Behaviour of LiF:Mg,Cu,P and LiF:Mg,Ti thermoluminescent detectors for electron doses up to 1 MGy

The behaviour of LiF:Mg,Cu,P and LiF:Mg,Ti detectors at ultra-high doses up to 1 MGy, has been investigated. The presence of the ultra-high-temperature peak (450 °C) of reproducible properties was observed in various batches of LiF:Mg,Cu,P, confirming earlier findings. The results indicate that this peak is not an effect of random impurities nor intrinsic effects of LiF, but it is rather connected with the doping.A parameter called ultra-high temperature ratio (UHTR) was defined in order to quantify the observed changes of LiF:Mg,Cu,P glow-curve shape at very high doses and very high temperatures. The use of this parameter allows to determine an absorbed dose in the range from 1 kGy to 1 MGy. This new method of high-dose dosimetry makes LiF:Mg,Cu,P a unique dosimeter, which is capable to cover at least 12 orders of magnitude of dose range: from a microgray to a megagray.     

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.     

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.     

Thermoluminescence glow-curve characteristics of LiF phosphors at high doses of gamma radiation

High doses of ionising radiation are becoming increasingly common for radiation-processing applications of various medical, agricultural and polymer products using gamma and electron beams. The objective of this work was to study thermoluminescence (TL) glow-curve characteristics of commonly used commercial LiF TL phosphors at high doses of radiation with a view to use them in dosimetry of radiation-processing applications. The TL properties of TLD 100 and 700 phosphors, procured from the Thermo-Scientific (previously Harshaw) company, have been studied in the dose range of 1–60 kGy. The shift in glow peaks was observed in this dose range. Integral TL responses of TLD 100 and TLD 700 were found to decrease as a linear function of dose in the range of 5–50 kGy. The paper describes initial results related to the glow-curve characteristics of these phosphors.     

Thermoluminescence of nanocrystalline LiF:Mg, Cu, P

Nanocrystalline LiF:Mg, Cu, P of rod shape (about 30-40 nm in diameter and 0.3-0.5 μm in length) has been prepared by the chemical co-precipitation method. Thermoluminescence (TL) and dosimetric characteristics of the nanocrystalline phosphor are studied and presented here. The formation of the material was confirmed by the X-ray diffraction (XRD). Its shape and size were also observed by transmission electron microscope (TEM). The TL glow curve of the nanocrystalline powder shows a single peak at 410 K along with four overlapping peaks of lesser intensities at around 570, 609, 638 and 663 K. The observed TL sensitivity of the prepared nanocrystalline powder is less than that of the commercially available “Harshaw TLD-700H hot-pressed chips” at low doses but it still around three times more than that of LiF:Mg, Ti (TLD-100) phosphor. The 410 K peak of the nanomaterial phosphor shows a very linear response with exposures increasing up to very high values (as high as 10 kGy), where all the other thermoluminesent dosimeters (TLD) phosphors show saturation. This linear response over a large span of exposures (0.1 Gy-10 kGy) along with negligible fading and its insensitivity to heating treatments makes the nanocrystalline phosphor useful for its application to estimate high exposures of γ-rays. The ‘tissue equivalence’ property of this material also makes it useful over a wide range of high-energy radiation.     

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.     

Spectral characteristic of high-dose high-temperature emission from LiF:Mg,Cu,P (MCP-N) TL detectors

High-temperature emission spectra of LiF:Mg,Cu,P (MCP-N) TL detectors, irradiated above the nominal saturation level, up to the hundreds of kGy, have been measured. Emission spectra integrated over the whole temperature range, as well as the spectra recorded at the temperatures corresponding to the TL peaks maxima, were analyzed. With increasing dose of γ-radiation no significant changes were observed in the short wavelength emission range (220–450 nm) of the measured spectra. For doses of 4 kGy and higher the long wavelength emission (450–800 nm) started to be visible. All recorded spectra have been expressed in a form of the sum of several Gaussian-shape bands in the energy domain, which parameters remain in a general agreement with the measurements of Mandowska et al. (2010). Spectra of the low-temperature, main, high-temperature and “B” TL peaks were investigated. In the ranges of the low-temperature and the main dosimetric peaks, that is 100–125 and 210–230 °C, respectively, the short wavelength emission disappeared with increasing dose and for the highest doses the long wavelength emission became dominant. Both the high-temperature (290–320 °C) and the “B” (370–425 °C) peaks emission spectra exhibited somewhat different behavior with increasing dose. Initially, an even growth of the whole spectrum was observed and for doses higher than 16 kGy the intensity of the spectrum decreased, but the short wavelength emission band fell significantly faster, in case of the high-temperature TL peaks. In case of the “B” peak emission spectra the long wavelength emission did not play any role in the analyzed dose range. The spectra measured at the TL peaks maxima were also fitted with several Gaussian-shape bands. Dose-intensity dependences for all Gaussian-shape bands fitted to the measured spectra are also included in this paper.     

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.     

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.     

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.     

Application of phototransferred thermoluminescence (PTTL) for dose re-assessment in routine dosimetry using MTS-N (LiF:Mg,Ti) thermoluminescent detectors

Erasure of the thermoluminescence (TL) signal on detector readout is considered to be a disadvantage of TL dosimetry, as post-readout dose reassessment is then impossible in principle. A method of dose reassessment based on phototransferred thermoluminescence (PTTL) has been developed at the Institute of Nuclear Physics, Polish Academy of Sciences (IFJ PAN) and applied to MTS-N (LiF:Mg,Ti) detectors. We demonstrate the possibility of applying PTTL for dose reassessment in MTS-N TL detectors routinely applied in the dosimetric service at IFJ PAN. Readings of TL detectors exposed to relatively high doses by the customers of our dosimetry service can now be reassessed using our automatic readers. A major obstacle in applying the PTTL method at lower exposures is the presence of residual dose accumulated in LiF:Mg,Ti detectors after many field exposure and readout cycles. Since most of the TL detectors in our service have been already used for a long time (e.g. for over 10 years in the case of some detector batches), we find that our PTTL method of dose reassessment is possible only in detectors which had received doses exceeding 5 mSv.     

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.     

The effect of pre-irradiation annealing on the thermoluminescence of LiF:Mg,Cu, P phosphor grown by EFG technique

LiF crystal doped with magnesium (Mg), copper (Cu) and phosphorous (P) was grown in the form of multicrystalline sheet using Edge-defined film-fed growth (EFG) technique for dosimetry application. These crystals were grown in argon gas atmosphere using graphite crucible and stainless steel die. Dosimetry peak was observed at 210 °C for as-grown crystal. As reported earlier LiF:Mg, Cu, P is a highly sensitive material but losses its sensitivity if annealed at temperature above 240 °C. In this paper, the effect of annealing temperature on thermoluminescence glow-curve structure, maximum peak temperature, peak height and integrated area of the glow peak of EFG grown samples was investigated in detail. Annealing temperature range from 220 °C to 500 °C was considered for the study. Experimental results of the obtained glow curve show that with increase in annealing temperature, glow peak shift towards higher temperature region with substantial increase in TL intensity. Annealing at 500 °C for 10 min gave maximum TL intensity with main dosimetry peak positioned at 233 °C. Change in the defect structure with different pre-annealing temperature was analysed using trapping parameters.     

Environmental gamma-ray dose rate measurement by using ultra-high sensitive LiF:Mg,Cu,Si TLD

Environmental gamma-ray dose rates were measured by using ultra-high sensitive LiF:Mg,Cu,Si thermoluminescence dosimeters (TLDs) during short-term burial. The resultant gamma-ray dose rates were compared with those measured by using optically stimulated luminescence (OSL) of Al₂O₃:C and assessed by using HPGe spectrometer. Regardless of the short-term burial and the change of environmental conditions as well as the mismatch of the effective atomic number (Z_eff) with the soil (mainly SiO₂), good agreement was observed between the doses measured by TLDs and OSL dosimeters or the assessed doses. LiF:Mg,Cu,Si TLD has been found to be an effective alternative for the measurement of environmental gamma-ray annual dose rates for use in optical dating.     

Comparison of commercial thermoluminescent readers regarding high-dose high-temperature measurements

Three different thermoluminescent measuring systems have been compared with respect to the differences in temperature profiles, spectral sensitivities, as well as characteristics for high intensities of TL light. The comparison was performed using the Harshaw 3500, Risø DA-20 and RA’94 TLD readers. The instruments were tested for the readouts of highly irradiated LiF:Mg,Cu,P (MCP) TL detectors, which require readout up to 550 °C, in case of doses exceeding 1 kGy. It was found that the Harshaw 3500 can be used, without any additional light attenuation, for the measurements of MCP detectors exposed to doses up to about 5 Gy. For the other two readers the upper dose limit is about 10 times lower. It was also found that the Harshaw 3500 shows the best thermal stability considering the peak maximum position. For the ultra-high doses the differences in the spectral characteristics of the applied optical filters and photomultipliers, in conjunction with an evolution of the MCP TL emission spectrum with increasing dose, significantly influence the shape of TL glow curves measured with the DA-20 reader. The detailed characteristic of the compared TLD readers at high-dose high-temperature range is discussed.     

High-dose high-temperature emission of LiF:Mg,Cu,P: Thermally and radiation induced loss & recovery of its sensitivity

Highly sensitive LiF:Mg,Cu,P (MCP) detectors enable measurements of radiation doses from tens of nanograys up to a few kilograys, where the saturation of the signal of the main dosimetric peak occurs. Thanks to the recently observed high-dose high-temperature emission of MCP detectors heated to temperatures up to 600 °C after exposures to radiation doses ranging from 1 kGy to 1 MGy, a new method of thermoluminescent measurement of radiation doses has been recently developed at the Institute of Nuclear Physics (IFJ). This method can measure doses ranging from micrograys up to a megagray. So far, high dose measurements were performed on fresh MCP samples and each detector was used only once, because as a result of these measurements, the detectors lose their sensitivity to a large extent. In this study, a specific thermal treatment intended to fully restore the loss of MCPs TL sensitivity was sought. We have investigated several annealing procedures, applying different temperatures (from 400 °C up to 700 °C) for different periods of time (10–30 min) in argon atmosphere. In this way we were able to recover MCP sensitivity fully, allowing for reuse of the samples after high-dose irradiation and high-temperature measurement.     

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.     

Influence of sintering temperatures on LiF:Mg,Cu,P with various magnesium concentrations

The dependence of LiF:Mg,Cu,P samples with various concentrations of Mg on sintering temperatures was investigated to find a new dosimeter. The influence of high sintering temperatures on LiF:Mg,Cu,P chips depends strongly on Mg concentrations. The height of the main peak versus the sintering temperatures exhibits a maximum, the position of which varies between 690 °C and 750 °C, depending on the Mg concentration in the range studied. The high temperature peaks of LiF:Mg,Cu,P for various Mg concentrations reduce basically when the sintering temperature is increased. LiF:Mg,Cu,P is much less sensitive than LiF:Mg,Cu,Si to sintering temperature. LiF:Mg,Cu,P with 0.6 mol% of Mg can be re-used at annealing temperature of 260 °C, regardless of the sintering temperature. It was found that the optimum concentration is Mg: 0.6 mol%, the optimum sintering temperature is 750 °C, considering that LiF:Mg,Cu,P with a low residual signal and good sensitivity can be re-used at annealing temperature of 260 °C and produced in a large scale. The new optimum LiF:Mg,Cu,P formation has 52 times higher than that of the TLD-100, and an extremely low residual signal of 0.07% without an initialization readout procedure.     

LiF:Mg,Ti (MTT) TL detectors optimised for high-LET radiation dosimetry

The properties of LiF:Mg,Ti (distributed as, e.g., TLD-100 or MTS-N), the most frequently used thermoluminescent detector, have been optimised for measurements of sparsely ionising radiation (gamma rays), typically encountered in radiation protection or clinical dosimetry. However, these detectors need also to be applied in conditions of mixed-field dosimetry with a high-LET component, such as those encountered in heavy ion beams or in space.

At the Institute of Nuclear Physics in Kraków a new type of LiF:Mg,Ti detector (named MTT) has been recently developed through modification of its dopant composition. This composition is intended to increase the detection efficiency after a dose of high-LET radiation. The concentration of dopants in the MTT material is: C_Mg=50 ppm, and C_Ti=120 ppm, i.e. about a three times less of magnesium and about 10 times more of titanium content, compared with the standard MTS-N. The MTT TL detectors feature an increased relative efficiency to high-LET radiation, which for 5 MeV alpha-particles is about twice that of standard LiF:Mg,Ti. The response of MTT detectors has been studied in charged particle beams of the HIMAC accelerator in Chiba, Japan and in Dubna, Russia. The main foreseen application of MTT detectors are dose measurements in space. The dose after high-LET exposure can be estimated from the difference of the response of MTS and MTT detectors. In the near future MTT detectors will be applied in the “Matroshka” experiment. Within this experiment a specially constructed human phantom will be exposed in free space (outside the International Space Station) for 1 year. The phantom will incorporate a few thousand measuring points enabling radiation doses to particular organs to be determined.     

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.