Evaluating two extremity dosemeters based on LiF:Mg,Ti or LiF:Mg,Cu,P

Evaluation of a new extremity dosemeter is presented. The dosemeter is a passive device that is easy to wear and features a permanent individual numerical ID with barcode, a watertight case, an automatic TLD reader and database management software. Two dosemeters were studied: the first consists of a 100 mg x cm(-2) 7LiF:Mg,Ti (TLD-700) chip and a 42 mg x cm(-2) cap, the other consists of a 7 mg x cm(-2) layer of 7LiF:Mg,Cu,P (TLD-700H) powder and a 5 mg x cm(-2) cap. Sensitivity, repeatability, lower limit detection, angular responses and energy responses for these dosemeters are studied and presented. The dose calculation algorithm is developed and its dosimetric performance accuracy is compared with the standard ANSI N13.32-1995, Performance Testing of Extremity Dosemeters.     

Comparison of two extremity dosemeters based on LiF:Mg,Cu,P thin detectors for mixed beta-gamma fields

Two types of thin LiF:Mg,Cu,P detectors, GR-200F and MCP-Ns, have been characterised for use in the design of an extremity dosemeter for mixed beta-photon radiation fields. Both detectors consist of an extremely thin layer of sensitive material with effective thicknesses of 5 and 8 mg cm(-2), respectively, held in a 5 mg cm(-2) PVC ring holder. Dosimetric performance was analysed according to the ISO 12794 standard and compared with 240 mg cm(-2) TLD-100 measurements. In particular, the energy response was obtained for ISO narrow X-ray spectra, (137)Cs, (60)Co, (204)Tl and (90)Sr/(90)Y. From these measurements a mean calibration factor was calculated to estimate H(p)(0.07). Subsequently, the performance of the dosemeters was checked for a set of 10 different mixed photon and beta-photon fields. The study shows that the proposed dosemeters can estimate H(p)(0.07) in a wide range of mixed beta-photon fields with a maximum deviation from the given dose of 30% and an overall uncertainty of the order of 25% (k = 1). However, the results also highlight a large variability among the different thin detectors and, thus, the standard TLD-100 material is recommended whenever the workplace does not include low-energy beta radiation.     

Individual monitoring in nuclear medicine therapeutic procedures using extremity dosemeters LiF(Mg, Cu, P)

Unsealed beta-gamma-emitting sources are used (15 GBq (90)Y each session) in nuclear medicine therapeutic procedures. Inside the manipulation cell and while giving the injection to the patient, the skin exposure is very high; electron radiation field is not homogeneous and thus the exposure of the extremities is not uniform. Particular individual monitoring is adopted: single thermoluminescence dosemeter, wrapped in polyethylene film and placed on an adhesive tape, is positioned on the tip of the fingers; 6-10 dosemeters are assigned to each operator per session. The energy and angle response is studied for X-ray spectra, (90)Sr/Y and (204)Tl--a unique mean calibration factor is calculated in order to estimate H(p)(0.07). Performance of dosemeter is analysed according to ISO 62387-1(2007) and the combined uncertainty (calculated using the Monte Carlo method) results lie in the order of 11 %. This method reveals the critical step of manipulation and administration and ensures that dose limits are not exceeded.     

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.     

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.     

Batch homogeneity of LiF(Mg,Cu,P)-GR200 and LiF(Mg,Cu,P)-MCP-NS TL detectors for use as extremity dosemeters at ENEA personal dosimetry service

The results of a study of two commercially available LiF(Mg,Cu,P) TL materials, a GR200 detector and a MCP-Ns thin detector, are described in order to use these phosphors for individual monitoring for the extremities. After a dosimetry system has been type tested, the implementation routine is not straightforward. Additional tests and software modification are needed to make the routine system work comply with the type test results. Not often can literature be found on the steps required to implement the results in a routine study. This paper reports the results of the individual calibration of about 15 000 extremity dosemeters, 12 000 containing a GR200 detector and 3000 an MCP-Ns thin detector. It describes the experimental procedure followed in order to assure reproducibility and stability of the results with proper accuracy and reliability. In particular, this is the first time that results on homogeneity of such a large batch of MCP-Ns detectors are reported.     

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.     

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).     

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.     

On the use of LiF:Mg,Ti thermoluminescence dosemeters in space--a critical review

The use of LiF:Mg,Ti thermoluminescence dosemeters (TLDs) in space radiation fields is reviewed. It is demonstrated in the context of modified track structure theory and microdosimetric track structure theory that there is no unique correlation between the relative thermoluminescence (TL) efficiency of heavy charged particles, neutrons of all energies and linear energy transfer (LET). Many experimental measurements dating back more than two decades also demonstrate the multivalued, non-universal, relationship between relative TL efficiency and LET. It is further demonstrated that the relative intensities of the dosimetric peaks and especially the high-temperature structure are dependent on a large number of variables, some controllable, some not. It is concluded that TL techniques employing the concept of LET (e.g. measurement of total dose, the high-temperature ratio (HTR) methods and other combinations of the relative TL efficiency of the various peaks used to estimate average Q or simulate Q-LET relationships) should be regarded as lacking a sound theoretical basis, highly prone to error and, as well, lack of reproducibility/universality due to the absence of a standardised experimental protocol essential to reliable experimental methodology.     

Extremity and whole-body dosemeters for beta and beta gamma fields based on LiF:Mg,Cu,P thin detectors

This study aims at proposing two TL dosemeters: one for the whole body and another for the extremities, for beta and gamma fields. Selected sensible material consists of 5 mg x cm(-2) LiF:Mg,Cu,P film (GR-200F) manufactured in China. Calibration was carried out according to ISO 4037-3, in terms of Hp(0.07), and dosimetric performance was analysed on the basis of IEC-1066 and ISO-12794 Standards. Experiments showed a satisfactory sensitivity of the proposed dosemeters for detecting beta radiation at protection levels and a very good energy response; thus, highly recommending their use for weakly penetrating radiation measurements. However, the homogeneity and the reproducibility of GR-200F are not found to be as reliable as in standard materials.     

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.     

The effects of ionisation density on the thermoluminescence response (efficiency) of LiF:Mg,Ti and LiF:Mg,Cu,P

In this paper, the various models dealing with the effects of ionisation density on the thermoluminescence (TL) response (efficiency) of TL LiF dosemeters are discussed. These include (i) the Unified Interaction Model (UNIM), which models photon/electron linear/supralinear dose response; (ii) the Extended Track Interaction Model (ETIM), which models heavy charged particle (HCP) TL fluence response; (iii) Modified Track Structure Theory (MTST), which models relative HCP TL efficiencies; and (iv) Microdosimetric Target Theory (MTT), which models both relative HCP efficiencies and photon energy response.     

Time-resolved spectroscopy of LiF:Mg,Cu,P

Time-resolved spectroscopy measurements of LiF:Mg,Cu,P luminescence are presented to obtain a better understanding of the emission characteristics of this material. The intensities and decay of the emission bands were studied as a function of annealing temperature and ionising radiation (gamma) dose. Two peaks in the emission were observed at 367 and 466 nm when excited by the 266 nm laser radiation. The luminescence spectrum under band-to-band X-ray excitation shows a dominant emission approximately 390-400 nm, which resembles the reported thermoluminescence emission and is clearly different from the spectrum obtained using the 266 nm pulsed laser excitation. Annealing of the material to 300 degrees C increases the intensity of the 367 and 466 nm emission bands by an order of magnitude as well as changes the relative intensity of the bands. Additional emission bands, which are not evident in the thermoluminescence emission spectra, are seen at longer wavelengths that also increase with dose. Possible explanations for the observed emission spectra are discussed in this paper.     

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.     

Low temperature radioluminescence of LiF:Mg,Cu,P

Low temperature radioluminescence spectra of LiF, variously co-doped with Mg, Cu and P, show highly unusual temperature dependencies which resemble thermoluminescence data. The signals include intense peaks and a relatively weak continuous background. One peak occurs below 30 K, together with a major peak near 125 K. The signals are highly sensitive to the dopants and slightly sensitive to X-ray dose rate. The role of donor acceptor pairs and the perturbations from intrinsic defects formed by ionisation can be used to describe all the observations. The 290 nm emission band is linked to H center annealing.     

Effects of high ambient temperature on glow-peak fading properties of LiF:Mg,Ti thermoluminescent dosemeters

The effects of a controlled high temperature environment on LiF:Mg,Ti thermoluminescent dosemeters (TLDs) were investigated. TLDs were exposed to ambient temperatures of 30, 40 and 50°C. Sensitivity changes before irradiation, typically called pre-irradiation fading, and signal loss after irradiation, called post-irradiation fading, were studied. Dosemeters were subjected to up to 33 d of pre-irradiation and 68 d of post-irradiation storage. For pre-irradiation fading, peak 5 showed a signal increase of ～30 % and peak 4 showed an ～30 % decrease in 20 d. The sum of the areas of peaks 4 and 5 remained relatively constant even for long pre-irradiation times, although at 50°C, losses in peak 5 signal were too significant to maintain the sum of peaks 4 and 5 constant. Peak 3 was still detectable even at 50°C and the longest irradiation times, but peak 2 was very difficult to detect after 15-20 d, especially with post-irradiation fading.