Dependence of the thermoluminescent high-temperature ratio (HTR) of LiF:Mg,Ti detectors on proton energy and dose

The high-temperature ratio (HTR) is a parameter quantifying changes of the shape of the high-temperature part of the LiF:Mg,Ti glow-curve after exposure to densely ionizing radiation. It was introduced in order to estimate the ‘effective LET’ of an unknown radiation field and to correct the decreased relative TL efficiency for high-Linear Energy Transfer (LET) radiation.In the present work the dependence of HTR on proton energy (14.5–58 MeV) and dose (0.5–30 Gy) was investigated. All measured HTR values were at the level of 1.2 or higher, therefore significantly different from the respective value for gamma-rays (HTR = 1), but HTR was found to be insensitive to changes of proton energy above 20 MeV. As a result the relationship between HTR and relative TL efficiency is not unequivocal. The HTR was found to be dependent on absorbed dose even for the lowest studied doses.     

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.     

Dose reassessment by using PTTL method in MTS-N (LiF:Mg,Ti) thermoluminescent detectors

The thermoluminescent (TLD) method is one of the most commonly used in dose measurements in radiation protection dosimetry. Due to its many advantages this method is widely spread. However, TLD has especially one disadvantage which is very inconvenient: the dose information in already read detectors is erased and in routine standard way the dose can not be reassessed. The positive is that this shortcoming can be eliminated by applying UV radiation. After first readout the same detector can be subjected to UV exposure and then read once again to reassess the dose.This method for reassessment of dose is based on phototransferred thermoluminescence (PTTL). In an irradiated TL detector deep traps are not emptied during the first readout. During exposure to UV, electrons are transferred from deep traps to shallower dosimetric traps. This TL signal emerging during the second readout following UV illumination is called phototransferred thermoluminescence.A method for reassessment of dose in a previously readout TLD is presented in this work. Experiments show that the method works well within region of doses between 5 and 50 mGy, but could be applied for higher doses as well. The efficiency of dose reassessment reaches about 17 percent of the first readout.The method could be a noticeable improvement in TLD dosimetry, giving more opportunities for better control and reliability of measurements.     

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.     

Advantages and challenges of high-dose thermoluminescent detectors

The current status of high-dose luminescent detector investigations is discussed. The examples of successful applications of various materials for medium and high-dose luminescent detector creation are presented. High-dose irradiation effect on luminescence of TLD-500 (α-Al₂O₃:C) detectors has been described. The challenges of detector properties restoration after high-dose irradiation are considered.     

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.     

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.     

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.     

Linear energy transfer dependence of Al2O3:C optically stimulated luminescence detectors exposed to therapeutic proton beams

Currently, there are no radiation detectors that can be used for routine measurements of linear energy transfer (LET) in particle therapy clinics. In this work, we characterized the LET dependence of Al₂O₃:C optically stimulated luminescence (OSL) detectors (OSLDs) exposed to therapeutic proton beams in order to evaluate their potential for clinical LET measurements. We evaluated OSLDs that were irradiated with an absorbed dose to water of 0.2 Gy in therapeutic proton beams with average energies ranging between approximately 25 MeV and 200 MeV, resulting in LET in water values between 0.45 and 2.29 keV/μm. We examined two properties of the OSL emission signal in terms of LET dependence: the signal intensities of the blue and ultraviolet (UV) emission bands, and the shapes of the OSL curves. We found that the signal intensity of the UV emission band increased consistently with LET within the range investigated, whereas the intensity of the blue emission band remained constant. Our results also demonstrated that the OSL curve shapes were more LET dependent for signals containing both the blue and UV emission bands than for signals containing only one of the bands. Both metrics we examined in this study – the relative UV/blue emission signal intensities and OSL curve shapes – show potential for LET detection in proton therapy.     

Characterization of YAlO3:Mn2+ thermoluminescent detectors

The paper contains characterization of the YAlO₃:Mn²⁺ thermoluminescent (TL) detectors. Namely, such TL parameters of the detectors as repeatability, thermal fading, effect of daylight exposure and dose dependence have been studied. The performed investigations confirm a high application potential of the detectors for middle- and high-dose dosimetry of ionizing radiation.     

Response of thermoluminescent detectors to charged particles and to neutrons

Thermoluminescent detectors (TLDs) are widely used for the dosimetry of photons and electrons. They are less used for the radiation with higher linear energy transfer (LET). One of the reasons for that is that their TL relative efficiency η decreases for the most of them with increasing LET.

The paper presents first a review of author's experimental results in which η was established for charged particles having LET of the order from 1 to 100 keV/μm in tissue. Among TLDs studied were known materials like LiF:Mn; Ti; Al–P glass; CaSO₄:Dy; Al₂O₃:Na; and Al₂O₃:C. It was found that the dependence of their η on LET is not the same for all TLDs studied.

The response of the same materials to neutrons was also studied. It was found that both η as the relative response (RR) defined in terms of absorbed dose in tissue are different, they depend critically also on the composition. When a TLD contains nuclei like ⁶Li and ¹⁰B, their RR would be rather high. As far as η is concerned, the same tendencies were observed as for charged particles, i.e. when average LET of secondary particles formed in a TLD increases, their η generally decreases.     

Ultra-thin LiF:Mg,Cu,P detectors for beta dosimetry

Standard thermoluminescent (TL) detectors, owing to their relatively large thickness, may seriously underestimate personal skin doses which are defined at the depth of 7 mg cm⁻². New TL ultra-thin, LiF:Mg,Cu,P-based detectors have been developed at the Institute of Nuclear Physics to fulfill simultaneously the requirements of flat energy response for beta rays and the ability to measure low beta ray doses. In our detectors a thin layer of MCP phosphor is bonded with a thick base of undoped LiF. We assess the effective thickness of this detector to be 8.5 mg cm⁻². Tests of these detectors exposed with and without covering foil to ¹⁴⁷Pm, ²⁰⁴Tl and ⁹⁰Sr/⁹⁰Y calibrated beta fields indicate that our detectors feature an essentially flat energy response and good angular characteristics. The sensitivity of our detectors permits doses in the microsievert range to be measured reliably.     

The efficiency of formation of primary radiation defects in LiF and MgF2 crystals

Processes of radiation formation of primary defects—F centers and self-trapped excitons—in lithium and magnesium fluorides, which have crystal lattices of different types and similar widths of the band gap and valence band, have been studied in a wide temperature range (11–500 K). It is shown that, along with qualitative similarity of the regularities of formation of the defects under study, LiF and MgF₂ crystals are characterized at low temperatures (11–100 K) by different relationships between the energy dissipation channels for self-trapping electronic excitations and the types of self-trapped excitons arising.     

System of pulsed solar energy supply of ionizing radiation detectors

The development of power supplies for ionizing radiation detectors is a complex and difficult problem, especially if this concerns with autonomous mobile measuring systems. In this article, we describe the experience of the implementation of the autonomous solar energy supply for semiconductor ionizing radiation detectors.     

Photoluminescence of gamma-, proton- and alpha-irradiated LiF detectors

Lithium fluoride (LiF), one of the most pervasive alkali halides in optical device research, is routinely used in optical data storage and radiation protection. LiF crystals may contain different aggregate defects produced by several types of ionizing radiation, with the number of defects being proportional to the cumulative radiation dose. Stimulation of irradiated LiF detectors by heating or with blue light causes thermoluminescence (TL) or photoluminescence (PL), respectively. We developed a new PL reader equipped with a blue light-emitting diode for stimulation and a Hamamatsu photomultiplier for registering green emissions, dedicated to examining LiF detectors as well as more broadly investigating TL/PL emission from standard LiF detectors irradiated with gamma rays, 60 MeV protons and alpha particles. The results confirmed very high efficiency PL signal from alpha-irradiated LiF detectors corresponding to their low efficiency after gamma irradiation, and vice versa for TL readout. Combining the TL and PL readouts permits us to discriminate between how different kinds of radiation affect efficiency in LiF detectors.     

Nonperturbative gluon radiation and energy dependence of elastic scattering

The energy dependence of the total hadronic cross sections is caused by gluon bremsstrahlung which we treat nonperturbatively. It is located at small transverse distances about 0.3 fm from the valence quarks. The cross section of gluon radiation is predicted to exponentiate and rise with energy as s(delta) with delta = 0.17+/-0. 01. The total cross section also includes a large energy independent Born term which corresponds to no gluon radiation. The calculated total cross section and the slope of elastic scattering are in good agreement with the data.     

Form factor dependence of proton induced deuteron breakup at low energy

Proton induced deuteron breakup has been studied experimentally and theoretically. The precision of the data makes it possible to distinguish between calculations made with different separable two-body forces. The possibility to observe off-shell effects and three-body forces is discussed.     

Measurement of radiation energy by spectrometric systems based on uncooled silicon detectors

The results of measurements of radiation energy for single-channel detection systems based on uncooled silicon planar detectors and spectrometric readout electronics, developed at the Kharkiv Institute of Physics and Technology National Scientific Center, are presented. Radiation sources of ⁵⁵Fe, ²⁴¹Am, ⁵⁷Co, ¹³⁷Cs, and ^99MTc and characteristic X-ray radiation (CXR) are used in the experiments. The radiation energy in the range of E _γ = 3–140 keV is measured by a spectrometer based on a Si PIN detector. The energy resolution (full width at half maximum, FWHM) in this energy range changes with increasing photon energy from 0.97 to 1.3 keV. The CXR of calcium (K _α = 3.69 keV) is measured by a Si planar detector with an input aluminum foil. Emission lines in the energy range of E _γ = 0.04–0.662 MeV are measured by a CsI(Tl) scintillator-silicon PIN-photodiode detection system. The energy resolution of the spectrometer changes with quantum energy increasing from 22 to 70 keV. The CXR from cesium and iodine with K _α 31 and 28.6 keV, respectively, is recorded.     

LiF Thickness dependence of Electron Injection Models for Alqa/LiF/A1 Cathode Structure

We present the experimental evidences showing that three different electron injection models play roles in Alq3 based organic light-emitting diodes in sequence when the thickness of LiF interlayer is changed. It is found that the device with a 0.2nm LiF layer displays the largest current with declined luminescence. However, the one with a 0.6 nm LiF layer displays the second largest current and the highest luminescence of all. Combining with the photoluminescent test results, three models, namely chemical reaction at ternary interface, dipole effect at binary interface and tunneling enhancement effect, are expected to play roles in sequence when the LiF thickness is increased from 0 nm to 4 nm.