent materials with various concentrations of the dopants (3-D measurement)

The thermoluminescence (TL) emission spectra from LiF TL materials, called KLT-300 (LiF:Mg,Cu,Na,Si) with various dopant concentrations are measured and analysed. These KLT-300 materials were developed by the Korea Atomic Energy Research Institute (KAERI) to achieve an enhancement of the thermal stability in TL readings. Six types of samples are prepared with different dopant concentrations in the following ranges; Mg (0-0.20 mol%), Cu (0-0.05 mol%), Na and Si (0-0.9 mol%). The spectra measurements are carried out for the six types of samples using a TL emission spectra measurement device. The spectra measurement device consists of a monochromator, photomultiplier tube and temperature control unit to thermally stimulate the samples. The measured data shows the light emission during heating of the sample as a function of temperature and wavelength (three-dimensional TL spectra). The spectra were analysed using a method of deconvolution based on gaussian curve. The wavelength of a main peak of the emission spectra changes depending on the existence of the Cu dopant, while intensity of the spectra rapidly changes with the Cu dopant concentrations. The 385 nm emission is mainly observed in all the spectra from the samples with the Cu dopant, but in those from the samples without the Cu dopant a very weak 401 nm emission is mainly observed. However, any change in the wavelength at a main peak of the TL emission spectra from the sample materials with Na and Si dopants is not observed but that in the intensity at a peak of the spectra is observed.     

Development of LiF:Mg,Cu,Si TL material (new KLT-300) with a low-residual signal and high-thermal stability

LiF-based thermoluminescence (TL) materials have been widely used for radiation dosimetry due to their attractive features. LiF:Mg,Cu,P is one of the most sensitive tissue-equivalent TL materials, approximately 40 times more sensitive than LiF:Mg,Ti (TLD-100), but it has two main drawbacks: a thermal loss of the TL sensitivity when annealed at temperatures >240 degrees C, and a relatively high-residual signal. Recently, LiF:Mg,Cu,Na,Si TL material was developed to overcome these drawbacks at the Korea Atomic Energy Research Institute, but it provided only marginal improvements in reducing the residual signal. The newly developed LiF:Mg,Cu,Si TL material has a significantly lower residual signal and a better stability to thermal treatments. In this article, the preparation method and some dosimetric properties (sensitivity and residual signal) of the new LiF:Mg,Cu,Si TL material are presented. At the end of the preparation procedures, a dual-step annealing method is introduced and this has proved as a very efficient method to reduce the high-temperature peak and is the cause of residual signal. Therefore, the high-temperature peak in the glow curve was significantly reduced. The sensitivity is approximately 20 times higher than that of TLD-100 and the residual signal was estimated to be approximately 0.04%.     

TL emission spectra from differently doped LiF:Mg detectors

There are two widely applied types of thermoluminescent detectors based on LiF:Mg luminophor: Lif:Mg,Ti and highly sensitive LiF:Mg,Cu,P. The role of luminescence centres in these materials is usually attributed to defects connected with, respectively, titanium and phosphorus dopants. In order to check how composition of dopants introduced into the LiF lattice influences emission spectra, measurements on a series of variously doped LiF:Mg samples were performed. Apart from LiF:Mg,Cu,P and LiF:Mg,Ti detectors with different concentration of activators, an experimental sample being a kind of a 'hybrid' between both standard materials was also prepared. It was synthesised with concentrations of magnesium and copper identical to those used for LiF:Mg,Cu,P preparation. but instead of phosphorus it was doped with titanium (LiF:Mg,Cu,Ti). The measurements of the emission spectra were performed by using a liquid nitrogen cooled CCD 1024E detector with an SP150 spectrograph. During the measurements the samples were placed inside a cryostat in a vacuum. Resulting data were numerically deconvoluted for individual peaks with respect to the wavelength and the temperature. The glow curve shape of this material resembles that of LiF:Mg,Cu,P, while sensitivity is at the level of LiF:Mg,Ti. Preliminary results indicate that emission of the LiF:Mg,Cu,Ti sample is similar to that of LiF:Mg,Cu,P rather than to LiF:Mg,Ti, showing a maximum for wavelengths well below 400 nm.     

Lithium fluoride: from LiF:Mg,Ti to LiF:Mg,Cu,P

Differences and similarities between LiF-based LiF:Mg,Ti and LiF:Mg,Cu,P are discussed, with respect to their dosimetric properties--sensitivity, non-linearity of dose response and heavy charged particle efficiency, as related to the concentration and the individual role of the Mg, Ti, Cu and P dopants. To study further the role of these dopants, the properties of some new, 'hybrid' phosphors: LiF:Mg,Cu,Ti and LiF:Mg,P, specially developed for this purpose, are also discussed. In the glow curve of LiF:Mg,Cu,P with a low concentration of Mg a new peak was found, which appears to be an analogue of peak 4 in LiF:Mg,Ti, Magnesium apparently controls most of the dosimetric properties of LiF-based phosphors. For instance, charged-particle efficiency appears to be anti-correlated with the concentration of Mg, being much less dependent on the content of other dopants. On the other hand, some properties of LiF-based systems seem to be correlated with changes in the emission spectra. It is suggested that Ti hampers the acceptance of any increased amount of Mg into more traps in LiF:MgTi. The absence of Ti, not the presence of P or Cu, is therefore a key to the high sensitivity of LiF:MgCuP.     

Thermoluminescence properties of LiF:Mg,Cu,Na,Si pellets in radiation dosimetry

Sintered LiF:Mg,Cu,Na,Si thermoluminescence (TL) pellets have been developed for application in radiation dosimetry. LiF:M,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 10(-6) - 10(2) 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 degrees C.s(-1). the main dosimetric peak of the glow curve appeared at 234 degrees C, its activation energy was 2.34 eV and the frequency factor was 1.00 x 10(23). 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.     

Pre- and post-irradiation fading effect for LiF:Mg,Ti and LiF:Mg,Cu,P materials used in routine monitoring

LiF is a well-known thermoluminescent (TL) material used in individual monitoring, and its fading characteristics have been studied for years. In the present study, the fading characteristics (for a period of 150 d) of various commercial LiF materials with different dopants have been evaluated. The materials used in the study are those used in routine procedures by the Personal Dosimetry Department of Greek Atomic Energy Commission and in particular, LiF:Mg,Ti (MTS-N, TL Poland), LiF:Mg,Cu,P (MCP-N, TL Poland), LiF:Mg,Cu,P (MCP-Ns, thin active layer detector, TL Poland) and LiF:Mg,Cu,P (TLD100H, Harshaw). The study showed that there is a sensitivity loss in signal of up to 20 % for the MTS-N material for a 150-d period in the pre-irradiation fading phase. The MCP-N has a stable behaviour in the pre-irradiation fading phase, but this also depends on the readout system. As far as the post-irradiation fading effect is concerned, a decrease of up to 20 % for the MTS-N material is observed for the same time period. On the other hand, the LiF:Mg,Cu,P material presents a stable behaviour within ± 5 %. These results show that the fading effect is different for each material and should be taken into account when estimating doses from dosemeters that are in use for >2 months.     

A comparative study on the susceptibility of LiF:Mg,Ti (TLD-100) and LiF:Mg,Cu,P (TLD-100H) to spurious signals in thermoluminescence dosimetry

It is well known that spurious signals can occur in thermoluminescence dosimetry (TLD) whenever contaminants (i.e. dirt, oil, dust) are present on the surface of the TLD card or crystal during the read-out process. For TLD cards, the Teflon material can also contribute to the background noise and this contribution has been found to depend on the material's light absorption. These non-radiation-induced signals contribute to the total light output during TLD read-out and can lead to incorrect dosimetry especially for low-dose measurements such as personal dosimetry. However, these spurious signals are generally in the low-temperature channels and are mostly accompanied by abnormal glow curves. Most of the published reports dealing with this type of spurious TL signal are on the LiF:Mg,Ti (TLD-100) material. The relatively new TLD material, LiF:Mg,Cu,P, is more sensitive and has higher signal-to-noise ratio than the traditional LiF:Mg,Ti. In this study, the effects of disturbing signals to the LiF:Mg,Cu,P (TLD-100H) cards used in personal dosimetry are investigated and compared with those of LiF:Mg,Ti (TLD-100).     

The application of computerised analysis of glow curves to personal dosimetry in LiF:Mg,Cu,P

In personnel monitoring services, it is important to omit the high-temperature annealing process so that large numbers of TL detectors can be produced economically. There are two efficient ways of reducing the residual signal of LiF:Mg,Cu,P. One is by increasing the maximum readout temperature and the other is by improving the preparation procedure (increasing the Cu concentration and the sintering temperature) but both reduce the TL sensitivity. In personal dosimetry the real dosimetric signals are separated from the residual signals by computerised analysis of glow curves. The adverse influence of the high residual signals of LiF:Mg.Cu.P TL material has been effectively eliminated and the sensitivity remains stable. A good dosimetric result using only reader measurement without pre-irradiation oven annealing is attained in a dose range of 50-80,000 microGy.     

OSL and TL in LiF:Mg,Ti following alpha particle and beta ray irradiation: application to mixed-field radiation dosimetry

The optically stimulated luminescence (OSL) of LiF:Mg,Ti (TLD-100) following irradiation by beta and alpha particles was investigated by the measurement of the excitation and emission spectra of OSL and comparison with thermoluminescence (TL) characteristics. Measurements were also carried out on nominally pure LiF monocrystals. The preferential excitation of OSL compared to TL following high-ionisation density (HID) alpha irradiation is naturally explained via the identification of OSL with the 'two-hit' F2 or F3+ centre, whereas the major component of composite TL glow peak 5 is believed to arise from a 'one-hit' complex defect. This discovery allows near-total discrimination between HID radiation and low-ionisation density radiation and may have significant potential in mixed-field radiation dosimetry.     

Role of impurities in the thermoluminescence of LiF:Mg,Cu,P

The thermoluminescent properties of lithium fluoride doped with magnesium, copper and phosphorous are reviewed. This material shows specific advantages over LiF (TLD-100) in that is has higher sensitivity, less energy dependence and no supralinear behaviour with dose. However, it suffers from an irreversible loss in sensitivity when heated above 270 degrees C. Possible causes for this loss in sensitivity are discussed and experimental results are shown that indicate potential roles for the Cu, P and O impurities in this process. The present work focuses on the understanding of the impurity contents, inclusions, cluster formation and valence changes in this material by using differential scanning calorimetry (DSC) and glow discharge mass spectroscopy (GDMS). This research concludes that phosphorus inclusions are formed and a valence change of the Cu ions occurs after annealing above 270 degrees C, which degrades the TL sensitivity of LiF:Mg,Cu,P phosphors. It is also shown that a high content of oxygen in the material and control of the oxygen content during processing are critical to maintaining the sensitivity of these phosphors.     

Comparative study of LiF:Mg,Cu,Na,Si and Li2B4O7:Cu,Ag,P TL detectors

Recently, two new types of 'tissue equivalent' thermoluminescent detectors (TLDs) have aroused attention: LiF:Mg,Cu,Na,Si and Li2B4O7:Cu,Ag,P. In this work the characteristics of both detectors were compared with the characteristics of the well-known type LiF:Mg,Ti detector, TLD-100. The following properties were investigated: the glow curve structures, relative sensitivity, batch homogeneity and uniformity, detection threshold, reproducibility of the response, linearity in the wide dose range and fading. Also, the energy dependence for medium and low energy X rays was determined in the range of mean energies between 33 and 116 keV. The results confirmed 'tissue equivalency' of both new types in the investigated range of photon energies. LiF:Mg,Cu,Na,Si detector has very high sensitivity (approximately 75 times higher than that of TLD-100) and is convenient for use in a very low range of doses. Li2B4O7:Cu,Ag,P detector shows some improvements in comparison with the previously prepared types of lithium borate. The most important is the five times higher sensitivity than that of TLD-100. This detector is also very promising, especially in medical dosimetry.     

Properties of the 4.45 eV optical absorption band in LiF:Mg,Ti

The optical absorption (OA) and thermoluminescence (TL) of dosimetric LiF:Mg,Ti (TLD-100) as well as nominally pure LiF single crystal have been studied as a function of irradiation dose, thermal and optical bleaching in order to investigate the role of the 4.45 eV OA band in low temperature TL. Computerised deconvolution was used to resolve the absorption spectrum into individual gaussian bands and the TL glow curve into glow peaks. Although the 4.45 eV OA band shows thermal decay characteristics similar to the 4.0 eV band its dose filling constant and optical bleaching properties suggest that it cannot be associated with the TL of composite peaks 4 or 5. Its presence in optical grade single crystal LiF further suggests that it is an intrinsic defect or possibly associated with chance impurities other than Mg, Ti.     

UV-induced bleaching of deep traps in Harshaw TLD LiF:Mg,Cu,P and LiF:Mg,Ti

The effects of UV-induced bleaching of deep traps on Harshaw thermoluminescent (TL) LiF:Mg,Cu,P and LiF:Mg,Ti materials were investigated. During a normal heating cycle, LiF:Mg,Cu,P is limited to a maximum temperature of 240 °C. LiF:Mg,Ti can be read to higher temperatures; however, encapsulation in polytetrafluoroethylene limits the maximum readout temperature to 300 °C. Generally, for both materials, these respective temperatures are sufficient for emptying traps corresponding to the main dosemetric peaks. However, when the dosemeters are subjected to a high dose level, such as 1 Gy (much higher than individual monitoring dose levels), higher temperature traps are filled that cannot be emptied without exceeding the above-mentioned maximum temperatures. These high temperature traps tend to be unstable during normal readout and can significantly increase the residual TL signal. The purpose of this study was to investigate the applicability of a UV-induced bleaching technique for emptying higher temperature traps following high-dose applications. In addition, in the case of LiF:Mg,Cu,P, where the maximum readout temperature is significantly lower, we investigated the possibility of reducing the residual signal using the application of repeated readout cycles. The optical bleaching approach was found to be effective in the case of LiF:Mg,Ti; however, for LiF:Mg,Cu,P, no reduction in the residual signal was observed. For this latter material, the application of repeatable readout cycles is very effective and residual signals equivalent to dose levels as low as 0.01 mGy were observed following an initial dose of 5 Gy. To the best of our knowledge, this work is the first attempt to apply an 'optical annealing' technique to the Harshaw thermoluminescent dosemeter (TLD) materials.     

The application of LiF:Mg,Cu,P to large scale personnel dosimetry: current status and future directions

LiF:Mg,Cu,P is starting to replace LiF:Mg,Ti in a variety of personnel dosimetry applications. LiF:Mg,Cu,P has superior characteristics as compared to LiF:Mg,Ti including, higher sensitivity, improved energy response for photons, lack of supralinearity and insignificant fading. The use of LiF:Mg,Cu,P in large scale dosimetry programs is of particular interest due to the extreme sensitivity of this material to the maximum readout temperature, and the variety of different dosimetry aspects and details that must be considered for a successful implementation in routine dosimetry. Here we discuss and explain the various aspects of large scale LiF:Mg,Cu,P based dosimetry programs including the properties of the TL material, new generation of TLD readers, calibration methodologies, a new generation of dose calculation algorithms based on the use of artificial neural networks and the overall uncertainty of the dose measurement. The United States Navy (USN) will be the first US dosimetry processor who will use this new material for routine applications. Until June 2002, the Navy used two types of thermoluminescent materials for personnel dosimetry, CaF2:Mn and LiF:Mg,Ti. A program to upgrade the system and to implement LiF:Mg,Cu,P, started in the mid 1990s and was recently concluded. In 2002, the new system replaced the LiF:Mg,Ti and is scheduled to start replacing the CaF2:Mn system in 2006. A pilot study to determine the dosimetric performance of the new LiF:Mg,Cu,P based dosimetry system was recently completed, and the results show the new system to be as good or better than the current system in all areas tested. As a result, LiF:Mg,Cu,P is scheduled to become the primary personnel dosimeter for the entire US Navy in 2006.     

Analysis of spectrally resolved thermoluminescence of LiF:Mg,Cu,P detectors by the surface fitting method using algorithm for unrestricted peak positions

Recently developed surface fitting technique is a natural replacement for the widely used glow curve deconvolution (GCD) technique. Surface fitting can be applied for advanced spectrally resolved thermoluminescence (TL) measurements. It combines both kinetic and emission-band analysis. Owing to greater number of parameters and data points the algorithm is more time-consuming than usual GCD. However, it offers greater reliability in determination of trap parameters. This is especially important for spectrally resolved measurements that are usually performed at low-light level conditions. This paper demonstrates an application of the surface fitting method to the analysis of TL-3D data from LiF:Mg,Cu,P detectors. The spectra were analysed using two different variants of surface fitting--for restricted and unrestricted peak positions.     

Energy responses of the LiF series TL pellets to high-energy photons in the energy range from 1.25 to 21 MV

The energy responses for the KLT-300(LiF:Mg,Cu,Na,Si, Korea), GR-200(LiF:Mg,Cu,P, China) and MCP-N(LiF:Mg,Cu,P, Poland) thermoluminescence(TL) pellets were studied for a photon radiation with energies from 1.25 MeV(60Co) to 21 MV (Microtron) to verify the usefulness of the calibration for the radiotherapy beams. The International Atomic Energy Agency (IAEA) and the World Health Organization (WHO) have performed thermoluminescence dosimetry (TLD) audits to verify the calibration of the beams by TL powder, but TL pellets were used in this study because the element correction factor (ECF), defined as the factor to correct the variations that all TL dosemeters cannot be manufactured to have exactly the same TL efficiency, for each TL pellet could be accurately derived and be handled conveniently when compared with the powder. Also several works for the energy response of the TLDs were done for the low-energy photon beams up to 60Co, but they will be extended in this experiment to the high photon energies (up to 20 MV), which are widely used in the therapy level of a radiation. The PTW 30006 ionisation chamber was calibrated by the Korea primary standards to establish the air-kerma rates and the TL pellets were irradiated in a specially designed waterproof pellet holder in a water phantom (30 x 30 x 30 cm3) just like the IAEA postal audits programme. This result was compared with that of another type of phantom [10 (W) x 10 (L) x 10 (H) cm3 PMMA Perspex phantom for the 60Co and 6 MV photon, and 10 x 10 x 20 (H) cm3 for the 10 and 21 MV photon] for its convenient use and easy handling and installation in a hospital. The results show that the differences of the responses for the water phantom and PMMA Perspex phantom were negligible, which is contrary to the general conception that a big difference would be expected. For an application of these results to verify the therapy beams, an appropriate energy correction factor should be applied to the energies and phantom types in use.     

Influence of background exposure on detection and determination limits for a TL dosimetry system based on LiF:Mg,Cu,P(GR-200A)

Thermoluminescence dosemeters are widely used to monitor personal doses. For these low dose range applications, it is important to determine the detection limit L(D) and the determination limit L(Q) of the dosimetric system. The influence of background exposure on these limits for LiF:Mg,Cu,P(GR-200A) based TL dosimetry was investigated. Both the conventional analysis and the glow curve analysis methods were used to determinate these limits. The detection limit L(D) was compared with the recording level and the investigation level. A systematic error can occur in the occupational dose evaluation when the detection limit L(D) is more than the recording level. It was found that the L(D) of the dosimetric system-based LiF:Mg,Cu,P(GR-200A) was less than the recording level for exposure time tau > or = 10 days considering an annual dose limit of 1 mSv for the public recommended in ICRP Publication 60.