title: Advances in Geophotonics: Multispectral Imaging of Quartz in Thermoluminescence Dating Applications
Creators: name: Borombovits, Daniel K
Publisher: name: Griffith University
Year: 2020
Subjects: subject: high-sensitivity
subject: multispectral
subject: thermoluminescence
subject: imaging system
subject: geophotonics
subject: luminescence dating

description: The work presented in this thesis details the design, construction, and calibration of a high-sensitivity multispectral thermoluminescence (TL) imaging system, with applications in geophotonics and luminescence dating. The system delivered enhanced spatially-resolved detection capabilities and sufficient sensitivity to accurately quantify the faint spectral emissions from single grains of quartz across multiple detection bands. Such measurement was previously unavailable using current techniques; with existing instrument architecture lacking the targeted detection, required sensitivity, and wavelength resolution made possible by the spatially-resolved imaging of thermoluminescence emissions. The advances presented herein provide i) a viable means of obtaining trapped-charge dose measurements from samples with luminescence emission characteristics for which current techniques are unsuited, and ii) a valuable research tool for investigating the temperature- and wavelength-resolved kinetic processes of thermoluminescence production and the mechanisms which govern these interactions. A design science research approach was adopted; requiring a comprehensive synthesis and understanding of the key elements underpinning instrument development and imaging techniques in luminescence dating applications. These elements included 1) luminescence production mechanisms and models, 2) the kinetic parameters of quartz governing these processes, 3) the current techniques in luminescence dating and existing instrument architecture used to detect and quantify the thermally and optically stimulated luminescence emissions of quartz, and 4) the application of advanced optical imaging systems and techniques to these areas. A comprehensive and systematic evaluation of the instrumentation used in luminescence dating was first undertaken, with a primary focus on the devices used for light collection and the spectral separation of luminescence emissions. These findings, along with fundamental considerations in optical imaging techniques, were used to inform the design and construction of the multispectral thermoluminescence imaging system. The key operational objectives relating to system performance were all met or exceeded, including specifications for quantum efficiency, optical throughput, spectral response, noise performance and sensitivity, as well as unit price and system integration cost. The resultant system delivered enhanced spatially-resolved multispectral detection capabilities previously unavailable using current techniques and existing instrument architecture. The system was developed around a Santa Barbara Instrument Group (SBIG) STT-8300M charge-coupled device (CCD) camera utilizing a Kodak KAF-8300 sensor, with peak quantum efficiency of 56% at approximately 550 nm. The sensor was thermo-electrically cooled to -20 oC during measurement to reduce thermal readout noise, producing dark current threshold values of 0.02e-/pixel/sec. A high-throughput custom-built lens configuration was designed for light collection and imaging, with lenses constructed from uncoated fused silica providing strong transmission (≥95%) across the visible spectrum and deep ultraviolet (UV). An Astrodon Sloan SDSS filter set was chosen for spectral separation of TL emissions, providing five discrete narrow bands of detection with extremely high peak transmission (≥95%), steep wavelength rejection characteristics and deep out-of-band blocking (≤0.03 %). Individual filters were mounted in a SBIG FW8S-STT filter wheel mounted below the STT-8300M camera. In addition, a single Hoya U-340 filter (25.4 mm diameter, 2 mm thickness) was mounted in a custom adapter within the filter wheel to be used for ‘standard’ signal detection analogous to current instrumentation. Total system transmittance was found to exceed 85% utilising the Astrodon SDSS filters (&gt;80% for the SDSS u’2 ultraviolet filter), with approximately 70% transmittance when using the Hoya U-340 filter. The optical column and STT-8300M camera were interfaced to the RISO DA-15 automated luminescence reader by means of a custom mounting plate and turret adapter. The RISO unit is equipped with a 90Sr/90Y beta irradiation source and facilities for the thermal and optical stimulation of mineral grains, allowing repeated luminescence measurement from quartz samples. Multiple calibration and evaluation cycles were undertaken in developing the high-sensitivity multispectral thermoluminescence imaging system. The relative detection efficiency and system response was evaluated, with an investigation of the signal:noise ratio (SNR) between the CCD imaging system and the standard EMI 9235QB photomultiplier tube (PMT) arrangement used on the RISO DA-15 automated luminescence reader. The noise performance of the PMT arrangement exceeded that of the CCD-based imaging system, with approximately twice the SNR measured at all given doses. A marginal improvement (approximately 20%) was observed in the SNR when using the SDSS u’2 filter compared to the Hoya U-340 for the imaging system, and overall detection efficiency increased slightly at higher administered radiation doses. The system was used to successfully recover a known irradiation dose from a calibration quartz sourced from the RISO Denmark Technical University (DTU) National Laboratory. The clear delineation of the luminescence signal required the development of novel image processing techniques. An aperture photometry tool was used for signal intensity calculations; a method typically used to determine stellar magnitudes in astrophotometry. The software is designed to measure point-source intensity and its uncertainty by summing all pixel values within a constrained region-of-interest (i.e. the aperture) and subtracting the background contribution. This is estimated from a nearby imaged region that surrounds, but excludes, the observed radiance from the TL emission (i.e. the source signal). An innovative suite of imaging and analysis protocols were also developed to maximize signal collection efficiency in high-sensitivity luminescence imaging applications. The well-known effects of varying heating ramp temperatures as a means to increase signal intensity was investigated, along with the potential to shift TL peak position relative to the detection band constrained by the optical filter; thereby providing a further proxy to test system response and calibration. This method was further adapted to maximize clean photon yield in the narrow UV detection band, with overall photon yield shown to increase at lower ramp temperatures (below 1.0 °C s-1) for both large-aliquot and single-grain samples. System noise performance was improved with the development of a calibrated background correction, which suppressed the signal arising from blackbody radiation emitted by the heater plate of the RISO luminescence reader. Ultraviolet thermoluminescence (UV-TL) photon yield was compared between the industry-standard Hoya U-340 optical filter and the high-transmittance SDSS astrophotometry optical filter set. Exposure correction techniques to mitigate the thermal and readout noise associated with charge-transfer processes in CCD sensors were also investigated. These approaches are vital in maximizing signal collection efficiency and are a necessary precursor for the use of advanced imaging systems in routine luminescence dating applications. The multispectral thermoluminescence imaging system was then used to characterize the spectral signature of individual quartz grains, allowing the inter- and intra-sample spectral diversity of quartz grains from contrasting geological context to be analysed. Luminescence emission behaviour was characterised for quartz grains (180-212 μm) for three samples of contrasting geological context, from which spectral profiles were collated. A clear distinction in the spectral signatures of individual grains originating from single deposits were identified within each provenance. This is a key finding of the work undertaken; and represents a significant advance in the understanding of the spectral variation in TL emissions from quartz, as well as in the characterization of geological provenance for individual quartz grains through luminescence studies. By better understanding the full spectral signature of individual grains, information on specific trapping sites (through temperature-resolved signal peaks) and on recombination centres (through wavelength-resolved signal peaks) may be extrapolated. Such findings demonstrate not only the viable collection of the luminescence dating signal beyond the detection range of current commercial instrumentation, but also highlights the potential for thermoluminescence imaging technique to be developed as a routine dating method, providing a new approach for extracting valuable provenance information from mineral grains natively with the luminescence dating signal. A key set of challenges that must be overcome in bridging current detection and analysis methods with spatially-resolved luminescence imaging techniques is the ability to generate accurate and precise equivalent dose (De) estimates and dose distributions in quartz. An innovative suite of imaging and analysis protocols were developed, which allowed investigation into the multispectral dose-response behaviour for natural quartz samples. This allowed, for the first time, the ability to generate multispectral equivalent dose distributions for quartz using solid-state luminescence imaging techniques. Dose response curves and De estimates were successfully generated across all four detection bands under investigation; providing encouraging results for the efficacy and validity of the TL system and luminescence imaging methods presented herein. The dose estimates compared favourably with the independent known dose of the sample in the UV emissions band, with overestimation of De in other detection bands. The overestimation was attributed to extrinsic factors such as the functional differences in detection efficiency and noise performance of solid-state imaging systems, and intrinsic factors relating to luminescence production such as the kinetic behaviour of TL traps and colour centres in quartz. The latter factors relating to luminescence production may be addressed with the development of advanced luminescence imaging and analysis protocols, which seek to account for the variability in the spectral emissions from natural quartz samples. While the development of such comprehensive analysis protocols is beyond the scope of the present work, much of the success in attempting to bridge routine detection and analysis methods with spatially-resolved luminescence imaging techniques is reliant on the further development and optimization of such procedures. A single-grain method for generating De estimates for quartz supergrains using in-situ luminescence imaging techniques was also presented. This technique investigated the viability of extracting luminescence signals from extremely bright individual grains contained within large-aliquot samples. Dose response was found to vary considerably between grains, with large errors in signal intensity measures attributed to sub-optimal background subtraction parameters in dense-field imaging applications. Such artefacts serve to further highlight the need for a true single-grain luminescence imaging method, providing increased accuracy and precision in dose estimates compared to large-aliquot and in-situ luminescence measures. This was achieved by the present study. A single-grain De distribution was successfully generated in the red detection band using dose estimates from twenty-six individual grains, providing a central De value in good agreement of the known independent dose of the sample. However, single-grain TL dose distributions could not be generated in detection bands other than the red transmission window constrained by the SDSS r’2 optical filter. This was attributed to a combination of 1) instrument limitations; namely those relating to detection efficiency, noise performance and spectral resolution, 2) sub-optimality of analysis protocols for specific detection windows and routine dating, and 3) the complexity and variability of the kinetic response and recombination processes which give rise to TL spectra in natural quartz samples. These limitations may be addressed with improved automation and the use of single-photon sensitive, back-illuminated, UV-enhanced electron-multiplying charge-coupled device (EM-CCD) cameras with appropriate high-throughput custom optics. Such a system was devised by the Author as the subject of a funding proposal under the Australian Research Council (ARC) Linkage, Equipment, Infrastructure and Facilities (LIEF; LE180100148) grant scheme in 2018. This proposal is currently pending resubmission. The work contained in this thesis has made significant advances in geophotonics and the multispectral imaging of quartz thermoluminescence. Despite the considerable and complex challenges inherent to the process of instrument development, particularly within the temporal and financial constraints of a PhD project, this work has overcome significant challenges in moving toward the routine use of spatially-resolved imaging systems and techniques in luminescence dating applications. With ongoing research and improvements in these key areas, the use of solid-state detectors for routine signal collection and age determinations will prove an invaluable tool in luminescence dating applications.