English heritage luminescence dating

Appendix 1: Sources of advice on. Scientific Dating from English Heritage Appendix 2: Luminescence laboratory contact details.
Table of contents

They all have the same number of protons, but the number of neutrons varies. The fact that they have varying numbers of neutrons makes no difference whatsoever to the chemical reactions of the carbon. Isotopes are atoms which have the same atomic number but different mass numbers. An atom in which the total number of electrons is not equal to the total number of protons, giving the atom a net positive or negative electrical charge.

The process by which an atom acquires a negative or positive charge by gaining or losing electrons to form ions. The transmission of energy in the form of waves or particles through space or through a material medium. There are two types of radiation: Non-ionizing radiation and Ionizing radiation. The spontaneous disintegration of atoms by emission of energy, including alpha and beta particles, and gamma rays.

The property of a material to decay and emit ionizing radiation is called radioactivity and the material is said to be radioactive. Isotopes such as 14C which emit radiation as they transform into a different isotope or a different element. Any type of electromagnetic radiation that does not carry enough energy to ionize atoms ,that is, to completely remove an electron from an atom. Radiation that carries enough energy to remove electrons from atoms , thereby ionizing them.

Ionizing radiation is capable of disrupting stable atoms and causing them to have an imbalance of charge. Such a radiation is composed of high energy subatomic particles, ions or atoms and has a higher frequency and shorter wavelength than nonionizing radiation. There are three main types of ionizing radiation: Alpha and beta radiations are particles, while gamma radiation is a wave similar to X-rays. Light emitted by some minerals when thermally or optically stimulated following to having been exposed to ionizing radiation.

The light is normally weak, invisible to the naked eye, but detectable in the laboratory using a photomultiplier tube a sensitive device for measuring very small amounts of light. Exposing a mineral to daylight in order to remove the trapped energy. A chronological method based on the emission of light luminescence , by commonly occurring minerals, principally quartz. The method can be applied to a wide range of materials that contain quartz or similar minerals.

A unit of energy, work, or amount of heat in the International System of Units.

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The International System unit used for absorbed radiation dose: The symbol of Gray is Gy. A sub- sample of the material whose luminescence signal is being measured. Typically, aliquots of sand-sized grains are 1—5mg of the sample. A device that measures exposure to ionizing radiation. The current and most recent of the three periods of the Cenozoic Era in the geologic time scale. It follows the Neogene Period and spans from approximately 2. The Quaternary Period is divided into two epochs: Luminescence dating is frequently used for age determination of Quaternary period materials such as archaeological artefacts, volcanic deposits and a variety of sediments from different environmental settings.

A common property of certain minerals such as quartz and feldspar is that when they are exposed to radioactivity, they are able to store within their crystal structure the energy delivered by the radiation. This energy accumulates in the minerals through time and can later be released in the form of light which is termed luminescence. Although a variety of minerals show the phenomenon of luminescence, the application of luminescence for dating purposes is mainly limited to quartz and in some cases also to feldspar.

This is due to its abundance in sediments and pottery as well as its ability to fulfil the requirements of sensitivity to radiation dose and behavioral characteristics. The OSL response to a test dose is measured between each regenerative OSL measurement to provide a quantifiable indication of sensitivity change. Murray and Wintle 4 suggested that the relationship between the OSL test dose and the OSL regenerative dose must be linear for sensitivity to be adequately corrected. The layers from which luminescence samples were collected include six that have associated radiocarbon dates Fig.

Prof IK Bailiff - Durham University

Each layer was discretely excavated and predominantly charcoal, but occasionally bone, were radiocarbon dated. The 'orange sand' comprises approximately a metre of deposit that has low artefact density, but it is nevertheless not entirely homogeneous as it retains some sedimentary features.

All luminescence samples were taken in dark conditions and subject to quartz isolation pre-treatment in a dark laboratory. Samples were treated with concentrated hydrochloric acid to remove the carbonates and iron, and organics were removed with NaOH. Magnetic particles were removed using a Franz magnetic separator. A sodium polytungstate solution of specific gravity 2. For each sample the quartz grains were mounted on 24 aluminium disks aliquots using silicone oil and the D e values determined according to the SAR protocol.

Luminescence dating

This instrument has both infrared and blue LED stimulation systems and infrared and green laser systems for single-grain measurement. A green long-pass GG filter was installed in front of the blue stimulation diode clusters. A systematic error for the reader was determined from the error of 10 sequential measurements of an aliquot subjected to a fixed irradiation and measurement protocol.

This gave a value of 1. Assessing the equivalent dose D e determinations. The OSL results in Tables 3 and 4 and are presented graphically in relation to the stratigraphy in Fig. The younger OSL samples can be compared with the corresponding 14 C dates, but the comparison is not direct.

Optically Stimulated Luminescence Thermochronometry

OSL ages are calculated in true calendar years, and the age is reported relative to the calendar date of the analysis. An additional level of complexity arises from natural fluctuations in the production of 14 C in the atmosphere. Both the secular variation in 14 C and the error resulting from the incorrect half-life used in 14 C dating are accommodated in the 'calibration' of 14 C dates.

In this process, the measured 14 C date is compared with 14 C dates from tree rings of known age in order to determine the calendar age of the sample. It became clear recently that differences in carbon cycling in the northern and southern hemispheres gives an approximate 40 year apparent age to the southern hemisphere. This has to be taken into account in the calibration step for 14 C dates because the tree-ring calibration data set derives entirely from the northern hemisphere. It has been shown 38 that using a fixed offset between the northern and southern hemispheres is erroneous and calibration of the southern hemisphere is therefore best achieved on dendro-chronologically dated wood.

Unfortunately, no such data set exists for the southern hemisphere for the period before years ago. Furthermore, the calibration record between 26 kyr and 50 kyr ago is constructed from a variety of data sets such as laminated lake sediments, corals and speleothems.

Prof Ian Bailiff

There are offsets greater than years between these data sets. In order to compare calibrated 14 C dates with OSL dates, it is necessary to convert them to the same reference. The term 'correlated 14 C age' in this study refers to calibrated 14 C ages expressed in calendar years relative to AD the year in which the OSL dates were performed in order to make a direct comparison with the OSL dates.

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Figure 2 presents a more detailed comparison between OSL and 14 C and identifies miscorrelations. Because the 14 C calibration can produce asymmetrical error margins, the one-sigma range is presented in Table 5. Without the benefit of high-resolution gamma spectrometer measurements HRGS we use the comparison between 14 C and OSL dates as an indication of the accuracy of the dosimetry.

The XRF-derived results are consistently better correlated with the radiocarbon chronology than the FGS derived measurement not shown , which suggests that there is a problem with the calibration of the FGS. This difference was not substantial and the conclusion is that the stripping of the gamma spectra was incorrect for the K channel, and correct for the U and Th channels. Only dates based on the XRF dosimetry were further analysed. Table 3 summarizes the final D e values and dose rates that were used to calculate the depositional age of the RCC. The result for RCC 19 is particularly unsatisfactory as the OSL analysis for this sample presents perhaps the most convincing case for an acceptable date.

The D e values appear to indicate a single population with minimal scatter around the central value. Such an age discrepancy may result from localized turbation such as pit digging that would have elevated older sediments to the surface without necessarily zeroing the OSL signal, or the problem may relate to sampling problems associated with fine stratigraphy the sampling tube was 30 cm long and 5 cm in diameter , but these seem unlikely.

A more likely scenario is that this sample was contaminated with feldspar in the same way that the single-aliquot analysis of RCC 21 was. In the case of RCC 21, the overdispersal of D e values For this sample either the dose rate is overestimated or the D e value is underestimated. RCC 7's is the only growth curve in this study that showed evidence of saturation at approximately Gy, so that the D e value of Gy may be an underestimate. Alternatively, the underestimate of age for RCC 7 may be the result of an overestimation of the dose rate. Previous studies 13 suggested that RCC has been a chemically dynamic environment.

No bone is preserved in these layers and the stratigraphic section shows that the deposit has deformed substantially through profile compaction and leaching. Several age underestimates are noted in other dating studies of the MSA layers using different luminescence techniques and datable materials Table 4 and it is possible that dosimetry presents a problem. This has not been demonstrated but the result for RCC 7 is rejected as an outlier. It should be noted that the age ranges obtained for RCC are the most probable. To fine-tune these results, further study will need to be conducted focusing on the dosimetry and effect of feldspars.

The values in Table 6 are rounded to the nearest years and exclude outliers from both the 14 C and OSL chronology. The latter interpretation is the simplest explanation of the data, and is supported by the similarity in stone tool assemblages from the MSA IV stone tool assemblage dated to between approximately 38 years ago and 27 years ago layers LYN to Ru with those defined as ELSA at Border Cave.

Published dates for Howiesons Poort assemblages in South Africa range between approximately 80 and 50 years ago. Unfortunately, a largely random scatter in the previous Howiesons Poort dates from RCC undermines their credibility, and it is likely that problems of feldspar contamination are the underlying cause. With the improvement in the SAR OSL dating technique, the results are significantly improved, and this study yields a coherent set of OSL dates that place the Howiesons Poort between 70 and 60 years ago.

The stranded beach-dune sequence of south-east South Australia: Assessing the completeness of quartz OSL in the natural environment. Radiation Measurements 40 , The use of measurement-time dependent single-aliquot equivalent-dose estimates from quartz in the identification of incomplete signal resetting. Radiation Measurements 37 , Luminescence dating of quartz using an improved single-aliquot regenerative-dose protocol.

Radiation Measurements 32 , Contact Prof Ian Bailiff email at ian. After reading physics at Sussex University Ian Bailiff joined the Research Laboratory for Archaeology and the History of Art at Oxford as a postgraduate student and subsequently submitted a research MSc on the development of new luminescence dating techniques. Following a further year working on an instrument development project he came to Durham in as a research assistant on a pottery dating project; following a series of appointments as a Research Fellow he was awarded an SERC Advanced Fellowship, and in was appointed a Lecturer.

He was Head of Department between and He has developed a luminescence dating laboratory with the capability to support both dating and methodological investigation, and his research has included the study of the luminescence properties of minerals, their application to dating and also to radiation dosimetry.