Optimization of OSL/IRSL experimental measurement parameters on various types of naturally occurring CaSO4 samples; sensitization pattern versus temperature, bleaching properties and slurry zeroing effect

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Abstract

Calcium sulphate (CaSO4) geological material is the main constituent of art objects and geological formations, mostly in the form of gypsum. Nevertheless, investigations of either optically or infrared stimulated luminescence (OSL or IRSL respectively) properties are scarce for this material towards age assessment applications. The present work studies for both IRSL and OSL signals of CaSO4 (a) the effect of the preheating to the sensitivity, towards selecting the optimum preheating conditions, (b) the bleaching ability of both signals via the corresponding residual TL signal levels, towards selecting the appropriate stimulation duration and (c) whether mixing with water acts as a zeroing mechanism for these signals. IRSL and OSL yield different sensitization threshold temperatures, being 250 °C or 300 °C, depending on the water content of the mineral (hemihydrate, anhydrous and the most usual dihydrate). However, for the cases of protocols involving either a single IRSL measurement or post IRSL OSL measurements, preheating temperature should not exceed 225 °C. The optimum stimulation duration is 100 s for the blue OSL and 60 s for the IRSL. Finally, mixing with water affects (i) insignificantly the zeroing of IRSL signal in anhydrites, (ii) partially the zeroing of OSL in all groups and (iii) substantially the zeroing of IRSL signal in gypsum and bassanite.

Introduction

Luminescence, in all forms of thermoluminescence (TL), optically stimulated luminescence (OSL) and infrared stimulated luminescence (IRSL) stands among the basic research tools in the fields of (a) ionizing radiation dosimetry, (b) retrospective dosimetry, namely geo-archaeological dating and accidental dosimetry along with (c) authenticity testing of archaeological artifacts [1], [2], [3], [4]. The great advantage of retrospective dosimetry using luminescence is the ability to use inorganic and naturally occurring materials. Both geo-archaeological dating as well as accidental dosimetry, involve the possible use of any geological material as dosimeter in order to evaluate the accumulated dose over a long time period. Despite the fact that quartz and feldspars have become the main minerals used for luminescence dating, the effective use of more minerals should be exploited. In general, any geological material could be a potential natural dosimeter in action, providing that a systematic characterization is performed in order to study whether its properties meet the requirements set by a specific dosimetric application. Ca-based materials such CaCO3 in all its forms such as chalk, ryzoliths as well as marbles and calcites, could be considered as effective TL chronometers [5], [6], [7], [8], [9], [10], [11], [12].

Another Ca-based mineral is calcium sulphate (CaSO4). It is a highly evaporative mineral which can be collected from many arid basins where the source geology does not provide adequate quartz or potassium feldspars for traditional OSL/IRSL applications, such as the South-West United States [13] and West Turkey [14]. Moreover gypsum, namely dihydrate CaSO4 is usually used as either decorative or sculpture material, plaster or a ground layer material in portable paintings. Initially the various forms (dehydrate, CaSO4·2H2O; hemihydrate, CaSO4 · ½ H2O, also known as plaster of Paris; or anhydrous, CaSO4 [15]) of calcium sulphate have not been considered as an effective naturally occurring TL phosphor, due to decomposition with temperature [16 and references therein]. Within the following years, very few studies were devoted to the luminescence (mostly TL and OSL) signals of gypsum for either dating [14], [17], [18], [19], [20], [21] or accidental dosimetry [22] applications. It is quite frequent to use only quartz OSL chronology, even for the cases of formations in arid environments, containing substantial amounts of anhydrite; examples can be found in [23], [24]. Nevertheless, Polymeris et al. [10] have presented preliminary TL glow curves measured for one commercial gypsum sample in the framework of a feasibility study on dating portable paintings using the ground layer materials. Recently, the relation between the optically stimulated luminescence (OSL) and the infrared stimulated luminescence (IRSL) signals was studied [25] for the same commercial gypsum sample that was previously studied by Polymeris et al. [10].

In a recent paper, Malletzidou et al. [26] have studied the TL properties of three different groups of calcium sulphate samples based on their level of hydration; one group of dihydrate CaSO4 (gypsum), one group of hemihydrate CaSO4 namely bassanite, along with one anhydrous CaSO4 sample. The presence of stable TL peaks, with delocalization temperatures around 280 °C, was monitored for all CaSO4 samples. However, these authors have reported that following one TL measurement, namely instantaneous heating up to 500 °C with moderate heating rates, the TL sensitivity of these peaks for both hemihydrate and the dihydrate forms increased substantially. Following 3 heating cycles at 500 °C, the sensitivity of all forms is stabilized, with tendency to attain a level which seems prevalent for all three groups of samples according to their hydration level. Based on FTIR measurements, this increase of the TL sensitivity was attributed to the transformation of the monoclinic forms of both gypsum and bassanite to the orthorhombic crystal form of anhydrite. The endothermic dehydration of gypsum takes place in two steps, according to the following reactions:

  • a)

    CaSO4·2H2O → CaSO4 · ½H2O + 1.5H2O

  • b)

    CaSO4 · ½H2O → CaSO4 + 0.5H2O

This dehydration is followed by the aforementioned transformation, to which the sensitivity enhancement is attributed. These two reactions take place between 150 °C and 250 °C, using a slow heating rate of 10 °C/min [27]. Similar sensitization of luminescence signals has also been reported for other calcium-based minerals such as CaCO3 [5], [10], [11], [28], [29], [30].

Based on these features, Malletzidou et al. [26] have concluded that the hemihydrate and the dihydrate forms of calcium sulphates should be excluded from effective dating applications using TL, unless multiple aliquot protocols might be applicable (for example refer to [14]). Moreover, for these forms of CaSO4, OSL and IRSL could possibly provide an alternative to TL. A possible correlation between these two former signals was reported by Angeli et al. [25]; however, in this latter work, both OSL and IRSL measurements were performed at room temperature (RT hereafter) without any previous (pre-)heating treatment. Nevertheless, isolation of stable OSL and IRSL signals requires (pre-)heating pre-treatments of the calcium sulphate samples, as these yield intense shallow peaks within the temperature range up to 220 °C. As sensitization due to thermal treatment is also expected for both the OSL and IRSL signals, Detschel and Lepper [31] and O’Connor et al. [32] have reported stable OSL signal characteristics from gypsum and large recoverable saturation doses without preheating when using a modified SAR procedure.

Therefore, optimization of the OSL/IRSL experimental measurement parameters is required. The term optimization refers to the selection of the most appropriate OSL/IRSL measurement parameters, including preheating and measurement temperatures as well as the optimum stimulation duration. Optimization of the OSL measurements has been accomplished when the studied material is quartz (the readers could refer to the review articles [33], [34]). Especially the former parameters, related to temperature, are vital. An appropriate heating pre-treatment in CaSO4 samples should be undertaken so that at the same time (a) the luminescence signal of the unstable, shallow traps be removed and (b) the luminescence signal sensitization is minimized, if possible even avoided. According to [27], the exact temperature profile of the phase transition that results in the sensitization is quite wide, overlapping with the range of the preheating temperatures. Moreover, the usually applied heating rates for the luminescence measurements are quite faster, being less than or equal to 5 °C/s compared to the heating rates of 10 °C/min of the thermal analysis measurements.

The present work follows on directly from the previous TL study [26], aiming to suggest the optimum conditions for both OSL and IRSL measurements in calcium sulphate samples, in terms of temperature pre-treatment and time stimulation. The purposes of the present work include the study of: (i) the temperature profile of the transformation that causes sensitization towards identifying the optimum heating pre-treatments before OSL and/or IRSL measurements and (ii) the bleaching ability towards identifying the optimum stimulation time. Finally, mixture with water was also studied, as a possible new zeroing luminescence mechanism on both IRSL and OSL signals. Previously, the same effect was studied only for the TL signal of the un-heated anhydrites; for the other sample groups, the low TL intensity was a major drawback.

Section snippets

Materials and experimental equipment

Α Risø TL/OSL reader (model TL-OSL-DA-20, Reader ID: 267, [35], equipped with a 0.115 ± 0.03 Gy/s 90Sr/90Y β-ray source was used for all OSL, IRSL and TL measurements. An EMI 9235QB bi-alkali photomultiplier tube was used for the detection of all luminescence signals. In all cases of heating, a low heating rate of 2 °C/s was applied, in a nitrogen atmosphere, in order to avoid significant temperature lag [36], without any previous annealing of the samples. Both OSL and IRSL measurements were

Experimental protocols & method of analysis

The following protocols were applied in the framework of the present study:

Protocol 3.1: Identifying the appropriate (pre-)heating treatment; OSL/IRSL signals versus previous heating temperature

3.1.1. OSL measurement at RT for 500 s in order to empty all optically active traps

3.1.2. Test dose

3.1.3. OSL or IRSL measurement for 500 s to get the signal corresponding to the test dose and not any previous heating (OSL0 or IRSL0 measurement)

3.1.4. Test dose

3.1.5. TL up to Ti (°C)

3.1.6. OSL or IRSL

OSL/IRSL signals versus previous heating

As the exact temperature profile of this aforementioned phase transition was not assessed, it is unknown which the maximum preheating temperature might be, without affecting the luminescence sensitivity. A similar increase in both OSL and IRSL signals is expected, as sensitization was reported for the high temperature TL peaks. Thus, protocol 3.1 aims towards identifying this temperature; this sensitization increase of OSL and/or IRSL signal will be considered as a probe for phase

Conclusions

The present study has reported for three different groups of calcium sulphate, depending on the water content of the mineral:

  • (a)

    the effect of the heating to the sensitivity and sensitization of both OSL and IRSL signals, revealing the optimum preheating conditions for the corresponding single aliquot measurement protocols,

  • (b)

    the relation between either aforementioned signal with the corresponding TL signals, towards selecting the appropriate stimulation duration,

  • (c)

    the effect of slurry on both signals.

CRediT authorship contribution statement

Georgios S. Polymeris: Conceptualization, Data curation, Formal analysis, Methodology, Validation, Visualization, Writing - original draft, Writing - review & editing.

Declaration of Competing Interest

The author declares that he has no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The author would like to thank Lamprini Malletzidou, PhD candidate, Physics Department, Aristotle University of Thessaloniki, for providing the samples and helping with the slurry procedure.

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