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Section III: Lab Prep and Equivalent Dose (DE) Analyses

A full explanation of USGS lab procedure may be read in USGS Open File Report 94-249. The USGS lab uses three grain sizes: a polymineralic 4-11 µ fine silt for both TL and OSL dating, 90-125 µ quartz for OSL (Blue Light-Optically Stimulated Luminescence) and 180-125 µ or 90-125 µ (depending on quantity and difficulty in calculating DR) K-feldspar for IRSL-OSL (Infrared Stimulated Luminescence). Before sieving for desired size fractions, each sample is treated with 4N HCl to remove carbonate, 30-35% H2O2 to destroy organic material and is dispersed in Na pyrophosphate solution. Wet sieving and Stoke's settling in de-ionized water are employed to achieve desired size fractions. Heavy liquid separation is used on the larger grains to separate quartz from feldspar by using lithium polytungstate (LST). Short sample etches in Hydrofluoric acid (HF) for the quartz fraction are be used to clean surfaces and impurities. HF of 45-50% is used for 40 minutes, followed by a 5-minute bath of 4N HCl. These samples are then run as loose grains on cups.

The silt size portion of the sample (4-11 µ) are then plated on 1 cm size aluminum disks from a grain suspension in methanol, while the larger fraction (90 µ and larger) are poured loosely into cups. All procedures are performed under reduced lighting conditions to minimize artificial bleaching of the samples. The intensity of the sodium vapor lights in the laboratory,which emit a single wavelength (589 nm), are adjusted to provide enough requisite sensitivity for the human eye but not bleach the samples.

The silt size sample disks are normalized for sample homogeneity by a 5-second exposure to infrared illumination and detection of the resulting luminescence by a photomultiplier tube covered with Schott BG-39 and Kopp 7-59 filters, as well as a Pyrex window. The TL signal is reduced by about 2% by this treatment and the IRSL signal by about 1%. The larger size fraction are measured using a single aliquot technique and thus require no normalization.

Samples are measured for TL and OSL separately, which requires the use of many disks or cups. TL is obtained by heating the disk to 500°C and the electronically intensified light emission is recorded as a function of the heating temperature. After each measurement the heating is repeated to record and subtract background. Most of the signal is due to TL from quartz and feldspars. The TL instrument records the signal as photons/5°C intervals and plots the resulting glowcurve as TL signal versus temperature. The glows are actually a composite of TL peaks from different traps. It is crucial to conduct TL measurements under the purest nitrogen (or argon) atmosphere to prevent unwanted chemiluminescence, which is energetically fed from exothermic chemical reactions.

Silt size, polymineralic IRSL is obtained by exposing the disk to a 30mA current to infrared LED's, 1 second dwell time per channel for 100 channels, 100 second total time exposure, sample temperature held to 30°C and a background count taken before and after a set of samples is exposed under the photomultiplier tube. (It is not necessary to obtain IRSL under nitrogen atmosphere). The coarser grains measured on the Riso System are done so under a steady light from either blue or infrared diodes. The blue light emission is given in the 400-550nm range (centered at 470 ± 30) and infrared emission is from the 800-900 nm range (centered on 880 ± 20 >nm).

Optical filters are inserted between the sample and photomultiplier tube to permit the recording of specific spectral regions. For optical filters the USGS lab uses Schott BG-39 (at the upper level) and Kopp 7-59 (at the lower level) for both TL and IRSL, on the Daybreak system analyses. On the Riso system analyses a U-340 detection window helps cut off spillover from blue light emission at the 350-400 nm range and BG-39 filters out 300-700 nm (helps to gain sensitivity). For Riso measurement of feldspars >U-340 and RG-715 are the common filters.

The fine-grain technique requires the determination of the a-value and thus the use of an alpha source as well as a beta source. These sources are used to artificially irradiate sets of disks so that a curve will result from increased TL growth induced by the laboratory "aging". This curve is then extrapolated back to its intersection with a residual baseline. The baseline is defined by the TL or IRSL signal from a natural disk. The disks are exposed to 8 to 16 hours of natural sunlight for TL or 5 to 10 minutes for OSL. The alpha source in use at the USGS lab is ²⁴¹Am at 0.5-mCi strength and two beta sources of ⁹⁰Sr at 100 mCi and 200 mCi respectively.

There are many methods a lab can use to generate DE. The methods in use by the USGS lab are the "total bleach" with a preheat of 124°C (62 hours) and a "total bleach" with a preheat of 140°C (6 hours) for TL. "Additive dose" of 124°C (62 hours) and "additive" dose of 140°C (6 hours) for IRSL, and single aliquot analysis for both blue light OSL and IRSL large grains at 220°C (5 minutes), fading tests at all preheats and a sunlight sensitivity test. Not all experiments are performed for every sample.

The traps of some minerals, particularly feldspars are afflicted by the malign phenomenon of anomalous fading. Such fading is anomalous in that observed stability is much less than predicted from kinetic considerations. The test for anomalous fading establishes the effectiveness of the preheat treatment for removing the less stable TL and OSL components generated by beta or alpha irradiation.

In the "total bleach" method, irradiating sets of natural disks for various lengths of time generates a growth curve. The longest of these times is chosen to produce about 8 times the best estimate of the DE for a particular sample. The resulting TL signals are plotted against the radiation dose. The intersection of this curve with a residual defines DE. IRSL "additive dose" technique is also approached in the same manner. Glowcurves illustrating this process are included in a sample report.

In order to test if the signal of natural TL has been stable over the age range in interest, the plateau test is applied. In the plateau test the DE or the TL age is calculated in dependence of the glow curve temperature. The start of the plateau value is indicative of the thermal stability of the analyzed TL signal. The plateau can be thermally "washed" through preheating to obtain better development of plateaus. This is why the client might note differently defined plateaus for samples at different preheat temperatures. The USGS lab uses two distinct preheats in an effort to fully test plateau placement. Experience and judgement can play a large role in choosing the DE for each sample. OSL DE does not have a plateau property; thus a TL run on the sample is usually done simply to judge the potential stability of luminescence within the sample, even if it is believed that the TL has been incompletely reset due to attenuated light exposure.

The TL and silt-size IRSL data are run and reduced using the Daybreak 1100SI TL Application software (registered trade name). This software computes a growth curve for the data points generated by passing a vertical line (single temperature) through the family of glowcurves. A least squares fit is made either to a line passing through the points or, for the saturating exponential, a line passing through a logarithmic transformation of the data. An iterative algorithm is employed for the latter fit. The natural (unirradiated) disks are given double weight. The Daybreak software estimates errors for DE using Rendell's equation #1 for the regression of TL signal on dose (Rendell, 1985).

OSL data is collected from the quartz grains of 90-125 µ size or occasionally the 125-150 µ size grains. These grains have been put through a further refined process using heavy liquids (LST) to separate them from the heavy minerals and feldspars. The quartz further undergoes a Hydrofluoric acid etch for 40 minutes (etching away the alpha radiated layer). Samples are run and reduced using a Riso TL/OSL-DA-15A/B (registered trade name) system. Riso system software was specially developed for the determination of the archeological and geological ages from quartz and feldspar in collaboration with Dr. Rainer Grun (Australian National University, Canberra, Australia). The special application software for the single aliquot regeneration (SAR) analyses were developed by G.A.T. Duller (University College of Wales, Aberystwth, UK) and Dr. Andrew Murray (Nordic Laboratory of Luminescence Dating at Riso, Roskilde, Denmark). Complete analysis and procedures followed in the USGS lab are taken from a 2000 paper of Andrew Murray and Ann Wintle (Luminescence Dating of Quartz Using an Improved Single-aliquot Regenerative-dose protocol, Radiation Measurements, 32 (2000), pp. 57-73).

Various combinations of these parameters furnish many variants of the TL and OSL technique of dating. In consequence the dating of a particular sample by several laboratories will not necessarily yield directly comparable ages---although we feel they should at least have overlapping age ranges. It is important to present this experimental procedure together with the age data. The results of the measurements can't be judged without this important information. Only then can ages be evaluated with a realistic quoted error.


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