Shigueo Watanabe

HIGH ENERGY SCATTERING OF DEUTERONS BY COMPLEX NUCLEI

Abstract An effective deuteron-nucleus potential consisting of an optical well plus a spin-orbit interactive was derived, starting from a Saxon potential plus a spin-orbit term for the interaction between each nucleon in the deuteron and the nuclens. The WKB method was used to calculate the angular distribution and the polarization of 94 MeV deuterons by carbon. The result of this calculation using well parameters which fit the scattering data of 40 MeV protons by carbon, is in good agreement with the measured values of the differential cross section. The predicted polarization shows a maximum of about 30 percent, but its sign is constant and does not oscillate as the experimental one does.

NIR to visible up-conversion, infrared luminescence, thermoluminescence and defect centres in Y2O3?:?Er phosphor

Er3+ doped Y2O3 phosphor was prepared by the solution combustion method and characterized using powder x-ray diffraction and energy-dispersive analysis of x-ray mapping studies. Room temperature near infrared (NIR) to green up-conversion (UC) emissions in the region 520–580 nm {(2H11/2, 4S3/2) →4I15/2} and red UC emissions in the region 650–700 nm (4F9/2 →4I15/2) of Er3+ ions have been observed upon direct excitation to the 4I11/2 level using ~972 nm laser radiation of nanosecond pulses. The possible mechanisms for the UC processes have been discussed on the basis of the energy level scheme, the pump power dependence as well as based on the temporal evolution. The excited state absorption is observed to be the dominant mechanism for the UC process. Y2O3 : Er exhibits one thermally stimulated luminescence (TSL) peak around 367 °C. Electron spin resonance (ESR) studies were carried out to study the defect centres induced in the phosphor by gamma irradiation and also to identify the centres responsible for the TSL peak. Room temperature ESR spectrum of irradiated phosphor appears to be a superposition of at least three distinct centres. One of them (centre I) with principal g-values g|| = 2.0415 and g⊥ = 2.0056 is identified as centre while centre II with an isotropic g-factor 2.0096 is assigned to an F+-centre (singly ionized oxygen vacancy). Centre III is also assigned to an F+-centre with a small g-factor anisotropy (g|| = 1.974 and g⊥ = 1.967). Additional defect centres are observed during thermal annealing experiments and one of them appearing around 330 °C grows with the annealing temperature. This centre (assigned to an F+-centre) seems to originate from an F-centre (oxygen vacancy with two electrons) and the F-centre appears to correlate with the observed TSL peak in Y2O3 : Er phosphor. The trap depth for this peak has been determined to be 0.97 eV from TSL data.

Limitation of peak fitting and peak shape methods for determination of activation energy of thermoluminescence glow peaks

This paper shows the limitation of general order peak fitting and peak shape methods for determining the activation energy of the thermoluminescence glow peaks in the cases in which retrapping probability is much higher than the recombination probability and the traps are filled up to near saturation level. Right values can be obtained when the trap occupancy is reduced by using small doses or by post-irradiation partial bleaching. This limitation in the application of these methods has not been indicated earlier. In view of the unknown nature of kinetics in the experimental samples, it is recommended that these methods of activation energy determination should be applied only at doses well below the saturation dose.

General order and mixed order fits of thermoluminescence glow curves—a comparison

Abstract General order (GO) and mixed order (MO) kinetics expressions are applied to a series of synthetic thermoluminescence (TL) glow peaks derived from different physical models. The correlation between kinetic order (KO) parameter b of the GO kinetics expression and the parameter α (=n 0 /(n 0 +M) where n0 is the initial filled concentration of the active traps and M that of the thermally disconnected deep traps) of the MO expression is checked. It is found that the correlation is not universal except at the limiting values of α and b, when α=0 and 1 always correspond to b=1 and 2 as expected. In the intermediate region the graphical plot does not produce a smooth line. An examination of all the results along with those from the published literature shows that the overall scatter in the values found for b corresponding to any value found of α is within about ±10%. This scatter is attributed to the influence of parameters other than α and b on the symmetry of the glow peak. A conclusion of greater significance reached from this study is that MO expression is a superior alternative to GO for the glow peak characterization. This is because the parameter α remains constant at all temperatures T in the glow curve whereas b, though assumed constant in the GO expression, changes with T when GO is applied to any physically plausible model of TL emission, except when b turns out to be 1 or 2. Due to this variation of b, the value found for E always has some error which increases systematically as the fitted b deviates farther away from 1 and 2. Parallel to the error in E, the figure of merit (FOM) of the fit also deteriorates. The paper also discusses the relevance of other fitted parameters, both in GO and MO fits.

General-order kinetics of thermoluminescence and its physical meaning

Thermoluminescence glow peaks are calculated numerically for a one-trap - one-recombination-centre model using a generalized approach. Except in extreme cases the peaks are seen to change in position and shape with a change in dose. These glow peaks are fitted to the general-order kinetics model and the values of the kinetic parameters, namely the activation energy, pre-exponential factor and order of kinetics are determined by finding the best fit. In this way an attempt is made to correlate the empirical parameters with the physically meaningful ones in the framework of the adopted model. The fitted value of the activation energy matches reasonably its input value used in the generalized approach model calculations. The fitted values of the order of kinetics and the pre-exponential factor parameters change with the initial occupancy of the traps (dose) in all cases in which the order of kinetics (KO) is found to be in the range in which 1< KO< 2. The KO decreases with increasing trap occupancy whereas the pre-exponential factor increases. The latter parameter undergoes a change also in its units. It is observed that, when the found value of the KO is such that 1< KO<2, the best fitted general-order kinetics peak deviates significantly from the computed peak. A better fit is found with two peaks, one of which is approximately of first order and the other approximately of second order. The KO parameter at saturation dose has been correlated with the ratio of the re-trapping and recombination cross sections. These theoretical results are discussed in the perspective of experimental observations in general. Plausible reasons for disagreements between theory and experiment are also discussed.

Optical absorption and electron spin resonance in blue and green natural beryl

A comparative study of blue and green beryl crystals (from the region of Governador Valadares, Minas Gerais, Brazil) using electron paramagnetic resonance (EPR) and optical absorption (OA) spectroscopy is reported. The EPR spectra show that Fe3+ in blue beryl occupies a substitutional Al3+ site and in green beryl is localized in the structural channels between two O6 planes. On the other hand the infrared spectra show that the alkali content in the blue beryl is mostly at substitutional and/or interstitial sites and in green beryl is mostly in the structural channels. The OA spectra show two types of Fe2+. Thermal treatments above 200° C in green beryl cause the reduction of Fe3+ into Fe2+ accompanied by a change of color to blue. The blue beryl color does not change on heating. The kinetics of the thermal conversion of Fe3+ into Fe2+ is composed of two first order processes; the first one has an activation energy ΔE 1=0.30 eV and the second one has an activation energy ΔE 2=0.46 eV.

A critical look at the kinetic models of thermoluminescence: I. First-order kinetics

Using a generalized scheme of multiple traps, thermoluminescence (TL) glow curves are calculated for different sets of systems parameters. In particular, the conditions under which glow peaks of first-order kinetics are produced are highlighted. The major findings and conclusions are as follows. (1) In the generalized scheme the glow peaks always reduce to first order at low trap occupancies. It is therefore suggested that the peak analysis to determine the parameters should be carried out only at low doses. (2) Glow peaks which follow first-order kinetics can be obtained irrespective of whether the recombination rate is faster, equal to or slower than the retrapping rate (Rret). (3) Quasi-equilibrium (QE) of free carriers in the delocalized band, which is the essential condition for the derivation of the conventional analytical expressions of TL and thermally stimulated conductivity, can be realized irrespective of whether RrecRret. (4) The realization of the QE condition depends on the concentrations of the traps and the recombination centres (RCs) and their cross sections for free carrier capture. It is discussed and shown that, in doped insulating and semiconducting materials, the values of these parameters are sufficiently high for the QE condition to be comfortably held. It is thus concluded that the doubts raised by earlier workers regarding the validity of the QE assumption in the derivation of the analytical expressions are unnecessary as far as these materials are concerned. (5) It is shown that a system in which some of the untrapped charge carriers recombine within the germinate centres and some become delocalized may satisfactorily explain the mechanism of TL emission in most of the phosphors. The properties of first-order, supralinearity and pre-dose sensitization may be easily explained under the framework of this system. (6) Conclusions (2) and (3) above disprove those of earlier workers who had concluded that QE and fast retrapping together do not form a consistent set of conditions and that the apparent dominance of first-order kinetics in nature is due to slow retrapping.

'Universal' curve of ionic conductivities in binary alkali silicate glasses

In the past 25 years, there has been an increasing interest in ionic-glass conductors, mainly motivated by the discovery of new glasses, attempts to produce new solid-state batteries, and the search for an ‘universal’ theory of iontransport in glassy materials. It is well-known that the ionic conductivity increases rapidly when a network glassformer is modified by the addition of an alkali metal. Despite considerable experimental and theoretical attempt, there is currently no consensus regarding the diffusion mechanism [1]. Numerous models have been proposed, and they vary from thermodynamics with principles in models for liquid electrolytes, such as the weak electrolyte model [2], to models based on solid state concepts such as the jump diffusion model [3], the strong electrolyte model [4], and the dynamic structure model [5]. Ionic conductivity σ in glass is a thermally activated process of mobile ions that overcome a potential barrier EA, of the form:

Test for quasi-equilibrium in thermally stimulated luminescence and conductivity

The uncertainty about the existence of quasi-equilibrium (QE) condition during the readout of thermoluminescence and thermally stimulated conductivity glow peaks in real materials causes doubts about the validity of applying the QE-based analytical expressions to analyse these glow peaks. In this paper a simple method is given to verify the QE condition during the readout of these glow peaks in real materials. The method is based on changes in the glow peak shapes caused by changing the heating rate in non-QE cases. It is illustrated by using synthetic glow peaks derived from two different physically meaningful models.

Anomalies in the determination of the activation energy of thermoluminescence glow peaks by general-order fitting

Thermoluminescence glow curves calculated from three physically meaningful models are fitted into the empirical general-order (GO) kinetics expression to find the fitted values of activation energy E and the kinetic order b. The results show that the figure of merit of the fit deteriorates progressively as the best fitted value of b deviates from 1 and 2. At the same time the fitted E value also departs from its true value. This result is used to reinterpret the observed change in E of glow peak 5 of LiF (TLD-100) in post-irradiation annealed samples. It is concluded that the application of GO kinetics to glow peak 5 of LiF (TLD-100) leads to inconsistencies and that the changes in E found by GO fitting may not be real.