T. Karali

A SIMPLE ROUTE TO SILICON-BASED NANOSTRUCTURES

.Powders of the nanoparticles were obtained from the colloidal solutions by removing the water with a rotary evaporator (bath temperature 50 C). Transmission electron micrographs of the samples were taken using a Philips CM 300 UT electron microscope, working at 300 kV acceleration voltage. A Philips Xpert system was used to measure the X-ray diffraction pattern of powder samples. UV-vis absorption spectra of the colloidal solutions were recorded with a Lambda 40 spectrometer (Perkin‐Elmer). Photoluminescence spectra were recorded with a Spex Fluoromax 2 spectrometer having a spectral resolution of 0.5 nm.

Rare-earth-size effects on thermoluminescence and second-harmonic generation

The substitution of rare-earth ions into insulating host crystals introduces lattice strains and, for non-trivalent sites, a need for charge compensation. Such effects alter the site symmetry and this is reflected in properties such as the wavelength, linewidth, lifetime and relative intensity of the rare-earth transitions. Equally clear, but less well documented, is the influence on second-harmonic generation (even from cubic crystal lattices). For example, in bismuth germanate, second-harmonic generation efficiency varies by factors of more than 100 as a result of different rare-earth dopant ions. The ions are variously incorporated as substitutional ions, pairs, clusters, or even as precipitates of new phases, but the detailed modelling is often speculative. This article summarizes some recent studies which explore the role of rare-earth ions in thermoluminescence and second-harmonic generation. There are numerous differences in glow peak temperature, for nominally the same defect sites, which are thought to indicate charge trapping and recombination within coupled defect sites, or within a large complex. Size and cluster effects can be modified by heat treatments. This review considers the similarity and trends seen between numerous host lattices which are doped with rare-earth ions. For thermoluminescence there are trends in the variation in glow peak temperature with ion size, with movements of 20 to 50 K. Examples are seen in many hosts with extreme effects being suggested for zircon, with peak shifts of 200 K (probably from precipitate phases).

Study of spontaneous deposition of 210Po on various metals and application for activity assessment in cigarette smoke

Abstract 210Po in environmental samples has been accumulated on various metals using the chemical electrodeposition technique. The 210Po was deposited spontaneously on metal discs after decomposition of the sample in HCl acid and then measured using a ZnS(Ag)α-particle detector. Silver was observed to have the highest deposition efficiency when 210Po deposition on different metals was studied. Copper discs were used in the application of activity assessment in cigarette smoke as this is more available and more economical although silver has a higher efficiency than that of other materials examined. The technique is applied for the analysis of 210Po in tobacco, ash and butt for several brands of imported and domestic cigarettes smoked in Turkey.

TL and EPR studies of Cu, Ag and P Doped Li2B4O7 Phosphor

Key characteristics of a newly prepared tissue-equivalent, highly sensitive thermoluminescence dosimeter, Li2B4O7:Cu,Ag,P, are presented. The material was developed at the Institute of Nuclear Sciences, Belgrade, in the form of sintered pellets. A new preparation procedure has greatly increased the sensitivity of the basic copper activated lithium borate and the glow curve of Li2B4O7 : Cu,Ag,P consists of a well-defined main dosimetric peak situated at about 460–465 K with a sensitivity which is about four to five times higher than that of LiF : Mg,Ti (TLD-100). The exceptionally good response features of Li2B4O7 : Cu,Ag,P are attributed to the incorporation of Cu as a dopant. Both low and high temperature emission spectra are presented and the origins of the various emission bands are considered. Additional data are provided from electron paramagnetic resonance measurements.

Spectral comparison of Dy, Tm and Dy/Tm in thermoluminescent dosimeters

Spectrally resolved thermoluminescence of co-doped Dy:Tm: shows that the maximum peak temperature for the nominal C dosimetry peak differs by C for Dy and Tm emission. This is interpreted as evidence that the rare earth ions form part of a complex defect which variously provides both the charge trapping and, during heating, the radiative decay. The peaks have the same activation energies but different pre-exponential factors. Modifications of the material by thermal treatments using furnace or laser pulse heating convert the state of dispersion of the rare earth ions between isolated, pair or defect clusters, which alter the dosimetry efficiency. In some cases the modified geometries are detectable via movement of the emission lines. For rapidly quenched materials, discontinuities in the thermoluminescence responses are suggested to be indicative of new microcrystalline phases. Slow cooling degrades the efficiency but also indicates the presence of further thermoluminescence glow peaks within the region of the main dosimetry signal. Pulsed laser heating with a UV laser altered the glow curve and resulted in strong signals. Mechanisms for this process are considered.

Influence of heating rate on thermoluminescence of Mg2SiO4 : Tb dosimeter

In this paper the influence of different heating rates on thermoluminescence (TL) kinetics, glow-peak temperatures, heights and integrated TL intensities of the main glow peak of recently produced Mg2SiO4 : Tb dosimeters are reported. The TL properties of the phosphor were investigated after β-irradiation at room temperature. Glow curve shape, peak shape, repeated initial rise and variable heating rate methods were used to analyse the TL kinetics of the main glow peak at 473 K. Results show that the TL glow peak of Mg2SiO4 : Tb phosphor obeys second-order kinetics. Analysis of the main dosimetric peak by using the methods mentioned above revealed that activation energy (E) is about 1 eV and the pre-exponential factor (s) is in the range 1010–1011 s−1. In addition it is ascertained that the glow-peak temperature increases with heating rate. The glow-peak height showed two maxima in the heating rate range investigated. The behaviour of the integrated TL intensities and the maximum glow-peak temperatures related to the heating rate have been discussed in terms of thermal quenching and clustering formation.

Radioluminescence and thermoluminescence of rare earth element and phosphorus-doped zircon

Abstract The radioluminescence and thermoluminescence spectra of synthetic zircon crystals doped with individual trivalent rare earth element (REE) ions (Pr, Sm, Eu, Gd, Dy, Ho Er, and Yb) and P are reported in the temperature range 25 to 673 K. Although there is some intrinsic UV/blue emission from the host lattice, the dominant signals are from the rare-earth sites, with signals characteristic of the REE3+ states. The shapes of the glow curves are different for each dopant, and there are distinct differences between glow peak temperatures for different rare-earth lines of the same element. Within the overall set of signals there are indications of linear trends in which some glow peak temperatures vary as a function of the ionic size of the rare earth ions. The temperature shifts of the peaks are considerable, up to 200°, and much larger than those cited in other rare-earth-doped crystals of LaF3 and Bi4Ge3O12. The data clearly suggest that the rare-earth ions are active both in the trapping and luminescence steps, and hence the TL occurs within localized defect complexes that include REE3+ ions.

Effect of heating rate on kinetic parameters of β-irradiated Li2B4O7:Cu,Ag,P in TSL measurements

The effect of heating rate on the thermally stimulated luminescence (TSL) emission due to the temperature lag (TLA) between the TSL material and the heating element has been investigated using Li2B4O7:Cu,Ag,P dosimetric materials. The TLA becomes significant when the material is heated at high heating rates. TSL glow curves of Li2B4O7:Cu,Ag,P material showed two main peaks after β-irradiation. The kinetic parameters, namely activation energy (E) and frequency factor (s) associated with the high temperature main peak of Li2B4O7:Cu,Ag,P were determined using the method of various heating rates (VHR), in which heating rates from 1 to 40 K s−1 were used. It is assumed that non-ideal heat transfer between the heater and the material may cause significant inconsistency of kinetic parameter values obtained with different methods. The effect of TLA on kinetic parameters of the dosimeter was examined.

Recent examples of cathodoluminescence as a probe of surface structure and composition

Abstract Cathodoluminescence (CL) provides a sensitive analytical probe of the near-surface region of insulating materials, and some new examples of the strengths of the technique are presented using recent data from the University of Sussex. Analysis of float glass shows that by spectral and lifetime resolved data it is possible to separate the emission bands from excitonic, intrinsic imperfections, and impurities in various valence states, as a function of their depth beneath the surface. Correlation of the CL data with those from Mössbauer, ion beam and other analyses then provides the basis for models of the defect sites. CL from a second glass, ZBLAN, reveals the presence of microcrystallites and growth defects, and the work underpins confidence in the high purity gas levitation method of ZBLAN production. New results on CL of wavelength shifts with crystal field of Mn in carbonates are presented, and of Nd emission from Nd:YAG. The effects are directly linked to surface damage and dislocations caused by sample preparation steps of cutting and polishing. Methods to minimise the damage, by furnace or pulsed laser annealing, and chemical routes, are mentioned. Such surface preparation damage has a profound effect on all CL monitoring, whether for fundamental studies or mineralogical applications. Finally, a route to eliminate such problems is demonstrated, with consequent improvements in luminescence, transmission and laser performance of surface waveguides. The implications of improved surface quality range widely from mineralogical CL imaging through improved photonic materials and epitaxial growth to elimination of surface damage, and additional information.

Luminescence spectra of CaSO4 with Ce, Dy, Mn and Ag codopants

Thermoluminescence (TL) and radio-thermoluminescence spectral analysis techniques have been applied to doped calcium sulphate samples designed for radiation measurements at elevated temperatures. CaSO4 :Dy, when co-doped with Ag, provides a TL dosimetric peak near 350 °C which is useful for radiation measurements at high temperatures. Dopants of Ce, Mn and Dy variously move the peak temperature from 400 °C to 200 °C. Each dopant ion gives a characteristic emission spectra, which for CaSO4 :Ce, Mn samples indicate that there is a systematic temperature difference of ~7 °C between the glow peaks from the Ce and Mn sites. The CaSO4 :Dy samples show a discontinuity in the emission wavelength from the Dy ions near T = 200 °C and a decrease in the radioluminescence fluorescence in the same temperature region. In each case it is proposed that the dopants form part of large, complex defects, instead of isolated trapping and recombination centres. The data offer further evidence for a localized phase transition of the defect complex at 200 °C. Low-temperature data, from 20 K, show similar differences in the peak temperature from the various dopants and additionally indicate reproducible discontinuities in the wavelength positions and intensities, for all samples, at T = 230 K. This again suggests structural phase adjustments of the defect sites.