Spatially-resolved measurement of optically stimulated luminescence and time-resolved luminescence

Spatially-resolved measurements of optically stimulated luminescence (OSL) were performed using a two-dimensional scanning system designed for use with planar samples. The scanning system employs a focused laser beam to stimulate a selected area of the sample, which is moved under the beam by a motorised stage. Exposure of the sample is controlled by an electronic shutter. Mapping of the distribution of OSL using a continuous wave laser source was obtained with sub-millimeter resolution for samples of sliced brick, synthetic single crystal quartz, concrete and dental ceramic. These revealed sporadic emission in the case of brick or concrete and significant spatial variation of emission for quartz and dental ceramic slices. Determinations of absorbed dose were performed for quartz grains within a slice of modern brick. Reconfiguration of the scanner with a pulsed laser source enabled quartz and feldspathic minerals within a ceramic sample to be differentiated using a single stimulation wavelength by measuring the time-resolved luminescence spectrum

Extending the range of optically stimulated luminescence using vein quartz and quartzite sedimentary pebbles

The feasibility of applying optically stimulated luminescence (OSL) techniques to obtain estimates of the burial age of lithic clasts in the form of pebbles of quartzose lithology is investigated in this study. We applied OSL measurement procedures to vein-quartz and quartzite pebbles from contexts at five sites with fluvial and coastal beach contexts of depositional age ranging from 500 ka to the mid Holocene. As in previous OSL studies with lithics, measured depth-dose profiles were interpreted to identify regions associated with key stages in the burial history of the pebbles, including the extent of pre-burial optical resetting, primary burial and any subsequent secondary stages of exposure to sunlight. While some of the pebbles had not been fully optically reset before burial, none were in field saturation, as commonly encountered with larger lithic clasts such as cobbles. The relatively low concentrations of radionuclides within vein-quartz and quartzite pebbles simplified the assessment of the radiation dose rate. OSL age estimates consistent with independent dating evidence were obtained for the three sites sampled in the UK (Langford Quarry, Leet Hill Quarry, Swanscombe Skull Site and Barnfield Pit) and one of two sites tested in Portugal (Forte Cão), whereas evidence of secondary optical bleaching detected in pebbles from a second site (Vale de Atela) underestimated the independent age for primary burial. The oldest depositional age of the contexts tested was ∼500 ka (Leet Hill Quarry), and for some of the pebbles tested there was the capacity to extend the potential dating range further as the age equivalence of the limiting value of De was in excess of 1 Ma. The advantages of working with smaller, portable, clasts in the form of pebbles and with a lithology of higher transparency opens up a wide range of potential applications, whether the depositional processes are environmental or anthropogenic in origin or modification

Luminescence dating of sediment mounds: associated with shaft and gallery irrigation systems

Optically stimulated luminescence (OSL) techniques, supported by geomorphological analysis, have been applied to date the construction of shaft and gallery irrigation systems, more commonly referred to as qanats, falaj and foggara. The approach developed was tested on four hydraulic systems located in semi-arid landscape settings, three in Murcia, Spain, and the fourth in the Sus Tekna region, Morocco. Excavation of the characteristic sediment mounds that surround each ventilation shaft enabled a detailed examination of strata containing upcast deposits and their assignment to the main stages in the construction and use of the hydraulic feature. OSL techniques with single grain resolution applied to samples taken from the key strata provided age estimates for their deposition on the mound and, from these, dating of the construction and use of the system. Of the four irrigation systems analysed, the OSL dates indicated that the youngest had been constructed in the 19th century AD and the oldest, located in Murcia, was dated to the Roman period. The latter is of archaeological significance because the introduction of this particular form of hydraulic technology to Spain is widely identified as an Islamic innovation of the early 8th century AD

Dating the setting of a late prehistoric statue-menhir at Cruz de Cepos, NE Portugal

The emergence of ‘standing stone’ monuments within the European Late Prehistoric landscape is considered to be associated with a pivotal human cultural transition from hunting and gathering to agriculture and permanent settlement, being the earliest monuments currently dated by radiocarbon to the 5th millennium BCE. However, many standing stones were first erected, subsequently collapsed, and then re-erected during the following three millennia. The excavation of the site of an apparently in situ statue-menhir at Cruz de Cepos in NE Portugal provided the rare opportunity in Iberian prehistory to apply radiocarbon and luminescence techniques to establish the date of construction. On the basis of the iconography, the standing stone was assigned to a sculptural tradition of north-western and western Iberia, loosely dated to the Early/Middle Bronze Age (ca. 2000/1900–1250 BCE). The optically stimulated luminescence (OSL) and dosimetry characteristics of quartz extracted from sediment samples taken from locations associated with the socket pit and surrounding backfilling deposits were examined, producing OSL single grain ages at eight locations. Comparison of the OSL and calibrated radiocarbon ages shows very good agreement, with the mid-3rd millennium BCE dates confirming original erection during the Copper Age and not a much later transformation of the monument. These encouraging results indicate that OSL has the potential to provide reliable dating of depositional processes related to the construction process and is suitable for wider application to megalithic monuments of this type

An examination of beta dose attenuation effects in coarse grains located in sliced samples

Previous work has demonstrated the feasibility of performing spatially resolved luminescence measurements with sliced solid materials (e.g., rock and ceramic) to determine the cumulative absorbed dose to individual luminescent grains in situ. In the present study, aspects of the dosimetry of individual grains that are truncated during slicing were examined using radiation transport simulations employing simplified geometries. The results of the simulations were applied to model the effect of grain truncation on the dosimetry of grains, in particular the beta attenuation factor and the laboratory beta source dose rate for whole and partial volumes of individual spherical grains. Where a material contains a wide range of coarse grain sizes there is the potential for misinterpreting the size of the parent grain on the basis of an examination of the shape and size of the truncated grain section exposed in the slice surface. If the original grain size is underestimated, which is likely, the simulations predict an overestimation of both the beta attenuation factor and the laboratory source beta dose rate, the maximum extent of which depends on the range of grain sizes present in the material. The simulations also indicate that by limiting the depth from the surface within which the average absorbed dose is determined, approximating the effects of opacity of the mineral, the magnitude of these deviations is reduced. However, a consequence of particular interest deriving from these results is that, when evaluating the age equation, the changes in the two quantities compensate, acting to moderate the overall effect on the calculated age for an individual grain, depending on the geometry of the grain and the composition of the sample material