Bleaching Wavelength Research Articles

OSL surface exposure dating of a lithic quarry in Tibet: Laboratory validation and application

Abstract Recent work has shown that the optically stimulated luminescence (OSL) signal can be used to determine the duration of daylight exposure for rock surfaces, complementing the surface exposure dating technique using cosmogenic nuclides. In this study we investigate the feasibility of using the newly developed OSL Surface exposure dating technique (OSL-Surf) to date flake scars at lithic quarry sites. We performed the first quantitative validation of the model describing the OSL-Surf dating technique using a controlled laboratory experiment. Our results show that longer laboratory bleaching durations yield deeper OSL-depth profiles, validating the use of OSL-Surf approach for relative dating of rock surfaces with different exposure ages. The OSL-surf model fitted to the OSL-depth profiles (excluding one outlier) yields accurate estimates of known exposure duration, thus confirming the method's usefulness as an absolute dating tool. Consequently, we used the OSL-Surf technique to determine an exposure duration of 117 ± 37 a for a previously unknown-age flake scar that is related to human exploitation of a lithic quarry site in Tibet. The problem of finding a known-age rock surface for parameter calibration was solved by revisiting the sampling site and collecting the scar remaining after earlier sample collection, which has a precisely known exposure age (1.667 a in this study) and identical lithology and irradiation aspect as the flake scar. The calibration sample yielded a measurable OSL-depth profile that could be used to calibrate the model to estimate the exposure duration of a flake scar associated with human exploitation of the area. Finally, we observe that the μ parameter of the OSL-Surf model varies considerably between the laboratory-bleached and two naturally daylight-bleached datasets, despite having identical lithologies. We thus infer that, in addition to lithological controls, the μ parameter is primarily sensitive to the daylight irradiation geometry and only weakly dependent on spectrum of the incident light; this interpretation implies a narrow effective bleaching wavelength band in quartzite. From the practical viewpoint, our results suggest that geometrical factors deserve a careful consideration both while designing the laboratory bleaching experiments as a surrogate of natural bleaching, as well as while choosing the field calibration samples.

The issue of laboratory bleaching in the infrared-radiofluorescence dating method

Measuring the infrared radiofluorescence (IR-RF) signal is methodologically simpler than measuring the IRSL or TL signals. However, unlike classical luminescence measurements, the IR-RF signal is totally resetting when its highest value is reached. Previous studies (Buylaert et al., 2012; Trautmann et al., 2000) reported that the proper bleaching level of an IR-RF signal is difficult to define and seems to be dependent 1) on the wavelength used for bleaching and 2) on exposure duration. The IR-RF signals from K-feldspar samples have been measured with various bleaching wavelengths and exposure time using a Lexsyg research luminescence unit. In this study, we propose suitable bleaching parameters adapted to IR-RF measurements.

Laser teeth bleaching: evaluation of eventual side effects on enamel and the pulp and the efficiency in vitro and in vivo.

Light and heat increase the reactivity of hydrogen peroxide. There is no evidence that light activation (power bleaching with high-intensity light) results in a more effective bleaching with a longer lasting effect with high concentrated hydrogen peroxide bleaching gels. Laser light differs from conventional light as it requires a laser-target interaction. The interaction takes place in the first instance in the bleaching gel. The second interaction has to be induced in the tooth, more specifically in the dentine. There is evidence that interaction exists with the bleaching gel: photothermal, photocatalytical, and photochemical interactions are described. The reactivity of the gel is increased by adding photocatalyst of photosensitizers. Direct and effective photobleaching, that is, a direct interaction with the colour molecules in the dentine, however, is only possible with the argon (488 and 415 nm) and KTP laser (532 nm). A number of risks have been described such as heat generation. Nd:YAG and especially high power diode lasers present a risk with intrapulpal temperature elevation up to 22°C. Hypersensitivity is regularly encountered, being it of temporary occurrence except for a number of diode wavelengths and the Nd:YAG. The tooth surface remains intact after laser bleaching. At present, KTP laser is the most efficient dental bleaching wavelength.

The effect of different bleaching wavelengths on the sensitivity of Al2O3:C optically stimulated luminescence detectors (OSLDs) exposed to 6 MV photon beams

To determine the effect of different bleaching wavelengths on the response of Al(2)O(3):C optically stimulated luminescence detectors (OSLDs) exposed to accumulated doses of 6 MV photon beams. In this study the authors used nanoDot OSLDs readout with a MicroStar reader. The authors first characterized the dose-response, fading, and OSL signal loss of OSLDs exposed to doses from 0.5 to 10 Gy. To determine the effect of different bleaching wavelengths on the OSLDs' response, the authors optically treated the OSLDs with 26 W fluorescent lamps in two modes: (i) directly under the lamps for 10, 120, and 600 min and (ii) with a long-pass filter for 55, 600, and 2000 min. Changes in the OSLDs' sensitivity were determined for an irradiation-readout-bleaching-readout cycle after irradiations with 1 and 10 Gy dose fractions. The OSLDs presented supralinearity for doses of 2 Gy and above. The signal loss rates for sequential readouts were (0.287 ± 0.007)% per readout in the reader's strong-stimulation mode, and (0.019 ± 0.002)% and (0.035 ± 0.007)% per readout for doses of 0.2 and 10 Gy, respectively, in the reader's weak-stimulation mode. Fading half-life values ranged from (0.98 ± 0.14) min to (1.77 ± 0.24) min and fading showed dose dependence for the first 10-min interval. For 10 and 55 min bleaching using modes (i) and (ii), the OSL signal increased 14% for an accumulated dose of 7 Gy (1 Gy fractions). For OSLDs exposed to 10 Gy fractions, the OSL signal increased 30% and 25% for bleaching modes (i) and (ii) and accumulated dose of 70 Gy, respectively. For 120 and 600 min bleaching using modes (i) and (ii), the OSL signal increased 2.7% and 1.5% for an accumulated dose of 7 Gy (1 Gy fractions), respectively. For 10 Gy fractions, the signal increased 14% for bleaching mode (i) (120 min bleaching) and decreased 1.3% for bleaching mode (ii) (600 min bleaching) for an accumulated dose of 70 Gy. For 600 and 2000 min bleaching using modes (i) and (ii), the signal increased 2.3% and 1.8% for an accumulated dose of 7 Gy (1 Gy fractions), respectively. For 10 Gy fractions, the signal increased 10% for mode (i) (600 min bleaching) and decreased 2.5% for mode (ii) (2000 min bleaching) for an accumulated dose of 70 Gy. The dose-response of nanoDot OSLDs read using the MicroStar reader presented supralinearity for doses of 2 Gy and above. The signal loss as a function of sequential readouts depended on dose. Fading also depended on dose for the first 10-min interval. For dose fractions of 1 and 10 Gy, OSLDs may be reused within 3% and 5% accuracies up to the maximum accumulated dose of 7 and 70 Gy investigated in this study, respectively. These accuracies were obtained after the OSLDs were bleached with a light source with wavelengths above about 495 nm. The authors also concluded that changes in sensitivity of OSLDs depended on bleaching time, accumulated dose, and wavelength spectrum of the bleaching source.

SU-E-T-90: Investigation of Different Bleaching Wavelengths on the Absorbed-Dose Sensitivity of NanoDot OSLDs Exposed to 6 MV X-Ray Beams.

To determine the effect of different bleaching wavelengths on the luminescence response of Al2 O3 :C optically stimulated luminescence detectors (OSLDs) exposed to accumulated doses of 6 MV photon beams. OSLDs of the nanoDot type were used and readout with a microStar InLight reader. To determine the effect of different bleaching wavelengths on the luminescence response of nanoDot OSLDs, we optically reset (bleached) the OSLDs with 26 W fluorescent lamps in two modes: (i) directly under the lamps for 10, 120 and 600 min; and (ii) with a long pass filter for 55, 600 and 2400 min. The long pass filter blocks wavelengths below 495 nm, hence the longer bleaching duration to attain equivalent OSL signals as bleaching directly under the lamps. Changes in the sensitivity of the nanoDot OSLDs were determined for an irradiation-readout-bleaching-readout cycle, after irradiations with 1 and 10 Gy dose fractions. The nanoDot OSLDs presented a linear response to dose up to 2 Gy and supra-linear response afterwards. They produced different OSL signal loss and fading behaviors for doses of 1 and 10 Gy. OSLDs bleached for 120 min and 600 min without and with the filter in 1 Gy fractions, did not exhibit any significant change in sensitivity over an accumulated dose of 7 Gy. The OSLDs exposed with 10 Gy fractions exhibited a small under-response when bleached with filter, and an over-response when bleached directly without filter. The nanoDot OSLDs may be reused without a significant sensitivity change for accumulated doses below 7 Gy. However, for accumulated doses beyond 10 Gy, nanoDot OSLDs show significant sensitivity change and hence, need to be recalibrated for reuse. We also concluded that the light spectrum used to reset the OSLDs has an influence on the sensitivity of the detectors. Natural Sciences and Engineering Research Council of Canada.

SU-E-T-87: The Effect of Bleaching Wavelengths on the Regeneration of the Optically Stimulated Luminescence Signal of NanoDot Dosimeters Pre-Exposed to High-Doses.

To investigate the effect of bleaching wavelengths on the regeneration of optically stimulated luminescence (OSL) signals in Al2 O3 :C nanoDot dosimeters pre-exposed to high doses. Regeneration is the increase in the OSL signal during storage of a bleached nanoDot that was previously pre-exposed to a high dose. This phenomenon affects the accuracy of a calibration protocol proposed by Jursinic 2010 (Med. Phys. 37:102) in which pre-exposure of nanoDots to a high-dose was used to minimize changes in the sensitivity of the detector as a function of accumulated dose. Al2 O3 :C OSLDs of the type nanoDot were used throughout this study. Readout was performed using the microStar reader. Bleaching of the OSLDs was performed with four 26 W fluorescent light bulbs in two modes: (i) directly under the lamps; and (ii) with the aid of a long-pass optical filter placed over the nanoDots, partially blocking wavelengths below 495 nm. Eighteen nanoDots were pre-exposed to 1 kGy dose. Then the pre-exposed nanoDots were bleached in two sets of 9 to very low residual OSL signals using bleaching modes (i) and (ii) for 12 h and 45 h, respectively. The nanoDots were then stored in dark and readout after various time intervals to monitor the regeneration of the OSL signal. We fitted the regeneration of the OSL signal using a saturation function and obtained rise-time values of 563 h and 630 h, for bleaching modes (i) and (ii), respectively. At the saturation level, the equivalent doses were about 1.18 Gy and 0.38 Gy for modes (i) and (ii), respectively. The regeneration rates of nanoDot OSLDs pre-exposed to high doses depend on the bleaching light wavelength used to reset the detectors. A bleaching source that has a low component of wavelengths below 495 nm can minimize the regeneration of the OSL signal. Natural Sciences and Engineering Research Council of Canada.

Energy levels of self-trapped holes in amorphous SiO2: fictive temperature dependence

Fictive temperature (Tf) dependence of self-trapped holes (STHs) in low temperature UV-irradiated silica glasses was systematically investigated by the electron spin resonance (ESR) method. The decay of the ESR signals was found to be bleaching photon energy dependent and to follow the stretched exponential decay function at each bleaching wavelength. When the one-photon process is dominant during the photo-bleaching process, where the recombination of electrons excited from the valence band with the holes in the STH band results in the decay of STH signals, the energy levels of STH1 were thus derived at 1.75 ± 0.02 eV, 1.61 ± 0.03 eV, 1.45 ± 0.05 eV and STH2 at 1.68 ± 0.02 eV, 1.44 ± 0.02 eV, 1.25 ± 0.04 eV, respectively, for Tf = 1500, 1350 and 1200 °C samples.

Characterisation of fulgides in a solid matrix as recording materials for optical components

The optical properties of solid samples of polymethyl-methacrylate doped with fulgide photochromic molecules were studied. The transmittance of a thin plate sample was measured when both the activation (UV) and bleaching wavelengths are employed. The samples were used as an optical switch where an UV pulse switches a visible signal beam. The behaviour of these compounds as holographic materials, studied by efficiency measurements at varying grating steps, is reported.

Bleaching of the thermoluminescence of feldspars by selected wavelengths present in sunlight

Feldspars occur in many sediments, and dating them by thermoluminescence (TL) requires a knowledge of how efficiently sunlight bleaches these minerals. The relative bleaching efficiencies of various wavelengths have been investigated for four alkali feldspars and an oligoclase, using the natural TL of the samples and the artificial TL induced by laboratory irradiation, bleached by wavelengths in the range 322–350 nm. Ultraviolet wavelengths are more efficient than the longer wavelengths at bleaching all the glow-curve peaks and are responsible for most of the phototransfer from high-temperature peaks to low-temperature peaks. Some feldspars show an initial increase in TL intensity with bleaching. Two sanidines of similar bulk composition but different TL glow curves different dependences on bleaching wavelength. In a sodic sanidine with a dominant 240°C peak the energy required to bleach to 50% was a thousand times greater at 550 nm than at 322 nm. In contrast, another sanidine with a complex glow curve between 150 and 450°C showed a much slower change in the 330°C peak with wavelength—a factor of 10 between 322 and 550 nm—with a region between 400 and 500 nm which was almost independent of wavelength. This behaviour is explained by competing mechanisms of phototransfer and bleaching which depend on wavelength in different ways.

Losses of foveal flicker sensitivity during dark adaptation following extended bleaches

Flicker sensitivity to a small foveal test stimulus can decrease appreciably during the period of dark adaptation that follows extinction of a bleach. For 20 min diameter, long wavelength tests that followed 70% L cone bleaches of various wavelengths, 18 Hz flicker sensitivity decreased precipitously (i.e. within about 45 sec) by about 1.5–2.0 log units beginning at about 100–200 sec following extinction of the bleach. For short wavelength tests that followed long wavelength bleaches, the corresponding precipitous losses of flicker sensitivity were relatively small and early; flicker sensitivity decreased by about 0.6–1.0 log units begining no later than 60 sec following extinction of the bleach. Whenever flicker sensitivity decreased precipitously, the losses of flicker sensitivity were followed by plateaus of M cone dominated flicker sensitivity. For combinations of test and bleaching wavelengths that did not cause flicker sensitivity to decrease precipitously, flicker sensitivity instead decreased gradually over a prolonged period of time, and incorporated a substantial L cone contribution. The precipitous suppression of flicker sensitivity found for certain combinations of bleaching and test wavelengths appears to depend on the action of a broadly tuned, red-green spectrally opponent process.

Wavelength-selective bleaching of the optical spectra of trapped electrons in organic glasses. II.

Further resolution of the inhomogeneous optical spectra of trapped electrons (e−t) in organic glasses has been obtained from wavelength selective bleaching and thermal decay studies on 3-methylpentane-d14(3MP-d14) and 2-methyltetrahydrofuran (MTHF) following γ irradiation in the temperature region of 20 K, and limits on the degree of resolution achievable have been indicated. Exposure of 3MP-d14 to light of wavelengths ≳2100 nm (from a tunable laser) reduces the optical densities at the bleaching wavelength and longer to zero, while ’’peeling off’’ a portion of the O.D. at all shorter wavelengths but leaving the remainder of the spectrum unaffected. The fraction of the integrated optical spectrum, ℱOD d (eV), removed by bleaching at each wavelength tested, and also by thermal decay, is equivalent to the fraction of the total e−t spins removed and measured by ESR. 1064 nm light bleaches the spectrum nearly uniformly, confirming that the spectra of all of the e−t have blue tails with similar ease of bleaching. Heretofore unobserved low temperature thermal decay of e−t occurs at 20 and 40 K (20% of the spin concentration in 30 min, 35% in 3h). The rate of decay of the optical spectrum decreases with decreasing wavelength of observation (2.5, 2.2, 1.8, and 1.5 μ), but at each wavelength is the same at 40 K as at 20 K, consistent with a tunneling mechanism. Unique inhomogeneities in the spectrum of e−t produced in MTHF at 20 K are revealed by bleaching at 1338 and 1556 nm. The effects of biphenyl solute and of exhaustive bleaching at these wavelengths are reported and discussed. The initial quantum yields of bleaching e−t in MTHF at 77 K with 1338 nm or 1064 nm light are less than 10−4 of the yields for e−t produced and bleached at 20 K.

Wavelength-selective bleaching (``burning holes'') in the optical spectra of trapped electrons in organic glasses

This paper presents experimental evidence confirming that the broad optical absorption spectra of trapped electrons in glassy 3-methylpentane and methyltetrahydrofuran (MTHF) are convolutions of spectra of electrons trapped with different energies. Laser light of wavelengths near or to the red of the et− optical absorption maximum selectively depopulates electrons which absorb at the bleaching wavelength, thus creating a hole in the et− spectrum. Some additional indications as to the spectral properties of the individual electrons are given by the observations that (1) illumination of et− produced in MTHF at 25°K with laser light to the blue of the absorption maximum results in nearly uniform bleaching throughout the spectrum; (2) et− produced in MTHF at 67°K and illuminated at 25°K with 1064 nm are bleached less efficiently then et− produced at 25°K and have a low probability of being shifted to traps typical of mobile electron trapping at 25°K; (3) the spectra of et− in MTHF are progressively red-shifted as the temperature at which they are produced is lowered from 97 to 10°K, and the resolution of the peaks observable in the spectrum produced at 72°K is lower for spectra of et− produced at higher (97°K) or lower (25 and 10°K) temperatures. Some implications of these observations are discussed.

Optical Absorption Characteristics and Photobleaching Behavior of Trapped Electrons in γ-Irradiated Alkaline Ice

Trapped electrons, et−, are produced by γ irradiation of alkaline ice (10M NaOH) at 77°K. The et− absorption band maximum at 590 nm shifts to shorter λ for bleaching at 700 nm and shifts to longer λ for bleaching at 400 nm. After partial bleaching, electrons can be shifted back and forth between traps characterized by long and short λ. Although this suggests at least two trap depths, there is probably a broad spectrum of trap depths. The quantum efficiency, φ, for photobleaching et− is 0.15 and independent of λ when the optical density is measured at 590 nm. For the optical density measured at the bleaching wavelength, φ increases to 0.52 at 400 nm and to 0.26 at 700 nm; φ also increases in the presence of electron scavengers. φ decreases as the fraction of et− bleached, F, increases and follows φ = 0.15 exp( − 3.7F) for F = 0–0.85. All of these changes are explained by retrapping and trap interconversion. φ is also independent of temperature between 4 and 77°K in agreement with photoconductivity measurements. ε(590 nm) = 1.94 × 104M−1·cm−1, and the oscillator strength is ≥ 0.86 for the et−band.