KINETIC ANALYSIS OF THERMOLUMINESCENCE GLOW CURVES IN FELDSPAR: EVIDENCE FOR A CONTINUOUS DISTRIBUTION OF ENERGIES

Abstract: The thermoluminescence (TL) glow curves from feldspars have been the subject of numerous studies, because of their importance in luminescence dating and dosimetry. This paper presents new experimental TL glow curves in a plagioclase feldspar, measured using the T max -T stop technique of glow curve analysis. Kinetic analysis of the experimental results is carried out for a freshly irradiated sample, as well as for a sample which has undergone optical treatment using infrared light for 100 s at 50°C. Application of the initial rise method of analysis indicates that the TL signals from both samples can be characterized by a continuous distribution of energy levels. By subtracting the TL glow curves measured at successive T stop values, a series of TL glow curves is obtained which are analyzed using the empirical general order kinetics. It is found that all TL glow curves obtained by this subtractive procedure can be described accurately by the same general order parameter b ~1.7. In a second attempt to analyze the same TL glow curves and possibly extract information about the underlying luminescence process, the shape of TL glow curves is analyzed using a recently proposed physical kinetic model which describes localized electronic recombination in donor-acceptor pairs. 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. This recent model has been used recently to describe successfully several types of luminescence signals. This paper shows that it is possible to obtain good fits to the experimental data using either one of these two approaches

An attempt to correct for the fading in million year old basaltic rocks

The use of feldspar for luminescence dating has been restricted because of anomalous fad-ing. This has made its application to several important geological problems such as volcanic terrains difficult. Presently, two correction procedures are used to correct for anomalous fading. The present study tests these correction procedures using volcanic samples of known ages spanning the time peri-od of 400 ka to 2.2 Ma. These correction procedures provided grossly underestimated ages (up to 60%). The possible causes for the underestimation are discussed

Mathematical characterization of continuous wave infrared stimulated luminescence signals (CW-IRSL) from feldspars

a b s t r a c t Continuous-wave infrared stimulated luminescence signals (CW-IRSL) from feldspars have been the subject of many experimental studies, due to their importance in luminescence dating and dosimetry. Accurate mathematical characterization of the shape of these CW-IRSL signals in feldspars is of practical and theoretical importance, especially in connection with "anomalous fading" of luminescence signals in dating studies. These signals are known to decay in a non-exponential manner and their exact mathematical shape as a function of stimulation time is an open research question. At long stimulation times the IRSL decay has been shown experimentally to follow a power law of decay, and previous researchers have attempted to fit the overall shape of these signals empirically using the well known Becquerel function (or compressed hyperbola decay law). This paper investigates the possibility of fitting CW-IRSL curves using either the Becquerel decay law, or a recently developed analytical equation based on localized electronic recombination of donor-acceptor pairs in luminescent materials. It is shown that both mathematical approaches can give excellent fits to experimental CW-IRSL curves, and the precision of the fitting process is studied by analyzing a series of curves measured using a single aliquot of a feldspar sample. Both fitting equations are solutions of differential equations involving numerically similar time dependent recombination probabilities k(t). It is concluded that both fitting equations provide approximately equivalent mathematical descriptions of the CW-IRSL curves in feldspars, and can be used as mathematical representations of the shape of CW-IRSL signals

Towards quantifying beta microdosimetric effects in single-grain quartz dose distribution

In luminescence dating, the single grain approach offers a promise for identification of the most bleached grains for age calculations. A proper interpretation of single grain dose distribution however is still awaited. Ideally the palaeodose distribution should be a sharply peaked distribution with its width determined by the experimental errors. However, a large range of values of the relative standard deviation (RSD), even for well-bleached samples, indicates the presence of additional effects, not considered so far in the literature. We suggest that microscopic fluctuations in the spatial distribution of feldspar containing 40K β emitters (termed as hotspots) can cause heterogeneous distribution of dose rate. This paper models these fluctuations and quantifies their effect on the distribution of doses in quartz. In this approach the dose to a quartz grain from a given spatial configuration of the hotspots within a sphere of maximum beta range is computed and this is then averaged over all possible configurations, to arrive at the dose rate/palaeodose distribution. The dose distribution is positively skewed. The net effect of such a distribution is estimated as a function of potassium concentration (lower the potassium, higher the hotspot heterogeneity and larger is the palaeodose distribution). For well bleached samples with low potassium content, the computations suggest that: (a) the probability of grains receiving zero doses is negligible and (b) distribution of doses is large. We suggest a minimum dose concept through a percentile analysis of the distribution function, viz., as the lowest doses received by a small fraction of the grains. This study therefore may imply a paradigm shift in the manner, in which the ages are computed from single grain paleodose distributions

Depositional environment and OSL chronology of the Homeb silt deposits, Kuiseb River, Namibia

Previous studies suggest that the Homeb silts of the Kuiseb valley, Namibia (i) accumulated in a dune-dammed lake, (ii) are end-point deposits, (iii) represent an aggrading river bed, and (iv) are slackwater deposits. Thus, they have been used alternatively as evidence of past drier conditions or past wetter conditions. Lithostratigraphic analysis of two sediment sequences at Homeb indicates sedimentation by aggradation of the Kuiseb River triggered by a transition from an arid to humid climate. OSL ages for the sequences were obtained by the SAR protocol on aliquots of 9.6-mm and 4.0-mm diameter and on single grains. Four-millimeter aliquot minimum ages closely approximate the single-grain minimum ages and are younger than 9.6-mm aliquot minimum and central ages. Based on these results, the small-aliquot (4-mm) approach appears to provide ages comparable to those obtained by the more laborious and time-consuming single-grain method. Minimum ages indicate rapid deposition of the Homeb Silts in at least two episodes centered at ~15 ka and ~6 ka during climate transitions from arid to humid. Flash floods eroded the valley fills during slightly more arid conditions

A 1000-year history of large floods in the Upper Ganga catchment, central Himalaya, India

10.1016/j.quascirev.2013.07.022Quaternary Science Reviews77156-166QSRE

Thermoluminescence of the meteorite interior: A possible tool for the estimation of cosmic ray exposure ages

Thermoluminescence of meteorites has been studied for nearly five decades and has provided useful information on meteorite classification and on terrestrial age estimation. In respect of natural luminescence measurement, studies so far have used TL emission in the blue region and the conventional protocols have yielded equivalent doses (De) of ~ 1000 Gy. Further, the annual cosmic ray dose to meteorites has been assumed as 0.1 Gy/year and, this does not take into account the depth dependence of cosmic rays inside meteorite interior and the efficiency of luminescence production that depends on linear energy transfer. Together, these De and dose rate estimates provide exposure ages of ~ 10,000 years, which are about two or more orders of magnitude lower compared to that estimated using cosmogenic radionuclides. Such abnormally low De's have been enigmatic as the analyses of kinetic parameters of luminescence of meteorites suggest a stability of luminescence signals of over a million years. The present study rigorously explored reasons for such low equivalent doses by, a) estimating De's for luminescence emission under different spectral emission windows, b) by using different protocols for the estimation of stability (particularly athermal) and c) by re-estimating the annual cosmic ray dose taking into account dose attenuation and the linear energy transfer effects on luminescence production efficiency. The result suggest significant improvements in understanding the relationship between natural luminescence and cosmic ray exposure ages via, a) probing the isothermal signal in the red emission band, b) a proper evaluation of the annual cosmic rays dose that was so far over estimated by a factor of ~2, and c) an estimation of the athermal fading (also termed as anomalous fading) rates. We found that athermal fading rate (g-value) decreased nonlinearly with cosmic ray exposure (CRE) ages and ranged from 9.4 ± 1.1 to 2.6 ± 1.1 (% per decade) for CRE age ranging from 2 to 110 Ma. We surmise that the decrease of fading rate is due to radiation damage by high energy heavy charged particle bombardment and consequent loss of defect centers. The equivalent doses increase with CRE ages accord with theoretical calculations, suggesting thereby the possibility of using a combination of g-values and De for constraining the CRE ages of meteorites