The Influence of Ultraviolet Light on Luminescence (700–1000 nm) of LiF and LiF:OH Crystals Irradiated in a Nuclear Reactor

Optics and Spectroscopy - The emission (700–1000 nm) and absorption (200–800 nm) spectra of LiF and LiF:ОН crystals irradiated in a reactor and by UV light, as well as...     

The effect of thermal annealing on defect structure and thermoluminescence in LiF:Mg,Cu,P

We have studied the behavior of the glow peaks in the thermoluminescence of LiF:Mg,Cu,P as a function of pre-irradiation annealing temperature in the range 80 to 170°C, and as a function of cooling rate following the 240°C/10 min anneal used for standardization in dosimetric procedures. The intensities of the major peaks in LiF:Mg,Cu,P (as well as in LiF:Mg,Ti—the current industrial standard) seem to be determined by the dynamics of clustering of (Mg²⁺−Li_vac) dipoles to dimers, trimers and a precipitate phase. The intensities of the thermal interactions, however, seem to be somewhat reduced in LiF:Mg,Cu,P compared to LiF:Mg,Ti. In addition, it seems plausible that phosphorus takes the role of titanium in LiF:Mg,Cu,P in the formation of a trapping center/recombination center spatially correlated complex.     

Investigation and simulation of the structural and electronic properties of LiF:Mg,LiF:Cu,LiF:P nano-layer dosimeters

In this study, electronic structure of lithium fluoride thin films in pure state and doped with magnesium (Mg), copper (Cu) and phosphorus (P) impurities was studied using WIEN2K Code. The structural and electronic properties of two LiF thin films with 1.61 and 4.05?nm thicknesses were studied and compared. Results show that the distance of atoms in the surface and central layers of pure LiF are 1.975 and 2.03?nm, respectively. Electronic density of the valence band around the surface atoms is greater than that around middle atoms of the supercell. The band gap of bulk LiF is 9?eV. But, in the case of thin films, it is reduced to 2?eV. Electronic and hole-traps were not observed in composition of LiF thin films doped with Mg and P with 1.61 and 4.05?nm thickness and in fact, metallic properties were observed. When Cu atoms were doped in composition of an LiF thin film, the thin film was converted to semiconductor.     

Thermoluminescence glow curve deconvolution of LiF:Mg,Cu,Si with more realistic kinetic models

Thermoluminescence (TL) glow curves of LiF:Mg,Cu,Si were deconvoluted with the introduction of enhanced physical model which envisages that both electrons and holes, produced by ionization radiation and trapped at the respective traps, can be thermally released into the conduction and the valence band, respectively and the holes may also radiatively recombine with electrons at the electron recombination centers. The model is more generalized than the ordinary trap interaction model which only permits the traffic of electrons through the conduction band. An effective numerical analysis method was developed to calculate the glow curve to be compatible with the measured curves. The validity of the numerical method was verified through artificially generated TL glow curves for a wide range of trap parameters. In order to identify TL kinetics of LiF:Mg,Cu,Si with higher accuracy, its glow curves were deconvoluted for two more generalized models, namely, the Schön–Klasens model and the Chen–Pagonis–Lawless model as well as the ordinary trap interactive model. The parameters in the more generalized multi-trap multi-recombination center (MTMR) model were found to be consistent with the quasi-static approximation(QSA) method.     

Ion-induced kinetic electron emission from 6LiF,7LiF and MgF2 thin films

<正>Secondary electron yields for Ar~+ impact on ~6LiF,~7LiF and MgF_2 thin films grown on aluminum substrates are measured each as a function of target temperature and projectile energy.Remarkably different behaviours of the electron yields for LiF and MgF_2 films are observed in a temperature range from 25℃to 300℃.The electron yield of LiF is found to sharply increase with target temperature and to be saturated at about 175℃.But the target temperature has no effect on the electron yield of MgF_2.It is also found that for the ion energies greater than 4 keV,the electron yield of ~6LiF is consistently high as compared with that of ~7LiF that may be due to the enhanced contribution of recoiling ~6Li atoms to the secondary electron generation.A comparison between the electron yields of MgF_2 and LiF reveales that above a certain ion energy the electron yield of MgF_2 is considerably low as compared with that of LiF.We suggest that the short inelastic mean free path of electrons in MgF_2 can be one of the reasons for its low electron yield.     

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.     

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.     

Vacuum ultraviolet spectroscopy of U:LiF, Cu, and U:NaF, Cu crystals

Luminescence and excitation spectra of doped LiF and NaF crystals are studied by time-resolved optical and luminescent vacuum ultraviolet (VUV) spectroscopy (2–40 eV energy range, T=10–295 K) with the use of synchrotron radiation of the X-ray and the VUV ranges and pulsed electron beams. Spectral kinetic parameters of luminescence and energies of excited states of U⁶⁺ ions are determined. The dominant role of the electron-hole mechanism for energy transfer to impurity centers is established. The effect of multiplication of electronic excitations is clearly manifested for E > 25 eV in NaF:U, Cu crystals and determines their high scintillation yield (137% relative to Tl:CsI when detected in the current regime).     

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.     

Enhanced electron injection in organic light-emitting devices using Al/LiF electrodes

The temperature dependence of the current-voltage-luminescence characteristics in organic light-emitting diodes (OLEDs) with varying thickness of LiF layers are studied to understand the mechanism of the enhanced electron injection by inserting a thin insulating LiF layer at the tris(8-hydroxyquinoline) aluminum (Alq₃)–Al interfaces. At room temperature, the LiF/Al cathode enhances the electron injection and the quantum efficiency (QE) of the electroluminescence (EL), implying that the LiF thin layer lowers the electron-injection barrier. However, at low temperatures it is observed that the injection-limited current dominates and the barrier height for the electron injection in the device with LiF/Al appears to be similar with the Al only device. Thus, our results suggest that at low temperatures the insertion of LiF does not cause a significant band bending of Alq₃ or reduction of the Al work function.     

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.     

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.     

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.     

Equation of state and thermal expansivity of LiF and NaF

The high-pressure and high-temperature behaviors of LiF and NaF have been studied up to 37 GPa and 1000 K. No phase transformations have been observed for LiF up to the maximum pressure reached. The B1 to B2 transition of NaF at room temperature was observed at ～28 GPa, this transition pressure decreases with temperature. Unit-cell volumes of LiF and NaF B1 phase measured at various pressures and temperatures were fitted using a P–V–T Birch–Murnaghan equation of state. For LiF, the determined parameters are: α₀ = 1.05 (3)×10^?4 K^?1, dK/dT = ?0.025 (2) GPa/K, V ₀ = 65.7 (1) Å³, K ₀ = 73 (2) GPa, and K′ = 3.9 (2). For NaF, α₀ = 1.34 (4)×10^?4 K^?1, dK/dT = ?0.020 (1) GPa/K, V ₀ = 100.2 (2) Å³, K ₀ = 46 (1) GPa, and K′ = 4.5 (1).     

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.     

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.     

Scintillation Characteristics of Lithium Fluoride Crystals Doped with Tungsten Oxide

Russian Physics Journal - Results of investigation of the spectral-kinetic parameters of a new LiF–WO3 luminophore in broad time interval 10–108 ns after the termination of the exciting...     

Efficient organic light-emitting diodes with zinc acetate as an effective electron injection layer

We report on the fabrication of organic light-emitting diodes (OLEDs) using a zinc acetate ((CH₃COO)₂Zn) layer as the cathode buffer layer. The results show that the device containing a (CH₃COO)₂Zn interlayer shows improved luminance and efficiency due to the Zn–N bond formation resulting in the occurrence of Alq₃ anion and also due to the band bending at the Alq₃/Al interface, which is beneficial to electron injection by lowering electron injection barrier. And the devices with structured cathodes (CH₃COO)₂Zn/LiF/Al and LiF/(CH₃COO)₂Zn/Al have a higher luminance and efficiency than the LiF/Al cathode-based device.     

LiF及AlF3对掺Tm3+氟磷玻璃光学、光谱性质、析晶性能及结构的影响

为了平衡掺铥氟磷玻璃对辐射寿命、发射截面和析晶稳定性的综合要求,研究了LiF及AlF₃对掺铥氟磷玻璃光学光谱性质和析晶性能的影响. 研究表明,高达30 mol%的LiF仍可保证氟磷玻璃具有较好的析晶稳定性能且其在提高Tm³⁺离子的辐射寿命上具有非常重要的作用;高AlF₃玻璃具有很好的析晶稳定性能,但Tm³⁺离子在其中的辐射寿命下降较快; 鉴于LiF和AlF₃含量的相对变化对发射截面的影响不大,因此 关键词： 氟磷玻璃 辐射寿命 发射截面 析晶稳定性