What is Luminescence? Optically Stimulated Luminescence (OSL) Sediment Context Collection Procedures

Facilities and Computational Resources

Laboratory Equipment

The Luminescence Dating Research Laboratory (LDRL) is housed in a recently refurbished 2000 ft2 facility within the Dept. of Earth and Environmental Sciences at the Univ. of Illinois at Chicago. This facility utilizes a variety of luminescence technology including two automated luminescence "Daybreak" Reader systems and an automated Risø Reader system. Laboratory irradiations used a calibrated 90Sr/90Y beta source coupled with the Risø reader, and the experimental sequences were executed using Risø TL/OSL software for Windows (version 4.0). The laboratory is fully capable to analyze fine-grained (4-11 microns) polymineral and quartz extracts and aliquots of coarse (100-200 microns) grained quartz. The readers have capabilities for thermoluminesence, infrared, red, blue and green stimulation. Our library of software provides flexibility to undertake single aliquot or multiple aliquot analyses utilizing regenerative or additive dose procedures. In addition to mounted and calibrated beta source (90Sr) on Risø Reader, the laboratory maintains three independent calibrated, automated alpha and beta irradiators that provide beta or alpha radiation exposure, for up to 20 samples sequentially, at individually prescribed periods ranging from seconds to hours. These sources have been calibrated to standards at the Laboratory for Art History Archaeology at Oxford University, England. A bank of eight automated intelligent alpha counters detect total, slow and fast scintillations. Counters are calibrated with uranium and thorium standards supplied by the U.S. Geological Survey. The data output is transferred via a LAN to calculate uranium and thorium content of sediments. Gamma and a portion of the beta contribution are calculated from potassium analysis of sediment. A Germanium gamma spectrometer well detector is available to assess equilibrium conditions in the U-Th Series Dating Laboratory of Dr. N. Sturchio, in the Department. A portable Na–I gamma spectrometer is also available for field measurements. The laboratory is illuminated by the indirect and diffuse light from sodium–vapor bulbs (590 nm). The intensity of the light is adjusted to prevent resetting of the OSL signal. This facility is equipped with ultrasonic baths; digital scales and precision preheat plates, IEC 2000 centrifuge, and automated grinders for the preparation of a variety of geological materials for luminescence analysis. The UIC Luminescence Dating Research Laboratory has access to electronics laboratory and machine shop within the Physics Department for the maintenance and construction of equipment.

Software

Luminescence Dating Analysis Software (or LDAS) is a 32-bit Windows-based software package developed by Tarun Agarwal and James Pierson to display, edit, process, and analyze both TL and OSL data. LDAS is compatible with Windows 95, 98, 98SE, NT 4.0, 2000, and possibly Windows ME and XP operating systems. Features can be activated by using the software menu, toolbar buttons, and hot keys.

Interface : LDAS uses an ASCII-text interface and can create image files from any graphic display, allowing easy portability between it and most commercially available word processing and spreadsheet packages.

Displaying and Editing Data : LDAS can display all processed and unprocessed data point values in graphic (linear and histogram) and tabular form. Corresponding variance data values can also be displayed, but only in tabular form. With the exception of age estimation, all analysis results are presented in both graphic (linear or scatter-plot) and tabular form. A zoom feature built into the graphic display allows the user to focus on any specified area of interest. LDAS supports multiple views of the same document, allowing simultaneous displays of unprocessed, processed, and analyzed data. The system parameters and user-defined data collection information can be displayed and edited with LDAS, although access to this feature is limited by password protections to prevent unauthorized changes to the data set.

Processing Data : LDAS processes all luminescence data using Poisson statistics, the probability distribution that describes any experimental data collected using scintillation counters operating in single photon mode (O'Conner and Phillips, 1984). Any pile-up distortions are corrected by using an iterative algorithm that can accurately compensate for undercounts even when the measured count rate exceeds 99% of saturation. LDAS removes any measured background light from the data set and supports all standard data normalization procedures, including the component-specific dose normalization procedure introduced by Jain et al. (2003). TL data sets can be temperature-shifted with LDAS, either by individual curve or by radiation group, using a seventh-order polynomial interpolation algorithm that can approximate 1°C resolution.

Analyzing Data : LDAS can interpolate (regeneration method) or extrapolate (additive dose and partial bleach methods) equivalent dose values from processed data sets using any one of ten operator-selected growth curve models, including a saturating exponential and a saturating exponential plus line. Model parameters are determined by using the inverse-variance weighted data set in a nonlinear least squares fitting routine based upon the Levenberg-Marquardt method (Marquardt, 1963; Press, et al., 1986). A weighted average of the results determines the equivalent dose value; a measure of the dispersion of the results around this value determines the absolute magnitude of the error. LDAS uses the radio-isotopic content and correction factors for water, organic material, and grain-size to estimate the natural dose rate for the analyzed data set. The a-value system is used to determine the alpha particle contribution to this value (Aitken, 1985). The ratio between the equivalent dose and the dose rate defines the age estimate for the data set. Additionally, LDAS can analyze anomalous fade test data to evaluate the temporal stability of the laboratory-induced luminescence signal.

 

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