Prompt isothermal decay of thermoluminescence in MgB4O7:Dy,Na and LiB4O7:Cu,In dosimeters

According to standard delocalized kinetic models of thermoluminescence (TL), when an irradiated sample is held at a high temperature T, the isothermal TL signal will decay with a characteristic thermal decay constant λ which depends strongly on the temperature T. This prediction of standard delocalized kinetic theory is investigated in this paper by studying two TL dosimeters, MgB₄O₇:Dy, Na and LiB₄O₇:Cu, In (hereafter MBO and LBO correspondingly). In the case of LBO it was found that the thermal decay constant λ of the main dosimetric TL peak follows exactly the predictions of standard delocalized kinetic theory. Furthermore, the thermal activation energy of the main peak evaluated by the isothermal decay method is in full agreement with values obtained from initial rise and glow curve fitting methods. However, in the case of MBO it was found that the thermal decay constant λ varies little with the isothermal decay temperature T. In order to explain these unusual results for MBO, the TL glow curves and isothermal decay curves were analyzed using analytical expressions derived recently from a radiative tunneling recombination model. Based on the different behavior of the two TL dosimeters, it is suggested that the isothermal decay of TL at high temperatures can be used to discriminate between radiative delocalized recombination and radiative localized recombination processes.     

Recombination centres and traps in ZnIn2S4

Luminescence intensity, phosphorescence decay and thermoluminescence glow have been measured for ZnIn₂S₄ and the results have been compared with previous data on the same material. Superlinear luminescence is found and it is interpreted in terms of the presence of several recombination centres in this crystal. From the analysis of the phosphorescence decay and thermoluminescence glow an exponential trap distribution is deduced with a slope of 70 meV/decade.     

A peak structure in isothermal luminescence signals in quartz: Origin and implications

Isothermal heating is commonly used in luminescence dosimetry and trap parameter studies. It is often observed that the isothermal luminescence signal has a peak shape instead of a monotonous decay form. We provide here evidence that this peak shape in quartz may equally result from a ‘thermal lag’ problem in contrast to the earlier propositions of non first-order kinetics. Temperature modelling suggests that the peak-shaped signal contains elements of both the ramped — and isothermal — thermoluminescence (TL). The modelled changes in the peak movement as a function of isothermal temperature and the ramp-rate show an excellent agreement with those obtained experimentally. This understanding of thermal lag is extended to optically stimulated luminescence (OSL) measurements for which the effects of isothermal TL contamination and changing thermal assistance during thermal equilibration are discussed. Appropriate methods are suggested to identify thermal lag on the basis of the peak structure, and to circumvent this effect in isothermal methods.     

The ντ-mass and the τ-decay modes

The behaviour of the different decay modes of the charged heavy lepton τ versus the neutrino ν_τ-mass is analyzed in detail. The τ→ν_τ A ₁ and τ→ν_τ K ^* decay rates have been evaluated using finite energy sum rules. The τ→ν_τ+ “Hadron Continuum” decay rate has been estimated within the framework of Quantum Chromodynamics (QCD). We find that the branching ratios of the semi-leptonic processes: τ→ν_τρ, τ→ν_τπ and τ→ν_τ+ “Hadron Continuum” are very sensitive to the value of the ν_τ-mass. Thus a more precise measurement of these branching ratios could provide an improved upper bound for the neutrino ν_τ-mass.     

Two-stage thermal stimulation of thermoluminescence

A thermoluminescence (TL) model of two-stage stimulation of electrons into the conduction band is discussed. This release of the carriers is assumed to take place via an intermediate localized excited state. Electrons are thermally stimulated from the trap into an excited state and then thermally released into the conduction band from which they may either be retrapped or recombine with holes in centers. The model resembles the previous “semi localized” model, but we concentrate only on recombination of electrons that go through the conduction band. It also bears similarity to the effect of thermally-assisted optically stimulated luminescence (OSL) previously discussed in the literature.The model is studied by solving the set of the relevant four simultaneous differential equations which govern the process during heating or isothermal decay. Using different sets of parameters, we can get pseudo-first-order, pseudo-second-order as well as intermediate cases, which are identified by their symmetry coefficient. Once the effective order is established, different analytical methods are used to determine the effective activation energy and frequency factor. We used the peak-shape methods, the various heating rate (VHR) method and the method based on the change of phosphorescence decay with temperature. The results are compared to the parameters used in the simulation. In many cases, the effective activation energy is equal to E₁ + E₂ where E₁ and E₂ are, respectively, the activation energies for the first and second stage of thermal stimulation. The numerical simulation results are accompanied by an analytical treatment using the usual quasi-steady assumption. Unusual cases, in which the effective frequency factor and the effective retrapping probability coefficient are temperature dependent, are identified. Some cases in which the effective activation energy is close to E₁ rather than E₁ + E₂ are identified and discussed. The relevance of this possible situation to the evaluation of the stability of TL signals is also considered, and a possible effect of anomalous stability is predicted.     

Polarization of thermoluminescence of various scheelites

The polarization of the thermoluminescence of various scheelite type single crystals being correlated with the thermal decay of intrinsic hole centres has been measured. The crystals show partial z-polarization of the thermoluminescence. The degree of polarization of the glow light increases with increasing compression of the anionic XO^2-₄ complexes in different crystals. A model explaining the results on the basis of the term diagram of a single oxocomplex is presented.     

Reconstruction of thermally quenched glow curves in quartz

The experimentally measured thermoluminescence (TL) glow curves of quartz samples are influenced by the presence of the thermal quenching effect, which involves a variation of the luminescence efficiency as a function of temperature. The real shape of the thermally unquenched TL glow curves is completely unknown. In the present work an attempt is made to reconstruct these unquenched glow curves from the quenched experimental data, and for two different types of quartz samples. The reconstruction is based on the values of the thermal quenching parameter W (activation energy) and C (a dimensionless constant), which are known from recent experimental work on these two samples. A computerized glow-curve deconvolution (CGCD) analysis was performed twice for both the reconstructed and the experimental TL glow curves. Special attention was paid to check for consistency between the results of these two independent CGCD analyses. The investigation showed that the reconstruction attempt was successful, and it is concluded that the analysis of reconstructed TL glow curves can provide improved values of the kinetic parameters E, s for the glow peaks of quartz. This also leads to a better evaluation of the half-lives of electron trapping levels used for dosimetry and luminescence dating.     

Migration and stabilization of hydrogen atoms in KCl

Stabilities of hydrogen atoms at the interstitial (H⁰_i), cation and anion sites (H⁰_sc and H⁰_sa) in KCl have been studied with ESR and thermoluminescence. The close pair between H⁰_i and S^? formed by UV-irradiation of KCl:SH^? recombines first with the activation energy of 0.17±0.02 eV following the emission of light and then H⁰_i becomes mobile with the energy of 0.20±0.02 eV. The energies determined from the decay of ESR signal intensity of H⁰_sc and H⁰_sa are 0.24±0.02 eV and 0.43±0.03 eV, respectively.     

Scintillation kinetics and thermoluminescence of SrI2:Eu2+ single crystals

Single crystal of strontium iodide doped with 1% europium (SrI₂:1% Eu²⁺) was grown by Vertical Gradient Freeze technique. UV excited emission spectra were studied as a function of temperature. Results indicate the thermal quenching of Eu²⁺ emission starts from ～400 K with a thermal activation energy of 0.39 eV. Gamma and UV excited decay measurements indicate that the scintillation decay time of SrI₂:Eu²⁺ is longer than the lifetime of Eu²⁺ luminescence center in the SrI₂ host. The thermoluminescence glow curve revealed a highly concentrated charge carrier trap at 50 K. Elimination of this trap is expected to enhance the energy migration of charge carriers and result in faster scintillation decay.     

Untersuchung der Lumineszenzabklingung von Wolframatleuchtstoffen nach Anregung mit UV-Licht und Elektronen

The decay of the luminescence of CaWO₄, CdWO₄, MgWO₄, and ZnWO₄ after excitation with short pulses of light and electron beams has been investigated at different temperatures. Excitation with UV-light yields an exponential decay, and from the temperature dependence of the decay time it can be concluded that at high temperatures non-radiative transitions are prevalent. After excitation with electrons, there are additional effects by the heating up of the “excitation channel”. In this case the decay is not purely exponential and thermoluminescence is observed. This indicates the existence of traps, which are assumed to be located in the centers of the luminescence.     

Impurity spin dynamics in the classical Kondo system Cu: Fe. NMR relaxation of impurity neighbour nuclei

The correlation time τ_i of the impurity electronic spin in the Kondo system Cu:Fe has been measured over the temperature range 4.2 K?T?300 K (T_K=27.6 K) by means of NMR relaxation of impurity neighbour nuclei. τ_i being in the sub-picosecond region varies with temperatures as predicted by the model calculation of Götze and Schlottmann.     

Energy relaxation of hot electrons and inelastic collision time in thin metal films at low temperatures

In a sandwich consisting of a thin Au film and a thin Bi film insulated by a 500 Å SiO layer the electrons of one film are heated above the lattice temperature while in the other film they are kept in thermal equilibrium temperature while in the other film they are kept in thermal equilibrium with the phonons. By measuring the magnetoresistance of both films we obtain the temperature difference between electrons and phonons from which we imply the energy-relaxation time τ_ϵ from 0.3 K to 2 K. We find that electron-phonon scattering determines τ_ϵ and, above 5 K, also the inelastic collision time τ_i obtained from weak localization theory in thermal equilibrium.     

Deformation luminescence in KCl irradiated at room temperature

It has been found that there is a relationship between the thermoluminescence and the deformation luminescence of irradiated pure KCl samples. It is concluded that the luminescent process in the deformation luminescence is the recombination of interstitial halogen atoms with vacancy centres (F, M) as well as in the thermoluminescence.     

Fluorescence of Mn2+ in CaCO3

The fluorescence of Mn²⁺ ion as impurity in CaCO₃ has been investigated. Emission bands from the ⁴D(E_g), ⁴D(T_2g) and ⁴G(T_1g) levels have been observed. The analysis of excitation and emission spectra has allowed to obtain the values of field strength (Dq) for the excited energy levels of Mn²⁺ in CaCO₃ lattice. The temperature dependence of excitation and emission spectra yield an activation energy for thermal quenching of luminescence very close to theoretical calculation. The behaviour of luminescence lifetime with temperature has also been obtained.     

Nanophosphor aluminum oxide: Luminescence response of a potential dosimetric material

This work reports on the investigation of the radiation dosimetry properties of Al₂O₃ nanopowders. Samples were produced by solution combustion synthesis using three different organic fuels to check for the effect of synthesis conditions on the properties of interest. Luminescence characteristics were studied by thermoluminescence and optically stimulated luminescence (OSL) techniques. We found that samples produced using urea have characteristics similar to bulk Al₂O₃:C and may be suitable for personal dosimetry, while samples produced using glycine and hexamethylenetetramine (HMT) may be more suitable for applications where fast OSL decay is advantageous. While these results are promising and warrant further investigation, much has to be done to overcome the greatly decreased luminescence intensity of the nanomaterials as compared to bulk Al₂O₃:C.     

Competition between different mechanisms of stability loss for the (C20)2 cluster dimer

The thermal stability of the (C₂₀)₂ cluster dimer consisting of two C₂₀ fullerenes has been numerically examined using the tight-binding method. The simulation of the dynamics of the (C₂₀)₂ dimer at temperatures of T = 2000–3500 K shows that the finite lifetime τ of this metastable system is determined by two fundamentally different processes: the decay of one of the C₂₀ fullerenes and the coalescence of two C₂₀ fullerenes to the C₄₀ cluster. The activation energies for these processes. E _a ≈ 3.4 and 2.7 eV, respectively, as well as their frequency factors, have been determined by analyzing the τ(T) dependence.     

Cr-related centers in Gd3Ga5O12 polycrystals

The luminescence excitation spectra, emission spectra under photo- and X-ray excitation, luminescence decay kinetics and thermostimulated luminescence (TSL) of Gd₃Ga₅O₁₂ garnet (GGG) polycrystalline samples have been investigated. It was established that the spectrum of Cr³⁺ ion emission were present in all TSL peaks. The activation energies of traps that are responsible for appearance of TSL in the region 295-600 K were estimated. It is shown that delocalization of electrons from the Cr³⁺e traps leads to the appearance of thermoluminescence (TL) glow peak at 390 K. The nature of other TSL peaks is discussed. The influence of visible light on the TSL intensity of the preliminary X-ray-irradiated samples is shown.     

Mean residence time of potassium ions on tungsten as measured with reference to ionization efficiency

The mean residence time, τ_i, of potassium ions on “clean” and oxygenated tungsten has been measured, together with the ionization efficiency, as a function of surface temperature T by using incident K beams of low intensity (10⁹–10¹² atoms cm^?2 s^?1). For T higher than ~900 K the observed τ_i followed Frenkel's equation $\text{τ}{}_{\mathrm{<mtext>i</mtext>}}\text{= τ}{}_{\mathrm{<mtext>i</mtext>}}{}^0\text{exp}\text{(}\text{Q}{}_{\mathrm{<mtext>i</mtext>}}\text{kT}\text{)}$ as usual and the agreement of the ionic desorption energy Q_i and of the pre-exponential factor τ_i⁰ with the corresponding values of previous experiments was quite satisfactory. Below 830 ~ 910 K, where a steep drop of ionization efficiency began to be noticeable, Arrhenius plots of τ_i deviated considerably from linearity. The apparent increment of the desorption energy was shown to be nearly equal to the decrement of thermionic work function of tungsten as obtained from the ionization efficiency and Saha-Langmuir equation. The increase of surface coverage by potassium was accordingly taken as the main cause of the departure of Arrhenius plots from linearity. Under certain conditions of incident beam intensity and surface temperature τ_i was observed to make an abrupt change from a higher to a lower value — a difference expressed as 100–140 meV in terms of the difference in ionic desorption energy. This peculiar phenomenon was attributed to the phase change of adsorbed potassium on tungsten.     

Anomalous heating rate effect in thermoluminescence intensity using a simplified semi-localized transition (SLT) model

During thermoluminescence (TL) measurements carried out with different heating rates, one expects the total number of emitted photons to be constant. However, for many luminescence materials one observes a decreased intensity of luminescence at elevated temperatures, due to the presence of the well-known phenomenon of thermal quenching. Recent experimental work on the dosimetric material YPO₄ double doped with lanthanides demonstrated the exact opposite behavior, in which the total luminescence intensity increases with the heating rate during the TL experiments. This anomalous TL behavior was recently explained by using the Mandowski model of semi-localized transitions (SLTs). In this paper it is shown that this anomalous heating rate or “anti-quenching” phenomenon can also be described by using a simplified SLT model of TL with approximated kinetic equations. The simulated glow curves show that as the probability of the non-radiative processes increases, the anomalous heating rate effect becomes dominant. The dependence of the anomalous heating rate effect on the values of the kinetic parameters in the model is examined by allowing random variations of the parameters in the model, within wide ranges of physically reasonable values covering several orders of magnitude. It is shown by simulation that the variable heating rate method can systematically underestimate the value of the activation energy E, while by contrast the initial rise method of analysis almost always yields the correct E value. These simulated results are discussed in relation to recent experimental work on the double doped dosimetric material YPO₄.     

Luminescence of Ca-doped (Y1-x, Gdx)2O2S

Broad-band yellow luminescence peaking around 575–595 nm has been found in Ca-doped (Y_1-x, Gd_x)₂O₂S. The doping of Ca into Y₂O₂S with the concentration up to 1 mol% is possible. At the optimum concentration (about 0.2 mol%), the cathodoluminescence brightness of Ca-doped Y₂O₂S is 10% of that of yellow-emitting (Zn, Cd)S: Ag.The emission peak is 575 nm in Y₂O₂S: Ca and 595 nm in Gd₂O₂S: Ca. The phosphor exhibits strong thermoluminescence after UV excitation at 77 K. In (Y_1-x, Gd_x)₂O₂S, neither emission spectra nor the temperatures of thermal glow peak depend on x, indicating localized characters of the traps concerned. The photoluminescence is slow in buildup and persistent in decay. These results are explained by a model: the substitution of Y(Gd) by Ca creates acceptor levels in which holes are captured, giving rise to subsequent radiative recombination with excited electrons. Trapped electrons recombine with mobile holes nonradiatively, but exhibit thermoluminescence with high efficiency when they are thermally released.