Photostimulated luminescence excitation spectroscopy of feldspars and micas

Abstract

Although much photostimulated luminescence (PSL) work has been with broad-band or monochromatic stimulation sources, both excitation and emission spectroscopy are fundamentally important to characterize the underlying physical processes and develop robust application methods. This paper discusses the results of excitation spectra obtained from a series of irradiated feldspar and mica samples stimulated from 450 to 1200 nm using a scanning spectrometer. Spectra from 450 to 950 nm were stimulated using a 300 W Cermax Xe lamp, supplemented from 700 to 1200 nm with a tungsten-halogen lamp. Samples were illuminated through beam production optics coupled with an f3.4 stepping motor-driven monochromator in conjunction with a series of long-pass filters. A filtered 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 monochromatic bleaching cycles to establish thermal stabilities and inter-band coupling. The instrument response was characterized by pyroelectric radiometry. Detailed spectroscopic studies were made to a pure microcline feldspar and an International Atomic Energy Agency (IAEA) secondary reference feldspar (AQCS F1). Additional measurements were taken from a range of 19 alkali feldspars, two plagioclase feldspars and nine mica samples of known geological origin and age. All samples showed a range of visible to infrared stimulation bands, in proportions that varied from sample to sample. After normalization for spectrometer response, using a pyroelectric radiometer, the positions, shapes and relative intensities of the broad stimulation bands were revealed. A typical sample had bands at 500–540 nm, 550–650 nm and 800–1000 nm. There was evidence of higher-sensitivity blue and near-ultraviolet stimulation bands. Spectra recorded at different sample temperatures, and during thermal ramps, provide clear evidence of thermal assistance to infrared and 550–650 nm visible bands. Activation energy estimates were obtained by Arrhenius analysis of the results. Pulsed annealing experiments and monochromatic bleaching experiments demonstrate coupling between infrared-stimulated PSL and the majority of the green-stimulated signal. They also raise questions about the long-term stability of infrared-stimulated signals. A more stable signal can be obtained within the green stimulation band, which is thermally separable from the infrared-associated component. Although the results reproduce some of the features of the Hütt and Jaek model for feldspars, there are additional features which require a more complex level scheme. The observation of stable centres available to the green band after thermal washing, and of more intense blue-ultraviolet bands suggests that these regions may be more promising than infrared stimulation for dating events of age greater than 10 4–10 5 years.