Application of microdosimetric concepts in CaCO3:Ce3+/Dy3+ for megalevel radiation dosimetry

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Highlights

Abstract

The 150 °C and 290 °C thermoluminescence (TL) peaks in CaCO3:Ce were attributed to generically related traps. TL process in calcites arises from O2− interstitial ions and anion vacancies created during pre-sintering which serve as hole traps and electron traps, respectively. The TL response of CaCO3:Ce is supralinear in the dose range 0.5–5 kGy due to secondary electron track interaction and sublinear beyond 5 kGy due to saturation of traps. The charge state surrounding a cation vacancy and Oi2− in CaCO3:Ce3+ can be considered as 3- (2- from oxygen and 1- from Ce3+) since Ce3+ is prone to oxidation to Ce4+ state which increases 2-track interaction during γ-irradiation at high doses and 2-hole trapping by densely ionizing radiations. TL of CaCO3:Dy is dominated by Dy3+ emission (480 and 570 nm) and a linear TL response with dose in the range 0.5–15 kGy since track interaction probability is less.

Introduction

Calcite is an interesting material owing to its abundance in nature and is an important mineral used in geological dating. A review by Sankaran et al. [1] has shown that the thermoluminescence (TL) of calcites and related polymorphs has attracted the maximum attention of the early workers. Medlin [2] studied the effect of different impurities in the synthetic form of calcite and found that except Mn and Pb, various other impurities introduced by him did not influence the TL in calcites. TL of fossil shells (calcite) has been reported by Johnson and Blanchard [3]. The dose rates observed in these shells were consistent with the known levels of radioactivity associated with these samples. TL measurements of calcite derived from shells or shell fragments were found to be relevant to the detection of irradiated shellfish for enforcement of food labeling regulations and to dating of archaeological and fossil materials. The main TL emission occurred around 600 nm associated with Mn2+ impurity [4]. The potential of natural TL dosimetry as a viable geological tool in Greek calcites was established by Liritzis et al. [5]. Bapat and Nambi [6] noted that above 0.3 mol % the Mn related luminescence decreased with impurity concentration. Studies on different natural carbonate samples, from several structural types, including rhombohedral, orthorhombic and monoclinic, from the National Museum of Natural Science in Madrid, also revealed concentration quenching of the TL when the Mn impurity levels exceed ~1000 ppm [7]. Soliman et al. [8] studied the green and red TL emission bands of natural dolomite irradiated with gamma rays and found it to be a promising phosphor for the retrospective dosimeter of gamma rays with the corresponding converters in the range of 200 to 104 Gy in the green and red emission bands. Down et al. [9] have reported that the heavily doped calcite has Mn clusters or precipitate leading to broad emission bands. At higher annealing temperatures or in lightly doped CaCO3, the Mn solubility is sufficient to produce line spectra from dispersed Mn impurity. Engin and Guven [10] studied three different types of natural calcites, and report that major TL peaks of stalactite occur around 280 °C, flowstone exhibit equally intense peaks at 140 and 260 °C, marble stone exhibit an intense TL peak at 220 °C and a less intense peak at 100 °C. Thermal annealing studies revealed no change in TL sensitivity up to 350 °C, an increase above 350 °C up to 600 °C and a sudden collapse at 700 °C. A 600 °C, 5 h heat treatment yielded optimal sensitivity for the TL peaks in stalactite with their TL response increasing linearly in the dose range 0.05–10 kGy. No satisfactory explanation was however, offered for the above temperature effect except for an analogy with Zimmerman’s [11] hole reservoir model for the 110 °C peak in pre-dosed quartz despite the fact even synthetic calcites which have not received any pre-dose show similar annealing temperature effect. An alternate explanation offered was a redistribution of impurities (Mn2+) in the powder lattice at high temperatures. Kalita and Wary [12] have recently reported that no phase change of natural CaCO3 mineral from Assam occurs in the 27–500 °C temperature range but the decomposition of CaCO3 to CaO appears from 600 to 660 °C. On x-irradiation, these samples showed three low temperature peaks at 50 °C, 87 °C and 135 °C which showed significant post-irradiation fading. The 300 °C pre-irradiation annealed sample showed a sublinear TL dose response from 10 mGy to 1 Gy and saturation beyond 3 Gy. Ponnusamy et al. [13] have reported that γ-irradiated blue colored natural calcite exhibit major TL peak at 140 °C and a minor peak at 260 °C with the emission peak at 610 nm characteristic of Mn2+ (4G → 6S transition). The TL response of 260 °C peak reportedly exhibit linearity in the 1–10 kGy dose range. The increase in TL sensitivity of this sample with annealing temperature up to 600 °C was explained on the basis of dispersion of Mn clusters at high temperatures although no explanation for the collapse of TL at 700 °C was offered. None of these studies identify the electron and hole traps causing the TL in calcites. Thus literature reports on glow peak temperatures, cause for the change in TL sensitivity with pre-annealing temperature and dose-response behavior of natural calcite vary considerably perhaps due to variation in impurity and its concentration in natural samples. Only studies on synthetic calcite samples with controlled impurity concentration would resolve these issues.

Sensitized luminescence of Mn2+ ions under 254 nm excitation in CaCO3 with the coactivators Pb2+, Tl+ and Ce3+ was reported originally by Schulman et al. [14]. Recent studies by Nara and Adachi [15] in CaCO3:Ce,Mn confirm that the intensity of Mn2+ PL emission centered at 640 nm on Ce3+ excitation at 325 nm is 200 times higher than that of the Mn2+ emission in CaCO3:Mn due to efficient resonant energy transfer from Ce3+ to Mn2+. The phosphors synthesized by coprecipitation without any pre-irradiation sintering treatment used in this study were found to be a mixture of the calcite and vaterite polymorphs. The XRD results indicated that the fraction of vaterite polymorph decreased and calcite increased as the amount of Ce3+ in the material was increased.

Radiation dose in conventional external radiotherapy is a macroscopic concept. Upon the properties of the short path length alpha emissions and the spatial distribution of the radionuclide relative to the small target volumes, microdosimetry is indispensible for cancer therapy involving targeted alpha particles or boron neutron capture etc to investigate the physical properties of radiation energy deposition in biological cells [16], [17]. In this regard, studies on the dose-response of small targets like the point defects in physical detectors can throw some insight into basic mode of interactions of radiation at nm level.

The TL studies on Mn doped CaCO3 is plagued by the difficulty of oxidation of Mn during high temperature sintering. Studies on lanthanide doped CaCO3 are scant because of their lower sensitivity when compared to CaF2 or CaSO4. Earlier work has brought out the importance of anion interstitials in the TL process of on CaSO4:Dy [18]. In the present work, TL and PL investigations were carried out on Ce3+ and Dy3+ doped carbonate hosts made by co-precipitation followed by sintering at different temperatures. The electron and hole traps causing TL and their mechanism of production by thermolysis and their destruction by decomposition of the host have been elaborated for the first time. The cause for the dependence of TL sensitivity on pre-irradiation sintering temperature, effect of ubiquitous Mn-impurity and Ce dopant concentration on TL and PL sensitivities, their emission spectra, TL dose response of CaCO3:Dy and CaCO3:Ce in the 0.5–15 kGy dose range, the application of microdosimetric concepts in CaCO3:Dy and CaCO3:Ce and the importance of charge state of the defects in secondary electron track interaction at high γ-doses will be highlighted.

Section snippets

Experimental details

The CaCO3 phosphor recipe involved co-precipitation in aqueous media followed by filtration and drying overnight the precipitate at room temperature (RT) ambience and then in an air oven for 180 °C for 2 h before sintering at higher temperatures in a muffle furnace in the range 300–700 °C for 30 min duration. The undoped CaCO3 was made with a 1: 1 M ratio of CaCl2 and Na2CO3, i.e.,CaCl2 + Na2CO3 → CaCO3↓ + 2NaClaq

Undoped (Ca0.5, Mg0.5)CO3 and (Ca0.9, Zn0.1)CO3 were also made using a similar

XRd

Fig. 1 shows that the XRD data of CaCO3:Dy synthesized by co-precipitation and pre-irradiation sintered at 400 °C and 500 °C for 30 min matches well with the Rhombohedral calcite structure (JCPDS 86-2340).

TL glow curves and sensitivities

Fig. 2 shows the TL glow curves of undoped CaCO3, CaCO3:Ce, CaCO3:Dy and CaCO3:Ce,Dy powders on x-irradiation, after a pre-irradiation sintering treatment of 500 °C for 30 min. Table 1 compares the TL sensitivities and peak temperatures of all carbonates after various pre-irradiation

Change in TL sensitivity with pre-annealing temperature

The increase in TL sensitivity of calcites with pre-annealing temperature was explained earlier on the basis of the hole reservoir model based on an analogy with pre-dosed quartz [10] or the dispersion of Mn2+ impurity clusters in heavily doped calcite at high temperatures [13]. High dose pre-irradiation is a pre-requisite for the former model but Table 1 and Fig. 3, Fig. 4 show results of calcites which have not undergone any such high pre-dose. The samples studied were also not heavily doped

Conclusions

All undoped carbonates made by co-precipitation route involving CaCl2 and Na2CO3 exhibit low-temperature TL peaks (140–180 °C) because in the absence of cation vacancies, the hole traps in them are less stable. Undoped calcite exhibited 600 nm emission due to the ubiquitous presence of Mn impurity. Ce and Dy doped CaCO3, however, exhibit a high-temperature TL peak near 290 °C due to the higher thermal stability of their hole traps in the presence of charge compensating cation vacancies and was

CRediT authorship contribution statement

Arunachalam Lakshmanan: Conceptualization, Methodology, Writing - review & editing. J. Nandhagopal: Data curation, Writing - original draft, Software, Validation. Bhaskar Sanyal: Visualization, Investigation. Bhushan Dhabekar: Supervision.

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

Authors are thankful to the Board of Research in Nuclear Sciences (BRNS), Department of Atomic Energy, Govt. of India, for providing financial assistance to carry out this work under research project (sanction letter No. 2013/36/47- BRNS/2422) on the Development of TL and RPL dosimeters for high dose region.

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