Techniques for spatially resolved measurements of infrared-stimulated luminescence (IRSL) and novel Infraredphotoluminescence (IRPL) emissions have recently been developed for applications of rock surface dating.
Radiation Measurements 155 (2022) 106783 Contents lists available at ScienceDirect Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas Investigating the potential of rock surface burial dating using IRPL and IRSL imaging E.L Sellwood a, *, M Kook a, M Jain a a Department of Physics, Technical University of Denmark, DTU Risø Campus, 4000, Denmark A R T I C L E I N F O A B S T R A C T Keywords: Infrared-photoluminescence Infrared-stimulated luminescence Luminescence-depth profile Rock surface burial dating Equivalent dose Techniques for spatially resolved measurements of infrared-stimulated luminescence (IRSL) and novel Infraredphotoluminescence (IRPL) emissions have recently been developed for applications of rock surface dating Such spatially resolved measurements overcome the need for separating out mineral fractions, speed-up sample preparation and measurement times, and data can be quickly processed provide high-resolution luminescencedepth profiles Here, we investigate the potential of using spatially resolved IRPL and IRSL measurements for rock surface burial dating using two large (cm-scale) rock samples with controlled exposure and surface dose histories We use a SAR style measurement protocol, with a test-dose normalisation step to monitor sensitivity changes, a preheat to remove unstable charges and a bleaching step to reset the IRPL signal Through establishing the response of IRPL and IRSL to dose, we are able to construct 2D maps of equivalent doses (Des) for each sample The results here indicate that spatially resolving IRSL and IRPL from large rock samples has the potential to be used for rock surface burial dating and offers a means to investigate the spatial distribution of dose and mineral-dependent sensitivity changes through cm-scale rock samples Introduction The potential of dating rock surfaces using optically stimulated luminescence (OSL) has gained ever increasing interest from geo scientists over the past few decades When a rock surface is exposed to sunlight, trapped charge is optically reset at the surface of the rock With increasing exposure time, the luminescence is bleached to progressively deeper depths from the surface (Polikreti et al., 2002; Sohbati et al., 2011) The exposure duration is recorded as a bleaching front within the rock and the exposure time can be determined by reconstructing the luminescence-depth profile and fitting the profile with a calibrated age model (Sohbati et al., 2012; Freiesleben et al., 2015) Should this exposed rock surface then be buried, the total burial duration can be determined through conventional OSL measurements from surface slices of the buried rock (e.g Theocaris et al., 1997; Vafiadou et al., 2007; Sohbati et al., 2015) The reliability of such dose measurements for rock surface burial dating is ascertained by reconstruction of the pre-burial luminescence-depth profile and through model fitting of the data (Freiesleben et al., 2015) Arguably one of the most advantageous as pects of OSL rock surface burial dating (RSBD) compared to OSL sedi ment dating is this easy validation of whether the rock surface was sufficiently bleached prior to burial (al Khasawneh et al., 2019; Souza et al., 2021) Applications of OSL rock surface dating (RSD) often favour the infrared-stimulated luminescence (IRSL) emission from feldspar because of its almost ubiquitous availability and relatively higher sensitivity compared to quartz (Simkins et al., 2016) However, the IRSL emission is known to suffer from anomalous fading (Winte, 1977; Spooner, 1994; Huntley and Lamothe, 2001), and although various methods have been developed to overcome this (e.g elevated temperature IRSL; Buylaert et al., 2009; Li and Li, 2011; Thomsen et al., 2011), other problems arise such as thermal transfer, poor bleaching, and changing sensitivity (Duller, 1991; Liu et al., 2016; Yi et al., 2016; Colarossi et al., 2018) Since the characterisation of the infrared-photoluminescence (IRPL) emissions at 955 nm (IRPL955) and 880 nm (IRPL880) from feldspar (Prasad et al., 2017; Kumar et al., 2020), there has been increasing hope in being able to overcome some of the limitations of IRSL Contrary to IRSL, IRPL is a steady state emission reliant on the transition of electrons between the excited and ground state within the principal trap (Prasad et al., 2017; Kumar et al., 2018, 2020) IRPL is thus a non-destructive emission with a higher sensitivity than IRSL and has a negligible fading component even at room temperature (Kumar et al., 2018, 2020) * Corresponding author Department of Physics, Technical university of Denmark, Frederiksborgvej 399, 201, 4000, Roskilde, Denmark E-mail address: el.sellwood@gmail.com (E.L Sellwood) https://doi.org/10.1016/j.radmeas.2022.106783 Received December 2021; Received in revised form 21 April 2022; Accepted May 2022 Available online 10 May 2022 1350-4487/© 2022 Published by Elsevier Ltd E.L Sellwood et al Radiation Measurements 155 (2022) 106783 Measurement times can be set for longer durations to increase the signal-to-noise ratio and thus it makes a more viable option for lumi nescence imaging (Kumar et al., 2018) To date, only a few applications have been attempted using IRPL for sediment or rock surface dating Sellwood et al (2019, 2021) recognised the suitability of using IRPL for spatially resolved measurements and demonstrated how luminescence-depth profiles can be reconstructed from naturally exposed rock slabs for rock surface exposure dating (RSED) Duller et al (2020) tested IRPL for determining equivalent doses (Des) using IRPL images of single sand-sized grains Kumar et al (2021) have described a suitable SAR-based protocol for determining equivalent doses without the need for a fading correction using IRPL emissions at 880 nm and 955 nm These latter two authors used a TL/OSL Risø reader adapted with an IRPL attachment (Kook et al., 2018) These promising results, as well as the development of appro priate measurement protocols through both imaging and reader-based measurements, have opened possibilities of using spatially resolved measurements of IRPL for rock surface burial dating (RSBD) Through imaging, the whole luminescence-depth profile can be rapidly assessed, and we can avoid the extensive sample preparation stages of coring and slicing which are required in conventional measurements We would also be able to recreate a dose map, presenting the 2D dose distribution of the whole rock sample, and investigate IRSL and IRPL characteristics (e.g sensitivity changes) from different locations across the sample Presented here is an exploration of the suitability of IRPL and IRSL imaging for rock surface burial dating We attempt to recover known “burial” doses from two rock samples with controlled exposure and surface dose histories using the Risø Luminescence Imager (Sellwood et al., 2022) The IRSL and IRPL at 880 nm and 955 nm was imaged from two granitic rock slabs, following a SAR-style protocol Pixel-wise analysis of the IRPL and IRSL dose response was used to construct 2D distributions of IRPL and IRSL Des We discuss sensitivity changes and residual IRPL levels across different regions of the rock samples, and their effect on the De estimates This study has implications for future applications where investigating and understanding dose distributions in rocks is crucial for obtaining reliable De estimates, and for under standing the response of different mineral constituents to dose, bleach ing and heating This method yielding high resolution luminescence-depth profiles is especially powerful when model fitting is deemed critical to ascertain the extent of bleaching prior to burial surface of G12 (200 Gy dose) and to the remaining exposed surface of G14 (500 Gy) using a cobalt-60 photon beam (1 Gy/min dose rate, DTU Health Tech department, Risø) The given doses were estimated to have been attenuated by up to 8% at a depth of 20 mm from the surface of the rocks (following dose attenuation factors presented in Fujita et al., 2011) Two different doses were chosen to investigate the response of IRPL and IRSL in rock to different doses, as well as to investigate whether IRPL is suitable for RSBD applications with samples of different ages From the irradiated cores of G12 and G14, sections for imaging were cut perpendicular to the now “buried” surface, labelled as G12B (~22 × 43 × 1.5 mm; ‘B’ notation is used to indicate a “buried” sample) and G14B (~30 × 39 × 1.8 mm) respectively Optical images of the three measured samples and a flow chart of the sample processing stages can be found in the supplementary information (S1) 2.2 Measurements Table outlines the measurement sequence for G12B and G14B using the Risø Luminescence Imager (Sellwood et al., 2022) A preheat at 200 ◦ C for was given in an oven Bleaching was achieved over 24 h ălne Solar simulator Samples were irradiated in the cobalt-60 in a Ho gamma cells at the High dose reference laboratory facilities at Risø Regeneration doses for G12B and G14B were 50, 250, 500, 1000 and 3000 Gy For G14E, the residual IRPL and IRSL was measured to determine the extent of bleaching from the 327 day exposure This was followed by measurement of IRPL and IRSL in response to a kGy saturation test dose for normalisation of the signals For all three sections, IRPL was integrated over s, and the whole IRSL decay curve was captured over 20 frames, each integrated over 10 s 2.3 Analysis All analyses was conducted in MATLAB using the Image Processing toolbox (The Mathworks, 2004) All images from each sample dataset were first registered onto one-another to allow pixel-wise analysis The area outside each sample was removed from the images The residual IRPL images (data from steps and 11 in Table 1) were subtracted from the respective Ln, Lx or Tx images for IRPL For IRSL, the final frame was subtracted from the first frame of the respective decay curve, after checking that residual levels had indeed been reached in the decay curves (decay curves are available in the supplementary information, S2) Luminescence-depth profiles were reconstructed by taking the mean and standard error of each column across the images, and plotted as a function of depth from the surface The equivalent dose calculation and analysis followed a three stage process First, Des were calculated for each pixel of the IRPL and IRSL images measured after the preheat stage This was achieved by inter polating the Ln/Tn of each pixel against its respective dose response curve and reconstructing the De map For all data sets, a double Methodology 2.1 Samples Two control samples were measured in this study: G14 and G12 These two samples were initially collected as cm Ø x 10 cm cores from a fine crystalline granite from an unknown location in China Prior to the experiments here, these cores were heated to 700 ◦ C for 24 h to anneal the luminescence, and were then given a saturation dose of 20 kGy using a cobalt-60 source at the Department of Health Technology at DTU, Risø campus For the experiments presented here, two further processing stages were followed to simulate a surface exposure and subsequent burial event First, the edges of the two cores were wrapped in light-proof black tape keeping the top surfaces of the cores exposed The cores were placed on a rotating table under four halogen lamps (Osram H7 70 W bulbs; 102 mW/cm2) where the exposed top surfaces were bleached for 327 days To explore the bleaching resulting from this exposure, a cm diameter core was drilled perpendicular to the bleached surface of core G14 Using a 0.35 mm thick diamond wire saw, a section (18.4 × 18 × 1.4 mm) was cut from the centre of this smaller core, perpendicular to the bleached surface IRPL and IRSL was measured from this section (hereafter named G14E; ‘E’ indicating exposure sample) to reconstruct the luminescence-depth profile resulting from the 327 day exposure At the second stage, a “burial” dose was administrated to the exposed Table Measurement sequence for IRPL and IRSL with the Risø Luminescence Imager Step Treatment Result 10 11 12 13 Natural or Regenerated IRPL Preheat (200 ◦ C, min) Natural or Regenerated IRPL and IRSL Bleach, 24 h IRPL for residual level Tx (100 Gy) Measure IRPL Preheat (200 ◦ C, min) IRPL and IRSL Bleach, 24 h IRPL for residual level Regeneration dose Repeat stages 1–12 Ln or Lx LnPH or LxPH LnBG or LxBG Tx TxPH TxBG E.L Sellwood et al Radiation Measurements 155 (2022) 106783 saturating exponential regression model was chosen for fitting the pixelwise dose response curves, and pixels which had an R2 value < 0.9 for the fit of the exponential model were rejected The second analytical stage followed the reasoning that it is only relevant to observe Des from pixels where IRPL or IRSL was actually detected To achieve this, a threshold mask was applied to each De map to select the luminescing regions of interest To create the masks, eight sections, each mm wide were defined in the full De maps parallel to the “buried” surface, at progressively deeper depths The pixel-wise De values from these sec tions were plotted against the corresponding pixels from the kGy regeneration dose (Lx) image We observed a very broad (sometimes bimodal) De distribution, with a peak present around the expected burial dose (see supplementary information, S3) From here, an optimum threshold value was defined based on Lx intensity to filter out pixels with low intensity or no luminescence, in order to narrow down the distributions The final De maps were constructed based on the selected pixels, after applying the binary masks to the IRSL De maps and to the IRPL De maps from both before and after preheat The third stage of analysis aimed to investigate where at the rock surfaces we could find the De values closest to the known doses For this the mean and standard error of the dose values were calculated from the mm sections parallel to the burial surface of the final De maps from the previously bleached regions of the slabs These values were plotted over depth from the surface, with depth defined as the mid point of the mm section “burial” dose) from after the preheat stage Panels a, b and c display the Ln/Tn ratio maps for IRSL, IRPL880 and IRPL955 respectively The white regions in the IRSL map (Fig 2a) indicate infinite values due to nonresponsive test dose regions (i.e minerals not emitting IRSL or IRPL) In Fig 2a–c, it is possible to view a gradual increase in Ln/Tn from the very surface of the rock to deeper depths for each signal The luminescence-depth profiles in Fig 2d show the expected sigmoidal form, with each of the profiles showing an expected raised plateau in Ln/ Tn ratio values near the surface due to the “burial” dose (note the IRSL data corresponds to the left y-axis, and the IRPL data to the right y-axis) The IRPL profiles (black circles and red triangles) show a slight valley shape with a relatively higher ratio value at the very surface, which then drop slightly before the profile progresses to the transition zone up to saturation This behaviour has been observed before in RSED profiles, and is attributed to slight sensitivity change (e.g see Sellwood et al., 2019) On the contrary a gradual increase in IRSL Ln/Tn is observed in the same region (from to mm depth) in the IRSL profile Fig 2e presents the G12B IRSL De map (Gy) The transition in apparent burial dose is observable from the surface to deeper depths (blue to green coloured pixels) Both the IRPL880 and IRPL955 (after preheat; Fig 2f and g respectively) De maps show a very narrow band of pixels presenting doses around our dose of interest (200 Gy) There is a more irregular distribution in apparent doses, with no clear progression in dose from the surface to the saturated region The average doses from the mm sections are presented in Fig 2h, which shows average doses from IRPL from both before and after the preheat stages The grey band marks the expected 200 Gy (±10%) burial dose profile through the rock (calculated from dose attenuation values from Fujita et al., 2011) The average IRSL burial doses from to 3.5 mm and from 5.5 to 7.5 mm all lie within this expected region, with an increase in dose at 4.5 mm Beyond 7.5 mm (the IRSL bleaching depth seen in the G14E profile in Fig 1), the De values begin to increase, where the initial IRSL was above residual level For the IRPL880 and IRPL955 from before the preheat stage (solid points in Fig 2h), the surface doses from to mm over estimate the known dose by up to 70% (IRPL880) The recovered IRPL Des from to mm from after the preheat stage (hollow points in Fig 2h) also over-estimate the known dose, but are slightly lower than those from before the preheat, with the very surface IRPL955 De almost falling within our expected range (see inset plot in Fig 2h for a closer view of the surface doses) The results from G14B (“burial” dose of 500 Gy) are found in Fig The IRSL and IRPL Ln/Tn maps (Fig 3a–c) are similar to those from G12B, with the surface regions clearly distinguishable from the satu rated region by the lower Ln/Tn values (blue – green colour scheme) The luminescence-depth profiles (Fig 3d) show comparably higher surface ratio values (IRPL Ln/Tn ~3.5) than in G12B (IRPL Ln/Tn ~2.2), with the IRPL profiles again showing an increase in sensitivity at the surface The IRSL De (Gy) map (Fig 3e) presents multiple regions at the “burial” surface with pixels representing doses ranging from ~200 to Results Presented below are the results from G14E, G12B and G14B The ratio and De maps are presented in false colour with colour bars repre senting Ln/Tn value or the De (Gy) The colour scaling has been adjusted to focus on the surface regions of interest Here, we focus on the De maps from G12B and G14B after the preheat stage The De maps from before the preheat stage can be found in the supplementary information (S4) 3.1 Equivalent doses The Ln/Tn ratio maps and luminescence-depth profiles from the bleached sample (G14E) are shown in Fig The IRSL ratio map (Fig 1a) presents a significant bleached region at the surface (left-hand side of the ratio map) The bleached regions in the IRPL880 and IRPL955 maps are smaller than IRSL; nonetheless translation of these signals into luminescence-depth profiles indicates that the 327 day exposure was sufficient to bleach the IRPL to residual levels to a depth of ~2.5 mm from the surface, and the IRSL down to ~8 mm from the surface It is within these bleached zones that investigation into the calculated De values will be focused The shapes of the IRPL luminescence-depth profiles are slightly different; this is attributed to the differences in bleachability of the two signals (greater for IRPL955; Kumar et al., 2020) Fig presents the data from G12B (residual from exposure + Fig a) IRSL Ln/Tn ratio map for G14E The exposed surface is indicated on the left-hand side of the ratio map b) IRPL880 Ln/Tn ratio map c) IRPL955 ratio map d) Luminescence-depth profiles from the IRSL and IRPL ratio maps The profile data here has been normalised to the saturation level E.L Sellwood et al Radiation Measurements 155 (2022) 106783 Fig Results from G12B a), b) and c) present the Ln/Tn ratio maps for IRSL, IRPL880 and IRPL955 after preheat respectively The “burial” surface is on the left-hand side of each ratio map d) Luminescence-depth profiles from the Ln/Tn ratio maps The IRPL profiles correspond to the right axis, and the IRSL to the left e) IRSL De map after masking to observe only the brightest luminescent regions f) IRPL880 De map g) IRPL955 De map h) Average De values from mm wide regions parallel to the “burial” surface from the IRSL De map, and the IRPL De maps from before and after the preheat stage The inset window shows a zoomed view of the data points from the surface two mm Error bars show standard error from the mean and the grey band is the expected dose-depth profile of the 200 Gy dose ( ± 20 Gy), based on attenuation factors from Fujita et al (2011) 600 Gy The calculated surface doses in the IRPL880 and IRPL955 De maps (Fig 3f and g, respectively) at first glance are slightly higher than the IRSL (yellow pixels) Observing Fig 3h, it is only within the surface to mm that the mean IRSL De values fall within the expected dose range (500 Gy ± 10%; grey band on Fig 3h), with a slight increase in dose between and mm, before increasing towards higher doses Across the whole rock slab, there is a significant difference between the IRPL880 Des from before (solid circles in Fig 3h) and after preheat (hollow circles), with an almost 50% overestimate of the known dose for the data after the preheat stage From to mm depth, the IRPL880 from before the preheat stage, and the IRPL955 results from both before and after preheat are within uncertainties consistent with the expected dose range before the preheat The data is plotted over each regeneration cycle in the measurement protocol Cycle corresponds to measurement of the initial IRPL and IRSL after receiving the “burial” dose The IRSL sensi tivity at the very surface of G12B (red circles in Fig 4a) varies within 5% from unity for each regeneration cycle, except for cycle (1 kGy regeneration dose), where the sensitivity decreases by over 25% This decrease is consistent for data from all the depths across the slab The G12B IRPL880 and G12B IRPL955 data (Fig 4b and c respectively) both before and after preheat show the biggest sensitivity changes across cycles in the surface mm (up to ~20%) The deeper slices show lesser variations of 2 mm) regions of the samples This suggests that IRPL sensitivity changes may be related to the initial bleaching period (resetting prior to burial) as such bleaching is most effective closest to Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper Acknowledgements The authors wish to thank Trine Freiesleben for providing the E.L Sellwood et al Radiation Measurements 155 (2022) 106783 samples used here We are also grateful to Mark Bailey, Arne Miller and Torben Esmann Mølholt, our colleagues at the at the Risø High Dose Rate Reference Laboratory who provided access to the large irradiation facilities Li, B., Jacobs, Z., Roberts, R., Li, S., 2013 Extending the age limit of luminescence dating using the dose-dependent sensitivity of MET-pIRIR signals from K-feldspar Quat Geochronol 17, 55–67 https://doi.org/10.1016/j.quageo.2013.02.003 Li, B., Li, S.H., 2011 Luminescence dating of K-feldspar from sediments: a protocol without anomalous fading correction Quat Geochronol 6, 468–479 https://doi org/10.1016/j.quageo.2011.05.001 Liu, J., Murray, A., Sohbati, R., Jain, M., 2016 The effect of test dose and first IR stimulation temperature on post-IR IRSL measurements of rock slices Geochronometria 43, 179–187 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Radiat Meas https://doi.org/10.1016/j radmeas.2018.06.018 ... Through imaging of the initial and regenerated IRSL and IRPL from the large rock samples, we were able to clearly observe the bleaching extent of the IRPL and IRSL, and validate the presence of the. .. ratio values near the surface due to the ? ?burial? ?? dose (note the IRSL data corresponds to the left y-axis, and the IRPL data to the right y-axis) The IRPL profiles (black circles and red triangles)... exploration of the suitability of IRPL and IRSL imaging for rock surface burial dating We attempt to recover known ? ?burial? ?? doses from two rock samples with controlled exposure and surface dose