1. Unresolved challenges in Luminescence dating

Besides the wide range of applications of luminescence dating in many multidisciplinary areas, age calculation using stimulated luminescence techniques still yields a number of currently unresolved challenges. Among these, the most important are the following:

1. Despite the fact that stimulated luminescence stands as a glowing reference among the other absolute dating methods of quaternary, the corresponding upper age limits do not extend 1 million years (1Ma) (Liritzis et al., 2013; Roberts et al., 2015 and references therein; Wintle and Adamiec, 2017). Thermoluminescence (TL) covers the entire range of the historical periods related to pottery, up to about 50.000 years (50 ka), while for the cases of geological materials ages up to 250.000 years (250 ka) have been reported. On the other hand, although laboratory experiments imply more than adequate thermal stability of the optically stimulated luminescence (OSL) signal for dating back to a million years, experimental ages determined in the laboratory are well below this upper limit. The (conventional) OSL age limits are extended up to 350.000 years (350 ka) although, occasionally higher ages, up to 600.000 years (600 ka) have also been reported (Erginal et al., 2022). Moreover, applications to independently dated sedimentary deposits from the last 250.000 years (250 ka) have yielded underestimated OSL ages for over 50 ka (Wintle and Adamiec, 2017). Specific protocols of infrared stimulated luminescence (IRSL) indicated ages with maximum limit of 1Ma and 15-20% corresponding relative errors (Roberts et al., 2015 and references therein; Li and Li, 2019). 

2. The limited number of minerals used for age assessment applications. From a theoretical point of view, the great advantage of luminescence age assessment is the ability to use inorganic and naturally occurring materials. Quartz and feldspars stand as the two (a) most frequent minerals at the Earth’s crust as well as (b) main minerals used for luminescence dating. Nevertheless, despite the fact that quartz has become a trustworthy luminescence chronometer, the use of luminescence signal from a number of minerals is excluded due to the presence of malign effects. The major drawback among these effects is the severe loss of signal that results in significant underestimation of the calculated age. It is the so called athermal or anomalous fading (AF) effect (Duller, 1995; Krbetschek et al., 1997), namely the rapid decay of the otherwise stable luminescence at room temperature, instead of the stability expected for it according to the basic luminescence theory. 

3. The time-consuming preparation and handling procedure following sampling. The majority of the steps of this procedure should be undertaken in dark (or red dim light) conditions inside the laboratory and include chemical and/or mechanical isolation of the appropriate mineral (Stokes et al., 2003; Nelson et al., 2015). 

Nevertheless, there is an increased interest for both archaeologists and geologists in multidisciplinary dating between 250 ka and 4 Ma, as several human’s milestones were directly linked to these dates. From geo-archaeological point of view, studying this period has attracted major interest in researchers of various specialties as a multidisciplinary approach. Nevertheless, age determination using science-based methods of cultural events, objects, and landscape changes that have taken place within this specific sub-era becomes even more and more difficult, as fewer, more expensive and less accurate absolute dating techniques are available (Walker, 2005); these include the techniques of the Uranium series, with limitation for ages beyond 1.3 Ma, Electron Paramagnetic Resonance (EPR) as well as ⁴⁰K/³⁹Ar and ⁴⁰Ar/³⁹Ar techniques. Moreover, these techniques are often questionable in terms of credibility or maturity for allpicability (Walker, 2005). Consequently, a robust and reliable dating technique will b emuch appreciated.

In fact, luminescence age limits can potentially be indeed extended in order to provide absolute ages within the Quaternary (namely up to 4 Ma). There are several ways to deal with the problem of age limit extension, including investigations of new methods of stimulating OSL as well as developments concerning other signals derived from trapped electrons; for a review over these aforementioned solutions, the readers could refer to a recent review article by Wintle and Adamiec (2017). Figure 1 presents an outline of the courent strategies/policies towards the direction of luminescence age limit extension over the luminescence scientific community worldwide. A short description of the research currently ongoing in our laboratory could be found below.

2. TA – OSL as an alternative tool for luminescence age limit extension

In general, the luminescence signal results from trapping levels with specific lifetimes. Temperature is a very important experimental parameter, being directly correlated to the stability of the signal and thus the ability to measure older ages. Trapping levels with higher excitation temperatures yield higher lifetimes and thus can provide with older ages (Wintle, 1997). For naturally occurring materials, such as quartz, feldspars, calcium carbonate, gypsum and apatites, heating from room temperature up to 500 °C has been a common, routine practice in TL dating (Polymeris et al., 2010; Polymeris, 2015; 2016), since most of dosimetric peaks are observed below 425 °C. Moreover, while applying OSL dating protocols for quartz samples, prior to any OSL measurement, preheating is required to remove thermally unstable signals; however, this preheating temperature usually does not exceed 260–280 °C, as the signal from this OSL component is directly related to an electron trap responsible for the TL glow-peak with maximum temperature at 325 °C (Spooner, 1994; Wintle and Adamiec, 2017). For the case of infrared stimulation, it was demonstrated that IRSL measurements at high temperatures could isolate a more stable luminescence signal (Thomsen et al., 2008). However, the maximum measurement temperature in these techniques does not exceed 300–350 °C. Consequently, currently, the trapping levels used for all luminescence dating applications in naturally occurring materials are exclusively those levels that can be thermally excited at moderate temperatures, namely below 400 °C. 

Nevertheless, most of luminescence phosphors are wide band gap materials and contain many trapping levels within temperatures ranging between room temperature and 1000 °C; these are known as very deep traps (VDTs hereafter). As VDTs we define the electron traps that are excited at temperatures above 500 ^°C. An alternative experimental technique was recently suggested in order to not only measure the signal of VDTs without heating the sample at temperatures greater than 500 °C using the available commercial luminescence readers, but also use this signal for high-dose-level dosimetry as well. The access to the charge originating from VDTs is achieved by a combined action of both thermal and optical stimulation, (namely a combination of simultaneous TL and OSL measurements) which is termed Thermally Assisted OSL (TA – OSL, Polymeris, 2016). Extension of luminescence age limits using TA – OSL signal from VDTs aims mostly at using signal from more stable traps, with simultaneous extension of the maximum detectable (paleo-)dose limit due to higher capacity to store trapped charges. From a theoretical point of view the long lifetimes expected for these traps provide from the early beginning one of the main pre-requirements for an extension of the age limits. In order to get an idea about their outstanding thermal stability, the lifetime of a trapping level at 500 °C was calculated of the order of the age of the Earth (Polymeris and Kitis, 2019). Traps that can be thermally stimulated by heating at temperatures beyond 500◦C have been rarely used for dating purposes (Polymeris and Kitis, 2019). Nevertheless, the outstanding stability of these VDTs is also further supported by the lack of instability effects such as AF for the TA – OSL signals of the majority of natural minerals (Polymeris et al., 2018). 

TA – OSL suggests a methodology which can respond to all three aforementioned challenges of luminescence dating, accounting the luminescence signal from VDTs. An important methodological issue deals with the feasibility of establishing more minerals as long-range chronometers besides quartz (Polymeris et al., 2015a). Special emphasis is and will be given to the K-feldspars (Polymeris et al., 2015b) and apatites, as the lack of the AF instability will be an important advantage for the TA – OSL signal. It is worth emphasizing the ability of these VDTs to retain the TA – OSL signal even for storage in room light conditions, due to the low values of photo-ionization cross-sections at room temperature. Thus, attempting to verify whether paleodose information can be also retained, even for storage and handling of the sample in room light conditions and at ambient temperatures becomes important towards avoiding the time-consuming chemical handling pre-treatments in dim light conditions. Thus, modification of the handling procedure in order to avoid handling in red dim light conditions might be possible. Ongoing research includes (a) experimental exploitation of the TA – OSL signal in various natural minerals that yield intense TA – OSL, (b) studying the prevalence of TA – OSL features for the cases of quartz and feldspars, (c) application and optimization of a SAR – TA – OSL protocol for age calculation in geological samples, (d) theoretical modeling, (e) correlation studies using other techniques such as Electron Spin Resonance (Meriç et al., 2018), Time Resolved OSL & Spectrally resolved luminescence and (f) enlarge the quite large already phosphors that indicate intense TA – OSL signal with dosimetric properties (Majgier et al., 2019).

3. IRPL

Both OSL and IRSL signals are dose dependent and typically measured in the anti-Stokes mode to avoid contamination from prompt fluorescence and phosphorescence emissions (Bøtter-Jensen et al., 2003). Moreover, the charge that recombine giving rise to both signals, decay very quickly. Thus, following one unique irradiation, conventional OSL and IRSL methods enable only one measurement from the same trapped electrons since the measurement is destructive. The full exploitation of IRSL dating technique over various feldspar types is limited, mostly by the AF effect, due to the fact that IRSL recombines mostly via localized transitions. Recently, the discovery of Infrared Photoluminescence (IRPL), where IR refers to the excitation energy and PL refers to the Stokes-shifted photoluminescence arising from the radiative relaxation within the electron trap (Prasad et al., 2017) opens up exciting new opportunities for archaeological and geological dating applications, as this specific signal is also dose dependent. Stimulation takes place using laser photons at ∼885 nm and recording the emission at ∼955 nm (Kumar et al., 2018). Another emission at ∼880 nm using a shorter wavelength excitation at 830 nm is associated with potassium rich feldspar. Due to the fact that IRPL does not involve release of charge from traps, nor does it require hole-centres to produce luminescence, it yields a number of interesting properties (Kook et al., 2018): (a) it does not deplete during measurement, making it possible to improve the precision of individual measurements by extending the period of stimulation, (b) this specific signal is thought not to suffer from AF, a major challenge for other methods based on analysis of stimulated luminescence from feldspars (Duller et al., 2020). While IRPL traps were proven to be the same as the traps responsible for radio-luminescence emissions, a correlation between the traps that feed IRSL and IRPL has not been established yet. Despite the fact that the signal can thus be accumulated over long periods by repeated excitation of the same trapped electron, without significant loss, initial observations indicated lack of rapid bleaching with exposure to daylight (Prasad et al., 2017). Ongoing research includes studying on these two aforementioned tasks, along with general dosimetric characterization of the IRPL signal in K-feldpars.

4. Remodelling of the dose response growth curve

Quartz has become a trustworthy mineral used for luminescence dating. Neverhtless, certain specific luminescence properties, such as low sensitivity in the case of un-fired, geological quartz, relatively low maximum dose detection limit as well as quickly saturating dose response behavior, act as drawbacks that impose major limitations in the direction of age limit extension. The lower and upper age limits of TL and OSL methods are set by the TL and OSL response to either natural or artificial dose. The dose response is a feature of great importance for the luminescence dating, for both archaeological and dosimetric aspects. In routine dating applications, it is highly desirable that entire or part of the dependence of luminescence signals on the dose be linear. Of course, there are several cases of dose responses yielding non-linearity. So far, there was no analytical single equation that could both explain and fit with great accuracy superlinear, linear, sublinear and the saturation stages of any material, namely fit the entire the dose response curves of different materials. Up to now, dose response curves in archaeometry applications could be described using specific saturation exponential expressions (Șahiner et al., 2017) or a combination of linear plus exponential functions; such alternatives, albeit empirical, are already included in the software that is attached to commercial luminescence readers.

Recently, Pagonis et al. (2020a; 2020b) have reported an effective way to fit all the dose response curves, being in the linear, but mostly supralinear or saturation regions, using the Lambert W function. As this specific equation is not included as special equation in ordinary commercial software packages, the implementation of the Lambert W function in Microsoft Excel, a well-established commercial spreadsheet program, was recently described for the first time in the literature (Konstantinidis et al., 2021). From a geo-archaeological point of view, the task of using the entire luminescence dose response curve and not only strictly the linear part, could possibly result in an extension of the upper limit of the detectable age in archaeological but mostly in geological findings. The present equations could possibly enable the possibility of using the saturation part of the luminescence dose response curve for the production of reliable ages. Using the current dose response empirical equations, when the behavior of the material reaches saturation, the errors of the indicated ages usually are of the order of at least 50% (Prevezanou et al., 2022). Flexibility on using the dose response spreadsheets and the corresponding W-Lambert equations can significantly reduce this uncertainty. Currently ongoing work on such analytical expressions could move our knowledge some further steps forward, as it is expected to improve the precision of ages within the saturation region for quartz (Wintle and Adamiec, 2017). 

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