PHOTOSTIMULATED LUMINESCENCE EXCITATION SPECTROSCOPY OF FELDSPARS ANDMICAS

Although much photostimulated luminescence (PSL) work has been with br oad-band or monochromatic stimulation sources, both excitation and emi ssion spectroscopy are fundamentally important to characterize the und erlying physical processes and develop robust application methods. Thi s paper discusses the results of excitation spectra obtained from a se ries of irradiated feldspar and mica samples stimulated from 450 to 12 00 nm using a scanning spectrometer. Spectra from 450 to 950 nm were s timulated from using a 300 W Cermax Xe lamp, supplemented from 700 to 1200 nm with a tungstem-halogen lamp. Samples were illuminated through beam production optics coupled with an f3,4 stepping motor-driven mon ochromator in conjunction with a series of long-pass filters. A filter ed bialkali photomultiplier in photon counting mode was used to record PSL. Spectra were recorded at various sample temperatures, at a range of doses from 1 Gy to 10 kGy, and after pulsed annealing and monochro matic bleaching cycles to establish thermal stabilities and inter-band coupling. The instrument response was characterized by pyroelectric r adiometry. Detailed spectroscopic studies were made to a pure microcli ne feldspar and an Internationalk Atomic Energy Agency (IAEA) secondar y reference feldspar feldspars and nine mica samples of known geologic al origin and age. All samples showed a range of visible to infrared s timulation bands, in proportions that varied from sample to sample. Af ter normalization for spectrometer response, using a pyroelectric radi ometer, th epositions, shapoes and relative intensities of the broad s timulation bands were revealed. A typical sample had bands at 500-540 nm, 550-650 nm and 800-1000 nm. There was evidence of higher-sensitivi ty blue and near-ultraviolet stimulation bands. Spectra recorded at di fference sample temperatures, an dduring thermal ramps, provide clear evidence of thermal assistance to infrared and 550-650 nm visible band s. Activation energy estimates were obtained by Arrhenium anylsis of t he results. Pulsed annealling experiments and monochromatic bleaching experiments demonstrate coupling between infrared-stimulated PSL and t he majority of the green-stimulated signal. They also raise questions about the long-term stability of infrared-stimulated signals. A more s table signal can be obtained within the green stimulation band, which is thermally separable from the infrared-associated component. Althoug h the results reproduce some of the features of the Hutt and Jack mode l for feldspars, there are additional features which require a more co mplex level scheme. The observation of stable centres available to the green band after thermal washing, and of more intense blue-ultraviole t bands suggests that these regions may be more promising than infrare d stimulation for dating events of age greater than 10(4)-10(5) years.