WO2016204634A1

WO2016204634A1 - The method of band shape stimulation of optically stimulated luminescence

Info

Publication number: WO2016204634A1
Application number: PCT/PL2015/000164
Authority: WO
Inventors: Alicja CHRUŚCIŃSKA
Original assignee: Nicolaus Copernicus University In Toruń
Priority date: 2015-06-18
Filing date: 2015-10-14
Publication date: 2016-12-22
Also published as: PL412756A1; PL229653B1

Abstract

What is characteristic for the method of band shape stimulation of optically stimulated luminescence is that luminescence (OSL) and selective detection of specific constituents of the signals produced occur when the spectrum and its intensity are changed at the same time, that is when at least two sources of light of different spectra are used together with simultaneous or consecutive exposure to light of the sample studied, while the intensity of the light of those two sources is changed by a set value.

Description

The method of band shape stimulation of optically stimulated luminescence

The subject matter of the innovation is a method of band shape stimulation of optically stimulated luminescence, which can be used in dosimetry to determine the dose of ionising radiation absorbed by persons operating various machines, which produce this type of radiation as well as to date sediments, minerals and ceramics in archaeology. The method, which is the subject of the innovation, may also be used in medicine for imaging used in radiological diagnostics.

The phenomenon of optically stimulated luminescence, hereinafter referred to as OSL, results from the fact that in the crystalline structure of non-conductive materials, there are electrons of various energy levels. They are the so-called traps as their energy level depends on the amount of radiation absorbed in the past. Electrons may be trapped in them even for millions of years. Those energy levels are responsible for OSL, a process during which light exposure results in changes of energy levels of electrons, followed by the production of radiation of a specific spectra content. The properties of the electrons describe each constituent of the OSL signal unambiguously. A basic physical quantity, which distinguishes a constituent of the OSL signal, is the optical cross-section of the trap responsible for such a constituent. In case of conventional measurements, this quantity may not described unambiguously as it depends on at least three characteristic parameters of the trap as well as on the stimulation energy and the measurement temperature.

Well-known methods of measuring optically stimulated luminescence (OSL) involve exposing a sample to a beam of light from a single source of a determined and stable spectrum by using light of a constant or linearly increasing intensity in a specific temperature and detection of the luminescence produced together with any changes that occur in it in another spectrum during the time of stimulation of the sample studied, especially disappearance of luminescence after the exposure was stopped or gradually decreased.

Such stimulation methods generate many signal constituents at the same time and do not allow for distinguishing between them effectively. The OSL curve (the decrease of OSL during stimulation) provides data on constituents of OSL, which makes it impossible to determine their properties, for example, to foresee the effects of measurements done with the use of another stimulation band or in a different temperature.

Unexpectedly, it turned out that luminescence (OSL) and selective detection of specific constituents of the signals produced occur when the spectrum and its intensity are changed at the same time, that is when at least two sources of light of different spectra are used together with simultaneous or consecutive exposure to the light of the sample studied, while the intensity of the Hght of those two sources is changed by a set value.

Such a method of changing the shape of the spectrum of the stimulation light results in sequential emptying of the traps and mmimisation of the time intervals, during which the process of emptying various types of traps occurs.

The innovation is illustrated with the following examples and diagrams, which do not limit the scope of its protection.

Example I.

A Band Shape Modulation OSL (BSM-OSL) measurement was done with the use of two sources of stimulation light of the maximal spectral bands of, respectively, 1.7 eV (Zl) and 2.2 eV (Z2), the width of 0.2 eV and the maximum photon bands of I_mas ⁼ 4 x 10¹⁷ photons per second per cm². Fig. 1 and Fig. 2 show the changes in the shape of the stimulation band (Φ), which is the sum of bands of both sources at the time of exposure. Fig. 1 shows the changes of intensity of each source of light standardised to the intensity of light of source Zl at the beginning of the experiment. Fig. 2 shows the spectrum of the total stimulation band (the sum of spectra of both sources at chosen moments). At the beginning of the stimulation, only source Zl gave light. Then, the intensity of its light decreased linearly (the top line in Fig. 1), and the intensity of source Z2 increased in accordance with the function marked with the bottom line in Fig. 1. The OSL detection is conducted with the use of a photomultiplier equipped with filters, which shut off the stimulation light. Detection sensitivity provided by the photomultiplier allows for obtaining OSL measurements of samples weighting a few milligrams.

Fig. 3 and Fig. 4 illustrate changes in the OSL intensity during the time of stimulation as well as changes occurring in the traps responsible for the luminescence observed. The results of the stimulation were modelled with the use of the system of differential equations, which constitute the kinetic model of the OSL phenomenon that assumes the coexistence of two different traps and one luminescent centre in the material studied. It is the simplest model allowing for presentation of the potential of the method of separation of OSL signals of various origins.

It turns out that after 150 s of the stimulation, when almost all traps of an optical depth of 2.5 eV were empty, over 75% of deeper traps remained full (Fig. 4). The OSL curve in Fig. 3 shows two peaks, which unambiguously show that there were at least two types of traps responsible for OSL. A further analysis of the curve allowed for determination of parameters determining the type of a given trap. The analysis involved adjusting the BSM-SOL curve obtained to the sum of two first-order kinetic curves, which are typical for traps of specific parameters.

The example shows that in case of identical light sources, it is possible to modify such parameters as the intensity of a light source (if it is increased, traps are emptied in a shorter period of time) and the tempo of changes in the stimulation band (slower changes result in a shift of the peaks in the direction of shorter time intervals). It is done in order to obtain the most optimal effects for given properties of the material studied. The most effective modification of the BSM-OSL process can be obtained by changing the spectra of the light sources, while maintaining a constant or changeable difference between the maximal limits of the bands.

Example Π.

Fig. 5 and Fig. 6 show measurements of an OSL signal coming from traps of a greater optical depth, obtained by changing the spectra of the stimulation light sources, that is increasing their energy. Two narrower peaks corresponding to the traps were obtained by narrowing the stimulation band and increasing the light intensity. Fig. 5 shows the changes of light sources of maximum spectra bands of, respectively, 2.1 eV (line Zl) and 2.6 eV (line Z2) with the band width of 0.1 eV. Figure 6 shows an OSL curve corresponding to such stimulation. The photon band at the begimiing of the exposure amounted to 8 x 10¹⁸ photons per second per cm². The OSL curve illustrated in Fig. 6 was a result of the OSL simulation process conducted for a model with two traps of a width of 2.9 width and 3.1 eV.

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