On the quasi-equilibrium problem in thermally stimulated luminescence and conductivity

Strong doubts havc been expressed about the validity of the quasi-equilibrium (QE) assumption used in the derivation of the analytical expressions of thermoluminescence (TL). So far there is no established method available to check if QE actually prevails during the emission of an experimental TL signal. The present study shows that the level of QE changes with a change in the heating rate beta. The change in the level of QE in its turn gets reflected in a change in peak shape when the system turns to a non-QE condition. This property is used as the first ever experimental method to test whether or not the emission of a given glow peak occurs under the QE condition. An essential condition for holding the QE condition is found to be T(R)/taum> or = 10(-3) where T(R) and taum are the glow peak recording duration and the maximum value of the free carrier lifetime, respectively. This relation between T(R) and taum is useful in finding the approximate value of taum. The value of taum being a function of the concentration and cross section of the TL related centres, one may be able to assess these basic parameters from the study of TL glow curves. The theoretical results are discussed in the perspective of LiF (TLD-100).     

Determination of ionisation energies and attempt-to-escape factors using thermally stimulated conductivity

A procedure for determining an arbitrary distribution of activation energies (E) and attempt-to-escape frequencies (s) from overlapping contributions to thermoluminescence (TL) or thermally stimulated conductivity (TSC) is described. For the case of no retrapping, i.e. first order kinetics, the glow curve can be described by a two-dimensional Fredholm equation representing a superposition of Randall-Wilkins first-order peak shapes. The solution to this equation gives the distribution of trapping energies and attempt-to-escape-frequency factors necessary to obtain the TL or TSC peak shape. Analysis of simulated TL/TSC data for trap distributions distributed in both E and s demonstrates that the arbitrary E and s values can be determined from the solution of the Fredholm equation. The procedure is demonstrated for experimental TSC data from gamma-irradiated Al2O3:C.     

Optically stimulated luminescence dosimetry performance of natural Brazilian topaz exposed to beta radiation

Optically stimulated luminescence (OSL) has become the technique of choice in many areas of dosimetry. Natural materials like topaz are available in large quantities in Brazil and other countries. They have been studied to investigate the possibility of use its thermoluminescence (TL) properties for dosimetric applications. In this work, we investigate the possibility of utilising the OSL properties of natural Brazilian topaz in dosimetry. Bulk topaz samples were exposed to doses up to 100 Gy of beta radiation and the integrated OSL as a function of the dose showed linear behaviour. The fading occurs in the first 20 min after irradiation but it is <6% of the integrated OSL measured shortly after exposure. We conclude that natural colourless topaz is a very suitable phosphor for OSL dosimetry.     

Dosimetric characterisation of Brazilian natural stones using the thermally stimulated exoelectron emission technique

A thermally stimulated exoelectron emission (TSEE) measuring system developed and constructed at IPEN was used to verify the feasibility of the use of Brazilian natural semi-precious stones, such as jasper. amethyst, agate (blue and rose) and quartz (rose and white) for gamma and X radiation detection. Its counting system consists of a 2pi windowless gas-flow proportional counter, and the heating system is formed by a temperature programmer that provides linear heating of the samples. The samples were tested in gamma and in X radiation beams and evaluated in relation to their main dosimetric characteristics, as TSEE glow curves, calibration curves and energy dependence. The TSEE results are compared with those of thermoluminescence. The results obtained show the usefulness of Brazilian natural stones as dosimetric materials, using the thermoluminescence and thermally stimulated exoelectron emission techniques.     

Structural characterization and thermally stimulated discharge conductivity (TSDC) study in polymer thin films

The electrical conductivity of naphthalene doped polystyrene (PS) films (≈61·58 μm thick) was studied as a function of dopant concentration and temperature. The formation of charge transfer (CT) complexes and strong concentration dependence of carrier mobility point out that the current carriers are transported through doped polymer system via hopping among sites associated with the dopant molecules. The activation energy, E _a, was calculated from the graph of log σ vs 10³/T plot within low and high temperature regions.     

Performance of pellets and composites of natural colourless topaz as radiation dosemeters

The aim of the present work is to investigate the possibility of using the properties of the thermoluminescent emission (TL) of Brazilian natural topaz for dosimetric applications. Topaz is an aluminium fluorsilicate with general composition of Al2(SiO4)(F,OH)2 found with relative abundance in Brazil and in other parts of the world. Topaz from Santo Antonio do Jacinto, Minas Gerais. Brazil, was used in this work, in the form of pellets of topaz mixed with Teflon and composites made with topaz embedded in a glass matrix. The TL sensitivity was tested between 10(-4) and 10(4) Gy. The TL peak intensity increases with the dose before saturation, which occurs around 2 kGy. The peak intensity showed a strong dependence with radiation energy. The effect of visible light and the behaviour of the TL signals after successive irradiation-reading-annealing cycles are presented and discussed.     

On the advantages and disadvantages of optically stimulated luminescence dosimetry and thermoluminescence dosimetry

The use of thermoluminescence as a method for the dosimetry of ionising radiation has been established for many decades and has been unquestionably successful. It is therefore difficult to imagine how any new technique could easily supplant it. Perhaps optically stimulated luminescence dosimetry should not be characterised as an entirely new technique, but rather a development of the well established technology that may be considered superior in some respects. As is obvious from the tenor of this debate, our two participants are longtime colleagues who have had numerous discussions on this topic. No doubt, many of us will have similar discussions in our own institutions as we weigh the advantages and disadvantages of TLD versus OSL.     

Thermal quenching of thermoluminescence in natural quartz

The thermal quenching properties of the thermoluminescence (TL) from several natural quartz specimens were examined, at two different wavelengths, using the heating rate dependence of the TL signal. As the heating rate varies the TL peaks shift to different temperatures and become affected by thermal quenching to different extents. In this work the heating rate was varied over several orders of magnitude and, through deconvolution of the TL glow curve the behaviour of the main TL peaks was followed as a function of the temperature at which the peak appeared in the glow curve. Through an analysis of the glow peak areas as a function of glow peak temperature the decrease in the efficiency of TL production with increasing temperature could be monitored, at the different emission wavelengths. The analysis supports the contention that the quenching phenomenon is a result of a Mott-Seitz mechanism.     

Methods for kinetic analysis of thermally stimulated processes

Several methods are known for the evaluation of the main kinetic parameters related to a thermoluminescence (TL) curve, namely the activation energy, the pre-exponential factor and the kinetic order. These methods can easily be applied under certain conditions to a series of related thermally stimulated phenomena which are governed by similar differential equations. These include thermally stimulated conductivity (TSC), thermally stimulated electron emission (TSEE), ionic thermocurrent (ITC), derivative thermogravimetry (DTG), differential thermal analysis (DTA), thermal desorption and other phenomena. The similarities and differences between these phenomena are considered and the applicability of the various methods to the different cases is discussed.     

Electrical conductivity of thermally grown titanium oxide films

TheV-I characteristics of thermally grown titanium oxide films on titanium are described and are explained using a model for current transport in the films.     

Analysis of thermally stimulated discharge data on amorphous semiconductors

This paper deals with the most common methods of interpretation of the thermally stimulated discharge (TSD) spectrum. TSD due to dipole disorientation, calculation of activation energies using initial rise method, graphical integration, and methods based on the variation of heating rates are described in some detail. It will be argued that calculation of activation energies from these methods strictly holds for relaxation of a single well-defined frequency. For a distributed relaxation, methods based on the variations of the heating rates are more appropriate, but certain difficulties may arise.     

Thermal conductivity and thermally induced charge imbalance in heavy-fermion systems

We study the effects of a magnetic field and a supercurrent flow on the thermal conductivity of nonconventional superconductors with an energy gap vanishing at either points or on a line or lines on the Fermi surface. A study of the thermally induced charge imbalance is also presented. Our calculations are done using a Boltzmann equation approach, and elastic scattering by nonmagnetic impurities is assumed to be the main source of relaxation; the relaxation rate is calculated in the low-temperature limit, assuming the phase shift of the scatterers to be /2 in the normal state. Our calculations indicate that the effects we predict are large enough to be experimentally observable.     

Characterisation and utilization of natural coconut fibres composites

Two thousand fibres were randomly taken from a coir fibre stack, characters of the fibres were analysed. It was shown that length of the fibres was in the range between 8 and 337 mm. The fibres amount with the length range of 15–145 mm was 81.95% of all measured fibres. Weight of fibres with the length range of 35–225 mm accounted for 88.34% of all measurement. The average fineness of the coir fibres was 27.94 tex. Longer fibres usually had higher diameters.Composite boards were fabricated by using a heat press machine with the coir fibre as the reinforcement and the rubber as matrix. Tensile strength of the composites was investigated.     

Theoretical considerations of thermally stimulated discharge techniques in amorphous semiconductors

Thermally stimulated discharge of a previously polarized and electroded dielectric, can generate a current with several peaks. The locations of the peaks along the thermally stimulated discharge current spectrum are characteristics of the particular mechanisms for the decay. Systematic analysis of the current peaks will yield information such as dipole relaxation characteristics and activation energies for intrinsic conduction or trapping parameters of electronic charges in the dielectric. When multilayer dielectrics such as amorphous semiconductor photoreceptors are subjected to an electret formation cycle, the heterogeneity in their structures may cause several polarization effects. For example, discontinuities in the intrinsic conductivities and dielectric constant in amorphous selenium (a-Se)-based multilayer photoreceptors can lead to the accumulation of space charges at the interfaces of the individual layers whenever the device experiences an electric stress for a period of time which is of the order of its effective dielectric relaxation time. Charge trapping by states associated with the heterogeneities of the structure cause an electrical polarization which can have a significant impact on the xerographic performance of the photoreceptor. The purpose of the present series of papers is firstly to describe the principles of thermally stimulated discharge techniques, the associated theories and interpretation of the current spectrum and secondly, to discuss applications of these techniques to a-Se:Te/Se double layer photoreceptors. The principles of thermally stimulated discharge and relevant theories are discussed.     

Optically and thermally stimulated luminescence characteristics of MgO:Tb3+

In this paper main optically stimulated luminescence (OSL) and thermoluminescence (TL) characteristics are presented of a newly synthesised material MgO doped with terbium (Tb) developed at the Institute of Nuclear Science, Vinca. A thermally stimulated emission spectrum showed the characteristic lines of Tb3+ in a wide range of wavelengths. The TL sensitivity of the main TL glow peak at 315 degrees C is 1.7 times higher than the TL of Al2O3:C. The highest OSL sensitivity was obtained under green lamp (500-570 nm) stimulation. The fast component in the OSL decay curve is 2.4 times faster than Al2O3:C. The OSL signal is linear with dose up to 10 Gy. The lower limit of detection was found to be 100 microGy. These first results show that the newly synthesised material has some promising properties for the application in radiation dosimetry.     

Performance of CVD diamond as an optically and thermally stimulated luminescence dosemeter

Diamond is a material with extreme physical properties. Its radiation hardness, chemical inertness and tissue equivalence qualify it as an ideal material for radiation dosimetry. In the present work, the optically stimulated luminescence (OSL) and thermoluminescence (TL) characteristics of a 10 microm thick CVD diamond (polycrystalline diamond films prepared by chemical vapor deposition) film were studied in order to test its performance as a beta radiation dosemeter. The TL response is composed of four main TL glow peaks; two of these are in the range of 150-200 degrees C and two additional peaks in the 250-400 degrees C temperature range. The integrated TL as a function of radiation dose is linear up to 100 Gy and increases with increasing dose exposure. The dose dependence of the integrated OSL exhibits a similar behavior. The observed OSL/TL behavior for the CVD diamond film clearly demonstrate its capability for applications in radiation dosimetry with special relevance in medical dosimetry owing to the diamond's intrinsic material properties.     

Studies of thermally stimulated currents and surface charge in polystyrene thin films

A careful study was undertaken of the initial charge developed on the non-metallized surface and of short-circuit thermally stimulated currents (TSCs) in unilaterally metallized polystyrene (PS) films 25 microm thick. The films had been negatively charged by Townsend breakdown at a voltage of −5 kV at various temperatures ranging from 90 to 130°C. The surface charge densities observed were of the order of 10^-8 C cm^-2, corresponding to full trap densities of the order of 10¹⁴ cm⁻³. The short-circuit TSC spectra of newly charged samples show a single peak at a temperature between 98 and 102°C. The thermal activation energy associated with this peak was found to be 1.2 eV. An analysis of TSCs indicates that electrons are subject to fast retrapping in PS. The ratio of the mean charge depth to the sample thickness is found to increase from 0.069 to 0.12 with the increase in the temperature of polarization, indicating a higher charge accommodation at higher temperatures of polarization. The trap-modulated mobility values for electrons at the TSC peak temperatures are the order of 10⁻¹² cm² V⁻¹s⁻¹. Such low mobility values at peak temperatures are considered to indicate the excellent charge storage properties of PS.     

Influence of thermal treatment on the structure and electrical conductivity of thermally expanded graphite

Thermally expanded graphite (TEG) is a promising filler beneficial to electrically conductive materials due to its high electrical conductivity, low density, and cost. In this work, the electrically conductive TEG was prepared by thermal treatment of the expandable graphite in the range of temperatures from 400 to 800 °C in air. Effects of the temperature treatment on the morphology and chemical structure of TEG were thoroughly characterized. Thermal treatment of the expandable graphite resulted in thermally expanded graphite formation with up to 6 times higher electrical conductivity than the precursor. Optimal conditions of thermal treatment were established at 600 °C providing material with the highest electrical conductivity, high expansion volume, and a well-ordered and defect-less structure.