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In this study, we explored the potential of a NaI(Tl) scintillator-based gamma spectrometer for the accurate determination of burial dose rates in natural geological samples using a full spectrum analysis (FSA) approach. In this method, an iterative reweighted least-square regression is used to fit calibration standard spectra (40K, and 238U and 232Th series in equilibrium) to the sample spectrum, after subtraction of an appropriate background. The resulting minimum detection limits for 40K, 238U, and 232Th are 4.8, 0.4 and 0.3 Bq·kg−1, respectively (for a 0.23 kg sample); this is one order of magnitude lower than those obtained with the three-window approach previously reported by us, and well below the concentrations found in most natural sediments. These improved values are also comparable to those from high-resolution HPGe gamma spectrometry. Almost all activity concentrations of 40K, 238U, and 232Th from 20 measured natural samples differ by ≤5% from the high resolution spectrometry values; the average ratio of dose rates derived from our NaI(Tl) spectrometer to those from HPGe spectrometry is 0.993 ± 0.004 (n=20). We conclude that our scintillation spectrometry system employing FSA is a useful alternative laboratory method for accurate and precise determination of burial dose rates at a significantly lower cost than high resolution gamma spectrometry.

Metastable states form by charge (electron and hole) capture in defects in a solid. They play an important role in dosimetry, information storage, and many medical and industrial applications of photonics. Despite many decades of research, the exact mechanisms resulting in luminescence signals such as optically/thermally stimulated luminescence (OSL or TL) or long persistent luminescence through charge transfer across the metastable states remain poorly understood. Our lack of understanding owes to the fact that such luminescence signals arise from a convolution of several steps such as charge (de)trapping, transport and recombination, which are not possible to track individually. Here we present a novel coupled RPL(radio-photoluminescence)/OSL system based on an electron trap in a ubiquitous, natural, geophotonic mineral called feldspar (aluminosilicate). RPL/OSL allows understanding the dynamics of the trapped electrons and trapped holes individually. We elucidate for the first time trap distribution, thermal eviction, and radiation-induced growth of trapped electron and holes. The new methods and insights provided here are crucial for next generation model-based applications of luminescence dating in Earth and environmental sciences, e.g. thermochronometry and photochronometry., Manuscript accepted in Scientific Reports (https://www.nature.com/srep/)

Prasad et al. (2017) recently developed a new method of measuring the dosimetric signal in feldspar, based on a Stokes-shifted photoluminescence emission (excitation energy ∼ 1.40eV (885 nm), emission energy ∼ 1.30eV (955 nm)). The new signal, termed as infrared photoluminescence (IRPL), was shown to arise from radiative relaxation of the excited state of the principle trap (dosimetric trap), and allows non-destructive probing of the dosimetric information. Thus, IRPL provides a unique tool to study physical characteristics of these metastable states in feldspar, e.g., number density and spatial distribution, trap depth, photo-ionisation and capture cross-section, excited state lifetime, and tunneling probabilities. The IRPL emission is apparently related to the infrared radioluminescence (IR-RL) in K-feldspar (Trautmann et al., 1998); in the latter, however, the electrons relax after being trapped as a result of exposure to ionising radiation, rather than as a result of excitation within the trap. In this study, we report the discovery of a second IRPL emission centred at ∼ 1.41eV (880 nm) in a K-feldspar which arises in response to excitation with 1.49eV photons. Based on the temperature- and dose-dependent behaviour of IRPL and IR-RL, we conclude that the same defect(s) participates in these two emissions. However, IRPL emission is governed by the characteristics of the principle trap (defect) alone, whereas IR-RL depends additionally on thermally assisted transport within the band-tail states. Since IRPL is a site selective technique, it does not, unlike IR-RL, suffer from contamination from higher energy emissions (e.g. from Fe 3 + ). This lack of contamination, and the possibility for thermal/optical pre-treatments and repeated measurements of the same trapped electrons, suggest that IRPL is a robust alternative to IR-RL.

It is desirable to be able to establish the relationship between the spatial distribution of luminescence and chemical composition/mineralogy. This is especially true in complex, heterogeneous samples such as solid coarse-grained rock slices and separated feldspar grains, where the internal dose rate is very dependent on the effective size of luminescent regions of the sample. The addition of spectral information would then help to give further insights into luminescence production mechanisms. A high sensitivity imaging attachment to the Riso TL/OSL Reader for investigating the spatial distribution of OSL and TL from natural and artificial phosphors has recently been developed (Kook et al., 2015). When combined with a Bruker μ-XRF facility and a high sensitivity spectrometer attachment, it is possible to investigate the relationship between the spatial distribution of luminescence, the distribution of chemical composition/mineralogy and the associated excitation and emission spectra. Here laboratory measurements undertaken using solid rock samples are presented and the observed relationships and their dosimetric implications are discussed.

Optical dating has revolutionized our understanding of Global climate change, Earth surface processes, and human evolution and dispersal over the last ~500 ka. Optical dating is based on an anti-Stokes photon emission generated by electron-hole recombination within quartz or feldspar; it relies, by default, on destructive read-out of the stored chronometric information. We present here a fundamentally new method of optical read-out of the trapped electron population in feldspar. The new signal termed as Infra-Red Photo-Luminescence (IRPL) is a Stokes emission (~1.30 eV) derived from NIR excitation (~1.40 eV) on samples previously exposed to ionizing radiation. Low temperature (7–295 K) spectroscopic and time-resolved investigations suggest that IRPL is generated from excited-to-ground state relaxation within the principal (dosimetry) trap. Since IRPL can be induced even in traps remote from recombination centers, it is likely to contain a stable (non-fading), steady-state component. While IRPL is a powerful tool to understand details of the electron-trapping center, it provides a novel, alternative approach to trapped-charge dating based on direct, non-destructive probing of chronometric information. The possibility of repeated readout of IRPL from individual traps will open opportunities for dating at sub-micron spatial resolution, thus, marking a step change in the optical dating technology.

Optically stimulated luminescence (OSL) dating of sediment, based on the accumulation of trapped charge in natural crystals since their last exposure to daylight, has revolutionised our understanding of the late Quaternary period. Recently, a complementary technique called luminescence rock surface dating (RSD), which uses differential spatial eviction of trapped charges in rocks exposed to daylight, has been developed to derive exposure and burial ages, and hard-rock erosion rates. In its current form, the RSD technique suffers from labour intensive sample preparation, uncertainties in the depth and dose rate estimates, and poor resolution of the luminescence-depth profile. Here, we develop a novel, 2D luminescence imaging technique for RSD of large rock slabs (3 × 5 cm) to overcome these challenges. We utilize the recently discovered infrared photoluminescence (IRPL) signal for direct, non-destructive imaging of the luminescence-depth profile in a sub-aerially exposed granitic rock, with an unprecedented spatial resolution of ~140 µm. We further establish a correlation between luminescence and geochemistry using micro X-ray fluorescence (µXRF) spectroscopy. Our study promises a substantial advancement in luminescence imaging and paves the path towards novel applications using 2D dating, micro-dosimetry in mixed composition samples, and portable instrumentation for in-situ luminescence measurements.

In contrast to the detailed investigations on the dosimetric electron trap in feldspar only little has been done to understand the luminescence centers. We use a comparison of multiple spectroscopic techniques, site selective photoluminescence spectroscopy and time resolved measurements to further our understanding of the luminescence mechanisms and recombination sites, in a sample of Na rich plagioclase feldspar (oligoclase). Both the UV and violet–blue emissions show resonant excitations arising from a distribution of energy levels. We propose, contrary to the general understanding, that the green emission may not arise from Mn 2+ in our sample and that photoionisation of this centre may be possible by excitation to the band tail states. The deep red emission is tied to the Fe 3+ , and the exponential rise in the UV excitation efficiency of this centre is discussed in the context of the band-tail model.

The measurement of OSL/TL relies on the initial trapping and subsequent release and recombination of charge in a suitable crystal structure. These measurements allow us to estimate the dose which the crystal has been subjected to and its age. During such processes, however, electrons can be emitted from the sample surface; the ability to measure such exo-electrons can provide information regarding the trapping process, as well as an alternative method of measuring trapped charge. Ideally, TL/OSL can be measured alongside exo-electrons. We will present a new design for an exo-electron detector compatible with the classic Riso reader stimulation head featuring multiple anode configurations and the option for high-voltage modulation to reduce the UV component during TL/OSL measurements.

A facility for the measurement of infrared photoluminescence (IRPL) has been developed for the Riso TL/OSL reader. The new IRPL measurement system uses an external laser light source at 1.49 eV (830 nm) and two photomultiplier tubes (PMT) for detecting emissions at ∼1.41 eV (880 nm) and ∼1.3 eV (955 nm) and an EMCCD. Pulsed IRPL measurement ensures a low background count rate by allowing the rejection of breakthrough from excitation light. We present the results of integrated IRPL measurements on both multiple- and single-grain aliquots, and finally demonstrate the potential of imaging natural K-feldspar samples and a granite rock slice.