Kristina Jørkov Thomsen

Stability of IRSL signals from sedimentary K-feldspar samples

Recent work has identified IR stimulated luminescence signals at elevated temperature from both potassium- and sodium-rich feldspars that have much lower anomalous fading rates than the conventional signal measured using IR stimulation at 50°C. This paper examines the stability of these signals for potassium-rich sedimentary feldspars. We show that the natural post-IR IRSL (pIRIR) signal from a 3.6 Ma old sample is in apparent saturation on a laboratory generated dose response curve, i.e. it does not show detectable fading in nature although a low fading rate is observed on laboratory time scales. We show that the pIRIR signal has a greater thermal stability than the IRSL signal and that the trend in increasing thermal stability is mirrored by a decreasing fading rate. We also investigate the effect of preheat temperature and IR stimulation power on the decay shape and conclude that the data can be explained in terms of either a single- or multiple-trap model. We present evidence that may suggest that at least part of pIRIR signal is derived from a high temperature trap (∼550°C thermoluminescence (TL) peak), although again the data can also be explained in terms of a single-trap model. Finally, we present dose response curves and characteristic curvature constants (D0) values for various IRSL signals and conclude that the more stable signals saturate more quickly than the less stable signals and that the initial and final signals saturate at approximately the same level.

Variation with depth of dose distributions in single grains of quartz extracted from an irradiated concrete block

Abstract Most attempts to apply retrospective dosimetry using luminescence methods to building materials have made use of heated (sensitised) items such as brick or tile ceramic. Unfired materials, such as mortar and concrete, are much more widespread in the industrial environment, but unfortunately these cannot be assumed to contain a negligible dose at the time of construction. We have analysed the dose distributions derived from optically stimulated luminescence (OSL) measurements of single grains of quartz extracted from two industrially produced concrete blocks, one unirradiated and the other irradiated normal to one face in the laboratory using 137 Cs gamma photons. The OSL dose-depth profile for the irradiated block was determined by measuring the dose distributions from single quartz grains extracted from slices taken across the block and compared with that predicted using Monte Carlo calculations. Despite measured doses in grains extracted from the unirradiated concrete of up to 140 Gy , we show that it is possible to detect accrued doses of about 100 mGy from grains extracted from the bulk concrete, and about 50 mGy in grains extracted from the surface of the block.

How confident are we in the chronology of the transition between Howieson's Poort and Still Bay?

Center for Nuclear Technologies, Technical University of Denmark, DTU Risø Campus, DK-4000 Roskilde, Denmark Nordic Laboratory for Luminescence Dating, Department of Geoscience, Aarhus University, DTU Nutech, Risø Campus, DK-4000 Roskilde, Denmark c Institut de Recherche sur les Archéomatériaux, UMR 5060 CNRS e Université de Bordeaux, Centre de Recherche en Physique Appliquée à l’Archéologie (CRP2A), Maison de l’archéologie, 33607 Pessac cedex, France

INVESTIGATING THE RESETTING OF OSL SIGNALS IN ROCK SURFACES

There are many examples of buried rock surfaces whose age is of interest to geologists and archaeologists. Luminescence dating is a potential method which can be applied to dating such surfaces; as part of a research project which aims to develop such an approach, the degree of resetting of OSL signals in grains and slices from five different cobbles/boulders collected from a modern beach is investigated. All the rock surfaces are presumed to have been exposed to daylight for a prolonged period of time (weeks to years). Feldspar was identified as the preferred dosimeter because quartz extracts were insensitive. Dose recovery tests using solar simulator and IR diodes on both K-feldspar grains and solid slices taken from the inner parts of the rocks are discussed. Preheat plateau results using surface grains and slices show that significant thermal transfer in naturally bleached samples can be avoided by keeping preheat temperatures low. Equivalent doses from surface K-feldspar grains were highly scattered and much larger than expected (0.02 Gy to >100 Gy), while solid surface slices gave more reproducible small doses (mean = 0.17±0.02 Gy, n = 32). Neither crushing nor partial bleaching were found to be responsible for the large scattered doses from grains, nor did the inevitable contribution from Na-feldspar to the signal from solid slices explain the improved reproducibility in the slices. By modelling the increase of luminescence signal with distance into the rock surface, attenuation factors were derived for two samples. These indicate that, for instance, bleaching at a depth of 2 mm into these samples occurs at about ∼28% of the rate at the surface. We conclude that it should be possible to derive meaningful burial doses of >1 Gy from such cobbles; younger samples would probably require a correction for incomplete bleaching.

An assessment of cumulative external doses from Chernobyl fallout for a forested area in Russia using the optically stimulated luminescence from quartz inclusions in bricks

Optically stimulated luminescence (OSL) has been used for estimation of the accumulated doses in quartz inclusions obtained from two fired bricks, extracted in July 2004 from a building located in the forested surroundings of the recreational area Novie Bobovichi, the Bryansk Region, Russia. The area was significantly contaminated by Chernobyl fallout with initial (137)Cs ground deposition level of approximately 1.1 MBq m(-2). The accumulated OSL doses in sections of the bricks varied from 141 to 207 mGy, of which between 76 and 146 mGy are attributable to Chernobyl fallout. Using the OSL depth-dose profiles obtained from the exposed bricks and the results from a gamma-ray-survey of the area, the Chernobyl-related cumulative gamma-ray dose for a point detector located in free air at a height of 1m above the ground in the study area was estimated to be ca. 240 mGy for the time period starting on 27 April 1986 and ending on 31 July 2004. This result is in good agreement with the result of deterministic modelling of the cumulative gamma-ray dose in free air above undisturbed ground from the Chernobyl source in the Bryansk Region. Over the same time period, the external Chernobyl-related dose via forest pathway for the most exposed individuals (e.g., forest workers) is estimated to be approximately 39 mSv. Prognosis for the external exposure from 1986 to 2056 is presented and compared with the predictions given by other investigators of the region.

High local ionization density effects in x-ray excitations deduced from optical stimulation of trapped charge in Al2O3:C

X-rays are used extensively in luminescence studies. Recent studies have shown a strong sample and material dependence in the response of the optically stimulated luminescence (OSL) signal to x-rays, which cannot be accounted for by differences in the photoelectric cross-sections of these materials. In this paper we give direct experimental evidence for a high local ionization density caused during low energy x-ray interactions; this is similar to the effect produced by high LET (linear energy transfer) radiations such as protons. This effect is deduced on the basis of optical stimulation of trapped charge in carbon doped aluminium oxide (Al2O3:C) after exposure to different radiations ( 90 Sr/ 90 Y beta, proton, 137 Cs gamma and x-rays), and making a comparative analysis of the changes in the initial OSL decay rates. The sensitive parameter λ (the decay constant) is >100% larger for irradiations carried out with x-rays in comparison to that for low LET radiation such as beta particles. We further show that the nature of the increase in λ with dose, λ(D) ,i s a saturating exponential function plus a constant (λ0), and that both λ(D) and λ0 are unique for a given radiation type or more specifically its LET. Such behaviour is explained by superimposition of OSL from several first-order traps or by the presence of a competing trap in Al2O3:C. The understanding of x-ray interaction with OSL traps presented here explains the x-ray dose rate calibration problem in different materials, and opens up the area of studying ionization density effects in low energy x-ray interactions using the OSL of Al2O3:C.

Counter-intuitive influence of Himalayan river morphodynamics on Indus Civilisation urban settlements

Urbanism in the Bronze-age Indus Civilisation (~4.6–3.9 thousand years before the present, ka) has been linked to water resources provided by large Himalayan river systems, although the largest concentrations of urban-scale Indus settlements are located far from extant Himalayan rivers. Here we analyse the sedimentary architecture, chronology and provenance of a major palaeochannel associated with many of these settlements. We show that the palaeochannel is a former course of the Sutlej River, the third largest of the present-day Himalayan rivers. Using optically stimulated luminescence dating of sand grains, we demonstrate that flow of the Sutlej in this course terminated considerably earlier than Indus occupation, with diversion to its present course complete shortly after ~8 ka. Indus urban settlements thus developed along an abandoned river valley rather than an active Himalayan river. Confinement of the Sutlej to its present incised course after ~8 ka likely reduced its propensity to re-route frequently thus enabling long-term stability for Indus settlements sited along the relict palaeochannel.The Bronze-age Indus civilisation (4.6–3.9 ka) was thought to have been linked to the development of water resources in the Himalayas. Here, the authors show that along the former course of the Sutlej River the Indus settlements developed along the abandoned river valley rather than an active Himalayan river.