Green Light Stimulation Research Articles

Optically stimulated luminescence and thermoluminescence in some Cu+ doped alkali fluoro-silicates

OSL and TL in some Cu doped alkali fluoro-silicates were studied. These investigations show that the materials possess good OSL properties and the sensitivity is comparable to that of the commercial α-Al2O3:C phosphor. Of the several fluoro-silicate phosphors studied Cu doped Na2SiF6 is found to be highly sensitive. The initial OSL intensity averaged over first second using blue and green light stimulation, were found to be 1.2 and 13.3 times respectively of Al2O3:C. The simple preparation procedure, high OSL sensitivity, fast decay, and high photoionization cross-section will make these phosphors suitable for on-line radiation dosimetry, particularly in medical dosimetry applications.

Optically stimulated luminescence at room temperature of hydrothermal K2YF5:Pr3+ crystals

AbstractIn this paper optically stimulated luminescence (OSL) properties of Pr3+‐doped double potassium yttrium fluoride (K2YF5) under green light stimulation at room temperature as a function of the doping concentration are reported. Besides, some characteristics concerning the possible application of this composition as a new OSL dosimeter are presented, namely, the dose–response and fading of the OSL signal. The residual thermoluminescence (TL) response of K2YF5:Pr3+ after the OSL experiment has been investigated in order to establish whether the traps responsible for OSL are also involved in the TL process. Spectral analysis of the OSL emission has been carried out to determine the role played by Pr3+ ions in the radiative relaxation process. Finally, the total OSL output of K2YF5:Pr3+ is compared to that of commercial Al2O3:C.

Altered motor cortex excitability by colour perception as a hint for cross-modal plasticity

Aims: Colour affects not only the emotional state, but also cognitive performance and perception of other sensory modalities such as tone or smell. However, very little is known about the neuronal mechanisms which account for these cross-modal interactions. To explore neurobiological mechanisms of these synaesthetic phenomena, we used transcranial magnetic stimulation (TMS) to assess the impact of colour perception on motor cortex excitability. Material and Methods: Transcranial magnetic stimulation allows to assess non-invasively the integrity and excitability patterns of cortical and subcortical networks. Using this method we studied ten healthy volunteers under three different conditions (no visual stimulation, stimulation with red light, stimulation with green light). Red and green light was matched for colour intensity and brightness. The colours were presented in a randomized order by video goggles. Measurements of cortical excitability started after 5 minutes adaptation and included the assessment of motor threshold (MT), cortical silent period (CSP), paired-pulse inhibition (PPI) and paired-pulse facilitation (PPF). Results: Intracortical inhibition (PPI) was significantly increased during green light stimulation as compared with the baseline condition, while red light had no significant effect on PPI. All other parameters of cortical excitability (PPF, MT, CSP) remained unchanged during red and green light stimulation. Conclusions: These data give first evidence that colour perception may lead to changes in motor cortex excitability as a direct hint for cross modal plasticity.

SU-FF-T-225: Evaluation of Optically Stimulated Luminescence (OSL) Dosimetry for High-Energy Photon and Electron Beam Measurements in Radiotherapy

Purpose: The precision and accuracy of the Optically Stimulated Luminescence(OSL) for determination of absorbed doses delivered by photon and electron beams from linear accelerator was investigated. The dependence of the OSLdose measurements with variations in temperature, field size,dose rate, and energy of the beam was also investigated. This study complements previous investigation on 6MV photons which showed that the OSL technique can provide dose estimates with a precision of 0.7% for a single measurement (1 dosimeter).Method and Materials:OSLdoses were obtained for various depths in a water phantom for photons (6 and 18 MV) and electrons (6 to 20 MeV) and compared to data obtained from ionization chamber. Al2O3:C OSLdosimeters (7mm diameter by 0.3mm thickness) were irradiated using a linear accelerator (Varian 21 EX S/N 2833) at SSD = 100 cm. The OSL measurements were carried out using a Riso TL/OSL‐DA‐15 reader with green light stimulation and using a readout procedure that eliminates dependences on the mass or sensitivity of the dosimeter.Results: Depth‐dose profile for 6 MV photon showed a high precision (0.5%) and accuracy, the difference between the OSLdose and the ionization chamber data being smaller than ±1.1% in all cases. The OSL measurements appear to be independent from variations in temperature, field size, and dose rate, with differences smaller than 1% compared to ionization chamber data for the 6MV photonbeam. The data for the 18 MV photonbeam and the electron beams are still being analyzed and will be presented. Conclusion: The results demonstrate that the OSL of Al2O3:C (dosimeters) provides reliable high‐precision and accurate dose estimates, with no dependence on temperature, field size, or dose rate. The OSL technique with automated readers offers a simple and effortless method, since no precise control of mass of dosimeter is required.

Altered motor cortex excitability by colour perception as a hint for cross-modal plasticity

Aims: Colour affects not only the emotional state, but also cognitive performance and perception of other sensory modalities such as tone or smell. However, very little is known about the neuronal mechanisms which account for these cross-modal interactions. To explore neurobiological mechanisms of these synaesthetic phenomena, we used transcranial magnetic stimulation (TMS) to assess the impact of colour perception on motor cortex excitability. Material and Methods: Transcranial magnetic stimulation allows to assess non-invasively the integrity and excitability patterns of cortical and subcortical networks. Using this method we studied ten healthy volunteers under three different conditions (no visual stimulation, stimulation with red light, stimulation with green light). Red and green light was matched for colour intensity and brightness. The colours were presented in a randomized order by video goggles. Measurements of cortical excitability started after 5 minutes adaptation and included the assessment of motor threshold (MT), cortical silent period (CSP), paired-pulse inhibition (PPI) and paired-pulse facilitation (PPF). Results: Intracortical inhibition (PPI) was significantly increased during green light stimulation as compared with the baseline condition, while red light had no significant effect on PPI. All other parameters of cortical excitability (PPF, MT, CSP) remained unchanged during red and green light stimulation. Conclusions: These data give first evidence that colour perception may lead to changes in motor cortex excitability as a direct hint for cross modal plasticity.

Time-resolved luminescence from annealed synthetic quartz under [formula omitted] pulsed green light stimulation

The influence of annealing on lifetimes has been studied in synthetic quartz annealed at temperatures up to 800°C using time-resolved optical stimulation in the range 20–200°C. Luminescence was stimulated using pulsed 525nm light-emitting-diodes. There is an increase with annealing temperature in the lifetime measured at 20°C for samples annealed beyond 500°C. Further, lifetimes decrease with measurement temperature in a manner consistent with the thermal quenching of luminescence.

The response of thermally and optically stimulated luminescence from Al2O3:C to high-energy heavy charged particles

The thermoluminescence (TL) and optically stimulated luminescence (OSL) response of Al2O3 dosimeters to high-energy heavy charged particles (HCP) has been studied using the heavy ion medical accelarator at Chiba, Japan. The samples were Al2O3 single-crystal chips, of the type usually known as TLD-500, and LuxelTM dosimeters (Al2O3:C powder in plastic) from Landauer Inc. The samples were exposed to 4He(150MeV/u), 12C (400MeV/u), 28Si(490MeV/u) and 56Fe(500MeV/u) ions, with linear energy transfer values covering the range from 2.26 to 189keV/μm in water and doses from 1 to 100mGy (to water). A 90Sr/90Y beta source, calibrated against a 60Co secondary standard, was used for calibration purposes. For OSL, we used both continuous-wave OSL measurements (CW-OSL, using green light stimulation at 525nm) and pulsed OSL measurements (POSL, using 532nm stimulation from a Nd:YAG Q-switched laser). The efficiencies (ηHCP,γ) of the different HCPs at producing OSL or TL were observed to depend not only upon the linear energy transfer (LET) of the HCP, but also upon the sample type (single crystal chip or LuxelTM) and the luminescence method used to define the signal—i.e. TL, CW-OSL initial intensity, CW-OSL total area, or POSL. Observed changes in shape of the decay curve lead to potential methods for extracting LET information of unknown radiation fields. A discussion of the results is given, including the potential use of OSL from Al2O3 in the areas of space radiation dosimetry and radiation oncology.

Optical dating of Holocene sediments from a variety of geomorphic settings using single grains of quartz

This paper presents an improved method for the optical dating of Holocene sediments from a variety of geomorphic settings. We have measured the equivalent dose ( D e) in individual grains of quartz, using green laser light for optical stimulation, and have simulated the D e distributions for multiple-grain ‘synthetic’ aliquots using the single-grain data. For 12 samples of known (independent) age, we show that application of a ‘minimum age model’ to the single-grain and ‘small’ (10-grain) aliquot D e data provides the most accurate estimate of the burial dose for nine of the samples examined (3 aeolian, 5 fluvial, and 1 marine). The weighted mean D e (as obtained using the ‘central age model’) gives rise to burial age overestimates of up to a factor of 10 for these nine samples, whether single grains, small aliquots, or ‘large’ (100-grain) aliquots are used. For the other three samples (two aeolian and one fluvial), application of either the minimum age model or the central age model to the single-grain, small aliquot, and large aliquot D e data yields burial ages in accord with the independent age control. We infer that these three samples were well bleached at the time of deposition. These results show that heterogeneous bleaching of the optical dating signal is commonplace in nature, and that aeolian transport offers no guarantee that the sample will be well bleached at the time of deposition. We also show that grains sensitive to infrared (IR) stimulation can give rise to low D e values, which will result in significant underestimation of the burial dose and, hence, of the age of deposition. We demonstrate that use of a modified single-aliquot regenerative-dose protocol incorporating IR stimulation prior to green light stimulation deals effectively with contamination by IR-sensitive grains. We conclude that application of the modified protocol to single grains or small aliquots of quartz, using the lowest D e population to estimate the burial dose, is the best means of obtaining reliable ages for Holocene sediments from a wide range of depositional environments.

Comparison of quartz OSL protocols using Lateglacial and Holocene dune sands from Brandenburg, Germany

We present the results obtained using various different protocols in the quartz optically stimulated luminescence dating of a Late Glacial and Holocene aeolian dune sequence near Eberswalde, north of Berlin, Germany. The Allerød `Finow’ palaeosol and the Laacher See tephra layer serve as chronostratigraphic markers, and further independent age control is provided by eight 14 C ages. The application of the single-aliquot regenerative-dose protocol (using blue light stimulation) and the multiple-aliquot regenerative-dose and additive-dose protocols (using broad-band blue and green light stimulation) are discussed. A comparison of radionuclide analyses using high-resolution gamma-spectrometry and neutron activation analysis is also presented. It is concluded that the two dosimetry methods give indistinguishable results, although the gamma spectrometry data are more precise. In contrast, the single-aliquot regenerative-dose protocol is between 2 and 10 times more precise than the multiple-aliquot protocols, and systematic differences in performance are also observed. The uncertainties in the dosimetry calculations are the largest contributor to the overall age uncertainties. The accuracy of the resulting ages is considered very satisfactory when tested against the independent age controls, over the entire age range from the present to 15 ka.

Photon-stimulated luminescence of quartz from loess and effects of sensitivity change on palaeodose determination

In an attempt to improve the accuracy and precision of optical dating for loess deposits, we have measured photon-stimulated luminescence (PSL) or optically stimulated luminescence (OSL) of fine-grain quartz extracted from loess using blue–green light stimulation. The luminescence signal of quartz is shown to have three components which decay with markedly different rates upon stimulation. The preheat procedures employed during the measurements gave rise to changes in luminescence sensitivity. The sensitivity changes monitored by the 110°C TL peak and OSL responses to a test dose are shown to be dependent on factors such as preheat temperature, number of cycles and radiation doses. The preheat-induced sensitivity changes occur in both single- and multiple-aliquot based methods. We show that with an OSL sensitivity correction in the single-aliquot regeneration method, high-precision palaeodose values with uncertainty smaller than 5% can be obtained for fine-grain quartz from loess.

Some aspects of single-grain luminescence dating

Single-grain methods of luminescence dating have been developed in order to circumvent problems of variability among aliquots of the sample being dated. Sufficient single grains must be separated in order to make an appropriate interpretation of the variability among such grains. We describe an automated apparatus for sorting grains for routine age determinations by green light stimulation. The sorted grains are then available for study of their collective luminescence physics characteristics. Examples are given of statistical frequency functions of grain brightness obtained with the apparatus and their interpretation. These frequency functions are lognormal or similar, covering up to four orders of magnitude, with consequent implications for luminescence dating application.

Investigations of the green- and infrared-stimulated luminescence using the recuperation effect

We used the recuperation effect (see also “ Recuperation of infrared stimulated luminescence of feldspars” in this issue) for investigating the connection between infrared- and greenlight-stimulated luminescence (IRSL and GLSL) of feldspars by performing a “double-bleach recuperation” experiment. A diode system was used for infrared (IR) stimulation, and a filtered-light unit for greenlight (GL) stimulation. Powdered feldspar samples (2 mg each) of known chemical characterisation from a mineral collection were used. After beta-irradiation with 180 Gy and storage in the dark for several weeks feldspar aliquots were bleached down to a residual level of ≈1% of the initial level, first with IR and subsequently with GL. For both stimulations, detection of the stimulated luminescence was carried out in the near ultraviolet region (around 260–360 nm, peaked at 340 nm). Other aliquots were bleached in reverse order (1st GL, 2nd IR). These bleaching sequences were also applied to recuperation measurements after 30 days storage time under controlled room-temperature conditions. Results of these experiments will be presented and a possible interpretation of the observed recuperation signals in terms of a two-trap optically simulated luminescence model will be given. The different interaction of the traps, depending on the specific feldspar, will be shown.

Factors controlling the shape of the OSL decay curve in quartz

The effects of stimulation temperature on the shape of the quartz OSL decay curve have been investigated using a green light stimulation source and a quartz sample from the Kimberley region of Western Australia. The relationship between the initial OSL (first 0.4 s), the OSL at the end of a 100 s stimulation period, and the full OSL integral, is presented as a function of stimulation temperature (25–500°C). It is deduced that a single trap/luminescence centre combination is responsible for the majority of the OSL at stimulation temperatures between 150 and 275°C, and thus that traps giving rise to TL peaks in this temperature range cannot contribute significantly to the OSL signal, either directly or indirectly. Comparison of the initial and integral OSL values in this temperature range permits the effects of thermal assistance and thermal quenching to be separated and the activation energies for each process to be calculated. Below 150°C the relationship changes markedly; this change cannot be explained in terms of only thermal assistance and thermal quenching. After 100 s of stimulation, the contribution to the OSL from deeper traps (>330°C) is deduced to be only about 1.7% of the average OSL over 100 s. All of this luminescence from deep traps appears to be thermally quenched, and it is concluded that the 375°C TL trap in particular does not contribute measurably to the slowly decaying component of the OSL from this material. Furthermore, shallow traps below about 150°C do not contribute a large fraction of the OSL signal by direct stimulation. However, we show that competition for charge by the 110°C trap may be significant during stimulation, and some limits are estimated for this process. The possible implications of our findings for dating procedures are also considered.

Single aliquot dating of ceramics by green light stimulation of luminescence from quartz

The single aliquot additive dose method of equivalent dose determination, and hence of dating, as applied to the stimulation of luminescence from quartz by green light has been investigated. The quartz was removed from pieces of ceramic, extracted from bore holes in an area of archaeological interest in the Aegialia region, Northern Peloponesse, Greece. Various tests and an appropriate single aliquot correction procedure are discussed. It is concluded that the additive dose single aliquot method of determining equivalent dose, with correction for re-use of the aliquot, can be successfully applied to the green light stimulation of quartz for the purpose of dating.

The relationship between quartz thermoluminescence, photo-transferred thermoluminescence, and optically stimulated luminescence

The effects of green light stimulation on the thermoluminescence (TL) of a sedimentary quartz from the Kimberley region of Western Australia are reported. The loss of TL at 310°C and the concomitant growth and decay of the photo-transferred TL (PTTL) at both 110 and 160°C are presented for optical stimulation of up to 200 s in a Risø TL/OSL reader. The optical decay lifetime for the 110°C peak was about 130 s, and we deduce that this decay probably arises from optical stimulation of electrons rather than loss of recombination centres. A linear correlation was found between the loss of TL from the rapidly bleaching peak at 310°C (following a natural (N) + 43 Gy dose) and the integrated optically stimulated luminescence (OSL), for stimulation at 125°C and for stimulation times between 0.1 and 100 s. For another N + 43 Gy sample, the OSL signal decreased by about 20% with increasing preheat temperature from 220 to 280°C, after correction for thermal activation of luminescence centres (monitored by an additional experiment). The TL signal associated with this OSL (i.e. the TL lost as a result of optical stimulation) decreased by about 30% with preheat temperature. For repeated 0.1 s stimulation at 25°C, there was a linear relationship between the OSL, and the PTTL at 110°C obtained after each stimulation. The effects of different doses and stimulation times on this relationship are reported, and the implications for dating protocols using the OSL signal are discussed.

The time and wavelength response of phototransferred thermoluminescence in natural and synthetic quartz

Phototransferred thermoluminescence (PTTL) properties from several types of crystalline quartz are presented. Several of the PTTL peaks display an increase, followed by a decrease, but not necessarily to zero. The data can be satisfactorily explained by a model which includes the recombination of electrons and holes during illumination, and two types of recombination center. Consideration of the rate equations describing the phototransfer of charge from deep traps to shallow traps, along with the wavelength dependence of the photoionization cross-section, leads to the realization that increasing the illumination intensity or decreasing the wavelength will have the same effect on the PTTL response. The wavelength dependence of the PTTL effect can be obtained by taking the initial slope of the PTTL-vs-illumination-time curve as a function of wavelength. Using this procedure reveals that only one source trap is emptying over the wavelength range of green light stimulation, but additional traps become active as the wavelength decreases. Over the whole wavelength range used three traps are active, with optical threshold energies of 2.37, 2.84 and 3.77 eV.

Study of sensitivity change of OSL signals from quartz and feldspars as a function of preheat temperature

Optically stimulated luminescence (OSL) signals from feldspar and quartz samples were studied using infrared (860 nm) and green light (420–575 nm) stimulation. A serious problem connected with the regeneration technique used for dating is associated with a change of OSL sensitivity to radiation in the course of the measurement process. A typical effect seen is a large increase of the apparent strength of our beta source when calibrated against a gamma source. If the regeneration procedure is used, it is shown that the sensitivity increases up to ∼50% during the measurement process and as a result, the equivalent dose (ED) would be underestimated. A study of sensitivity changes in feldspars and quartz was carried out with emphasis on the effect of preheat and annealing on the OSL signal. Measurement results obtained are presented, and possible elimination of errors in dating caused by sensitivity changes is discussed.

BLUE AND GREEN LIGHT EFFECTS, AND THEIR PHOTOREVERSIBILITY, UPON ZOOSPORE PRODUCTION AND THE SYNCHRONIZED CYCLE IN PROTOSIPHON BOTRYOIDES KLEBS*

Abstract—The synchronized asexual cycle in Protosiphon botryoides Klebs is described. A blue light (430 nm) inhibition of zoospore production can be photoreversed by green light (550 nm). Similarly, a green light (550 nm) stimulation of zoospore production can be photoreversed by blue (430 nm) light. 430 nm light also shortens the period of zoospore motility, and 550 nm light lengthens this period.