Measuring modulated luminescence using non-modulated stimulation: ramping the sample period

Conventional methods of recording linearly modulated (LM) optically stimulated luminescence (OSL) require control over either the exciting light intensity, or the ability to pulse the source. For many light sources (e.g. constant-power CW lasers, arc lamps and synchrotrons) this can be problematic. Directly analogous results to LM-OSL can, however, be achieved with non-modulated excitation sources, by ramping the sample period (RSP) of luminescence detection. RSP-OSL has the distinct advantage over LM-OSL in that, since the excitation remains at full power, data accumulation times (that can be considerable) can be reduced by typically 50%. RSP methods are universally applicable and can be employed, for example, where the excitation source is constant heat, rather than light: here, iso-thermal decay of phosphorescence becomes recorded as a sequence of peaks, corresponding to de-trapping of charge from different defect levels, and is particularly useful for analysing shallow-trap effects. RSP methods are also useful in providing significant compaction of data sets, where signal analysis is required of overlapping systems having a wide range of decay kinetics.     

Thermo-optical properties of optically stimulated luminescence in feldspars

Optically stimulated luminescence processes in feldspars are subject to competing thermal enhancement and quenching processes: this article describes the thermal enhancement effects for orthoclase, albite and plagioclase feldspars. It is demonstrated that certain lattice vibrational modes can be selectively probed at specific optical excitation energies. The results are described in terms of the Bohr hydrogen model of the OSL donor defects.     

An optically stimulated luminescence study of flint related to radiation dosimetry

This article describes the essential features of photo-stimulated luminescence in flint. The emission is shown to be complex, arising from a number of different aspectsof the material. Under constant illumination, dose-dependent time-decaying anti-Stokes signals are superimposed on dose-independent, time-stable emission that is both Stokes and anti-Stokes shifted in energy. It is argued that the dose-dependent signals arise from the crystalline quartz phase of the material, whereas the dose independent signals originate from an amorphous-like phase of SiO₂. The microcrystallinity of the material leads to contrasting behaviour when compared with bulk crystalline quartz.     

Luminescence sensitivity changes in quartz as a result of annealing

Retrospective dosimetry using optically stimulated luminescence (OSL) on quartz extracted from (for example) bricks needs to account for strong OSL sensitivity changes that are known to occur depending on the previous thermal treatment of the sample. Non-heated quartz exhibits OSL orders of magnitude less per unit radiation than that for heated material. The reason these temperature-induced sensitivity changes occur in quartz is presently not well understood. This phenomenon is also seen in the related area of luminescence dating in which sedimentary quartz and quartz from heated archaeological samples show very different OSL sensitivities. In this paper we report on studies of the effects of high temperature annealing on the OSL and phototransferred TL (PTTL) signals from sedimentary and synthetic quartz. A dramatic enhancement of both OSL and PTTL sensitivity was found especially in the temperature range 500–800°C. Computer simulations of the possible effects are shown to produce data that agree in all essential details with the experimental observations. It is further demonstrated that the enhanced OSL sensitivity as a function of annealing temperature is not a pre-dose effect.     

Excitation and emission spectrometry of stimulated luminescence from quartz and feldspars

A new scanning monochromator was designed to investigate the wavelength-resolved luminescence excitation and emission characteristics of quartz and feldspar samples; results are presented, together with detailed measurements of the thermal activation energies required for luminescence stimulation in feldspar over the wavelength range 400–1000 nm. When coupled to automated dating equipment, we demonstrate that optical characteristics of samples selected for dating can be routinely measured, or more detailed analysis of physical processes can be made.     

Steady vortical flow around a finite plate

An explicit solution for a layer of fluid with constant vorticitysurrounding a thin plate of finite length is obtained usingelementary conformal mapping methods. In the limit of largeplate length the behaviour of the solution near the ends ofthe plate tends to that of the previously known solution fora semi-infinite plate. Contour dynamics is used to investigatethe stability of the steady solutions.     

A continuous OSL scanning method for analysis of radiation depth-dose profiles in bricks

This article describes the development of a method for directly measuring radiation depth-dose profiles from brick, tile and porcelain cores, without the need for sample separation techniques. For the brick cores, examples are shown of the profiles generated by artificial irradiation using the different photon energies from ¹³⁷Cs and ⁶⁰Co gamma sources; comparison is drawn with both the theoretical calculations derived from Monte Carlo simulations, as well as experimental measurements made using more conventional optically stimulated luminescence methods of analysis.     

Electronic and optical properties of magnesium phthalocyanine (MgPc) solid films studied by soft X-ray excited optical luminescence and X-ray absorption spectroscopies

In a study to link the optical and structural properties of solid films of magnesium Phthalocyanine (MgPc), a range of synchrotron based spectroscopic methods have been used. These include X-ray excited optical luminescence (XEOL) together with X-ray absorption spectroscopy (XAS) measured both by total electron yield methods (TEY) and by using the optically detected photoluminescence yield method (PLY). XEOL spectra below K shell threshold show a broad emission peak at approximately 860 nm which can be attributed to the optical Q-band of these organic systems, which is then suppressed above the threshold. The shift to higher wavelength compared to optical emission spectra from MgPc in solution is consistent with intermolecular coupling of the excited states in the loosely intermolecular bonded phthalocyanine crystal structure. Zero order total PLY spectra at both C and N K edges are compared to TEY spectra where at the C K edge an inversion of intensity ratios between features is observed. Wavelength-specific PLY absorption spectra taken at 860 nm at the N K edge show a role for sigma* states participating in the luminescence process possibly through the sigma-like lone pair of bridging nitrogen atom, denoted the n --> pi* transition.     

Luminescence emission from metastable Sm2+ defects in Y PO4:Ce,Sm

Ce(3+) and Sm(3+) both form stable defect centres in Y PO(4), and their emission properties are well known. However, by irradiating co-doped Y PO(4):Ce,Sm with x-rays or UV light the charge states of the defects can be modified to become Ce(4+) and Sm(2+), which are metastable, and their behaviour acts as a model system for understanding carrier dynamics in charge storage phosphors. Here we report on the luminescence emission behaviour of the Sm(2+) defects that can be observed after x-irradiation. Under suitable excitation conditions, emission from both the stable Sm(3+) and metastable Sm(2+) can be monitored simultaneously. The Sm(2+) luminescence is found to be comprised of a series of narrow lines in the energy range 1.5-1.8 eV, identifiable as internal 4f-4f transitions, accompanied by a series of phonon replicas (phonon energy, 20.4 meV). The intensity of the metastable Sm(2+) emission increases in proportion to the x-irradiation time as their population is increased; under 1.92 eV laser stimulation, the PL is found to fully quench at temperatures above 150 K due to photo-thermal ionization of the defect.     

An optically stimulated luminescence study of porcelain related to radiation dosimetry

This article describes the essential features regarding the photo-stimulated luminescence of porcelain: both the main ceramic and glazing materials are studied. In each case, radiation dose dependent signals are observed, superimposed on dose independent luminescence transitions that are both Stokes and anti-Stokes shifted in energy. Glazing is shown in some cases to be considerably more sensitive as a radiation dosemeter than the main porcelain ceramic. By comparison with the properties of artificial phosphors, the principal luminescent matrix is identified as being Al₂O₃, and the recombination centres in one glazing type are shown to be F-centres, Cr³⁺ and Dy³⁺.