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Thermoluminescence and infrared light stimulated luminescence of limestone (CaCO3) and its dosimetric features
Applied Radiation and Isotopes 154 (2019) 108888
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Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Thermoluminescence and infrared light stimulated luminescence of limestone (CaCO3) and its dosimetric features
T
J.M. Kalitaa,b,∗, M.L. Chithamboa a b
Department of Physics and Electronics, Rhodes University, P O Box 94, Grahamstown, 6140, South Africa Department of Physics, Assam Down Town University, Guwahati, 781026, India
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
TL and IRSL of limestone (CaCO ) • have been studied. shows three composite TL • Sample glow peaks at 92 °C (P1), 165 °C (P2) 3
and 239 °C (P3).
has continuum trap distribu• Sample tion with E between 0.40 and 1.12 eV. response is linear for P1 • Dose (10‒1000 Gy) and P3 (10–100 Gy & 100–1000 Gy).
dose response is linear in two • IRSL separate ranges within 10–100 Gy & 100–1000 Gy.
A R T I C LE I N FO
A B S T R A C T
Keywords: Limestone (CaCO3) Thermoluminescence (TL) Infrared light stimulated luminescence (IRSL) Radiation dosimetry
Thermoluminescence (TL) and infrared light stimulated luminescence (IRSL) of limestone (CaCO3) collected from the Mawsmai Cave, India is reported. Structural and compositional analyses show that the sample has a rhombohedral crystal structure and contains 33.45% of CaO. TL measured at 1 °C/s from a sample irradiated to 600 Gy produces three composite glow peaks P1, P2 and P3 at 92, 165 and 239 °C respectively. The nature of the glow peaks is suggestive of the presence of a continuum trap distribution with activation energy between 0.40 eV and 1.12 eV. As regards to dose response, the TL intensity of P1 increases at a uniform rate with dose between 10 and 1000 Gy. Interestingly, the intensity of P3 increases with dose through two uniform regions, one within 10–100 Gy and the other between 100 and 1000 Gy. The IRSL measurement produces ill-shaped decay curves. The IRSL intensity also increases with dose at two different uniform rates within 10–100 Gy and 100–1000 Gy. Residual TL recorded after each IRSL measurement shows similar dose response as that under the conventional TL. Regarding fading, P1 fades by 88% and P3 by 14% within 12 h of irradiation.
1. Introduction Limestone is a sedimentary rock composed of calcium carbonate (CaCO3). Most limestone form from shells and animal skeleton in shallow, warm marine waters. The limestone formed this way is biological sedimentary rock. Limestone also develops by direct precipitation of calcium carbonate or through evaporation. Limestone occurring
∗
through precipitation and evaporation are chemical sedimentary rocks and are less abundant than biological sedimentary rock (King, 2019). Limestone has numerous industrial applications including use as a building material, chemical feedstock for the production of lime or as a soil conditioner. Despite its abundance, there are only few studies concerning stimulated luminescence of limestone and any potential applications (Ninagawa et al., 2001). In contrast, calcite, a variety of
Corresponding author. Department of Physics and Electronics, Rhodes University, P O Box 94, Grahamstown, 6140, South Africa. E-mail address: [email protected] (J.M. Kalita).
https://doi.org/10.1016/j.apradiso.2019.108888 Received 8 August 2019; Received in revised form 6 September 2019; Accepted 6 September 2019 Available online 06 September 2019 0969-8043/ © 2019 Elsevier Ltd. All rights reserved.
Applied Radiation and Isotopes 154 (2019) 108888
J.M. Kalita and M.L. Chithambo
were carried out in a nitrogen atmosphere. For a given aliquot, 40 mg of sample was used. IRSL measurements were carried out in a continuous-wave mode using a set of 870 nm infrared LEDs set at an optical power density of 130 mW/cm2 using the following protocol:
calcium carbonate with excellent luminescence properties has been studied more extensively with aims including mechanisms of its luminescence (Townsend et al., 1994), luminescence dosimetry (Engin and Guven, 2000; Ponnusamy et al., 2004; Soliman and Metwally, 2006; Kalita and Wary, 2016) and dating (Ninagawa et al., 1992). Calcite is the most pure form of calcium carbonate whereas limestone has a lower percentage of calcium carbonate. Numerous thermoluminescence (TL) studies show that calcite commonly shows three glow peaks in its TL glow curve (Wintle, 1978; Debenham, 1983; Debenham and Aitken, 1984; Down et al., 1985; Liritzis et al., 1996; Stirling et al., 2014; Kalita and Wary, 2014; Kalita and Wary, 2015). Natural orange calcite typically has three types of electron traps of activation energy 0.60, 0.70 and 1.30 eV (Kalita and Wary, 2014, 2015). Regarding radiation dosimetry, Engin and Guven (2000) and Ponnusamy et al. (2004) studied the effect of heat treatment on the TL of natural calcite and discussed its use as a gamma-ray dosimeter. Heating at 600 °C for 5 h and cooling in ambient air was found to enhance the TL sensitivity (Engin and Guven, 2000; Ponnusamy et al., 2004). The dose response of the annealed calcite was found to be linear between 0.05 and 104 Gy under gamma irradiation (Engin and Guven, 2000). Soliman and Metwally (2006) also observed good TL response from calcite within a gamma dose range of 0.01–104 Gy. Wintle (1978) carried out a preliminary TL study on calcite and showed that it could be used to date geologically relevant samples. Chithambo et al. (2014) studied TL spectral properties of X-ray irradiated manganiferous carbonatite (whose the main content is calcite) and reported that the dose response of a peak at around 86 °C is sublinear and that of a high temperature peak at 261 °C is linear from 200 to 1400 Gy. Kalita and Wary (2016) studied TL of annealed and un-annealed orange calcite for application in TL dosimetry. This study showed that a glow peak corresponding to an electron trap of activation energy 1.30 eV is the most effective for dosimetric application. The peak has a linear dose response from 10 mGy to 5.40 Gy, low fading and good reproducibility. The literature shows that calcite can be used as a natural dosimeter but studies concerning dosimetric features of limestone are limited. Galloway (2002) carried out a study of optically stimulated luminescence from limestone under 370 nm ultraviolet light stimulation. Luminescence was observed although of weak intensity. Galloway (2002) noted that a detailed study of stimulated luminescence properties of limestone is important owing to its potential application in dosimetry and dating. Dubey et al. (2015, 2017) carried out preliminarily TL studies on limestone exposed to UV, beta and gamma radiation. The sample produced a composite glow peak at around 300 °C. To date no study has been carried out on dosimetric properties of limestone. In this work, we study stimulated luminescence properties of limestone using TL and infrared light stimulated luminescence (IRSL). The aim of this study is to explore the dosimetric properties of limestone including its dose response and fading.
Step (a): Sample irradiated to a dose D Step (b): IRSL measured at 25 °C for 500 s Step (c): TL measured at 1 °C/s to 500 °C Step (d): Background IRSL measured at 25 °C for 500 s Step (e): Steps (a) to (d) repeated for doses D = 10, 20, 40, 60, 80, 100, 200, 400, 600, 800 and 1000 Gy In this protocol, the TL measured in step (c) is intended to record any residual-TL remaining after the IRSL measurement. The IRSL measured in step (d) is the background signal used to correct the value in step (b).
3. Results and discussion 3.1. Crystallinity and chemical composition In order to study the crystallinity and crystal structure of the material used, an XRD measurement was made. Fig. 1 shows the XRD pattern recorded between 5 and 90° at a scan rate of 0.02°/s. There is a sharp intense peak at 29.63° (in 2θ scale) and several other diffraction peaks of moderate and low intensity between 22° and 85°. The presence of the diffraction peaks indicate that the sample is crystalline. The diffraction pattern matches that of the ‘Joint Committee on Powder Diffraction Standard’ (JCPDS) data number JCPDS 85–1108 shown for comparison in Fig. 1. The latter means that the sample has a rhombohedral crystal structure. The peaks are labelled with reference to the corresponding reflection planes in the crystal. The intense peak at 29.63° is due to reflection from the (1 0 4) plane whereas the other moderate and low intensity peaks are due to reflection from other planes as identified in Fig. 1. The XRD confirms that the sample has no amorphous phase. In addition, as all the diffraction peaks refer to limestone only, the presence of any other mineral phase in the sample is negligibly low. The chemical composition of the sample was determined using XRFS. Table 1 shows the quantitative XRFS analysis. It is evident that the concentration of CaO at 33.45%, is the highest of any other oxides. Moderate amounts of two other oxides, MgO at 2.69% and SiO2 at 2.38% are also found in the sample. The presence of SiO2 is easy to understand because this constitutes the major elemental oxide in the earth's crust and is thus always present in other minerals. The other
2. Experimental details Samples used were limestone collected from Mawsmai Cave, India (latitude: 25°18′00″N, longitude: 91°42′00″E). The samples were cleaned and ground to fine powder. X-ray diffraction (XRD) and energy dispersive X-ray fluorescence spectroscopy (XRFS) were carried out on samples using a Phillips X'Pert Pro Powder X-ray Diffractometer and an Axios, PANalytical spectrometer respectively. The measurements were made to determine the crystalline structure and chemical composition of the limestone. The XRFS instrument is capable of detecting ten major oxides, namely SiO2, Al2O3, Fe2O3, MgO, CaO, Na2O, K2O, TiO2, MnO and P2O5 and a few trace elements. TL and IRSL were measured using the RISØ TL/OSL DA-20 Luminescence Reader from samples irradiated at ambient temperature using a90Sr/90Y beta source at a dose rate of 0.10 Gy/s. The luminescence was detected by an EMI 9235QB photomultiplier tube through a Schott BG 39 filter (transmission band 330–690 nm). All measurements
Fig. 1. XRD pattern of a CaCO3 powder sample recorded between 5 and 90° at a scan rate of 0.02°/s. JCPDS data number 85–1108 is shown for comparison. 2
Applied Radiation and Isotopes 154 (2019) 108888
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1994; Down et al., 1985). The sample has impurities including Mn, Fe, Mg etc. Therefore, we speculate that some of these impurities contribute to the emission. There are various methods used to evaluate kinetic parameters including the initial rise- (McKeever, 1985), whole glow peak- (Pagonis et al., 2006), peak shape- (Pagonis et al., 2006), variable heating rate(Pagonis et al., 2006), phosphorescence- (McKeever, 1985), glow curve deconvolution method (Pagonis et al., 2006; Kitis et al., 1998), etc. for first order-, second order- or general order kinetics. In addition, GomezRos et al. (2006) reported a method for the analysis of glow curves assuming arbitrary recombination-retrapping rates. Most of these methods, except the initial rise method, phosphorescence and glow curve deconvolution, are applicable to an isolated glow peak. The phosphorescence and glow curve deconvolution methods can be utilised for a complex glow curve if any overlapping peaks within it are distinct. On the other hand, the initial rise method, which uses only the rising edge of a glow peak, can be used for evaluating activation energy of electron traps from closely spaced overlapping glow peaks (McKeever, 1985).
Table 1 Major and minor oxides present in the limestone sample. All values are in wt.%. Element oxides
wt. %
SiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2 O TiO2 P2O5
2.380 0.890 0.390 0.242 2.690 33.450 0.000 0.190 0.040 0.050
oxide MgO, is the most common impurity present in all naturally occurring calcium carbonate minerals, for example, in calcite (Kalita and Wary, 2014) or manganiferous carbonatite (Chithambo et al., 2014) etc. There are also some other minor oxides such as Al2O3, Fe2O3, MnO, K2O, TiO2 and P2O5 present in the sample. This analysis shows that the sample has impurities including Mg, Si, Al, Fe, Ti, Mn etc.
3.2.2. Tm‒Tstop analysis In order to determine the order of kinetics of each of the peaks and whether each peak is single or composed of several components, the Tm‒Tstop procedure (McKeever, 1985) was carried out. A sample irradiated to 600 Gy was heated at 1 °C/s up to a temperature Tstop corresponding to a point on the low temperature part of a peak. The sample was then cooled down and reheated to obtain the complete glow peak and its position Tm noted. This procedure was repeated several times, re-irradiating the same sample anew each time and Tstop increased to a slightly higher value. Fig. 3(a) shows examples of glow curves measured this way whereas Fig. 3(b) shows a plot of Tm against Tstop. Interestingly, a new peak at 165 °C was noted after preheating to 140 °C. For ease of reference, the peaks observed at 92, 165 and 239 °C corresponding to the dose of 600 Gy are labelled as P1, P2 and P3 respectively. Since the position Tm of a first order glow peak is independent of the initial concentration of the trapped charge, its position is expected to be independent of Tstop (McKeever, 1985). On the other hand, for second order kinetics or for closely overlapping peaks, the peak temperature should increase with Tstop (McKeever, 1985). Fig. 3(b) shows that peaks P1, P2 and P3 shift to higher temperature with Tstop temperature. This implies that none of the peaks follow first order kinetics. It must be then that the peaks are subject to second order kinetics or that they are each a composite of several closely spaced overlapping peaks.
3.2. Thermoluminescence (TL) 3.2.1. Glow curve characteristics and kinetics features As a start, an attempt was made to monitor any natural TL but none was observed. Even with test doses of say, 1 Gy or 5 Gy, no TL was seen. Appreciable TL was only observed when the sample was irradiated to 10 Gy or more. In order to obtain proper TL for analysis, the sample had to be irradiated up to 600 Gy and the resulting glow curve is shown in Fig. 2. Despite the extensively high dose used in this test, the intensity of the emission is only moderate. There are two TL glow peaks apparent at 92 and 239 °C. Nevertheless, a study of the kinetic features of each peak was carried out and is explained here. It should be noted that previous attempts to study limestone also showed its stimulated luminescence to be of low intensity (Galloway, 2002). Regarding the emission band, we note that the TL was measured using a Schott BG 39 filter that has a transmission band of 330–690 nm. In a test measurement, TL was also measured from an irradiated sample using a Hoya U-340 filter that has a transmission band of 290–380 nm. In the latter, no TL was observed from the sample after irradiation to 600 Gy. This implies that the emission from the sample falls between 380 and 690 nm. Literature shows that other natural calcium carbonates such as calcite (Townsend et al., 1994; Down et al., 1985; Kalita and Wary, 2014), manganiferus carbonatite (Chithambo et al., 2014) etc. show dominant emission bands between 380 and 690 nm due to Mn impurity. However, other impurities such as Fe, Mg etc. may also contribute to emission in calcium carbonate minerals (Townsend et al.,
3.2.3. Dependence of Tm on delay between irradiation and TL measurement In this study, the position Tm of a peak was monitored as a function of delay between irradiation and measurement of TL. This was done to study whether the glow peaks follow second order kinetics or whether the shift of Tm with Tstop reflects a peak being composite. In this experiment, each time the sample was irradiated to 600 Gy, the TL was measured after some delay following the irradiation. Fig. 4 shows examples of glow curves recorded at 1 °C/s from a sample irradiated to 600 Gy and measured after delays between 0 and 43200 s. The inset to Fig. 4 shows, for peak P1, the change of Tm with the delay. Peak P1 fades with delay between irradiation and measurement and its apparent position shifts to higher temperature. This means that peak P1 is likely a combination of several closely spaced overlapping glow peaks. However, no such conclusion could be made for peaks P2 and P3 as they show negligible fading with the delay up to 43200 s. 3.2.4. Dependence of Tm on dose Furthermore, to assess the order of kinetics of the peaks, the Tm‒dose analysis was used. TL was measured at 1 °C/s from a sample irradiated each time to a dose between 10 Gy and 1000 Gy. The position of the peak corresponding to a particular dose was noted. Fig. 5(a)
Fig. 2. A glow curve measured at 1 °C/s after beta irradiation to 600 Gy. 3
Applied Radiation and Isotopes 154 (2019) 108888
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Fig. 3. (a) Examples of glow curves measured from the sample each time freshly irradiated to 600 Gy and preheated to different Tstop temperatures. (b) A plot of Tm against Tstop. The error bars in (b) represent the standard deviation (σ) of the data from three identical measurements and the errors are within 2σlevel.
Fig. 5. (a) Examples of glow curves measured after irradiated to different doses. (b) The dependence of the position of peaks P1 and P3 on dose. The error bars represent the standard deviation of the data from three identical measurements and the errors are within 2σ-level.
76 to 96 °C when the dose is increased from 10 to 1000 Gy. In the same dose range, the position of P3 decreases consistently from 239 to 237 °C. The shift of peak P1 to higher temperature probably reflects the successive appearance of overlapping components. This is consistent with the earlier estimation from the Tm‒Tstop study as well as from the dependence of Tm on delay between irradiation and TL measurement for this particular peak. Further, the shift of peak P3 to lower temperature indicates that the peak follows either second order kinetics or general order kinetics or may be due to the peak being composite as noted from the Tm‒Tstop analysis. 3.2.5. Activation energy of electron traps As stated earlier, most methods of kinetic analysis are suitable for an isolated glow peak. In our study, it is not possible to determine the total number of overlapping peaks as well as their position thus phosphorescence and deconvolution methods are unsuitable. Therefore, we used the initial rise method to evaluate the activation energy of the corresponding electron trap (McKeever, 1985). The initial rise method was applied to glow peaks recorded after preheating as well as those measured after delays between irradiation and measurement. Fig. 6(a) shows examples of plots of ln(I) against 1/ kT corresponding to glow peaks measured after different preheating Tstop temperatures. Values of the activation energy obtained from such plots are shown in Fig. 6(b) against the preheating temperature Tstop (squares) or against delay between irradiation and measurement (triangles). The error bars are associated with each value of the activation energy estimated from the least square fitting of the ln(I) versus 1/kT
Fig. 4. Examples of glow curves recorded at 1 °C/s from a sample irradiated to 600 Gy and measured after delays between 0 and 43200 s. The inset shows the variation of Tm of peak P1 with delay. The error bars in the inset represent the standard deviation (σ) of the data from three identical measurements and the errors are within 2σ-level.
shows examples of glow curves measured after different doses. Fig. 5(b) shows the dependence of the position of peaks P1 and P3 on dose. Peak P2 was not analysed as its position could not be reliably determined. The error bars in Fig. 5(b) represent the standard deviation from three identical measurements. According to theory, the position of a first order isolated TL peak should be independent of dose whereas that of a second order or general order peak is expected to shift to lower temperatures with dose (McKeever, 1985). The position of P1 changes from 4
Applied Radiation and Isotopes 154 (2019) 108888
J.M. Kalita and M.L. Chithambo
Fig. 6. (a) Examples of the plots of ln(I) against 1/kT corresponding to glow curves measured after preheating to some Tstop temperatures. (b) A plot of activation energy versus preheating temperature Tstop (squares) and delays (triangles) between irradiation and measurement. The dotted lines in (b) are visual guides used to bracket the consistent range of the activation energies.
Fig. 7. (a) Examples of IRSL decay curves (solid lines) measured after irradiation to different doses. The open circles represent the background signal. (b) Examples of residual-TL measured at 1 °C/s after the measurement of IRSL.
heated to 500 °C at 1 °C/s to remove any residual TL. IRSL was measured thereafter from the sample using the protocol stated earlier (section 2). Fig. 7(a) shows examples of IRSL decay curves measured after irradiation to different doses. Fig. 7(b) shows examples of residual-TL following the measurement of IRSL. The IRSL decay curves have been corrected for background. The IRSL shows ill-shaped decay curves of low intensity. This characteristic is due to low concentration of electron traps involved in stimulated luminescence in the sample. Similar low intensity emission are seen for thermally assisted optically simulated luminescence from beta irradiated α-Al2O3:C, Mg also due to low concentration of (deep) electron traps (Kalita et al., 2017). Although the IRSL is weak, its intensity increases with dose. During IRSL stimulation, owing to low energy of IR light, electrons are probably ejected from shallower traps (corresponding to TL peaks P1 and P2) and recombine with holes via the conduction band. In addition, some electrons perhaps also transfer from relatively deeper traps to shallower traps during IR illumination and move to conduction band thereafter. The transfer of trapped electrons between localised energy levels aided by illumination is known as ‘phototransfer’. This is not to be confused with phototransferred thermoluminescence. Phototransfer is possible in this sample as the localised energy levels are closely spaced. Apart from the transitions from delocalised energy level, there may also be some localised transitions where electrons recombine at recombination centre without making transition from the conduction band. Such localised transition under IR stimulation was observed in feldspar (Sfampa et al., 2015). When an excited state of a recombination centre is at a similar energy depth as that of electron traps, electrons can move to a nearby excited level of the recombination centre and thereby produce luminescence without involvement of the conduction band.
plots. It is evident in Fig. 6(b) that the activation energy falls into three distinct regions as a function of Tstop. In the first region for Tstop = 30‒100 °C, the values of the activation energies vary from 0.60 to 0.68 eV. In the second region for Tstop = 110‒170 °C, the activation energies vary from 0.40 to 0.46 eV whereas that in the third region for Tstop = 180‒240 °C, the values vary from 0.86 to 1.12 eV. The dotted lines in Fig. 6(b) are visual guides used to bracket the minimum and maximum values of activation energy in the three regions. Further, the activation energies estimated from the glow curves measured after the delays between 0 and 43200 s during which the first peak P1 fades, varies from 0.60 to 0.72 eV. Note that the peak temperature corresponding to peaks P1, P2 and P3 are 92, 165 and 239 °C respectively. Therefore, taking the preheating and peak temperatures into account, it can be summarised that the sample has a continuum electron trap distribution with activation energy between 0.40 eV and 1.12 eV responsible for the three composite peaks. The activation energies corresponding to peak P2 are less than those of peak P1 although the latter appears at a lower temperature than peak P2. This is because the position of a TL peak depends simultaneously on activation energy, frequency factor and heating rate. Literature shows that in calcite, there is also a glow peak at a higher temperature with a relatively lower activation energy (Kalita and Wary, 2014, 2015; Chithambo et al., 2014).
3.3. Infrared light stimulated luminescence (IRSL) The luminescence properties of limestone were further studied by infrared light stimulation. Before any measurement, the sample was 5
Applied Radiation and Isotopes 154 (2019) 108888
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The residual-TL glow curves measured after the IRSL measurements show similar structure as that of the conventional TL. Three residual-TL peaks are found at 104 °C (labelled as P1/), 175 °C (P2/) and 248 °C (P3/ ) corresponding to the dose of 600 Gy. Note that the position of conventional TL peaks P1, P2 and P3 corresponding to the dose of 600 Gy were 92, 165 and 239 °C respectively. The residual-TL peaks have all shifted by about 10 °C towards high temperature. This is expected since the position of the peaks are affected by dose as reported in section 3.2.4. The analysis of the dependence of Tm on dose showed that the position of the peaks changes with irradiation. Thus the position of the peaks depends on concentration of electrons at the electron traps. Since the residual-TL was measured after the measurement of IRSL, the concentration of the electrons at the electron traps would have changed. The glow peaks in the residual-TL then occur at slightly different positions compared to those under the conventional TL. We note that the intensity of the residual-TL peaks is about half that of the conventional TL peaks. Therefore, the sample is expected to produce intense IRSL. However, the sample only produces weak IRSL. This discrepancy can be explained considering the effect of fading. The study on fading (to follow in section 3.5) shows that the sample fades rapidly. Most electrons thus escape from the traps before the measurement of residual-TL resulting in weak TL intensity. 3.4. Dose response The dose response was studied using TL as well as IRSL for doses between 10 and 1000 Gy. The intensity (peak-height) of peaks P1 and P3 was monitored as a function of dose. Peak P2 was excluded from this analysis as its position and hence the intensity could not be reliably monitored at low doses. Fig. 8(a) shows the dose response of peaks P1 and P3. The intensity of P1 increases at a uniform rate with dose from 10 to 1000 Gy whereas that of P3 shows two different regions of uniform increase within 10 to 100 Gy and 100 to 1000 Gy respectively. The dose response was also studied by IRSL measured using the protocol describe earlier. The IRSL intensity measured as the area under the decay curves has been plotted as a function of dose. Fig. 8(b) shows the variation of IRSL intensity as a function of dose. The IRSL intensity also increases with dose in two different uniform regions within 10 to 100 Gy and 100 and 1000 Gy. This is a significant result of the present study that indicates some relation between the TL of peak P3 and the IRSL from the sample. The dose response analysis was further extended for residual-TL peaks P1/and P3/. Fig. 8(c) shows the variation of intensity of peaks P1/ and P3/with dose. The intensity of P1/increases linearly from 10 to 1000 Gy whereas that of P3/increases as two different linear regions within 10 to 100 Gy and 100 to 1000 Gy. The dose responses of peaks P1/and P3/are very similar to those of conventional TL peaks P1 and P3 respectively. This is a very important result as it shows that the sample could be used to measure an unknown dose in two different ways: first by measuring IRSL and by subsequently measuring residual-TL without any further irradiation.
Fig. 8. Variation as a function of dose of (a) intensity of conventional TL peaks P1 and P3, (b) IRSL intensity and (c) intensity of residual-TL peaks P1ʹ and P3ʹ. The doted lines are only visual guides. The error bars in all the figures represent the standard deviation of the data from three identical measurements and the errors are within 2σ-level.
3.5. Fading In the context of radiation dosimetry, fading means the loss of luminescence with delay between irradiation and measurement. Fading occurs due to escape of trapped electrons from electron traps by many processes including charge hopping (Kalita and Chithambo, 2017), tunnelling (McKeever, 1985) etc. In the present case, fading was studied by irradiating a sample to 600 Gy and measuring TL at 1 °C/s using delays of 0 to 42300 s between irradiation and TL measurement. Fig. 4 shows examples of glow curves measured after some specific delays. The intensity of peaks P1 and P3 was each studied as a function of delay. Peak P2 was excluded from the analysis due to its weak intensity. Fig. 9 shows variation of intensity of peaks P1 and P3 against the delay. It was observed that the intensity of peak P1 decreases by about 88%
Fig. 9. Variation of intensity of peaks P1 against the delay. The inset shows the same feature of peak P3. The dotted lines are only visual guides. The error bars represent the error estimated by Poisson statistics.
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and that of peak P3 decreases by about 14% in 12 h after irradiation. We can now deduce that it is as a result of such a high rate of fading in ambient conditions that no natural-TL was observed in the sample. With regards to any use as a radiation dosimeter, the fading as measured for peak P1 is high, however peak P3 could be useful for radiation dosimetry.
analysis of UV, beta and gamma induced limestone collected from Amarnath holy cave. J. Radiat. Res. Appl. Sciences 8, 126–135 2015. Dubey, V., Kaur, J., Dubey, N., Pandey, M.K., Suryanarayana, N.S., Murthy, K.V.R., 2017. Kinetic and TL glow curve analysis of UV-, β- and γ-irradiated natural limestone collected from Chunkatta mines. Radiat. Eff. Defects Solids 172, 866–877. Engin, B., Guven, O., 2000. The effect of heat treatment on the thermoluminescence of naturally-occurring calcites and their use as a gamma-ray dosimeter. Radiat. Meas. 32, 253–272. Galloway, R.B., 2002. Does limestone show useful optically stimulated luminescence? Ancient TL 20, 1–5. Gomez-Ros, J.M., Furetta, C., Correcher, V., 2006. Simple methods to analyse thermoluminescence glow curves assuming arbitrary recombination–retrapping rates. Radiat. Prot. Dosim. 119, 339–343. Kalita, J.M., Chithambo, M.L., 2017. A comparative study of the dosimetric features of of α-Al2O3:C,Mg and α-Al2O3:C. Radiat. Prot. Dosim. 177, 261–271. Kalita, J.M., Chithambo, M.L., Polymeris, G.S., 2017. Thermally-assisted optically stimulated luminescence from deep electron traps in α-Al2O3:C,Mg. Nucl. Instrum. Methods Phys. Res. B 403, 28–32. Kalita, J.M., Wary, G., 2014. Thermoluminescence study of X-ray and UV irradiated natural calcite and analysis of its trap and recombination level. Spectrochim. Acta A. 125, 99–103. Kalita, J.M., Wary, G., 2015. Thermal quenching in calcite and evaluation of quenching parameters from composite glow curve by a computerized resolved peak technique. J. Lumin. 160, 134–137. Kalita, J.M., Wary, G., 2016. X-ray dose response of calcite—a comprehensive analysis for optimal application in TL dosimetry. Nucl. Instrum. Methods Phys. Res. B 383, 93–102. King, Hobart M., 2019. Limestone. https://geology.com/rocks/limestone.shtml. Kitis, G., Gomez-Ros, J.M., Tuyn, J.W.N., 1998. Thermoluminescence glow-curve deconvolution functions for first, second and general orders of kinetics. J. Phys. D Appl. Phys. 31, 2636–2641. Liritzis, I., Guibert, P., Foti, F., Schvoerer, M., 1996. Solar bleaching of thermoluminescence of calcites. Nucl. Instrum. Methods Phys. Res. B 117, 260–268. McKeever, S.W.S., 1985. Thermoluminescence of Solids. Cambridge University Press, Cambridge. Ninagawa, K., Adachi, K., Uchimura, N., Yamamoto, I., Wada, T., Yamashita, Y., Takashima, I., Sekimoto, K., Hasegawa, H., 1992. Thermoluminescence dating of calcite shells in the pectinidae family. Quat. Sci. Rev. 11, 121–126. Ninagawa, K., Kitahara, T., Toyoda, S., Hayashi, K., Nishido, H., Kinjo, M., Kawana, T., 2001. Thermoluminescence dating of the ryukyu limestone. Quat. Sci. Rev. 20, 829–833. Pagonis, V., Kitis, G., Furetta, C., 2006. Numerical and Practical Exercises in Thermoluminescence. Springer, Berlin. Ponnusamy, V., Ramasamy, V., Dheenathayalu, M., Hemalatha, J., 2004. Effect of annealing in thermostimulated luminescence (TSL) on natural blue colour calcite crystals. Nucl. Instrum. Methods Phys. Res. B 217, 611–620. Sfampa, I.K., Polymeris, G.S., Pagonis, V., Theodosoglou, E., Tsirliganis, N.C., Kitis, G., 2015. Correlation of basic TL, OSL and IRSL properties of ten K-feldspar samples of various origins. Nucl. Instrum. Methods Phys. Res. B 359, 89–98. Soliman, C., Metwally, S.M., 2006. Thermoluminescence of the green emission band of calcite. Radiat. Eff. Defects Solids 161, 607–613. Stirling, R.J., Duller, G.A.T., Roberts, H.M., 2014. Developing a single-aliquot protocol for measuring equivalent dose in biogenic carbonates. Radiat. Meas. 47, 725–731. Townsend, P.D., Luff, B.J., Wood, R.A., April–July 1994. Mn2+ transitions in the TL emission spectra of calcite. Radiat. Meas. 23 (2–3), 433–440. https://doi.org/10. 1016/1350-4487(94)90076-0. Wintle, A.G., 1978. A thermoluminescence dating study of some Quaternary calcite: potential and problems. Can. J. Earth Sci. 15, 1977–1986.
4. Conclusions TL and IRSL properties of limestone have been studied under beta irradiation. The sample has rhombohedral crystal structure and contains 33.45% of CaO. It produces three composite glow peaks P1, P2 and P3 at 92, 165 and 239 °C respectively. Kinetic analysis shows that the sample has a continuum trap distribution with activation energy from 0.40 to 1.12 eV. A study of dose response shows that the TL intensity of P1 increases at constant rate with dose from 10–1000 Gy whereas that of P3 shows two different rate of increase of intensity within 10–100 Gy and 100–1000 Gy. IRSL measurement produces illshaped decay curves. The dose response studied from IRSL intensity shows similar variation as that of P3. Further, residual-TL measured after the measurement of IRSL shows similar response as that under the conventional TL. Regarding fading, P1 fades by 88% whereas P3 by 14% within 12 h after irradiation. The TL and IRSL mechanisms have been discussed considering various possibilities including localised and delocalised transitions as well as phototransfer process under IR illumination. This study shows that limestone can be used as a potential natural radiation dosimeter. Acknowledgement J.M. Kalita acknowledges with gratitude for financial support from Rhodes University and the National Research Foundation of South Africa. Authors also thank the Sophisticated Analytical Instrument Facility (SAIF), Department of Instrumentation & USIC, Gauhati University, India for the XRD and XRFS reports. References Chithambo, M.L., Pagonis, V., Ogundare, F.A., 2014. Spectral and kinetic analysis of thermoluminescence from manganiferous carbonatite. J. Lumin. 145, 180–187. Debenham, N.C., 1983. Reliability of thermoluminescence dating of stalagmitic calcite. Nature 304, 154–156. Debenham, N.C., Aitken, M.J., 1984. Thermoluminescence dating of stalagmitic calcite. Archaeometry 26, 155–170. Down, J.S., Flower, R., Strain, J.A., Townsend, P.D., 1985. Thermoluminescence emission spectra of calcite and Iceland spar. Nucl. Tracks Radiat. Meas. 10, 581–589. Dubey, V., Dubey, V.P., Tamrakar, R.K., Upadhyay, K., Tiwari, N., 2015. TL glow curve
7
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Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Thermoluminescence and infrared light stimulated luminescence of limestone (CaCO3) and its dosimetric features
T
J.M. Kalitaa,b,∗, M.L. Chithamboa a b
Department of Physics and Electronics, Rhodes University, P O Box 94, Grahamstown, 6140, South Africa Department of Physics, Assam Down Town University, Guwahati, 781026, India
H I GH L IG H T S
G R A P H I C A L A B S T R A C T
TL and IRSL of limestone (CaCO ) • have been studied. shows three composite TL • Sample glow peaks at 92 °C (P1), 165 °C (P2) 3
and 239 °C (P3).
has continuum trap distribu• Sample tion with E between 0.40 and 1.12 eV. response is linear for P1 • Dose (10‒1000 Gy) and P3 (10–100 Gy & 100–1000 Gy).
dose response is linear in two • IRSL separate ranges within 10–100 Gy & 100–1000 Gy.
A R T I C LE I N FO
A B S T R A C T
Keywords: Limestone (CaCO3) Thermoluminescence (TL) Infrared light stimulated luminescence (IRSL) Radiation dosimetry
Thermoluminescence (TL) and infrared light stimulated luminescence (IRSL) of limestone (CaCO3) collected from the Mawsmai Cave, India is reported. Structural and compositional analyses show that the sample has a rhombohedral crystal structure and contains 33.45% of CaO. TL measured at 1 °C/s from a sample irradiated to 600 Gy produces three composite glow peaks P1, P2 and P3 at 92, 165 and 239 °C respectively. The nature of the glow peaks is suggestive of the presence of a continuum trap distribution with activation energy between 0.40 eV and 1.12 eV. As regards to dose response, the TL intensity of P1 increases at a uniform rate with dose between 10 and 1000 Gy. Interestingly, the intensity of P3 increases with dose through two uniform regions, one within 10–100 Gy and the other between 100 and 1000 Gy. The IRSL measurement produces ill-shaped decay curves. The IRSL intensity also increases with dose at two different uniform rates within 10–100 Gy and 100–1000 Gy. Residual TL recorded after each IRSL measurement shows similar dose response as that under the conventional TL. Regarding fading, P1 fades by 88% and P3 by 14% within 12 h of irradiation.
1. Introduction Limestone is a sedimentary rock composed of calcium carbonate (CaCO3). Most limestone form from shells and animal skeleton in shallow, warm marine waters. The limestone formed this way is biological sedimentary rock. Limestone also develops by direct precipitation of calcium carbonate or through evaporation. Limestone occurring
∗
through precipitation and evaporation are chemical sedimentary rocks and are less abundant than biological sedimentary rock (King, 2019). Limestone has numerous industrial applications including use as a building material, chemical feedstock for the production of lime or as a soil conditioner. Despite its abundance, there are only few studies concerning stimulated luminescence of limestone and any potential applications (Ninagawa et al., 2001). In contrast, calcite, a variety of
Corresponding author. Department of Physics and Electronics, Rhodes University, P O Box 94, Grahamstown, 6140, South Africa. E-mail address: [email protected] (J.M. Kalita).
https://doi.org/10.1016/j.apradiso.2019.108888 Received 8 August 2019; Received in revised form 6 September 2019; Accepted 6 September 2019 Available online 06 September 2019 0969-8043/ © 2019 Elsevier Ltd. All rights reserved.
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were carried out in a nitrogen atmosphere. For a given aliquot, 40 mg of sample was used. IRSL measurements were carried out in a continuous-wave mode using a set of 870 nm infrared LEDs set at an optical power density of 130 mW/cm2 using the following protocol:
calcium carbonate with excellent luminescence properties has been studied more extensively with aims including mechanisms of its luminescence (Townsend et al., 1994), luminescence dosimetry (Engin and Guven, 2000; Ponnusamy et al., 2004; Soliman and Metwally, 2006; Kalita and Wary, 2016) and dating (Ninagawa et al., 1992). Calcite is the most pure form of calcium carbonate whereas limestone has a lower percentage of calcium carbonate. Numerous thermoluminescence (TL) studies show that calcite commonly shows three glow peaks in its TL glow curve (Wintle, 1978; Debenham, 1983; Debenham and Aitken, 1984; Down et al., 1985; Liritzis et al., 1996; Stirling et al., 2014; Kalita and Wary, 2014; Kalita and Wary, 2015). Natural orange calcite typically has three types of electron traps of activation energy 0.60, 0.70 and 1.30 eV (Kalita and Wary, 2014, 2015). Regarding radiation dosimetry, Engin and Guven (2000) and Ponnusamy et al. (2004) studied the effect of heat treatment on the TL of natural calcite and discussed its use as a gamma-ray dosimeter. Heating at 600 °C for 5 h and cooling in ambient air was found to enhance the TL sensitivity (Engin and Guven, 2000; Ponnusamy et al., 2004). The dose response of the annealed calcite was found to be linear between 0.05 and 104 Gy under gamma irradiation (Engin and Guven, 2000). Soliman and Metwally (2006) also observed good TL response from calcite within a gamma dose range of 0.01–104 Gy. Wintle (1978) carried out a preliminary TL study on calcite and showed that it could be used to date geologically relevant samples. Chithambo et al. (2014) studied TL spectral properties of X-ray irradiated manganiferous carbonatite (whose the main content is calcite) and reported that the dose response of a peak at around 86 °C is sublinear and that of a high temperature peak at 261 °C is linear from 200 to 1400 Gy. Kalita and Wary (2016) studied TL of annealed and un-annealed orange calcite for application in TL dosimetry. This study showed that a glow peak corresponding to an electron trap of activation energy 1.30 eV is the most effective for dosimetric application. The peak has a linear dose response from 10 mGy to 5.40 Gy, low fading and good reproducibility. The literature shows that calcite can be used as a natural dosimeter but studies concerning dosimetric features of limestone are limited. Galloway (2002) carried out a study of optically stimulated luminescence from limestone under 370 nm ultraviolet light stimulation. Luminescence was observed although of weak intensity. Galloway (2002) noted that a detailed study of stimulated luminescence properties of limestone is important owing to its potential application in dosimetry and dating. Dubey et al. (2015, 2017) carried out preliminarily TL studies on limestone exposed to UV, beta and gamma radiation. The sample produced a composite glow peak at around 300 °C. To date no study has been carried out on dosimetric properties of limestone. In this work, we study stimulated luminescence properties of limestone using TL and infrared light stimulated luminescence (IRSL). The aim of this study is to explore the dosimetric properties of limestone including its dose response and fading.
Step (a): Sample irradiated to a dose D Step (b): IRSL measured at 25 °C for 500 s Step (c): TL measured at 1 °C/s to 500 °C Step (d): Background IRSL measured at 25 °C for 500 s Step (e): Steps (a) to (d) repeated for doses D = 10, 20, 40, 60, 80, 100, 200, 400, 600, 800 and 1000 Gy In this protocol, the TL measured in step (c) is intended to record any residual-TL remaining after the IRSL measurement. The IRSL measured in step (d) is the background signal used to correct the value in step (b).
3. Results and discussion 3.1. Crystallinity and chemical composition In order to study the crystallinity and crystal structure of the material used, an XRD measurement was made. Fig. 1 shows the XRD pattern recorded between 5 and 90° at a scan rate of 0.02°/s. There is a sharp intense peak at 29.63° (in 2θ scale) and several other diffraction peaks of moderate and low intensity between 22° and 85°. The presence of the diffraction peaks indicate that the sample is crystalline. The diffraction pattern matches that of the ‘Joint Committee on Powder Diffraction Standard’ (JCPDS) data number JCPDS 85–1108 shown for comparison in Fig. 1. The latter means that the sample has a rhombohedral crystal structure. The peaks are labelled with reference to the corresponding reflection planes in the crystal. The intense peak at 29.63° is due to reflection from the (1 0 4) plane whereas the other moderate and low intensity peaks are due to reflection from other planes as identified in Fig. 1. The XRD confirms that the sample has no amorphous phase. In addition, as all the diffraction peaks refer to limestone only, the presence of any other mineral phase in the sample is negligibly low. The chemical composition of the sample was determined using XRFS. Table 1 shows the quantitative XRFS analysis. It is evident that the concentration of CaO at 33.45%, is the highest of any other oxides. Moderate amounts of two other oxides, MgO at 2.69% and SiO2 at 2.38% are also found in the sample. The presence of SiO2 is easy to understand because this constitutes the major elemental oxide in the earth's crust and is thus always present in other minerals. The other
2. Experimental details Samples used were limestone collected from Mawsmai Cave, India (latitude: 25°18′00″N, longitude: 91°42′00″E). The samples were cleaned and ground to fine powder. X-ray diffraction (XRD) and energy dispersive X-ray fluorescence spectroscopy (XRFS) were carried out on samples using a Phillips X'Pert Pro Powder X-ray Diffractometer and an Axios, PANalytical spectrometer respectively. The measurements were made to determine the crystalline structure and chemical composition of the limestone. The XRFS instrument is capable of detecting ten major oxides, namely SiO2, Al2O3, Fe2O3, MgO, CaO, Na2O, K2O, TiO2, MnO and P2O5 and a few trace elements. TL and IRSL were measured using the RISØ TL/OSL DA-20 Luminescence Reader from samples irradiated at ambient temperature using a90Sr/90Y beta source at a dose rate of 0.10 Gy/s. The luminescence was detected by an EMI 9235QB photomultiplier tube through a Schott BG 39 filter (transmission band 330–690 nm). All measurements
Fig. 1. XRD pattern of a CaCO3 powder sample recorded between 5 and 90° at a scan rate of 0.02°/s. JCPDS data number 85–1108 is shown for comparison. 2
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1994; Down et al., 1985). The sample has impurities including Mn, Fe, Mg etc. Therefore, we speculate that some of these impurities contribute to the emission. There are various methods used to evaluate kinetic parameters including the initial rise- (McKeever, 1985), whole glow peak- (Pagonis et al., 2006), peak shape- (Pagonis et al., 2006), variable heating rate(Pagonis et al., 2006), phosphorescence- (McKeever, 1985), glow curve deconvolution method (Pagonis et al., 2006; Kitis et al., 1998), etc. for first order-, second order- or general order kinetics. In addition, GomezRos et al. (2006) reported a method for the analysis of glow curves assuming arbitrary recombination-retrapping rates. Most of these methods, except the initial rise method, phosphorescence and glow curve deconvolution, are applicable to an isolated glow peak. The phosphorescence and glow curve deconvolution methods can be utilised for a complex glow curve if any overlapping peaks within it are distinct. On the other hand, the initial rise method, which uses only the rising edge of a glow peak, can be used for evaluating activation energy of electron traps from closely spaced overlapping glow peaks (McKeever, 1985).
Table 1 Major and minor oxides present in the limestone sample. All values are in wt.%. Element oxides
wt. %
SiO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2 O TiO2 P2O5
2.380 0.890 0.390 0.242 2.690 33.450 0.000 0.190 0.040 0.050
oxide MgO, is the most common impurity present in all naturally occurring calcium carbonate minerals, for example, in calcite (Kalita and Wary, 2014) or manganiferous carbonatite (Chithambo et al., 2014) etc. There are also some other minor oxides such as Al2O3, Fe2O3, MnO, K2O, TiO2 and P2O5 present in the sample. This analysis shows that the sample has impurities including Mg, Si, Al, Fe, Ti, Mn etc.
3.2.2. Tm‒Tstop analysis In order to determine the order of kinetics of each of the peaks and whether each peak is single or composed of several components, the Tm‒Tstop procedure (McKeever, 1985) was carried out. A sample irradiated to 600 Gy was heated at 1 °C/s up to a temperature Tstop corresponding to a point on the low temperature part of a peak. The sample was then cooled down and reheated to obtain the complete glow peak and its position Tm noted. This procedure was repeated several times, re-irradiating the same sample anew each time and Tstop increased to a slightly higher value. Fig. 3(a) shows examples of glow curves measured this way whereas Fig. 3(b) shows a plot of Tm against Tstop. Interestingly, a new peak at 165 °C was noted after preheating to 140 °C. For ease of reference, the peaks observed at 92, 165 and 239 °C corresponding to the dose of 600 Gy are labelled as P1, P2 and P3 respectively. Since the position Tm of a first order glow peak is independent of the initial concentration of the trapped charge, its position is expected to be independent of Tstop (McKeever, 1985). On the other hand, for second order kinetics or for closely overlapping peaks, the peak temperature should increase with Tstop (McKeever, 1985). Fig. 3(b) shows that peaks P1, P2 and P3 shift to higher temperature with Tstop temperature. This implies that none of the peaks follow first order kinetics. It must be then that the peaks are subject to second order kinetics or that they are each a composite of several closely spaced overlapping peaks.
3.2. Thermoluminescence (TL) 3.2.1. Glow curve characteristics and kinetics features As a start, an attempt was made to monitor any natural TL but none was observed. Even with test doses of say, 1 Gy or 5 Gy, no TL was seen. Appreciable TL was only observed when the sample was irradiated to 10 Gy or more. In order to obtain proper TL for analysis, the sample had to be irradiated up to 600 Gy and the resulting glow curve is shown in Fig. 2. Despite the extensively high dose used in this test, the intensity of the emission is only moderate. There are two TL glow peaks apparent at 92 and 239 °C. Nevertheless, a study of the kinetic features of each peak was carried out and is explained here. It should be noted that previous attempts to study limestone also showed its stimulated luminescence to be of low intensity (Galloway, 2002). Regarding the emission band, we note that the TL was measured using a Schott BG 39 filter that has a transmission band of 330–690 nm. In a test measurement, TL was also measured from an irradiated sample using a Hoya U-340 filter that has a transmission band of 290–380 nm. In the latter, no TL was observed from the sample after irradiation to 600 Gy. This implies that the emission from the sample falls between 380 and 690 nm. Literature shows that other natural calcium carbonates such as calcite (Townsend et al., 1994; Down et al., 1985; Kalita and Wary, 2014), manganiferus carbonatite (Chithambo et al., 2014) etc. show dominant emission bands between 380 and 690 nm due to Mn impurity. However, other impurities such as Fe, Mg etc. may also contribute to emission in calcium carbonate minerals (Townsend et al.,
3.2.3. Dependence of Tm on delay between irradiation and TL measurement In this study, the position Tm of a peak was monitored as a function of delay between irradiation and measurement of TL. This was done to study whether the glow peaks follow second order kinetics or whether the shift of Tm with Tstop reflects a peak being composite. In this experiment, each time the sample was irradiated to 600 Gy, the TL was measured after some delay following the irradiation. Fig. 4 shows examples of glow curves recorded at 1 °C/s from a sample irradiated to 600 Gy and measured after delays between 0 and 43200 s. The inset to Fig. 4 shows, for peak P1, the change of Tm with the delay. Peak P1 fades with delay between irradiation and measurement and its apparent position shifts to higher temperature. This means that peak P1 is likely a combination of several closely spaced overlapping glow peaks. However, no such conclusion could be made for peaks P2 and P3 as they show negligible fading with the delay up to 43200 s. 3.2.4. Dependence of Tm on dose Furthermore, to assess the order of kinetics of the peaks, the Tm‒dose analysis was used. TL was measured at 1 °C/s from a sample irradiated each time to a dose between 10 Gy and 1000 Gy. The position of the peak corresponding to a particular dose was noted. Fig. 5(a)
Fig. 2. A glow curve measured at 1 °C/s after beta irradiation to 600 Gy. 3
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Fig. 3. (a) Examples of glow curves measured from the sample each time freshly irradiated to 600 Gy and preheated to different Tstop temperatures. (b) A plot of Tm against Tstop. The error bars in (b) represent the standard deviation (σ) of the data from three identical measurements and the errors are within 2σlevel.
Fig. 5. (a) Examples of glow curves measured after irradiated to different doses. (b) The dependence of the position of peaks P1 and P3 on dose. The error bars represent the standard deviation of the data from three identical measurements and the errors are within 2σ-level.
76 to 96 °C when the dose is increased from 10 to 1000 Gy. In the same dose range, the position of P3 decreases consistently from 239 to 237 °C. The shift of peak P1 to higher temperature probably reflects the successive appearance of overlapping components. This is consistent with the earlier estimation from the Tm‒Tstop study as well as from the dependence of Tm on delay between irradiation and TL measurement for this particular peak. Further, the shift of peak P3 to lower temperature indicates that the peak follows either second order kinetics or general order kinetics or may be due to the peak being composite as noted from the Tm‒Tstop analysis. 3.2.5. Activation energy of electron traps As stated earlier, most methods of kinetic analysis are suitable for an isolated glow peak. In our study, it is not possible to determine the total number of overlapping peaks as well as their position thus phosphorescence and deconvolution methods are unsuitable. Therefore, we used the initial rise method to evaluate the activation energy of the corresponding electron trap (McKeever, 1985). The initial rise method was applied to glow peaks recorded after preheating as well as those measured after delays between irradiation and measurement. Fig. 6(a) shows examples of plots of ln(I) against 1/ kT corresponding to glow peaks measured after different preheating Tstop temperatures. Values of the activation energy obtained from such plots are shown in Fig. 6(b) against the preheating temperature Tstop (squares) or against delay between irradiation and measurement (triangles). The error bars are associated with each value of the activation energy estimated from the least square fitting of the ln(I) versus 1/kT
Fig. 4. Examples of glow curves recorded at 1 °C/s from a sample irradiated to 600 Gy and measured after delays between 0 and 43200 s. The inset shows the variation of Tm of peak P1 with delay. The error bars in the inset represent the standard deviation (σ) of the data from three identical measurements and the errors are within 2σ-level.
shows examples of glow curves measured after different doses. Fig. 5(b) shows the dependence of the position of peaks P1 and P3 on dose. Peak P2 was not analysed as its position could not be reliably determined. The error bars in Fig. 5(b) represent the standard deviation from three identical measurements. According to theory, the position of a first order isolated TL peak should be independent of dose whereas that of a second order or general order peak is expected to shift to lower temperatures with dose (McKeever, 1985). The position of P1 changes from 4
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Fig. 6. (a) Examples of the plots of ln(I) against 1/kT corresponding to glow curves measured after preheating to some Tstop temperatures. (b) A plot of activation energy versus preheating temperature Tstop (squares) and delays (triangles) between irradiation and measurement. The dotted lines in (b) are visual guides used to bracket the consistent range of the activation energies.
Fig. 7. (a) Examples of IRSL decay curves (solid lines) measured after irradiation to different doses. The open circles represent the background signal. (b) Examples of residual-TL measured at 1 °C/s after the measurement of IRSL.
heated to 500 °C at 1 °C/s to remove any residual TL. IRSL was measured thereafter from the sample using the protocol stated earlier (section 2). Fig. 7(a) shows examples of IRSL decay curves measured after irradiation to different doses. Fig. 7(b) shows examples of residual-TL following the measurement of IRSL. The IRSL decay curves have been corrected for background. The IRSL shows ill-shaped decay curves of low intensity. This characteristic is due to low concentration of electron traps involved in stimulated luminescence in the sample. Similar low intensity emission are seen for thermally assisted optically simulated luminescence from beta irradiated α-Al2O3:C, Mg also due to low concentration of (deep) electron traps (Kalita et al., 2017). Although the IRSL is weak, its intensity increases with dose. During IRSL stimulation, owing to low energy of IR light, electrons are probably ejected from shallower traps (corresponding to TL peaks P1 and P2) and recombine with holes via the conduction band. In addition, some electrons perhaps also transfer from relatively deeper traps to shallower traps during IR illumination and move to conduction band thereafter. The transfer of trapped electrons between localised energy levels aided by illumination is known as ‘phototransfer’. This is not to be confused with phototransferred thermoluminescence. Phototransfer is possible in this sample as the localised energy levels are closely spaced. Apart from the transitions from delocalised energy level, there may also be some localised transitions where electrons recombine at recombination centre without making transition from the conduction band. Such localised transition under IR stimulation was observed in feldspar (Sfampa et al., 2015). When an excited state of a recombination centre is at a similar energy depth as that of electron traps, electrons can move to a nearby excited level of the recombination centre and thereby produce luminescence without involvement of the conduction band.
plots. It is evident in Fig. 6(b) that the activation energy falls into three distinct regions as a function of Tstop. In the first region for Tstop = 30‒100 °C, the values of the activation energies vary from 0.60 to 0.68 eV. In the second region for Tstop = 110‒170 °C, the activation energies vary from 0.40 to 0.46 eV whereas that in the third region for Tstop = 180‒240 °C, the values vary from 0.86 to 1.12 eV. The dotted lines in Fig. 6(b) are visual guides used to bracket the minimum and maximum values of activation energy in the three regions. Further, the activation energies estimated from the glow curves measured after the delays between 0 and 43200 s during which the first peak P1 fades, varies from 0.60 to 0.72 eV. Note that the peak temperature corresponding to peaks P1, P2 and P3 are 92, 165 and 239 °C respectively. Therefore, taking the preheating and peak temperatures into account, it can be summarised that the sample has a continuum electron trap distribution with activation energy between 0.40 eV and 1.12 eV responsible for the three composite peaks. The activation energies corresponding to peak P2 are less than those of peak P1 although the latter appears at a lower temperature than peak P2. This is because the position of a TL peak depends simultaneously on activation energy, frequency factor and heating rate. Literature shows that in calcite, there is also a glow peak at a higher temperature with a relatively lower activation energy (Kalita and Wary, 2014, 2015; Chithambo et al., 2014).
3.3. Infrared light stimulated luminescence (IRSL) The luminescence properties of limestone were further studied by infrared light stimulation. Before any measurement, the sample was 5
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The residual-TL glow curves measured after the IRSL measurements show similar structure as that of the conventional TL. Three residual-TL peaks are found at 104 °C (labelled as P1/), 175 °C (P2/) and 248 °C (P3/ ) corresponding to the dose of 600 Gy. Note that the position of conventional TL peaks P1, P2 and P3 corresponding to the dose of 600 Gy were 92, 165 and 239 °C respectively. The residual-TL peaks have all shifted by about 10 °C towards high temperature. This is expected since the position of the peaks are affected by dose as reported in section 3.2.4. The analysis of the dependence of Tm on dose showed that the position of the peaks changes with irradiation. Thus the position of the peaks depends on concentration of electrons at the electron traps. Since the residual-TL was measured after the measurement of IRSL, the concentration of the electrons at the electron traps would have changed. The glow peaks in the residual-TL then occur at slightly different positions compared to those under the conventional TL. We note that the intensity of the residual-TL peaks is about half that of the conventional TL peaks. Therefore, the sample is expected to produce intense IRSL. However, the sample only produces weak IRSL. This discrepancy can be explained considering the effect of fading. The study on fading (to follow in section 3.5) shows that the sample fades rapidly. Most electrons thus escape from the traps before the measurement of residual-TL resulting in weak TL intensity. 3.4. Dose response The dose response was studied using TL as well as IRSL for doses between 10 and 1000 Gy. The intensity (peak-height) of peaks P1 and P3 was monitored as a function of dose. Peak P2 was excluded from this analysis as its position and hence the intensity could not be reliably monitored at low doses. Fig. 8(a) shows the dose response of peaks P1 and P3. The intensity of P1 increases at a uniform rate with dose from 10 to 1000 Gy whereas that of P3 shows two different regions of uniform increase within 10 to 100 Gy and 100 to 1000 Gy respectively. The dose response was also studied by IRSL measured using the protocol describe earlier. The IRSL intensity measured as the area under the decay curves has been plotted as a function of dose. Fig. 8(b) shows the variation of IRSL intensity as a function of dose. The IRSL intensity also increases with dose in two different uniform regions within 10 to 100 Gy and 100 and 1000 Gy. This is a significant result of the present study that indicates some relation between the TL of peak P3 and the IRSL from the sample. The dose response analysis was further extended for residual-TL peaks P1/and P3/. Fig. 8(c) shows the variation of intensity of peaks P1/ and P3/with dose. The intensity of P1/increases linearly from 10 to 1000 Gy whereas that of P3/increases as two different linear regions within 10 to 100 Gy and 100 to 1000 Gy. The dose responses of peaks P1/and P3/are very similar to those of conventional TL peaks P1 and P3 respectively. This is a very important result as it shows that the sample could be used to measure an unknown dose in two different ways: first by measuring IRSL and by subsequently measuring residual-TL without any further irradiation.
Fig. 8. Variation as a function of dose of (a) intensity of conventional TL peaks P1 and P3, (b) IRSL intensity and (c) intensity of residual-TL peaks P1ʹ and P3ʹ. The doted lines are only visual guides. The error bars in all the figures represent the standard deviation of the data from three identical measurements and the errors are within 2σ-level.
3.5. Fading In the context of radiation dosimetry, fading means the loss of luminescence with delay between irradiation and measurement. Fading occurs due to escape of trapped electrons from electron traps by many processes including charge hopping (Kalita and Chithambo, 2017), tunnelling (McKeever, 1985) etc. In the present case, fading was studied by irradiating a sample to 600 Gy and measuring TL at 1 °C/s using delays of 0 to 42300 s between irradiation and TL measurement. Fig. 4 shows examples of glow curves measured after some specific delays. The intensity of peaks P1 and P3 was each studied as a function of delay. Peak P2 was excluded from the analysis due to its weak intensity. Fig. 9 shows variation of intensity of peaks P1 and P3 against the delay. It was observed that the intensity of peak P1 decreases by about 88%
Fig. 9. Variation of intensity of peaks P1 against the delay. The inset shows the same feature of peak P3. The dotted lines are only visual guides. The error bars represent the error estimated by Poisson statistics.
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Applied Radiation and Isotopes 154 (2019) 108888
J.M. Kalita and M.L. Chithambo
and that of peak P3 decreases by about 14% in 12 h after irradiation. We can now deduce that it is as a result of such a high rate of fading in ambient conditions that no natural-TL was observed in the sample. With regards to any use as a radiation dosimeter, the fading as measured for peak P1 is high, however peak P3 could be useful for radiation dosimetry.
analysis of UV, beta and gamma induced limestone collected from Amarnath holy cave. J. Radiat. Res. Appl. Sciences 8, 126–135 2015. Dubey, V., Kaur, J., Dubey, N., Pandey, M.K., Suryanarayana, N.S., Murthy, K.V.R., 2017. Kinetic and TL glow curve analysis of UV-, β- and γ-irradiated natural limestone collected from Chunkatta mines. Radiat. Eff. Defects Solids 172, 866–877. Engin, B., Guven, O., 2000. The effect of heat treatment on the thermoluminescence of naturally-occurring calcites and their use as a gamma-ray dosimeter. Radiat. Meas. 32, 253–272. Galloway, R.B., 2002. Does limestone show useful optically stimulated luminescence? Ancient TL 20, 1–5. Gomez-Ros, J.M., Furetta, C., Correcher, V., 2006. Simple methods to analyse thermoluminescence glow curves assuming arbitrary recombination–retrapping rates. Radiat. Prot. Dosim. 119, 339–343. Kalita, J.M., Chithambo, M.L., 2017. A comparative study of the dosimetric features of of α-Al2O3:C,Mg and α-Al2O3:C. Radiat. Prot. Dosim. 177, 261–271. Kalita, J.M., Chithambo, M.L., Polymeris, G.S., 2017. Thermally-assisted optically stimulated luminescence from deep electron traps in α-Al2O3:C,Mg. Nucl. Instrum. Methods Phys. Res. B 403, 28–32. Kalita, J.M., Wary, G., 2014. Thermoluminescence study of X-ray and UV irradiated natural calcite and analysis of its trap and recombination level. Spectrochim. Acta A. 125, 99–103. Kalita, J.M., Wary, G., 2015. Thermal quenching in calcite and evaluation of quenching parameters from composite glow curve by a computerized resolved peak technique. J. Lumin. 160, 134–137. Kalita, J.M., Wary, G., 2016. X-ray dose response of calcite—a comprehensive analysis for optimal application in TL dosimetry. Nucl. Instrum. Methods Phys. Res. B 383, 93–102. King, Hobart M., 2019. Limestone. https://geology.com/rocks/limestone.shtml. Kitis, G., Gomez-Ros, J.M., Tuyn, J.W.N., 1998. Thermoluminescence glow-curve deconvolution functions for first, second and general orders of kinetics. J. Phys. D Appl. Phys. 31, 2636–2641. Liritzis, I., Guibert, P., Foti, F., Schvoerer, M., 1996. Solar bleaching of thermoluminescence of calcites. Nucl. Instrum. Methods Phys. Res. B 117, 260–268. McKeever, S.W.S., 1985. Thermoluminescence of Solids. Cambridge University Press, Cambridge. Ninagawa, K., Adachi, K., Uchimura, N., Yamamoto, I., Wada, T., Yamashita, Y., Takashima, I., Sekimoto, K., Hasegawa, H., 1992. Thermoluminescence dating of calcite shells in the pectinidae family. Quat. Sci. Rev. 11, 121–126. Ninagawa, K., Kitahara, T., Toyoda, S., Hayashi, K., Nishido, H., Kinjo, M., Kawana, T., 2001. Thermoluminescence dating of the ryukyu limestone. Quat. Sci. Rev. 20, 829–833. Pagonis, V., Kitis, G., Furetta, C., 2006. Numerical and Practical Exercises in Thermoluminescence. Springer, Berlin. Ponnusamy, V., Ramasamy, V., Dheenathayalu, M., Hemalatha, J., 2004. Effect of annealing in thermostimulated luminescence (TSL) on natural blue colour calcite crystals. Nucl. Instrum. Methods Phys. Res. B 217, 611–620. Sfampa, I.K., Polymeris, G.S., Pagonis, V., Theodosoglou, E., Tsirliganis, N.C., Kitis, G., 2015. Correlation of basic TL, OSL and IRSL properties of ten K-feldspar samples of various origins. Nucl. Instrum. Methods Phys. Res. B 359, 89–98. Soliman, C., Metwally, S.M., 2006. Thermoluminescence of the green emission band of calcite. Radiat. Eff. Defects Solids 161, 607–613. Stirling, R.J., Duller, G.A.T., Roberts, H.M., 2014. Developing a single-aliquot protocol for measuring equivalent dose in biogenic carbonates. Radiat. Meas. 47, 725–731. Townsend, P.D., Luff, B.J., Wood, R.A., April–July 1994. Mn2+ transitions in the TL emission spectra of calcite. Radiat. Meas. 23 (2–3), 433–440. https://doi.org/10. 1016/1350-4487(94)90076-0. Wintle, A.G., 1978. A thermoluminescence dating study of some Quaternary calcite: potential and problems. Can. J. Earth Sci. 15, 1977–1986.
4. Conclusions TL and IRSL properties of limestone have been studied under beta irradiation. The sample has rhombohedral crystal structure and contains 33.45% of CaO. It produces three composite glow peaks P1, P2 and P3 at 92, 165 and 239 °C respectively. Kinetic analysis shows that the sample has a continuum trap distribution with activation energy from 0.40 to 1.12 eV. A study of dose response shows that the TL intensity of P1 increases at constant rate with dose from 10–1000 Gy whereas that of P3 shows two different rate of increase of intensity within 10–100 Gy and 100–1000 Gy. IRSL measurement produces illshaped decay curves. The dose response studied from IRSL intensity shows similar variation as that of P3. Further, residual-TL measured after the measurement of IRSL shows similar response as that under the conventional TL. Regarding fading, P1 fades by 88% whereas P3 by 14% within 12 h after irradiation. The TL and IRSL mechanisms have been discussed considering various possibilities including localised and delocalised transitions as well as phototransfer process under IR illumination. This study shows that limestone can be used as a potential natural radiation dosimeter. Acknowledgement J.M. Kalita acknowledges with gratitude for financial support from Rhodes University and the National Research Foundation of South Africa. Authors also thank the Sophisticated Analytical Instrument Facility (SAIF), Department of Instrumentation & USIC, Gauhati University, India for the XRD and XRFS reports. References Chithambo, M.L., Pagonis, V., Ogundare, F.A., 2014. Spectral and kinetic analysis of thermoluminescence from manganiferous carbonatite. J. Lumin. 145, 180–187. Debenham, N.C., 1983. Reliability of thermoluminescence dating of stalagmitic calcite. Nature 304, 154–156. Debenham, N.C., Aitken, M.J., 1984. Thermoluminescence dating of stalagmitic calcite. Archaeometry 26, 155–170. Down, J.S., Flower, R., Strain, J.A., Townsend, P.D., 1985. Thermoluminescence emission spectra of calcite and Iceland spar. Nucl. Tracks Radiat. Meas. 10, 581–589. Dubey, V., Dubey, V.P., Tamrakar, R.K., Upadhyay, K., Tiwari, N., 2015. TL glow curve
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