Towards rapid, high-resolution measurement of luminescence-depth profiles using 2D InfraredPhotoluminescence imaging

Rock surface dating (RDS) with optically stimulated luminescence (OSL) utilises the principal that upon exposure to light, the luminescence at the rocks surface will be reset. Over time, the OSL is bleached to greater depths from the surface, and from measuring the luminescence-depth profile and fitting it with age models one can determine the total exposure duration.

If the exposed surface is then buried, dose will start to accumulate, and the burial age of the rock can be determined following conventional OSL methods. However, in its current form, RSD with OSL demands laborious sample collection and processing, and data sets are often of low resolution. The current age models are also limited in their representation of the true luminescence kinetics (especially of feldspar) and of the interaction of light with matter.

Recently, two infrared-photoluminescence (IRPL) emissions at 880 nm and 955 nm, have been characterised in feldspar. Contrary to IRSL, these signals arise from radiative relaxation of the electrons from the excited to ground states within the principal trap, with thousands of photons able to be generated per second, and high stability due to more distal location with respect to recombination centres.

These characteristics make it suitable for luminescence imaging purposes. Imaging of luminescence would be highly beneficial for rock surface dating applications, increasing the data resolution and speeding up sample preparation and measurement times. The main purpose of this PhD was to improve on instrumentation for imaging of luminescence from feldspar, with a focus on the development of methods for rock surface dating.

A new EMCCD-based instrument titled the Risø Luminescence Imager is described. The novelties of this instrument include its suitability for measuring both IRPL at 880 nm and 955 nm, as well as IRSL at high resolution (~170 μm). Images can be taken of large cm-scale rock samples, up to ~7 × 7 cm in size.

Full IRSL decay curves can be obtained through time-lapse measurements of the IRSL. Several applications using the Risø Luminescence Imager are presented. I demonstrate several alternative methods for normalising IRPL and IRSL luminescence-depth profiles. These include: taking the ratio between the IRPL 955/880 nm data, calculating the ΔIRPL, and normalising natural IRSL by a later part of the decay curve.

These normalisation methods open up the possibility of RSD to laboratories which lack irradiation facilities. A dose recovery study is presented in chapter 4, describing measurement and analysis protocols for rock surface burial dating using imaging. Two samples with known exposure and "burial" histories were measured.

The IRSL burial doses were able to be recovered from the surface ~5 mm of both rock samples, but from the IRPL data, the dose recovery was only successful for the sample with the higher "burial" dose (500 Gy). A novel application where the principles of OSL RSD are applied to three cracks of known chronological formation is presented in chapter 6.

Through imaging a plane perpendicular to the crack surface, the extent of IRSL and IRPL bleaching around the crack can be assessed. Through estimating the maximum bleached regions for the three cracks, a relative chronology of crack formation was established, consistent with field observations. Until now, age models for determining the exposure durations have not been applied to IRPL data sets.

Chapter 5 presents a novel in-depth study of the development of IRSL and IRPL luminescence-depth profiles as a function of time and specific wavelength, and explores the suitability of first-order and general-order age models for representing the data. The IRPL-depth profiles progress deeper from the surface with increasing exposure time, along with a decrease in attenuation coefficient due to the preferential attenuation of higher-energy wavelengths.

It was established that IRPL luminescence-depth profiles can be represented adequately by first order kinetics through fitting of the first order model. The research presented in this thesis will benefit the field of rock surface dating by offering easily adaptable instrumentation for high-resolution imaging of multiple luminescence signals from feldspar.

The results present measurement protocols and data analysis procedures suitable for applications of rock surface burial and exposure dating using imaging. This research also paves the road for further development of novel dating methods for cracks and cracked landscapes, as well as for developing field instrumentation which would allow in situ measurement of luminescence.