Lithium Fluoride Thermoluminescent Detectors Research Articles

Evaluating the effect of low-density inhomogeneities: A comparative study of lithium fluoride detectors, radiochromic films, and collapsed cone algorithm in 6 MV photon beams

In recent years, there has been significant advancement in the development of physical simulators for dose evaluation. Many dosimetric studies employ solid materials, equivalent to human tissues, to evaluate dose distribution. This study aims to evaluate the effectiveness of inhomogeneity correction carried out by the Monaco/Elekta radiotherapy planning software. To achieve this goal, a physical simulator was created using cork boards to simulate lung tissue and solid water to represent other tissues. This simulator was combined with a dosimetric system that utilized lithium fluoride thermoluminescent detectors - RADOS MTS-N (LiF:Mg,Ti). The thermoluminescent detectors were positioned at various depths using a precisely drilled 2.0 mm thick acrylic plate, and they were placed at predefined positions. The irradiation of the simulator was conducted using an Elekta Synergy® Platform accelerator, employing a 6 MV photon beam with a field size of 15 × 15 cm2 and a source-surface distance (SSD) of 97.5 cm. A radiation dose of 200 cGy was applied for this study. In addition to the dosimetric assessment using thermoluminescent detectors, GAFCHROMIC™ EBT-3 Dosimetry Films were utilized to evaluate the dose at the same measurement points. The dose distribution data measured with the detectors were compared with the values provided by the planning system (TPS) and the inhomogeneity correction was verified. The results support the hypothesis that there is a lack of precision in the analytical simulations performed by the TPS, particularly in cases involving dose distribution at interfaces with varying densities.

Evaluation and modelling of the lithium fluoride based thermoluminescent detector response at the CERN-EU high-energy reference field (CERF)

Lithium fluoride (LiF) thermoluminescent detectors (TLDs) are commonly used for measurements in mixed high energy radiation fields such as space, hadron radiotherapy and other high energy accelerator facilities. However, the results of these measurements are often challenging to interpret since the radiation field compositions are not known a priori. The goal of this work was to model and understand the LiF TLD response in the CERN-EU high-energy Reference Field (CERF).Six different LiF TLD types, namely MTS (LiF:Mg,Ti) and MCP (LiF:Mg,Cu,P) TLDs with different Li abundances, were irradiated at CERF. For the modelling of their response, CERF reference fluence energy spectra from FLUKA simulations and TLD relative luminescence efficiencies according to the Microdosimetric d(z) Model were collected from previous works and supplemented with new relative luminescence efficiency calculations for neutrons with energies above 0.01 MeV. Dedicated MCNP6.2 simulations were performed to obtain the required fluence to absorbed dose conversion factors.Results demonstrated that the response of LiF detectors can be well predicted even for complicated, mixed radiation fields. The modelling allowed to assess the dose contributions from different radiation types in mixed fields. Based on the results of this work, a methodology was proposed to perform LiF TLD measurements in well-characterized mixed fields in terms of absorbed dose in water or tissue. In unknown mixed radiation fields MTS-7 TLDs are recommended to assess the non-neutron component of the absorbed dose in water or tissue. Assessing the dose from neutrons requires dedicated neutron detectors.

Evaluation of different types of lithium fluoride thermoluminescent detectors for ring dosimetry in nuclear medicine

Nuclear medicine staff is primarily exposed to their fingers during radionuclide manipulation. About 20% of these workers have risk to exceed the hand dose limit. Therefore, monitoring with ring dosimeters in terms of Hp(0.07) is advised. Most ring dosimeters make use of lithium fluoride (LiF) thermoluminescent detectors (TLDs). Different types of LiF TLDs exist, each with their advantages and disadvantages. The goal of this work was to evaluate the performance of three different LiF TLD types (MCP-N, MTS-N and MCP-Ns) for ring dosimetry during manipulation of the more novel radionuclides Lu-177, Ga-68 and I-131 within radionuclide imaging and therapy, and to provide recommendations for the most accurate dosimetric follow-up of the dose to the hands. This was done by combining measurements with TLDs and theoretical modelling of the TLD responses using Monte Carlo radiation transport simulations and microdosimetric modelling of the TLD relative luminescence efficiency. It was concluded that MTS-N TLDs are best suited for measuring doses from Lu-177 and I-131 if more than 1 mm polycarbonate equivalent shielding is provided. However, MCP-N TLDs have less than 20% under-response and are thus also acceptable in case high sensitivity is important. MCP-Ns TLDs were found to be best suited for measuring doses from Ga-68, and from Lu-177 and I-131 if less than 1 mm polycarbonate equivalent shielding is provided. The recommended TLD type strongly depends on both the manipulated radionuclide and the amount of shielding between the radionuclides and the fingers (syringe thickness and potential additional shielding around the syringe). But the approach presented here for the theoretical modelling of the Hp(0.07) response can easily be applied to select the most appropriate TLD type for other radionuclides or other amounts of shielding.

Development and validation of a model for assessing neutron fluence with lithium fluoride thermoluminescent detectors

Lithium fluoride thermoluminescent detectors are commonly used for neutrons measurements. However, the calibration is usually performed with readily available gamma sources in terms of gamma equivalent air kerma instead of the more meaningful neutron fluence. Neutron calibration coefficients in terms of neutron fluence depend strongly on the experimental conditions and can thus usually not be found in literature. Therefore, the goal of this work was to develop and validate a model to calculate such neutron calibration coefficients and to use this model to quantify the main sources of uncertainty. Comparison of the neutron calibration coefficients predicted by the model with experimental data from literature and this work showed reasonable agreement. However, a slight systematic deviation was found, probably caused by the significant uncertainty on the relative luminescence efficiency values predicted by the recently developed Microdosimetric d(z) Model for the low energy charged particles released during the 6Li(n,α)3H reaction. Therefore, it was decided to use relative luminescence efficiency values based on the experimental results instead. The model also identified the neutron energy and angular distribution and the luminescence reader light collection geometry as the main sources of uncertainty. Imprecise knowledge of these experimental input parameters can easily lead to uncertainties of the order of tens of percent on the calculated neutron calibration coefficients. The developed model is applicable for neutron energies below 0.01 MeV, but could be extended to higher energies in the future.

A novel methodology to assess linear energy transfer and relative biological effectiveness in proton therapy using pairs of differently doped thermoluminescent detectors

A new methodology for assessing linear energy transfer (LET) and relative biological effectiveness (RBE) in proton therapy beams using thermoluminescent detectors is presented. The method is based on the different LET response of two different lithium fluoride thermoluminescent detectors (LiF:Mg,Ti and LiF:Mg,Cu,P) for measuring charged particles. The relative efficiency of the two detector types was predicted using the recently developed Microdosimetric d(z) Model in combination with the Monte Carlo code PHITS. Afterwards, the calculated ratio of the expected response of the two detector types was correlated with the fluence- and dose- mean values of the unrestricted proton LET. Using the obtained proton dose mean LET as input, the RBE was assessed using a phenomenological biophysical model of cell survival.The aforementioned methodology was benchmarked by exposing the detectors at different depths within the spread out Bragg peak (SOBP) of a clinical proton beam at iThemba LABS. The assessed LET values were found to be in good agreement with the results of radiation transport computer simulations performed using the Monte Carlo code GEANT4. Furthermore, the estimated RBE values were compared with the RBE values experimentally determined by performing colony survival measurements with Chinese Hamster Ovary (CHO) cells during the same experimental run. A very good agreement was found between the results of the proposed methodology and the results of the in vitro study.

Influence of cosmic radiation spectrum and its variation on the relative efficiency of LiF thermoluminescent detectors – Calculations and measurements

Lithium fluoride thermoluminescent detectors (TLD) were used for cosmic radiation dosimetry already in early 1960s. Since that time they have been constantly applied in numerous space missions for personal dosimetry, area monitoring, phantom measurements and dosimetry for biological experiments. The relative efficiency of TLDs, defined as the ratio of their response to a given radiation and to a reference radiation, is not constant, but depends on ionization density. This raises a question about the relative efficiency of TLDs exposed to the complex cosmic radiation spectrum encountered in Earth's orbit, which consists of a variety of particles, including heavy ions, the spectrum of which covers an extremely broad energy range. The present work is an attempt to find an answer to this question.The particle energy spectra were calculated for realistic flight conditions of the International Space Station (ISS). The calculation of the Galactic Cosmic Ray (GCR) component was based on the input spectra generated with the DLR model for solar minimum (2009) and solar maximum (2000) conditions. Contributions of trapped protons were estimated based on the AP8 model for solar minimum and maximum taking into account the altitude variations of the ISS. The interactions of the primary particles with the ISS were simulated with GEANT4 using a shielding geometry derived from the mass distribution of the Columbus Laboratory of the ISS and several constant aluminum shieldings. The calculated spectra were convoluted with the experimental data on the relative TL efficiency measured for ions ranging from H to Xe at various particle accelerators for two commonly applied TL-materials, namely LiF:Mg,Ti and LiF:Mg,Cu,P.The results showed the differences in the average TL-efficiency for these two TL-materials. For LiF:Mg,Ti the relative efficiency is within a few percent from unity for any of the analyzed values of shielding, altitude and solar cycle conditions. This means that one can assume cosmic radiation doses measured in Low Earth Orbit (LEO) with LiF:Mg,Ti detectors to be correct within such uncertainty. LiF:Mg,Cu,P underestimates the cosmic radiation doses by more than 15% in all cases. Altitude and solar cycle were found to have a very weak influence on the TL efficiency. In contrast, the influence of shielding thickness is quite significant. The reason for this is a change of contributions of radiation field components: trapped protons dominate at low shielding (97% of dose at 1 g/cm2), but are negligible above 60 g/cm2, as well as changes within GCR spectrum (increase of dose due to lower LET secondaries for higher shielding). Shielding thickness affects both TLD types in different ways: the efficiency of LiF:Mg,Cu,P increases with increasing shielding thickness, while the efficiency of LiF:Mg,Ti shows some fluctuations, with a weak minimum for 60 g/cm2. The response ratio of these TLDs decreases monotonically with the shielding thickness and could be used as an indicator for the average shielding conditions in which the TLDs were exposed.

SU‐E‐T‐35: Extracting a Neutron Dose Using Only TLDs (Thermoluminescent Detectors)

Purpose: To test the use of TLDs for out‐of‐field neutron measurements and demonstrate the calibration and calculations necessary to extract a neutron dose by using TLD 600 chips, which measure photons plus neutrons, and TLD 700 chips, which measure only photons. For treatments requiring high‐energy photons or protons, secondary neutron doses may become a factor and TLDs provide many advantages in a clinical setting over conventional neutron dosimeters. Methods: A set of lithium fluoride TLDs (thermoluminescent detectors) consisting of 100, 600 and 700 series Rexon TLDs were arranged at set distances in and out of field of 6MV and 18MV photon beams from an Elekta Precise Treatment System. All TLD chips were read out using a Rexon System 310 reader and raw glow output data was exported for data analysis. Results: Examination of the integrated TLD glow curves shows an energy dependent response and a higher reading for TLD 600 chips as the distance or angle from the beam field increases for the 18MV beam. At a low neutron dose, correct integration interval must be used extracted a signal above noise. Conclusion: This work demonstrates the feasibility of neutron measurements using only TLDs as opposed to TLDs with CR39, bubble detectors, diamond detectors or WENDI detectors. TLDs are commercially available in different sensitivities and relatively cheap. We speculate that the TLD 600 chips will continue to read neutron doses at large distances to its neutron cross section. Furthermore they allow for spatial resolution and in vivo measurements. With calibration for neutron doses and photon energy ranges, a quantitative neutron dose is possible. This integration and subtraction method will be used in future investigation to directly quantify neutron doses spatially with respect to the beam and for different beam characteristics.

Photoluminescence measurements of LiF TL detectors

Abstract The properties of standard lithium fluoride (LiF) thermoluminescent (TL) detectors, which are routinely used in radiation protection systems, were investigated under light stimulation. The luminescence of different types of LiF detectors, which were irradiated with gamma rays of energy up to 300 Gy and alpha particles with a fluence up to 5*109 cm−2, were stimulated by a blue light and were heated up to temperature of 240 °C or 400 °C, depending on the type of detectors. The irradiated LiF detectors during the blue-light (460 nm) stimulation emit green photoluminescence (PL) with a wavelength of 530 nm. The LiF detectors showed a PL effect of much higher efficiency after they were irradiated with alpha particles than after they were irradiated with gamma rays. However, in contrast to PL, the TL readout showed a significantly lower efficiency of LiF detectors after alpha particle irradiation. These effects result from the different trap mechanisms that are responsible for TL and PL phenomena. The temperature stability of the traps responsible for the PL effect for both types of LiF detectors was studied.

TH-E-BRB-10: Examination of General Cavity Theory for Lithium Fluoride TLDs in Bone and Lung Heterogeneities

Purpose: To compare the measured and theoretical dose response of Lithium Fluoride Thermoluminescent Detectors (TLD-100) of different thickness in bone and lung heterogeneities. Methods: After determining individual TLD sensitivities, a total of 45 TLD chips (3.2×3.2 mm and depths of 0.15mm -thin, 0.38mm -medium and 0.89mm -thick TLDs) were placed in bone and lung slab phantoms and irradiated with a 6MV photon beam. According to the Burlin cavity theory, the proportion of the dose in the cavity that is due to secondary electrons generated in the medium is given by the parameter d which depends on β and g. Four different methods of calculating β and three formulas to calculate g were tested in this study which resulted in twelve different versions of the d parameter for both lung and bone. The mean stopping power ratios and mean mass energy absorption coefficients of the 6 MV photon spectrum in bone and lung heterogeneities were calculated with BEAMnrc Monte Carlo simulations. Then the ratio of dose in TLD to dose in medium (f) was calculated and compared against the measured values. Results: The Burlin cavity theory resulted in a good agreement with measurements for the TLDs in lung, where it overestimated the f factor by 1.9%, 2.2% and 3.0%, for thin, medium and thick TLDs respectively. However, the discrepancy between the calculated and measured f factors for bone were higher and the f factors were underestimated by 5.6%, 5.4% and 3.5% with the Burlin cavity theory for thin, medium and thick TLDs respectively. Conclusions: Better agreement of the theoretical and measured response of LiF TLDs was observed in lung than the bone heterogeneities. Furthermore all twelve versions of the Burlin cavity theory used in this study agreed to within 0.5% of each other in both lung and bone.

Dosimetry of mixed radiation field at the Kraków cyclotron using lithium fluoride TL detectors

Abstract The AIC-144 isochronous cyclotron at the Institute of Nuclear Physics (IFJ) in Krakow can be used for acceleration of light ions. Protons can be accelerated to maximum energy of 60 MeV in the form of a focused ion beam. In October 2009 the AIC-144 isochronous cyclotron at the IFJ was dedicated to the Krakow Centre for Hadron Radiotherapy. Then, the cyclotron was modernized and adapted for proton radiotherapy of eye cancer. The aim of our work was to validate suitability of different thermoluminescent (TL) detector types for dosimetry of a mixed radiation field at the cyclotron unit. LiF:Mg,Ti and LiF:Mg,Cu,P detectors are well known standards of TL dosimetry; however, several of their properties are significantly different. In this study four types of LiF-based TL detectors were used to measure spatial dose distribution of a mixed radiation field in the Cyclotron Hall during operation of the machine. A program named the MapFly was developed in order to visualize radiation levels in the Cyclotron Hall in 2D and 3D.

Thermoluminescent method for the measurements of tritium production in neutronics experiments

A European nuclear-fusion technology program is currently, except of other acivities, on the stage of validating the capability of neutronic codes and nuclear data for use in the ITER project. This requires a technique for reliable measurements of tritium produced by a neutron flux and these estimates must have a qualified uncertainty. The application of LiF TL detectors, calibrated by the direct measurement of tritium activity using the Liquid Scintillation Counting (LSC) technique, is a new approach to the assessment of the tritium production rate in fusion related benchmark experiments. Lithium Fluoride Thermoluminescent Detectors (TLD) are commonly used for measurements of thermal neutron dose using the 6 Li(n,α) 3 H reaction. The accumulation of radioactive tritium in the LiF detectors is usually considered as an undesired effect to the detectors, increasing intrinsic background after high-dose neutron exposure. The same effect may be exploited for the measurement of generated tritium activity. A TL signal corresponding to the energy deposited in the − β decay of 3 H, expressed as mGy/h to be recalculated to Bq/mg, requires calibration by an independent measurement of 3 H activity. This calibration can be performed by applying e.g. LSC-technique. A good correlation was obtained in the tested range of tritium activity: 17–25 Bq/mg of LiF (0.080–0.118 mGy/h) with a slope of regression line: 1.75 × 10 −4 [mGy/h]/[Bq], r 2 = 0.94. The thermal treatment of the TLDs after irradiation (annealing: 400 °C, 1 h) leads to a loss of 37% of the initial tritium content. Prolonged storage of the irradiated TLDs at room temperature, pressure and humidity causes the escape of less than 5% of the accumulated tritium after 100 days. Thus LiF TLDs after calibration can be used as one of techniques for the assessment of tritium production in neutronic experiments.

Thermoluminescent detectors applied in individual monitoring of radiation workers in Europe—a review based on the EURADOS questionnaire

Among the activities of EURADOS Working Group 2 formed by experts from several European countries is the harmonisation of individual monitoring as part of radiation protection of occupationally exposed persons. Here, we provide information about thermoluminescent detectors (TLDs) applied by the European dosimetric services and the dosimetric characteristics of dosemeters in which these detectors are applied. Among 91 services from 29 countries which responded to the EURADOS questionnaire, 61 apply dosemeters with TLDs for the determination of personal dose equivalent H(p)(10) for photons and beta radiation, and 16 services use TLDs for neutron albedo dosemeters. Those most frequently used are standard lithium fluoride TLDs (mainly TLD-100, TLD-700, Polish MTS-N and MTS-7, Russian DTG-4), high-sensitive lithium fluoride (GR-200, MCP-N) and lithium borate TLDs. Some services use calcium sulphate and calcium fluoride detectors. For neutron dosimetry, most services apply pairs of LiF:Mg,Ti TLDs with (6)Li and (7)Li. The characteristics (energy response) of individual dosemeters are mainly related to the energy response of the detectors and filters applied. The construction of filters in dosemeters applied for measurements of H(p)(10) and their energy response are also reviewed.

Experimental determination of dosimetry functions of Ir-192 sources.

Experimental data related to the dosimetric characteristics of Ir-192 brachytherapy sources are limited. The aim of this work was to obtain the dosimetry functions required by the American Association of Physicists in Medicine Task Group 43 for both a low and a high dose-rate iridium-192 brachytherapy source through dose measurements in a water-equivalent phantom. Dose measurements have been performed using lithium fluoride thermoluminescent detectors positioned in a polystyrene phantom at distances from the source that vary from 1 to 10 cm, with 1 cm intervals, and at angles that vary from 0 degree to 170 degrees with 10 degrees intervals. The anisotropy functions, radial dose functions, and dose rate constants were determined for both brachytherapy sources. The precision of results obtained on those relatively fine intervals of angles and distances provides clinics with the possibility to use and interpolate the complete data sets for treatment planning.

A flexible fiber optics scintillation probe for intracavitary dosimetry.

In order to investigate suspected fluctuations in dose rate to the rectum and/or bladder of patients during uterocervical radium therapy, it was desired to make continuous intracavitary recordings in both locations. A number of commercially produced radiation-monitoring instruments with probes slender enough for transurethral insertion were available, but in general they are so handicapped by rigid construction and the weight of attached apparatus that their use would appear likely to introduce considerable anatomical distortion. Some in vivo measurements have also been made with the miniature Sievert type condenser ion chambers, phosphor glass microdosimeter needles, and lithium fluoride thermoluminescent detectors. In most cases such technics have yielded a fair degree of agreement with the theoretically calculated values, but since all are inherently integrating methods they were not suitable for registering the postulated variations of dose rate with time. A continuously recording ratemeter with a detecting probe of a maximum diameter of not more than a few millimeters and sufficiently light and flexible to avoid any mechanical displacement of the surrounding organs seems to be essential for adequate evaluation of this phenomenon. It was therefore decided to undertake modification of equipment already on hand in this laboratory, to permit incorporation of the features considered most important. The Curtis Nuclear Corporation's “Probitron” (Fig. 1) is a scintillation type detector-ratemeter with six-decade ranges of dose-rate sensitivity, with additional adjustments for empirical calibration and background suppression. Recorder operation is provided through a plug connector. The radiation-sensing element is a plastic phosphor, located in the aluminum-capped tip of a cylindrical stainless-steel probe approximately 17 cm long. Light excited by the incident gamma radiation is conducted via a solid Lucite “light pipe” through the central axis of the probe to an attached photomultiplier tube, where it is converted to an electrical analog signal. Connections from the photomultiplier to the main amplifier chassis, and from thence to the recorder, are by shielded coaxial cables. Three interchangeable probes were originally provided: one straight cylinder with a nominal diameter of 1/4 inch (6.4 mm); and two of smaller gauge, one straight and the other with a 45° angulated tip, each 1/8 inch (3.2 mm) in diameter. The probe to be used is secured in a socket fitting, near one end of and perpendicular to the axis of the photomultiplier housing, by means of a threaded metal collar. In the left lower foreground of Figure 1, the larger probe is shown attached ready for use, with the two smaller versions lying on either side of it. It is apparently intended that the photomultiplier housing, which has a diameter of slightly more than 5 cm and a length of 18 cm, be held in a sort of pistol grip technic while measurements are made.