The dose and X-ray energy response of LiF:Mg,Cu,P and CaSO4:Dy thermoluminescent foils

Within a systematic study of a novel system enabling 2D readout of TL foils, the X-ray energy and dose response was investigated in TL foils containing LiF:Mg,Cu,P (MCP-N) or CaSO4:Dy as activators. Foils were exposed to broad X-ray beams of mean energies ranging between 45 keV and 208 keV (ISO 4037 standard), with reference to 662 keV ¹³⁷Cs gamma rays. The MCP-N foils, of about 380 nm emission wavelength, show a flat X-ray energy response, but low sensitivity. Due to poor TL light detection efficiency of the CCD (charge-coupled device) camera over this range of wavelengths, only doses exceeding 500 mGy can be reliably measured. In the case of CaSO4:Dy foils, their TL light emitted around 450 nm wavelengths is registered by the CCD camera with no loss of efficiency, enabling X-ray doses as low as 100 mGy to be evaluated. Unlike that of MCP-N, the dose response of CaSO4:Dy foils is highly supralinear. Nevertheless, within experimental conditions applied in this study, the 2D-TL technique may be applied to determine Entrance Surface Dose or Maximum Skin Dose in radiology. The more sensitive CaSO4:Dy foils could be used only in a well-specified radiation field (e.g., in mammography) or in qualitative dose mapping.     

Evaluation of TL response and intrinsic efficiency of TL dosimeters irradiated using different phantoms in clinical electron beam dosimetry

The TL response of LiF:Mg,Ti microdosimeters and CaSO₄:Dy dosimeters were studied for 12 MeV electron beams using PMMA, liquid water and solid water (SW) phantoms. The different phantom materials affect the electron spectrum incident on the detector and it can alter the response of dosimeters to different radiation types, so this fact should be considered in clinical dosimetry. The dosimeters were irradiated with doses ranging from 0.1 up to 5 Gy using a Varian Clinac 2100C linear accelerator of Hospital Israelita Albert Einstein – HIAE using a 10 × 10 cm² field size and 100 cm source-phantom surface distance, with the dosimeters positioned at the depth of maximum dose. The TL readings were carried out 24 h after irradiation using a Harshaw 3500 TL reader. This paper aims to compare the TL response relative to ⁶⁰Co of the dosimeters for different phantoms used in radiotherapy dosimetry. CaSO₄:Dy dosimeters presented higher TL sensitivity relative to ⁶⁰Co and intrinsic efficiency than microLiF:Mg,Ti dosimeters for all phantoms.     

Relative efficiency of TL detectors to energetic ion beams

The relative TL efficiency of LiF:Mg, Ti and LiF:Mg, Cu, P was evaluated for several ion beams, ranging from helium to xenon ions. Irradiations were realized at the HIMAC accelerator in Chiba, Japan, partly within the ICCHIBAN intercomparison project. The covered LET range was extending from about 2 keV/μm to 1500 keV/μm.Both tested TLD types exhibited a decrease of relative response with increasing ionization density – stronger for LiF:Mg,Cu,P detectors. The relationship between efficiency and LET was found to follow unique trend lines, as nearly all data points lied within 5% around the fitted empirical functions. Values of TL efficiency measured for various batches of same type TLDs agree within a few percent. The measured relationships between relative TL efficiency and LET will be used in the analysis of data obtained from space dosimetric experiments.     

The reusability characteristics of LiF:Mg,Cu,P with a maximum intensity at 280 °C in an environment with elevated temperature

Efforts are aimed at finding a method that could serve TL dosimetric measurements in the range of low-dose but carried out in an environment with elevated temperature. The temperature at the position of the maximum intensity of LiF:Mg,Cu,P was about 280 °C when annealed at 460 °C. LiF:Mg,Cu,P with a maximum intensity at 280 °C should present good thermal stability. The TL intensity of LiF:Mg,Cu,P with a maximum intensity at 280 °C was about 54% of the standard LiF:Mg,Cu,P, it should have a minimum measurable dose in the range of micro-Gy. LiF:Mg,Cu,P with a maximum intensity at 280 °C could be re-used by the 660 °C/30 min annealing, followed by 270 °C/20 min, 240 °C/10 min and 460 °C/30 min. It's possible for LiF:Mg,Cu,P to be extended application for low dose test in an environment with elevated temperature.     

The dependence of thermoluminescence of LiF:Mg,Cu,Si on sintering temperatures and dopants concentrations

The dependence of thermoluminescence (TL) of LiF:Mg,Cu,Si on sintering temperatures and dopants concentrations were investigated. The dependency of the TL in LiF:Mg,Cu,Si on sintering temperature exhibits a very sharp maximum at 830 °C. LiF:Mg,Cu,Si is much too sensitive than LiF:Mg,Cu,P to sintering temperature. The glow curve and the TL sensitivity depend on the concentration of Mg, Cu and Si, showing a distinct maximum for certain concentrations of these impurities. Mg seems to be the most essential dopant, as very small changes of the Mg content strongly influence both the glow curve and the TL sensitivity. Si is the main activator responsible for TL emission. The stability to heat treatments in LiF:Mg,Cu,Si was influenced greatly by Mg concentrations. The thermal instability in LiF:Mg,Cu,Si is caused not by Cu and Si but Mg ion state change. It was found that the optimum concentrations are Mg:0.6 mol%, Cu:0.03 mol% and Si:0.9 mol% for this material, which showed the best stability to heat treatment.     

Optimization of preparation procedure of LiF:Mg,Cu,Si TLD for improving the reusability

Several thermal treatments in the temperature range from 270 °C to 320 °C (each of 10 min) were tested as a final preparation procedure of LiF:Mg,Cu,Si to improve the protocol of TL readout with less residual signal for the LiF:Mg,Cu,Si TLD. This high sensitivity LiF:Mg,Cu,Si TLD exhibited thermal stability much better than that of the well known LiF:Mg,Cu,P. For LiF:Mg,Cu,Si, a readout temperature up to 300 °C did not affect the TL sensitivity and glow curve structure for 12 cycles of exposure and readout following an initial thermal treatment at 295 °C for 10 min. The residual TL signal also remained negligible.     

Newly developed highly sensitive LiF:Mg,Cu,Si TL discs with good stability to heat treatment

The preparation method and some dosimetric properties of the new LiF:Mg,Cu,Si discs are presented. The effect of heat treatments on LiF:Mg,Cu,Si was investigated. The shape of the glow curve for LiF:Mg,Cu,Si is similar to that for standard LiF:Mg,Cu,P (GR-200A), and shows minimal differences when annealed in the range from 260 °C to 290 °C for 10 min. The TL sensitivity for LiF:Mg,Cu,Si is much lower than that for GR-200A, but is 35 times larger than that for TLD-100 and is slightly higher than that for HMCP. The height of the high-temperature peaks for LiF:Mg,Cu,Si is not only lower than that for GR-200A, but also lower than that for HMCP. The glow curve shape of LiF:Mg,Cu,Si annealed at 260 °C for different times shows minimal differences and TL response remains stable. These results indicate that the new LiF:Mg,Cu,Si disc has a good stability to thermal treatments and a lower residual TL signal.     

Relative thermoluminescent efficiency of LiF detectors for proton radiation: Batch variability and energy dependence

The available experimental data on the relative thermoluminescent efficiency of the LiF:Mg,Ti dosimetric peaks for protons are contradictory. There are several reports showing that the efficiency exceeds unity by even more than 30%, however, many others show the efficiency close to unity or even lower. These contradictory data might be a result of the real variability of TLD properties or of not perfectly reproduced experimental conditions.In an attempt to resolve this issue, the efficiency of 16 batches of LiF:Mg,Ti (MTS) detectors for 60 MeV protons produced at the IFJ Kraków over the last 20 years was measured. All values of the relative TL efficiency were found to exceed unity significantly, with an average of 1.09. Dispersion between different batches was very low, all data were within 4% of the mean value.In second part of experiment the dependence of the relative efficiency of LiF:Mg,Ti and LiF:Mg,Cu,P detectors on proton energy was determined. The efficiency for LiF:Mg,Ti dosimetric peaks was found to have a maximum of 1.20 at about 20 MeV. For LiF:Mg,Cu,P the relative efficiency decreases systematically with decreasing proton energy, from 0.96 at 56 MeV, to 0.61 at 11 MeV.     

Investigations of thermoluminescence properties of multicrystalline LiF: Mg,Cu, Si phosphor prepared by edge defined film fed growth technique

Thermoluminescence (TL) properties of LiF: Mg, Cu, Si phosphor prepared in multicrystalline form using edge defined film fed growth (EFG) technique has been investigated. The effect of preparation route on TL properties and thermal stability has been studied. To improve the TL dosimetry properties, phosphor is subjected to different annealing temperatures ranging from 250 °C to 450 °C. The shape of the glow curve structure and peak temperature remains similar at different annealing temperatures, however peak intensities vary. The consistency in the glow curve structure with annealing temperature elucidate that TL trapping states are stable in nature. Thermal annealing at 300 °C for 10 min gives maximum TL intensity with main dosimetry peak at 209 °C. The TL intensity of the main dosimetry peak is increased by a factor of five as compared to as-grown crystal. The thermal stability of LiF: Mg, Cu, Si is found to be better than LiF: Mg, Cu, P. Trapping parameters are calculated to have an insight study of defect states. A simple glow curve structure, tissue equivalency, thermal stability, low residual signal, linear response and reusability makes LiF: Mg, Cu, Si a suitable phosphor for radiation therapy, radio diagnostics and personnel dosimetry applications.     

Thermoluminescence dosimetry in evaluation of liquid 32P sources for intravascular brachytherapy

Several new types of miniature (1 and 2 mm diameter) thermoluminescence (TL) detectors were elaborated and tested aiming at dosimetry of liquid [³²P] sodium phosphate sources suggested for intravascular brachytherapy. They were prepared using LiF:Mg,Cu,P and LiF:Mg,Ti TL materials in a two-layer technique with the effective thickness below 0.1 mm. Dosimetric measurements were carried out in tissue-equivalent polymethyl methacrylate (PMMA) phantoms, with TL detectors placed at various distances around angioplasty balloons filled with aqueous solution of Na₂H³²PO₄. Of particular interest was the response of detectors placed at shortest distances from the balloon surface. All types of the TL detectors used in trials showed very good spatial resolution and dose readings were consistent for all detector types, even at the steepest parts of the dose–distance curve. In our opinion, the new TL detectors can be recommended for measurements of dose distribution around liquid radiation sources.     

OSL signal of lithium fluoride and its relationship with TL glow-curves

The widely known LiF TL detectors: LiF:Mg,Ti (MTS-N) and LiF:Mg,Cu,P (MCP-N), were investigated with respect to their OSL properties. It was found that both materials exhibit quite substantial OSL sensitivity. In particular, in the case of LiF:Mg,Cu,P this sensitivity was very high, significantly exceeding that of BeO, the standard OSL dosimetric material. LiF:Mg,Cu,P could be a very promising candidate for application in dosimetry, if not for the fading, which was found to be quite high, reaching nearly 80% loss of the signal within 60 h. The OSL signal intensity shows a correlation with the peak 2 of the TL glow curves indicating that the same trapping sites are responsible for both processes. Peak 2 of LiF:Mg,Ti shows a peculiar property, that blue light stimulation removes only about half of its initial intensity, disregarding the duration of stimulation. This suggests, that this peak may have a composite structure and originates from both light-sensitive and light-insensitive trapping centres.     

A study on thermal degradation of LiF:Mg,Cu,P and LiF:Mg,Cu,Si

We investigated the thermal degradation of LiF:Mg,Cu,P (NTL-250) and LiF:Mg,Cu,Si (MCS) for the development of TL sheet. By thermogravimetry and differential scanning calorimetry (TG-DSC), the exothermic reaction was observed between 320 °C and 400 °C in MCS as well as NTL-250. The heat value of MCS was twice as large as that of NTL-250. This ratio corresponded with that of Mg amount in these TL materials measured by ICP-OES (inductively-coupled plasma optical emission spectrometry). X-ray diffraction (XRD) measurements were also carried out, and the peaks of MgF₂ phase were also observed in degraded MCS sample as well as NTL-250. Moreover, X-ray absorption near-edge structures (XANES) of Cu in these LiF TLDs were measured. The valences of Cu did not change before and after degradation. It indicates that the thermal degradation is caused by not Cu but Mg ion state change. The exothermic reaction is possible caused by the stabilization reactions, and then it was expected to correspond with MgF₂ precipitation. From these results, we concluded that the thermal degradations of these LiF TLDs are caused by the precipitation of MgF₂.     

Study of a new Lif:Mg,Cu,P formulation with enhanced thermal stability and a lower residual TL signal

This paper presents results obtained for a new LiF:Mg,Cu,P (HMCP) preparation with modified Mg and Cu concentrations. The shape of the HMCP glow curve shows minimal differences for annealing in the range from 523 to 543 K for 10 min. The thermoluminescence (TL) readout value remained stable when annealed in the range from 513 to 543 K for 10 min. The new formula allows heating of the material to higher temperatures than that originally employed for the well-known GR-200A dosemeter, practically without losses in sensitivity. The TL sensitivity is approximately half of that for the GR-200A, and still 29-fold greater than that for the TLD-100 dosemeter, and the residual signal is approximately five-fold lower than for the GR-200A. These results indicate that the new TL material shows enhanced thermal stability and a lower residual TL signal at a small TL sensitivity cost. The heat treatment temperatures are related to concentrations of Mg and Cu in LiF:Mg,Cu,P.     

Spectrally resolved thermoluminescence of highly irradiated LiF:Mg,Cu,P detectors

Well known, widely applied high sensitive thermoluminescence (TL) detector LiF:Mg,Cu,P (MCP-N) was investigated. This paper analyses changes of the TL emission spectrum of MCP-N after irradiation with ultra high doses (up to 500 kGy). Spectral dependence of TL on dose is very complex especially in the region of very high doses (>1 kGy). As a general trend we found that the number of peaks increases with dose in the long-wavelength region indicating new types of recombination centres (RCs). Wavelength peak positions for increasing doses are quite stable. Only some of them show slight red-shift.     

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.     

Radio-photoluminescence of highly irradiated Lif:Mg,Ti and Lif:Mg,Cu,P detectors

The radio-photoluminescent (RPL) characteristics of LiF:Mg,Ti (MTS) and LiF:Mg,Cu,P (MCP) thermoluminescent detectors, routinely used in radiation protection dosimetry, were investigated after irradiation with ultra-high electron doses ranging up to 1 MGy. The photoluminescence of both types of LiF detectors was stimulated by a blue light (460 nm) and measured within a spectral window around 530 nm. The RPL dose response was found to be linear up to 50 kGy and sublinear in the range of 50 kGy to 1 MGy for MCP detectors and linear up to 3 kGy and next sublinear in the range from 5 kGy to 1 MGy for MTS detectors. For both type of LiF detectors RPL signal is saturated for doses higher than 100 kGy. The observed differences between MCP and MTS may suggest, that the RPL effect in LiF is not entirely governed by intrinsic defects (F₂ and F₃⁺ centers), but dopants may also have a significant influence. Due to the non-destructive character of the RPL measurement, it is suggested to apply combined RPL/TL readouts, what should improve accuracy of high-dose dosimetry.     

Development of a personal dosimeter badge system using sintered LiF:Mg,Cu,Na,Si TL detectors for photon fields

The badge system of personal thermoluminescence (TL) dosimeter for photon fields using LiF:Mg,Cu,Na,Si TL material, which was developed by Korea Atomic Energy Research Institute (KAERI) a few years ago, was developed by taking advantage of its dosimetric properties including energy dependencies. A badge filter system was designed by practical irradiation experiments supported by computational modeling using Monte Carlo simulation. Design properties and dosimetric characteristics such as photon energy response and angular dependence of new TL dosimeter system examined through the irradiation experiments are presented. Based on the experiments for the developed dosimeter, it is demonstrated that the deep dose response of dosimeter provided the value between 0.78 and 1.08, which is within the design limit by ISO standard. This multi-element TL dosimeter badge system allows the discrimination of the incident radiation type between photon and beta by using the ratios of the four TL detectors. Personal TL dosimeter using sintered LiF:Mg,Cu,Na,Si TL detectors has the ability to measure a personal dose equivalent H_p(d) for a wide range of photon energies.     

Dose response of F center optical absorption in LiF:Mg,Ti (TLD-100)

Optical absorption (OA) of nominally pure single crystal LiF following beta irradiation was measured in order to estimate, the energy and width of the dominant F-band with minimum interference from dopant-related bands. The OA dose response of LiF:Mg,Ti was measured to 30,000 Gy, a level of dose sufficiently high to observe total saturation of the F band, which, we believe, reduces uncertainty in the estimation of the dose filling constant. The dose filling constants for the OA bands associated with the trapping center (4 eV) and competitive center (5.45 eV) responsible for the major dosimetric TL glow peak 5 were also determined. The results of these studies will be used in the framework of a kinetic model which includes the effects of radiation created defects and which will aid in the investigation of the capability of Track Structure Theory to predict OA heavy charged particle (HCP) relative efficiencies.     

High-dose characterization of different LiF phosphors

The dose response of three LiF TLDs: standard LiF:Mg,Ti (denoted MTS), high-sensitive LiF:Mg,Cu,P (MCP) and a recently developed in Kraków version of LiF:Mg,Ti with modified activator composition (MTT) and increased high-LET response was measured. The TLDs have been exposed to ⁶⁰Co gamma-rays, up to dose of 10 000 Gy, i.e. beyond saturation dose of the main dosimetric peaks, which corresponds to ca. 1000 Gy. The measured glow-curves were deconvolved into separate peaks with first order kinetic function (using self-developed GlowFit software). The dose response of the main peaks was found to be supralinear for MTS and sublinear for MCP detectors, as expected. The dose response of MTT was found to be even more supralinear than that of MTS. An interesting effect has been observed with regard to glow-curve shape of MCP detectors. Up to a dose of 1 kGy it remains practically unchanged, while for higher doses a strong growth of high-temperature peaks is observed. In the same dose region a decrease of the main peak of MCP with increasing dose is observed, unlike LiF:Mg,Ti detectors.