Luminescence Dating Laboratory

The luminescence dating started in our Department on the break of 70' and 80'. The early work on thermoluminescence was inspired by the late Prof. Mieczysław Pazdur and consisted mainly in measuring TL signal from various samples using an old equipment for mineralogical studies. At that time when basic experimental experience was gained an effort was made towards designing and constructing TL reader more suitable for measuring faint light signals from small aliquots of quartz, feldspar or calcite grains. The first TL reader was constructed as a manual unit with analogue controls. The gained photomultiplier output was directed to the XY recorder and recorded thermoluminescence glow curves were digitised prior to further calculations. At this early stage collaboration with the Physics Department in the Institute of Oncology in Gliwice gave access to calibrated gamma ray sources used to additional irradiation of grains extracted from geological samples. The three-channel gamma scintillation spectrometer was available in the Solid State Physics Department and used for samples natural radioactivity assessment. This equipment enabled first TL dating to be performed in 1983. Later works were concentrated on refining laboratory procedures for sample preparation, automated data taking and storing and developing software tools data reduction and calculations. The new version of the TL reader was built in 1985. It was completely redesigned. The light collection was improved making possible measurements of TL from calcite samples and from quartz grains extracts from very young sediments. The TL reader was directly controlled by the computer system consisted of the CAMAC crate with an autonomous controller and several specialised digital units. The linear heating was achieved through applying a digital to analogue converter driven by the system controller. Temperature and PMT current signals were digitised and stored in the core memory. The CAMAC system included a serial communication unit through which it was connected to the main PC computer responsible for the whole measurement process. The PC sent primitive commands to and received data from the CAMAC controller. This computer controlled version had much better stability and reproducibility, and was much more convenient in routine use. TL glow curves were now stored in a digital form as computer files and were available to the originally developed computer programme which performed all the calculations and made graphic presentations of original and reduced data.
Research was focused on TL dating of aeolian sediments. The most important works considered comparison of TL and ¹⁴C dates obtained on very young dune sands and comparison of TL dates obtained on loess samples in different TL laboratories. Other subjects of interest included influence of repeated freezing - melting cycles on TL signal recorded from quartz grains and detecting changes in sensitivity of quartz grains due to sunlight bleaching. An important work was also done to improve data processing procedures and function fitting algorithms.
The measurement system was soon replaced by still more useful one with a CAMAC - PC interface instead of the autonomous crate controller. This made the whole system more flexible and easily adaptable to various other needs.
In the meantime the laboratory became equipped with a 90Sr beta source and with a multichannel gamma scintillation spectrometer. This allowed confining the whole dating process including irradiation and radioactivity measurements on the Department's premises.
The measurement equipment remained essentially the same till early 90'. Changes in the system of financing scientific researches and intensive efforts towards gaining financial support from international bodies resulted in significant improvement of laboratory hardware. In the period of 1990-95 the laboratory was equipped with a commercial automated TL reader Daybreak 1150 and a gamma ray spectrometry system with a high purity germanium detector manufactured by Canberra. Later the TL reader was converted into TL/OSL reader with an internal beta source and separate automated alpha and beta irradiators were bought. This investment changed the laboratory throughput capacity and improved the quality of the routine laboratory work.
The total number of samples processed in the laboratory is close to 1000, out of which 660 was successfully dated by TL or OSL method (the number includes also 17 dates obtained on calcite samples by the ESR method). The dated samples were submitted by archaeologists (210 samples total: 98 ceramic or fired artefacts and 112 sediment samples) and geologists (450 samples), from Poland (540 samples) and from abroad (120 samples).

Some solved important problems

Investigations into the efficiency of mechanism of zeroing pregenetic TL in aeolian sediments. The studies were carried out on samples of dune sands dated independently by ¹⁴C method. The findings of this research project indicated possible overestimation of absorbed dose value and consequently the TL age for aeolian sediments up to 5 Gy or 10 ky.

Study of thermoluminescence sensitivity changes of mineral grains caused by optical bleaching. The study was carried out on a set of geological samples of different origin. The bleaching was done by exposure to natural unfiltered sunlight and to artificial filtered light from the mercury lamp. The changes in sensitivity were detected by means of the Snydecor's F-variable statistics in 20% of studied samples.

Comparison of TL dates obtained for a set of samples from the well known loess profile in Odonów, Southern Poland. Multiple samples taken from the same loess layers were dated independently in three TL laboratories by different techniques. The results of this interlaboratory comparison project showed that TL dating results could be severely biased by specific laboratory procedures adapted in the laboratory while differences found for soil layers may be related to soil development processes. The issue of comparing results of dating geological objects by different absolute methods was further investigated within the project where ¹⁴C, ESR and TL dating methods had been applied to speleothems from caves in the Tatras.

Bootstrap method in statistical analysis of dates. The possibility of bootstrap method application in an interpretation of radiometric dating results had been investigated. The research concerned date assemblages obtained for single geological or archaeological objects as well as aggregates of such objects. The total frequency distribution of greater number of dates usually shows multiple maxima and minima. In a case of geological samples of appropriate type these minima and maxima may be interpreted as corresponding to periodic climate changes. In a case of archaeological samples the periods of characteristic cultures or periods of certain area inhabitation may be established in an objective way. The bootstrap method enabled assessment of statistical significance of the observed distribution maxima.

Influence of disequilibrium in the uranium series on TL and ESR dates of speleothems. The goal of this study was to find the relationship between actual radiation dose absorbed by the speleothem from the moment of its formation till now, and that calculated from th
e gamma spectrometric measurements of 214Bi isotope in the sample with the assumption of equilibrium in the uranium series. Both doses differ from each other, since during precipitation of calcite in growing speleothem, the radioactive equilibrium in the uranium series is seriously disturbed. The problem was resolved by calculation of the so-called GH curve, which shows direct relationship between actual age of speleothem and that obtained in TL of ESR dating when no disequilibrium is taken into account.

Measuring very small doses of ionising radiation using natural quartz as dosimeter. It was shown that using natural quartz grains extracted from selected samples and making TL or OSL measurements following single aliquot protocols absorbed dose values of the order of 1 mGy or below may be assessed with a good accuracy. These investigations were of a great importance for retrospective dosimetry of accident doses.

Combined OSL/TL dating of geological samples was proposed as a new method giving possibility of assessing the degree of bleaching the mineral grains in the dated sediment underwent during its formation. The method takes advantage of the fact that the OSL measurement does not reduces significantly TL signal measured directly after it. It was shown that the same set of aliquots may be first measured by OSL method and then by the TL method, yielding to estimates of absorbed dose value. Taking into account the fact that OSL signal is much more efficiently bleached by light (matter of minutes instead of hours or days as in the case of TL signal) the difference in both dose estimates may be expected if sample underwent a relatively short exposure to light. Long exposures should result in well bleached grains and consequently no difference should be found between OSL and TL derived values of absorbed dose. Because OSL and TL are measured exactly for the same aliquots there is no other possible sources of divergence and the observed difference of absorbed dose values may be interpreted in terms of bleaching efficiency and conditions under which sedimentation took place.

OSL dating of geological sediments by the single aliquot regenerative and additive dose protocols. The OSL dating of geological sediments using single aliquot additive dose and regenerative dose protocols has been successfully introduced into the laboratory practice. By using specially designed measurement protocols one is able to perform all the necessary OSL measurements and laboratory irradiation on a single aliquot (the limit of aliquot size is eventually a single grain) and finally obtain the absorbed dose value. This method enables investigations of distribution of absorbed dose if a number of single aliquots is measured. The shape of the distribution enables conclusions regarding the origin of the sediment and processes involved in its formation.