Optical excitation of LiF:Mg,Ti following alpha and beta irradiation

It is demonstrated experimentally that optical excitation of irradiated LiF:Mg,Ti (TLD-100) by 4 eV photons has the same effect for both alpha particle (high-ionisation density) irradiation and photon/electron irradiation. In both cases, peak 5a converts to peak 4 causing peak 4 to increase following the bleach. Such an observation is consistent with the major premise of track structure theory that radiation effects following heavy changed particle (HCP)/neutron irradiation are due exclusively to the interaction of the secondary electrons created by the HCP slowing down.     

Determination of LiF:Mg,Ti and LiF:Mg,Cu,P TL efficiency for X-rays and their application to Monte Carlo simulations of dosemeter response

The simulation of response of a new passive area dosemeter for measuring ambient dose equivalent H*(10) for photons has been performed using the Monte Carlo code MCNP and experimentally determined responses of LiF:Mg,Ti and LiF:Mg,Cu,P thermoluminescent (TL) detectors for hard-filtered X-ray spectra from 20 to 300 keV and for 137Cs and 60Co gamma radiation. Relative TL efficiency for both types of detectors, determined in experiments with bare detectors and similar Monte Carlo simulations, compared favourably with prediction of microdosimetric models for proposed microdosimetric target sizes in the range of 20-40 nm. The concluding verification experiment showed small deviations between measured and simulated dosemeter energy response values in the range of a few percent.     

On neutron-gamma mixed field dosimetry with LiF:Mg,Ti at radiation protection dose levels

The possibility of using the specific responses of the high temperature Peaks 6 and 7 and Peaks 4 and 5 to different LET radiations was mentioned in the past mainly for very high doses. The applicability of the two regions method for thermal neutrons--gamma ray mixed field dosimetry was investigated by analysing the response of LiF:Mg,Ti dosemeters irradiated to different ratios of thermal neutrons and gamma rays at radiation protection dose levels encountered in routine work conditions, up to approximately 50 mSv. The Region of Interest method was used to define the areas of the Peaks 4 + 5 and 6 + 7. We found that a simple algorithm can be used to determine with good accuracy the separate contributions of neutron and gamma doses.     

Optimisation of the readout parameters when evaluating thermal neutron doses by TL dosimetry with LiF:Mg,Ti

The location of the glow peaks depends on the heating rate. It takes some time until the crystal reaches the heater temperature, and this time lag has a direct effect on the shift of peaks towards higher temperatures. Some information on the high-temperature peaks may be lost if the readout conditions (mainly length of time) are not properly chosen. Step heating profiles to a varying final temperature between 300 degrees C and 125 degrees C were used to study the time dependence of the collected information in the glow curves of (6)LiF:Mg,Ti crystals, and the minimal heating time for evaluation of thermal neutron doses was determined.     

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.     

Uncertainty assessment of a two element LiF:Mg,Ti TL personal dosemeter using Monte-Carlo techniques

This paper presents the results of an uncertainty assessment and a comparison study of dose algorithms for H(p)(10) and H(p)(0.07) used for evaluating a routine two-element thermoluminescent whole-body dosemeter. Due to the photon-energy response of the two different filtered LiF:Mg,Ti detector elements, the application of a dose algorithm is necessary to assess the relevant photon doses over the rated energy range with an acceptable energy dependence. A linear dose algorithm with two different sets of parameters was designed to assess the personal dose equivalent in the relevant quantity H(p)(10) and H(p)(0.07). Based on the experimental results from calibrations on the ISO water slab phantom, a detailed uncertainty analysis was performed by means of Monte-Carlo (MC) techniques and other analytical methods. The uncertainty contribution of the individual detector element signals was taken into special consideration. For this analysis, realistic energy and angular distributions were applied to calculate the dosemeter response. It is concluded that the MC method is an appropriate tool to perform uncertainty calculations. The possibility to assign arbitrary probability density functions to the input quantities, as well as to define a complex model function (dose algorithm) allows the simulation of irradiation conditions close to reality.     

Comparison of the effects of exposure to light in Harshaw LiF:Mg,Ti and LiF:Mg,Cu,P

The response of thermoluminescence dosemeters (TLDs) to light, in various conditions, has been studied. TLD cards containing both conventional lithium fluoride (LiF:Mg,Ti) and the high-sensitivity material LiF:Mg,Cu,P were available, so permitting a comparison between the two types. Also available for the tests were Harshaw(TM) extremity EXT-RAD (LiF:Mg,Cu,P) dosemeters. The LiF:Mg,Ti body TLD cards and the EXT-RAD extremity dosemeters both showed some response to fluorescent light, while the LiF:Mg,Cu,P cards showed no significant response. It is therefore concluded that LiF:Mg,Cu,P body cards need no special precautions to protect them from the effects of light. For LiF:Mg,Ti cards and extremity dosemeters, effects are small, but steps to avoid excessive light exposure should be considered.     

The thermoluminescence dose-response and other characteristics of the high-temperature TL in LiF:Mg,Ti (TLD-100)

Various characteristics of the high-temperature thermoluminescence (HTTL) in the glow curve of LiF:Mg,Ti (TLD-100) are reviewed. The proposed applications of the HTTL to mixed-field radiation dosimetry are outlined with special emphasis on the question of the linearity/supralinearity of the HTTL dose-response at low dose levels from 2.5 to 250 mGy. Recent measurements of the HTTL dose-response using non-linear hot-gas heating and linear planchet heating are discussed in detail. It appears that a mild HTTL supralinearity of approximately 15-50% for each dose decade may be present, followed by an abrupt and rapid increase in the supralinearity >250 mGy. However, difficulty in the estimation of background and the great variability in the protocols of measurement do not allow a definitive conclusion. There is much work to be done in the areas of protocol standardisation, materials selection, methods of data analysis and especially the details of background behaviour, and subtraction before the HTTL can become a reliable dosimetric tool.     

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 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.     

Energy response of different types of RADOS personal dosemeters with MTS-N (LiF:Mg,Ti) and MCP-N (LiF:Mg,Cu,P) TL detectors

The photon energy response of different RADOS (Mirion Technologies) personal dosemeters with MTS-N (LiF:Mg,Ti) and MCP-N (LiF:Mg,Cu,P) thermoluminescence (TL) detectors was investigated. Three types of badges were applied. The irradiation with reference photon radiation qualities N (the narrow spectrum series), and S-Cs and S-Co nuclide radiation qualities, specified in ISO 4037 [International Organization for Standardization (ISO). X and gamma reference radiations for calibrating dosemeters and doserate meters and for determining their response as a function of photon energy. ISO 4037. Part 1-4 (1999)], in the energy range of 16-1250 keV, were performed at the Dosimetry Laboratory Seibersdorf. The results demonstrated that a readout of a single MTS-N or MCP-N detector under the Al filter can be used to determine Hp(10) according to requirements of IEC 61066 [International Electrotechnical Commission (IEC). Thermoluminescence dosimetry systems for personal and environmental monitoring. International Standard IEC 61066 (2006)] for TL systems for personal dosimetry. The new RADOS badge with the experimental type of a holder (i.e. Cu/Al filters) is a very good tool for identifying the radiation quality (photon energy).     

Search for ionisation density effects in the radiation absorption stage in LiF:Mg,Ti

Optical absorption (OA) dose-response of LiF:Mg,Ti (TLD-100) is studied as a function of electron energy (ionisation density) and irradiation dose. Contrary to the situation in thermoluminescence dose-response where the supralinearity is strongly energy-dependent, no dependence of the OA dose filling constants on energy is observed. This result is interpreted as indicating a lack of competitive process in the radiation absorption stage. The lack of an energy dependence of the dose filling constant also suggests that the charge carrier migration distances are sufficiently large to smear out the differences in the non-uniform distribution of ionisation events created by the impinging gamma/electron radiation of various energies.     

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 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.     

Ionisation density effects following optical excitation in LiF:Mg, Ti (TLD-100)

The TL signal following 5 eV photon excitation of previously irradiated and readout material has been studied as a function of ionisation density and various experimental parameters: (i) maximum temperature of the first readout; (ii) photon fluence; (iii) photon energy and (iv) beta ray dose. Following alpha particle irradiation, the ratio of the second-readout to first-readout TL signal, epsilon(alpha,) has been found to be 10-20 times higher than that following beta irradiation, indicative of the possibility of using the double ratio epsilon(alpha)/epsilon(beta) as a mixed-field discriminator. The beginning of an attempt to explain this unusual effect is offered in the framework of the track structure theory and kinetic modelling of the beta ray dose-response of the first and second readouts.     

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.     

Comparison of time effects, decision limit and residual signal in Harshaw LiF:Mg,Ti and LiF:Mg,Cu,P

The personal dosimetry service of the UK Health Protection Agency-formerly of the National Radiological Protection Board (NRPB)-is currently commissioning a body thermoluminescence dosemeter (TLD) system based on the use of Harshaw(TM) 8800 readers and two-element cards. As part of the process, studies have been carried out into the long-term time dependence of response, the limit of detection and the magnitude of the signal remaining after recommended processing. TLD cards containing both conventional lithium fluoride (LiF:Mg,Ti) and the high-sensitivity material LiF:Mg,Cu,P were available, thus allowing a comparison between the two types of material.