Optical dating of sediments
Optical dating of sediments - Videobedava xx caml sohbet
The introduction of a standard test dose (Tn) following the measurement of the natural and laboratory doses allows the calculation of a ratio with which to monitor, and correct for, any change in sensitivity.
Heterogeneous sediments and radioactive disequilibria will increase errors on Dr, while incomplete bleaching of the sample prior to burial, anomalous fading in feldspars, and the estimation of past sediment moisture content may all also add to increased errors.It is recognised that the luminescence generated following laboratory irradiation contains a proportion of electrons that are considered thermally unstable over geological time and are not present when measuring the natural signal in a sample.To remove this element a preheat is administered, which in turn may increase the sensitivity of the sample to further irradiation.It is important to observe certain conventions when collecting samples in order to reduce errors as much as possible.By taking samples from well-sorted sediment structures problems with heterogeneous dose rates may be avoided, and all grains are more likely to have undergone the same depositional history.Increased moisture content and carbonate coating of grains can lead to an attenuation of the radiation reaching the sample, and so ultimately to an age overestimation if not identified.
Chemical and radiometric assessment of the surrounding sediment, through in-situ gamma spectrometry or laboratory high-resolution gamma spectrometry, identifies the proportions of each element together with the contribution from the alpha, beta and gamma components.
LEDs are now widely used for optical stimulation, with the photons emitted being measured by a photomultiplier tube (PMT) attached to a luminescence reader (see Figure 3).
By placing optical filters in front of the PMT it is possible create detection windows to isolate particular bandwidths of emissions, and also to avoid the influence of backscattering of the stimulation source.
The use of longer wavelength stimulation light to that of the emissions helps to ensure reliable evaluations and blue or green light is now most widely used for the stimulation of quartz.
As feldspar can be measured using the longer wavelengths of infrared (IR) stimulation, a broader range of emissions are available for measurement (see Figure 4).
The use of fine-grain dating for samples such as pottery, loess, burnt flint and lacustrine sediments, and coarse-grain dating of aeolian, fluvial and glacial sediments is regularly undertaken.