Narrow gated Raman and luminescence of explosives

Narrow gated Raman spectroscopy is used to detect Raman signals of explosives, which are usually screened by their intrinsic or background luminescence. It was found that the Raman/luminescence ratio is improved by 2-10 times with gate width of 500 ps compared to the 10 ns gate. It enables in certain cases to combine the luminescence suppression by gating with higher identification ability of Raman signals achievable with green excitation.     

Luminescence dating: laboratory procedures and protocols

In the last 30 years, from 1967 to 1997, the use of luminescence signals from naturally occurring minerals has gone though a major metamorphosis, from thermoluminescence (TL) dating of pottery to optically stimulated luminescence (OSL) dating of sediments. Laboratory procedures for dating sediments have been adapted from those for pottery and new procedures have been developed as the need arises.

The majority of sediment dating applications are carried out on quartz and potassium-rich feldspars and the general characteristics of the TL and OSL signals from these minerals are reviewed. For sediments some new problems were encountered, with some grains perhaps not being completely bleached at deposition. For OSL signals there is no simple procedure for the selection of a thermally stable signal, as there had been in the case of pottery.

Many different laboratory protocols have been developed as our understanding of the fundamental behaviour of luminescence signals from quartz and feldspar has improved. These protocols are explained and discussed, giving the advantages and disadvantages of each procedure as applied to different types of sediment.

This review is presented as a guide to the selection of the most appropriate procedure for a particular dating application.     

OSL properties of NaCl relative to dating and dosimetry

The luminescence properties of NaCl are discussed. Attention is focussed predominantly upon the optically stimulated luminescence (OSL) signal (UV emission) of NaCl (420–560 nm stimulation). The results from experiments relevant to geological dating applications, such as signal resetting, dose response and thermal stability, are described. The behaviour of the OSL signal observed was found to be favourable for dating, with both the thermal stability and dose response suggesting a range of at least 10 ka. Signal resetting via optical exposure was found to be extremely rapid and no evidence of either thermoluminescence or OSL signals was found following re-crystallization. The potential for dating natural salt deposits, as found in various arid environments, appears to be good.     

Na-rich feldspar as a luminescence dosimeter in infrared stimulated luminescence (IRSL) dating

One of the challenges in dating rock surfaces is the choice of the luminescence mineral. Although quartz is the preferred dosimeter in sediment dating, it is often not sufficiently sensitive when extracted from solid rocks. The intensity of signals from feldspars tends to be much less dependent on geological origin and erosion history, but the dosimetry of K-rich feldspar grains extracted from rocks is complicated because the internal dose rate is very dependent on the original feldspar grain size. The in situ grain size information is lost during the crushing process used to separate the grains for measurement. This latter problem does not apply to Na-rich feldspar because of the absence of internal radioactivity.The potential application of Na-rich feldspar as a luminescence dosimeter for the IRSL dating of rock surfaces is investigated using a variety of sediment samples from different geological settings for which independent age control is available. The blue and yellow luminescence emissions are measured for IR stimulation at 50 °C (IR₅₀), and post-IR IR stimulation at 290 °C (pIRIR₂₉₀). Thermal stability experiments imply that the corresponding signals in both emissions have comparable thermal stabilities and that all signals have similar recombination kinetics and are thermally stable over geological timescales. The IR₅₀ doses measured using blue and yellow emissions are similar to or lower than quartz doses while pIRIR₂₉₀ blue doses are higher than those from yellow emission and quartz doses. The fading rates measured for the IR₅₀ signals are ∼3%/decade larger than those measured for the pIRIR₂₉₀ signals in both yellow and blue emissions. Furthermore the average fading rates of both yellow signals are ∼3%/decade higher than the corresponding fading rates of the blue signals. However, there is no detectable correlation between fading rates and the measured D_e values. The residual doses measured from the laboratory-bleached samples and a modern analogue suggest that the IR₅₀ signals in both blue and yellow emissions bleach to the same degree, as do the corresponding pIRIR₂₉₀ signals, and that there is no significant naturally-unbleachable residual dose observed using these signals. Neither anomalous fading nor incomplete bleaching explains the observed dose discrepancy between the two emissions. Eight uncorrected and fading-corrected ages are calculated for each sample based on all four signals, using the dose rate relevant to Na-rich feldspar extracts (i.e. ∼3% K). The IR₅₀ and pIRIR₂₉₀ blue ages were also calculated assuming a dose rate based on 12.5% internal K (i.e. assuming that the blue signals were mainly derived from contamination by K-rich feldspar). The latter pIRIR₂₉₀ blue ages are in agreement with the expected age control, raising the possibility that this signal originates mainly from K-rich feldspar contamination in our Na-rich fractions, and thus is not so useful in the luminescence dating of rock surfaces. On the other hand, the pIRIR₂₉₀ fading-corrected ages based on the yellow emission are consistent with the independent age controls; higher preheat and stimulation temperatures may result in more stable yellow signals from Na-rich feldspar extracts from rocks, and so reduce the size of the fading correction. We conclude that, because this signal avoids the dosimetry difficulties of K-rich feldspar extracts, it has considerable potential in the IRSL dating of rock surfaces.     

Testing the reliability of ESR dating of optically exposed buried quartz sediments

Optical dating of quartz by optically stimulated luminescence has a time range that is generally less than about 500 ka, due to relatively rapid saturation of the available luminescence defects in quartz. We test here a new method, electron spin resonance (ESR) optical dating of quartz, in which radiation-sensitive defects at aluminum and titanium atoms on silicon sites give rise to signals which can only be detected near liquid nitrogen temperature and which have a much higher capacity to absorb radiation dose before saturating than optical luminescence-detected signals. Our results show this method yields agreement with independent age control out to about 2.5 million years, extending here the dating range of optically exposed quartz in sediments in along-shore sediments (aeolian and waterlain) by a factor of about 5. Three sites in along-shore lacustrine and marine aeolian environments yielded very good agreement with independent age control. Details of single saturating exponential fitting in relation to agreement with expected burial doses and annealing of Al and Ti signals provide additional data to consider the best approaches to the dating method. Furthermore, we propose a new criterion for ESR optical dating: both the Al signal and Ti signal ages must agree to insure accurate burial ages. Moreover, when Al signal ages are lower than Ti signal ages, then the Al signal may be taken as the minimum burial age.     

Relationship between the giant enhancement of the Raman scattering and luminescence on nanostructured metallic surfaces

The relationship between the factors of giant enhancement of luminescence and inelastic (Raman) scattering of light at planar silver nanostructures is investigated. For this purpose, the variation of the enhanced luminescence and Raman signals with the distance between the nanostructure surface and a layer of test molecules is examined. It is found that, for lines whose spectral position is close to the wavelength of the excitation laser, the enhancement factor for the Raman scattering is proportional to the square of that for the luminescence signal. As the spectral shift between the Raman lines and the laser position increases, the above dependence becomes subquadratic. It is established that the spatial scale on which the enhancement is manifested is the same for both effects and amounts to 25–30 nm.     

Raman spectroscopy of magnetoplasma excitations in a system of two-dimensional electrons in inclined and parallel magnetic fields

The dispersion of magnetoplasma excitations of two-dimensional electrons in oblique and parallel magnetic fields is investigated by the method of Raman scattering of light. The inclination of the field is used to distinguish the signals due to volume hot luminescence from the signals due to Raman scattering by two-dimensional electrons. The dependence of the magnetoplasmon energy on the inclination angle of the field is measured for different momentum transfers. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 12, 974–978 (25 June 1996)     

Behavioural studies of stimulated luminescence from feldspars

Since the work of Mejdahl in the mid-1980s feldspars have been used widely for palaeodosimetry. They have proved particularly popular because of the technical ease with which an optically stimulated luminescence signal can be obtained, and the potential for their use in dating over a wider time range than quartz. However, while the use of infrared stimulated lminescence has been a great success, the realisation of the potential for dating over a wide time range has been hampered by a series of behavioural problems, in particular concerned with the stability of the luminescence signal. This paper surveys the current state of knowledge of a range of aspects of the stimulated luminescence behaviour of feldspar. Particular attention is paid to the thermal and optical properties of the thermoluminescence (TL) and optically stimulated luminescence (OSL) signals from feldspars since these are of greatest relevance to their use as palaeodosimeters. Results obtained from well-characterised museum specimens demonstrate the variety of responses that are seen from different types of feldspar, particularly in their TL behaviour. Less variation is seen in their OSL response. In contrast, the response of potassium-rich feldspars separated from Quaternary sediments tends to be very consistent when many grains are analysed simultaneously, suggesting that averaging of the signal simplifies the behaviour of the system.     

Time-resolved infrared stimulated luminescence signals in feldspars: Analysis based on exponential and stretched exponential functions

Time-resolved infrared-stimulated luminescence (TR-IRSL) signals from feldspar samples have been the subject of several recent experimental studies. These signals are of importance in the field of luminescence dating, since they exhibit smaller fading effects than the commonly employed continuous-wave infrared signals (CW-IRSL). This paper presents a semi-empirical analysis of TR-IRSL data from feldspar samples, by using a linear combination of exponential and stretched exponential (SE) functions. The best possible estimates of the five parameters in this semi-empirical approach are obtained using five popular commercially available software packages, and by employing a variety of global optimization techniques. The results from all types of software and from the different fitting algorithms were found to be in close agreement with each other, indicating that a global optimum solution has likely been reached during the fitting process. Four complete sets of TR-IRSL data on well-characterized natural feldspars were fitted by using such a linear combination of exponential and SE functions. The dependence of the extracted fitting parameters on the stimulation temperature is discussed within the context of a recently proposed model of luminescence processes in feldspar. Three of the four feldspar samples studied in this paper are K-rich, and these exhibited different behavior at higher stimulation temperatures, than the fourth sample which was a Na-rich feldspar. The new method of analysis proposed in this paper can help isolate mathematically the more thermally stable components, and hence could lead to better dating applications in these materials.     

Luminescence techniques: instrumentation and methods

This paper describes techniques, instruments and methods used in luminescence dating and environmental dosimetry in many laboratories around the world. These techniques are based on two phenomena – thermally stimulated luminescence and optically stimulated luminescence. The most commonly used luminescence stimulation and detection techniques are reviewed and information is given on recent developments in instrument design and on the state of the art in luminescence measurements and analysis.     

Optically stimulated luminescence (OSL) study of synthetic stishovite

Optically stimulated luminescence (OSL) of synthetic stishovite was investigated for a future dating technique of meteor impact craters. Luminescence around 330 nm was measured on the γ-ray irradiated stishovite under two stimulating light sources of infrared laser (830 nm) and blue light emitting diode set (470 nm). Thermoluminescence (TL) studies before and after the OSL measurements showed the intensities around 100–200°C and 220–350°C to increase and those around 350–450°C to decrease. This indicates that a part of deep-trapped charges excited during the OSL measurements were retrapped by shallower traps. The infrared stimulated luminescence (IRSL) after the TL measurement up to 450°C could not be detected, while the blue light stimulated luminescence (BLSL) after TL had about one-tenth of the intensity before TL. This indicates that a part of the charges in shallower traps were detrapped thermally and returned to the deeper traps which were related to BLSL. The result implies that some of the BLSL-related traps are quite stable at room temperature and could be used for geological dating. In addition, two paramagnetic centers produced by sudden release of high pressure in synthesis process were found in the unirradiated stishovite by electron spin resonance (ESR). Their g-factors are g=2.00181 and g=2.00062 for an axial signal and g=2.00305 for the other isotropic signal. These signals could be used for an evidence of impacts if those signals could be stored in geological time.     

Spectral information from minerals relevant for luminescence dating

The paper reviews basic spectral features of luminescence from minerals used in dating and allied research. Luminescence production is a result of multiple interactions within the imperfect crystal lattice and spectral information is not limited to the emission of light. Results of spectral investigations of luminescence emission during thermal stimulation (TL) or optical stimulation (OSL) form the main part of the paper. However, information on luminescence excitation and light absorption spectroscopy is also presented and possible links between luminescence production in minerals and particular lattice defects are considered. Quartz and feldspars, the most commonly used minerals, receive special attention, but the review includes other materials such as polymineral fine-grained fractions from sediments, zircon, calcite and other salts (halite, sulfate), meteorites, flint, volcanic materials (obsidian, tephra), ceramics and metallurgical slags. Although a wide range of different luminescence emission wavebands occur, it can be shown that certain emissions dominate in particular materials. Basic dosimetric properties are often known just for single emission wavebands of a particular mineral, and are listed in this case. The paper also aims to provide a starting point and inspiration for the study of other TL and OSL emissions, with particular regard to their potential and suitability for dating and related dosimetry tasks. These investigations, involving palaeodose determination based on an emission waveband with known characteristics, need careful separation of the particular emission peak, which may be influenced by its behaviour during the dating procedure (sample preparation, irradiation, preheat treatments, luminescence measurements, etc.). Spectral information available in this context and some technical remarks on the experimental conditions will be given to pave the way for conventional TL or OSL measurements in luminescence dating and dosimetry using natural or semi-natural materials.     

Further investigations of tunneling recombination processes in random distributions of defects

The shape of infrared stimulated luminescence signals (IRSL) from feldspars has been the subject of numerous studies in the field of luminescence dating. Specifically linearly modulated IRSL signals (LM-IRSL) are commonly assumed to consist of several first order components corresponding to distinct optical stimulation cross sections. This paper models the shape of LM-IRSL signals using a recently proposed kinetic model, which describes localized electronic recombination in donor–acceptor pairs of luminescent materials. Within this model, recombination is assumed to take place via the excited state of the donor, and nearest-neighbor recombinations take place within a random distribution of centers. The model has been used previously successfully to describe both thermally and optically stimulated luminescence (TL, OSL). This paper shows that it is possible to obtain approximate solutions for the distribution of donors in the ground state as a function of two variables, time and the distance between donors and acceptors. Approximate expressions are derived for several possible modes of optical and thermal stimulation, namely TL, OSL, linearly modulated OSL (LM-OSL), LM-IRSL and isothermal TL (ITL). Numerical integration of these expressions over the distance variable yields the distribution of remaining donors at any time t during these experimental situations. Examples are given for the derived distributions of donors in each experimental case, and similarities and differences are pointed out. The paper also demonstrates how LM-IRSL signals in feldspars can be analyzed using the model, and what physical information can be extracted from such experimental data. The equations developed in this paper are tested by fitting successfully a series of experimental LM-IRSL data for Na- and K-feldspar samples available in the literature. Finally, it is shown that the equations derived in this paper are a direct generalization of an equation previously derived for the case of ground state tunneling.     

Steps towards surface dating using luminescence

Whereas luminescence dating applications usually avoid gradients and therefore surfaces, this study explores the requirements and possibilities of dating surfaces using infrared stimulated luminescence. The basic prerequisite for dating applications is a dose-dependent, bleachable and stable luminescence signal. It was found to be fulfilled for crushed granite rock material. Depth profiles below polished surfaces of granites indicate that a well bleached layer below the actual surface solely contributes to the infrared stimulated luminescence signal. A new measurement device, designed for in situ surface dating applications, is presented along with first experimental results.     

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.     

Investigating signal evolution: A comparison of red and UV/blue TL,and UV OSL emissions from the same quartz sample

Samples from plutonic geological settings emit strong and stable UV, blue and red luminescence. These samples provide a unique opportunity to investigate the evolution of luminescence emissions by comparing their origin, intensity, trapping and recombination characteristics. In addition, the D_e derived from certain techniques can be used to investigate D_e consistency. Thus, UV and red signals emitted by the same sample of grantic quartz have been compared according to the strength and shape of the emissions, response to annealing and dose, growth characteristics and the resulting D_e derived from single-aliquot regeneration (SAR – UV emissions) and the dual-aliquot protocol for red TL (DAP – red emissions). This comparison of UV and red D_e values can be used as an alternative measure of accuracy for depositional situations where independent age control is not available. Furthermore, this comparison has been extended to a single-grain level to determine if UV and red signals are emitted by the same quartz grain. The resulting comparisons demonstrate that both signals are strong, reproducible, are suitable for dating, and a comparison of D_e is a valid internal check of consistency. The single-grain analysis demonstrates that the grains contain UV and red recombination centres rather than the typical characteristics of just beta or alpha quartz. Thus it has been suggested that the emissions relate to post-origin modifications of the signal rather than its thermal origin. This type of comparative analysis provides clues as to the source and evolution of the signals, provides a greater understanding of the complexities of the luminescence signal and a possible solution for accuracy testing in challenging depositional contexts.     

A new irradiated quartz for beta source calibration

For luminescence dating to be an accurate absolute dating technique it is very important that we are able to deliver absolutely known radiation doses in the laboratory. This is normally done using a radiation source (alpha, beta, X-ray) calibrated against an absolutely known reference source. Many laboratories have used the various different batches of Risø calibration quartz for the calibration of beta and X-ray sources, but these have been largely undescribed. Here we describe in detail the preparation and luminescence characteristics of a new quartz standard, based on a North Sea beach sand collected from south-western Denmark (Rømø). Two grain sizes (4–11 μm and 180–250 μm) have been examined in detail. These were pre-treated (annealed, dosed and annealed again) to sensitise and stabilise the luminescence signals before being given an absolutely known gamma dose from a point ¹³⁷Cs source in scatter-free geometry. The luminescence characteristics are described; the very intense blue-light stimulated signal is dominated by the fast OSL component and the IR-stimulated signal is negligible. The material is shown to be suitable for measurement using SAR, and the dose recovery ratio is indistinguishable from unity with a standard deviation of <2% for multi-grain aliquots. The material is also shown to be suitable for single-grain calibration, with >80% of the grains giving a useful signal. Although there is an unexplained dispersion in our calibration data of ∼3% (which we cannot attribute to instrument variability), we nevertheless conclude that this material is very suitable for transferring absolute known doses from a standardised gamma source to in-built irradiation sources.     

Raman scattering and exciton luminescence at low temperatures

A theoretical study is made of the effect of Raman scattering of light in crystals on their exciton luminescence. The polariton concept is used to show that Raman scattering may play a determinative role in the low-temperature luminescence of excitons with a large oscillator strength. The luminescence spectrum calculated on a computer by the Monte Carlo method is in good agreement with experimental data. This enables a number of experimental facts concerning the luminescence of crystals at low temperatures to be ascribed to the influence of Raman scattering.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 1, pp. 100–103, January, 1973.     

The basic principle of radioluminescence dating and a localized transition model

The IR signal of the radioluminescence of potassium feldspars is caused by the luminescent transition of electrons into optically active traps. This allows the direct determination of the density of trapped electrons and therefore a method of sediment dating with higher precision and accuracy than conventional luminescence dating. The principle behind it and its advantages are presented, in particular the fact that it is a real single aliquot dating technique. The explanation of both radioluminescence and IR-optically stimulated luminescence (IR-OSL) in terms of a band model is possible after the introduction of a localized transition. In contrast to previous models, the process of dose accumulation in the sediment was simulated using a dose rate as low as in real sediments. Preheat experiments indicate that the recombination centres are the unstable part of the luminescence process. The parameters of these centres are equal to those previously assigned to thermally unstable electron traps. Furthermore, the sources of systematic errors in conventional IR-OSL dating are discussed.     

Identifying well-bleached quartz using the different bleaching rates of quartz and feldspar luminescence signals

When dating older sedimentary deposits using quartz, there are no unambiguous methods for identifying the presence of incomplete bleaching. Current statistical analysis of dose distributions depends entirely on the assumption that incomplete bleaching and mixing are the main causes of any excess dispersion in the distribution; the only existing way to test this assumption is using independent age control. Here we suggest a new approach to this question, based on the differential bleaching rates of quartz and feldspar luminescence signals. We first present data that confirm the differences in relative bleaching rates of quartz optically stimulated luminescence (OSL) and feldspar luminescence stimulated at 50 °C by infrared light (IR₅₀) and feldspar luminescence stimulated at 290 °C by infrared light after a stimulation at 50 °C (pIRIR₂₉₀), and use recently deposited samples to determine the likely significance of the difficult-to-bleach residual feldspar signals in non-aeolian samples. For a set of mainly Late Pleistocene non-aeolian sediments, large aliquot quartz doses are then used to predict feldspar doses (based on a knowledge of the sample dose rates). The differences between observed and predicted feldspar doses as a function of the quartz dose, combined with a conservative assumption concerning the relative feldspar and quartz residual signals after natural bleaching prior to deposition, are used to identify those samples for which the quartz is very likely to be well bleached (20 out of 24). Two of these apparently well-bleached samples are then examined using single-grain quartz dose distributions; one of these is consistent with the well-bleached hypothesis, and one indicates poor bleaching or a multi-component mixture. However, independent age control makes it clear that the large aliquot data are more likely to be correct. We conclude that a comparison of quartz and feldspar doses provides a useful independent method for identifying well-bleached quartz samples, and that it is unwise to apply statistical models to dose distributions without clear evidence for the physical origins of the distributions.