A comparative study of the luminescence characteristics of polymineral fine grains and coarse-grained K- and Na-rich feldspars

The IRSL and post-IR IRSL (pIRIR) signal characteristics of polymineral fine grains are investigated and compared with those of K- and Na-rich feldspar extracts. TL signal loss after IR and pIRIR stimulations occurs mainly at around 320 °C for polymineral and Na-feldspar samples and around 410 °C for K-feldspar samples, when a preheat temperature of 250 °C for 60 s is used. After preheating to a higher temperature (320 °C for 60 s) all samples show a TL reduction around 410 °C in the blue detection window. Pulse annealing experiments for IRSL and pIRIR signals for preheats between 320 °C and 500 °C indicate that the signal stabilities are similar among the different feldspar types, when a higher preheat temperature (>320 °C) is used. Thermal activation energies for IRSL and pIRIR signals are largest in K-feldspar and smallest in polymineral fine grains, in both blue and UV detection windows for both fast time-resolved (TR) and continuous wave (CW) signals. These results suggest that IRSL and pIRIR signals in polymineral fine grains originate mainly from Na-feldspar grains; these signals are less thermally stable than those from K-feldspar, but a more stable signal (presumably from K-feldspar grains) can be obtained using a higher preheat temperature.     

Bleaching of the post-IR IRSL signal from individual grains of K-feldspar: Implications for single-grain dating

Post-IR IRSL (pIRIR) signals from K-feldspar grains measured at elevated temperatures are increasingly being used for dating sediments. Unfortunately the pIRIR signal from K-feldspars bleaches more slowly than other signals (e.g. OSL from quartz) upon exposure to daylight, leading to concerns about residual signals remaining at deposition. However, earlier studies have not assessed whether the pIRIR signal bleaches at the same rate in all feldspar grains. In this study laboratory bleaching experiments have been conducted and for the first time the results show that the rate at which the pIRIR signal from individual K-feldspar grains bleach varies. To determine whether grain-to-grain variability in bleaching rate has a dominant control on equivalent dose (D_e) distributions determined using single grains, analysis was undertaken on three samples with independent age control from different depositional environments (two aeolian and one glaciofluvial). The D_e value determined from each grain was compared with the rate at which the pIRIR₂₂₅ signal from the grain bleaches. The bleaching rate of each grain was assessed by giving a 52 Gy dose and measuring the residual D_e after bleaching for an hour in a solar simulator. There is no clear relationship between the rate at which the pIRIR₂₂₅ signal of an individual grain bleaches and the magnitude of its D_e. It is concluded that variability in the bleaching rate of the pIRIR₂₂₅ signal from one grain to another does not appear to be a dominant control on single grain D_e distributions.     

Natural variations in the properties of TL and IRSL emissions from metamorphic and volcanic K-feldspars from East Africa: Assessing their reliability for dating

The elevated temperature infrared stimulated luminescence (IRSL) and post-IR IRSL signals of potassium (K)-feldspars have recently garnered attention for their minimal rates of anomalous fading. The post-IR IRSL signal has been used to obtain age estimates for geological deposits, mostly in Europe. Studies on the behaviour of the IRSL and post-IR IRSL signals of K-feldspars from a wider range of geographic regions and depositional contexts are needed, particularly for regions where the OSL signal from quartz is poorly behaved. Discrepancies in the literature regarding the behaviours of the IRSL and TL signals of K-feldspars also highlight the need to characterise the behaviours of samples from a wide variety of contexts. This paper begins to address this problem by characterising and comparing the IRSL signals of a metamorphic and a volcanic K-feldspar sample from two sites in East Africa, a region in which the OSL signal from quartz has generally proven problematic for dating. We demonstrate that the metamorphic and volcanic K-feldspars have substantially different TL glow curves that respond differently to IR stimulation. The sample of metamorphic K-feldspar from Tanzania (MR9) has a peak at 430 °C that is associated with the IRSL signal and an optically less-sensitive peak at 350 °C, while the sample of volcanic K-feldspar from Ethiopia (MB3) exhibits a single broad TL region centred at ～230 °C that responds differently to IR stimulation. Differences in the change of IRSL decay curve shape with stimulation temperature suggest that the processes of IRSL production many vary between the two samples. Using dose recovery tests, we demonstrate that the IRSL (50 °C), IRSL (225 °C) and post-IR IRSL (50 °C, 225 °C) signals of sample MR9 are suitable for dose and age estimation using the single-aliquot regenerative-dose procedure, while those of sample MB3 are less suitable. The post-IR IRSL signal of the latter sample performs poorly in tests of SAR suitability and the three signals exhibit extremely high fading rates over laboratory timescales (g_2days > 19%/decade).     

Investigations of the post-IR IRSL protocol applied to single K-feldspar grains from fluvial sediment samples

The post-IR IRSL protocol with single K-feldspar grains was applied to three samples taken from a fluvial sedimentary sequence at the archaeological site of the Dali Man, Shaanxi Province, China. K-feldspar coarse grains were extracted for measurement. Approximately 30–40% of the grains were sufficiently bright to measure, and after application of rejection criteria based on signal strength, recuperation, recycling ratio and saturation dose, ～10–15% of the grains were used for D_e calculation. The relationship of signal decay rate and form of D_e(t) with the recovery dose were investigated. The dose recovery ratios of the samples after initial bleaching with the four different light sources were within uncertainties of unity. No anomalous fading was observed. The over-dispersion of the recovered dose and D_e values were similar, suggesting neither incomplete resetting of the post-IR IRSL signals nor spatially heterogeneous dose rates significantly affected the natural dose estimates. The values of D_e obtained with the single K-feldspar grain post-IR IRSL protocol were in the range ～400–490 Gy. Combining all of the measured single-grain signals for each of the individual samples (into a ‘synthetic single aliquot’) increased the D_e estimates to the range ～700–900 Gy, suggesting that the grains screened-out by the rejection criteria may have the potential to cause palaeodose over-estimation, although this finding requires a more extensive investigation. Thermally transferred signals were found in the single K-feldspar grains post-IR IRSL protocol, and the proportion of thermally transferred signal to test-dose OSL signal (stimulation at 290 °C) from the natural dose was higher than from regenerative doses, and the proportion was grain- and dose-dependent. As such, TT-post-IR IRSL signals at 290 °C have the potential to cause dose underestimation, although this may be reduced by using larger test-dose irradiations. Our study demonstrates considerable potential in the post-IR IRSL method in providing chronological control in studies relevant to human evolution in the later-Pleistocene.     

Anomalous fading of the IRSL signal of polymineral grains in Chinese loess

Laboratory storage and preheating experiments were carried out to study anomalous fading of infrared stimulated luminescence (IRSL) signals derived from polymineral grains extracted from Chinese loess. Results of laboratory storage at 150 ^°C and higher temperature preheating experiments showed that such thermal treatments could lessen the effect of fading and indicated the presence of both thermal and non-thermal fading. In addition, the behavior of natural fading over the past 9–170 ka was investigated. By comparing with independent ages (obtained from fine-grain quartz using the optically stimulated luminescence (OSL) signal for the past 130 ka and the thermally transferred OSL (TT-OSL) signal in the age range of 130–170 ka) for the same samples, equivalent doses obtained from the IRSL signals were found to be underestimated by different amounts since the penultimate glacial; there was a linear dependence when the age underestimation was plotted against geological time.     

Comparison of the properties of various optically stimulated luminescence signals from potassium feldspar

Various optically stimulated luminescence signals from K-feldspar have been used to determine the equivalent doses of sediment samples. Understanding the properties of these optical signals is critical to evaluate their applicability and limitations to optical dating. In this paper, some properties of IRSL, post-IR OSL and post-IR IRSL signals (detected in the UV region using U-340 filters) from a museum sample of K-feldspar were investigated by analyzing the relationships between optical and TL signals, and the effect of optical bleaching and heating on optical signals. The trap parameters of the different optical signals were calculated using the pulse annealing method. The results show that this sample exhibits two regenerated TL peaks at ～140 and ～330 °C. Corresponding to the low temperature TL peak, the OSL and post-IR OSL signals appear to be more associated with lower temperature TL than the IRSL signal measured at 50 °C. Corresponding to the high temperature TL peak, the post-IR IRSL signals mainly originate from the more thermally stable traps associated with the high temperature TL, compared with the IRSL and post-IR OSL signals. However, the post-IR IRSL_225 °C signal is shown to be hard to be bleached by blue light and simulated sunlight, compared with the IRSL_50 °C and low temperature post-IR IRSL signals. The implication for optical dating is that the elevated temperature post-IR IRSL signals can be preferentially applied over other signals from K-feldspar, but it is desirable that the effectiveness of the pre-depositional zeroing of these signals is assessed.     

Assessment of diagnostic tests for evaluating the reliability of SAR De values from polymineral and quartz fine grains

In this study, we applied optically stimulated luminescence (OSL) dating to two fine grain sediment samples collected at Jeongokri, Korea. A single aliquot regenerative dose (SAR) procedure was applied to both polymineral grains and to chemically isolated (H₂SiF₆) quartz grains of 4–11 μm diameter. For polymineral fine grains, the OSL IR depletion ratio and the equivalent dose (D_e) plateau test appear to be equally sensitive indicators of appropriate IR stimulation time for use in the ‘double SAR’ protocol. Additionally, the OSL IR depletion ratio test gives an indication of the relative mineral composition of the samples, hence providing an assessment of the likelihood of obtaining a quartz-dominated [post-IR] OSL signal. Use of higher preheat temperatures would assist in thermally eroding the non-quartz component of the [post-IR] OSL signal from polyminerals. For the quartz fine grains, data from both natural D_e determinations and laboratory dose recovery tests are required to identify the appropriate preheat temperatures for dating, due to problems of thermal transfer. This phenomenon is particularly exaggerated for these samples due to the large D_e values (≥350 Gy) and hence low slope of the dose–response curve. The double SAR method cannot be applied ubiquitously, even after careful and rigorous study of one sample from a section. Quartz OSL dating using a range of preheat temperatures is suggested to be the most suitable method for OSL dating of fine grain sediments.     

Yellow stimulated luminescence from potassium feldspar: Observations on its suitability for dating

Yellow stimulated luminescence (Y-OSL) is the light detected from potassium-rich feldspars at 410 nm under stimulation by a yellow light source emitting 590 nm. The investigation of this study aimed at understanding basic luminescence physics of Y-OSL in order to assess the suitability of the technique for dating. The Y-OSL signal properties tested were signal intensity, thermal assistance, thermal stability, sensitivity to daylight and the suitability of a single aliquot regenerative (SAR) protocol to be employed for equivalent dose (D_e) estimation. D_e measurements were conducted on samples of Holocene, last glacial and Tertiary age. The tests were undertaken on sedimentary feldspar separates extracted from aeolian, fluvial and coastal deposits.Results from experiments show that the signal intensity increases by measuring Y-OSL at elevated temperature suggesting thermal assistance characteristics similar to infrared stimulated luminescence (IRSL). The yellow stimulated signal remains unaffected by preheat temperatures up to ～200 °C suggesting higher thermal stability than the IRSL signal. The Y-OSL signal is less light sensitive than the IRSL signal and D_e residuals obtained from modern samples are up to 7 Gy indicating suitability of the technique for ‘older’ and well-bleached sediments. The dose recovery tests successfully recovered the given dose if the specific light sensitivity of Y-OSL is taken into account. For every sample Y-OSL D_e values obtained by a single aliquot regenerative dose protocol (SAR) are higher than those obtained by an IRSL SAR approach. From these results we infer high thermal stability and a minimal anomalous fading of the Y-OSL signal. We conclude that Y-OSL has a high potential to date Quaternary sediments that were sufficiently bleached in nature.     

Testing Post-IR IRSL protocols for minimising fading in feldspars,using Alaskan loess with independent chronological control

Concern over anomalous fading has been the biggest single factor responsible for deterring the widespread use of the infra-red stimulated luminescence (IRSL) or thermoluminescence (TL) signal from feldspars for luminescence dating. There has therefore been great interest in the use of the recently proposed Post-IR IRSL signal, because it has been shown to significantly reduce the degree of anomalous fading observed in feldspars and therefore potentially provides a means of circumventing the issue. This study undertakes a systematic investigation into various preheat and Post-IR IRSL measurement conditions proposed in the literature, by using two samples from the Halfway House loess section in Alaska which bracket the Old Crow tephra which has been dated using fission track methods. Preheat plateau tests show a dramatic change in equivalent dose with Post-IR IRSL measurement conditions, and further tests reveal that these changes are driven by preheat temperature rather than Post-IR IR stimulation temperature. Dose recovery tests on laboratory-bleached material mimic the findings of the natural preheat plateau test data, and sensitivity change between the first and second Single Aliquot Regenerative dose (SAR) measurement cycle is found to be responsible. Comparison of the Post-IR IRSL ages with the independent age control shows that, for the samples in this study, the Post-IR IR signal stimulated at 290 °C is inappropriate for dating. However, use of lower preheat (250–300 °C) and Post-IR IR stimulation temperatures from 225 to 270 °C gave rise to ages which were in agreement with the independent age control.     

Application of pulsed OSL to polymineral fine-grained samples

Pulsed OSL is applied to nine fine-grained sediment samples from Sichuan province, China, using stimulating pulses of 10 μs on and 240 μs off, with an infrared exposure prior to each OSL measurement. Comparison of fading rates between pulsed and non-pulsed signals, the latter also obtained with a preceding IR exposure, shows that fading is significant for mainly the non-pulsed signals. Presence of a pulsed IRSL and the magnitudes of b-value to correct for lower alpha efficiency suggest that pulsing does not fully remove a significant feldspar signal, only a fading component. Comparison with ages of quartz extracts shows that pulsed OSL ages are consistent, while CW-OSL ages are slightly older and CW-IRSL ages are much older. The older ages suggest a less well-bleached feldspar component.     

Effects of dielectric barrier discharge plasma on pathogen inactivation and the physicochemical and sensory characteristics of pork loin

This study aimed to evaluate the use of a dielectric barrier discharge (DBD) plasma system to improve the safety of pork loins. When pork loin was exposed to DBD plasma with the input gases He and He + O₂, the population of Escherichia coli was reduced by 0.26 and 0.50 log cycles following a 5-min treatment and by 0.34 and 0.55 log units following a 10-min treatment, respectively. That of Listeria monocytogenes was also reduced from 0.17 to 0.35 and 0.43 to 0.59 log cycles when the samples were exposed to DBD for 5 and 10 min using He and He + O₂, respectively. The pH and L*-values (lightness) of the samples decreased significantly with DBD plasma treatment, but a*- (redness) and b*-values (yellowness) exhibited no obvious changes. Lipid oxidation, measured by TBARS values, was greater in samples with He + O₂ than in other samples. Significant reductions in sensory quality parameters (appearance, color, odor, acceptability, etc.) were observed in DBD-treated samples. These results indicate that the DBD plasma system has potential for use in sanitizing pork loins by inactivation of foodborne pathogens, although the effect was limited. In order to meet market requirements, however, a method to overcome sensory deterioration of pork loins should be developed and applied.     

Luminescence from NaCl for application to retrospective dosimetry

The alkali halide NaCl (Common salt) is an environmentally-abundant phosphor of considerable potential for retrospective dosimetry and radiological event analysis due to its high sensitivity to ionising radiation when analysed by Thermoluminescence (TL), Optically-stimulated luminescence (OSL) or Infrared-stimulated luminescence (IRSL). We report here aspects of luminescence from NaCl relevant to the development of valid protocols for measurement of recent ionising radiation exposure. The timescale of interest in this application is from days to decades, hence our emphasis is on detection and characterisation of TL emission in the 100–300 °C range, and of OSL and IRSL emissions measured following only low temperature preheating (160 °C). A collection of 19 salt samples was assembled, including samples of rock salt and domestic salt produced by evaporation from brine. Analysis of TL emission spectral changes, together with previously reported TL, OSL and IRSL sensitivity changes, confirmed activation of sensitivity change by exposure to temperatures exceeding 160 °C. Kinetic analysis using Chen's method found E = 0.943 eV and s = 5.1 × 10¹¹ s^?1 for the 100 °C TL peak, giving a lifetime at 20 °C consistent with previous calculations and in the range of 7–14 h.     

Standardised growth curves for optical dating of sediment using multiple-grain aliquots

The single aliquot regenerative dose (SAR) protocol offers the opportunity of exploring, relatively simply, the existence of a ‘universal’ growth curve for use in dating using optically stimulated luminescence (OSL). A test dose is used in the SAR procedure to monitor and correct for sensitivity change occurring either over the burial period, or as a result of thermal pretreatments during the measurement procedure. However, this test dose can also be used to correct for variations in signal intensity between individual aliquots, thus enabling the comparison of growth response with dose for many different aliquots and samples, even for measurements made using different instruments.In this paper, the growth characteristics of coarse-grained quartz and polymineral fine-grains are examined using data obtained as part of the SAR procedure. Following standardisation of these data for differences in the test dose response and the magnitude of the test dose used, distinct and reproducible patterns of growth are observed and mineral-specific standardised growth curves (SGCs) are defined for multiple-grain aliquots. An equivalent dose for a sample can then be determined based only on measurements of the natural luminescence signal intensity (L_n) and the response to an artificial irradiation dose (T_n). This equivalent dose, determined using the SGCs defined for quartz and polymineral fine-grains, is compared to that determined using a conventional SAR measurement procedure, for a large number of samples and aliquots. Using an SGC, accurate estimates of equivalent dose may be made based solely on measurements of L_n and T_n, thus potentially speeding up the measurement process. This has obvious benefits where it is necessary to examine a large number of aliquots.     

Neubestimmung der absoluten Oszillatorenstärke der Eisen I-Resonanzlinieλ=3720 Å durch optische Doppelresonanz

The natural width of the strong Fe I spectral lineλ=3719,9 Å was determined in a double resonance experiment with an atomic beam as resonance cell and a hollow cathode as light source by the observation ofrf-induced magnetic dipole transitions between the Zeeman sublevels of the excited state in a weak static magnetic fieldH ₀. The optical line was isolated from disturbing neighbours in the same multiplet either roughly by an interference filter or strictly by a monochromator. From the widthΔν=5,35(14) MHz of the double resonance signal (monochromator value, including a small correction of 1.5% for the hyperfine structure of the 2.2% natural abundance of iron-57) the lifetimeτ of the (z ⁵ F ₅ ⁰ )-level is found to beτ=5,95 (16)×10^?8 sec. Thef-value of the transition to the ground state (a ⁵ D ₄) is calculated, by using the known relativef-values for the weak alternative transition possibilities to the (a ⁴ F ₅) and (a ⁵ F ₄) levels (λ=5012 and 5127 Å), to bef _λ3720=4,21 (11)×10^?2.     

Post-IR IRSL production in perthitic feldspar

A museum sample of perthitic feldspar was used to study the production of post-IR IRSL signals. It was found that traps responsible for low temperature (∼230 °C) TL peaks play an unexpectedly important role in post-IR IRSL production. During the production of the IRSL signal during low temperature IR stimulation (100 °C), electrons are optically transferred from IRSL traps into these TL traps which have been emptied by the preceding preheat at 320 °C. Subsequent heating to 300 °C causes thermal transfer of these electrons from these traps back into previously emptied IRSL traps which are related to the high temperature TL peaks. IR stimulation of these electrons results in post-IR IRSL. Thus the initial source of the post-IR IRSL signal is the same as the IRSL signal, with a role being played by intermediate traps that give rise to TL signals between 200 and 250 °C, and the final source is similar to that of the IRSL signal. Therefore the post-IR IRSL signal is a by-product of the production of the IRSL signal. It was also found that post-IR IRSL production with high post-IR IR stimulation temperatures (e.g. >230 °C) additionally includes a small contribution from the post-IR isothermal decay of high temperature TL peaks that are not sensitive to IR stimulation at low stimulation temperatures.     

Investigating the contribution of recuperated TL to post-IR IRSL signals in a perthitic feldspar

Using a museum specimen of perthitic feldspar, the characteristics of post-IR IRSL production at 200 °C after different prior IR bleaching at 100 °C were investigated. It was found that the post-IR IRSL signal had an isothermal TL contribution that was unexpected following a previous preheat at 320 °C; this is the result of isothermal decay of recuperated TL peaks resulting from photo-transfer that occurred when the previous IRSL signal was measured at a lower temperature. The isothermal TL contribution to the post-IR IRSL signal depends on prior IR bleaching conditions. Since the recuperated TL signal comes from photo-transfer during IRSL production, this signal should also suffer from anomalous fading. Thus, it is suggested that this isothermal TL contribution to the measured post-IR IRSL is removed by the inclusion of an additional step, a cut-heat to 300 °C, in the post-IR IRSL dating protocol.     

Assessing the impact of IR stimulation at increasing temperatures to the OSL signal of contaminated quartz

The purpose of the present study is to identify the effect of the increasing temperature IR stimulation to the component-resolved OSL luminescence signal of mixed quartz-feldspars material. Post IR OSL signals measured at 110 °C were analysed via only general order kinetic terms, while IR signals obtained at increasing temperatures were de-convolved using the sum of general order kinetics plus a tunnelling component. By increasing stimulation temperature, it was demonstrated that IRSL at temperatures above 50 °C does not only stimulate feldspar but also stimulates both fast and medium quartz OSL components. In the temperature range between 175 and 250 °C, the IRSL initial intensity is dominated by the fast OSL component. Estimated equivalent doses using either Post-IR₁₇₅.OSL₁₁₀ as well as IRSL₁₇₅ (with the indices indicating the measurement temperature) are in good agreement between each other, due to both stimulating quartz. Finally, the physical meaningfulness of the fitting parameters for the tunnelling component is also discussed.     

Soft chemistry synthesis and characterizations of fully oxidized and reduced NdBaCo2O5+δ phases δ = 0, 1

The double ordered perovskites NdBaCo₂O₅ and NdBaCo₂O₆ were prepared by soft chemistry. The samples were characterized from a structural and chemical point of view, concomitantly with their physical properties. Upon reduction, NdBaCo₂O₅ is formed with a tetragonal unit cell (a = b = 3.94 Å, c = 7.57 Å) and presents an antiferromagnetic behavior. Upon oxidation, a complete stoichiometric ordered phase NdBaCo₂O₆ with a tetragonal unit cell (a = b = 3.88 Å, c = 7.63 Å) could be obtained with a ferromagnetic and a metallic behavior. Finally it is shown that these phases are able to reversibly catch and release oxygen, suggesting a high anionic conductivity.     

Electron paramagnetic resonance in the system CaO-SiO2-Al2O3 containing sulfur

An EPR absorption was detected in the system CaO-SiO₂-Al₂O₃ doped with sulfur. The signal consisted of six isotropic hyperfine lines and demonstrated isotropic fine structure and forbidden doublets. The signal was attributed to an Mn²⁺ impurity displacing Ca²⁺ in fine CaS crystallites formed in the matrix of the ternary system. The spectrum was well accounted for with the parameters g = 2.0021 ± 0.0001 and A = ?77.1 ± 0.6 × 10^?4 cm^?1. Sample preparation conditions did not affect the hyperfine splitting, although the fine structure and forbidden lines proved to be dependent on the conditions of preparation. It is suggested that the appearance of the fine structure is a result of the second effect of the hyperfine structure when the cubic field parameter a approaches zero and, consequently, the structure is free of superposition caused by an anisotropic shift determined by a. We ascribe the change in fine structure to the variation in a and discuss the nature of the energy splitting of the ${}^6\text{S}{}_{\mathrm{<mtext>5</mtext><mtext>2</mtext>}}$ state in a cubic field. We also note a correlation between the fine structure and the forbidden lines. This finding suggests that a is not necessarily specific to the host lattice and, furthermore, that the consideration of superposition of axial or lower symmetry upon the original cubic field of the central Mn²⁺ ion is vital for the determination of a.     

Thermal annealing of GaSb quantum dots in GaAs formed by droplet epitaxy

We investigate effects of annealing on GaSb quantum dots (QDs) formed by droplet epitaxy. Ga droplets grown on GaAs are exposed to Sb molecular beam and then annealed at T_a=340–450 °C for 1 min to form GaSb QDs. An atomic force microscope study shows that with the increase of T_a, the average diameter of dots increases by about 60%, while their density decreases to about 1/3. The photoluminescence (PL) of GaSb QDs is observed at around 1 eV only for those samples annealed above T_a=380 °C, which indicates that the annealing process plays an important role in forming high quality GaSb QDs.