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Thermoluminescent (TL) properties of the perovskite KMgF3 activated by Ce and Er impurities
Applied Radiation and Isotopes 55 (2001) 533–542
Thermoluminescent (TL) properties of the perovskite KMgF3 activated by Ce and Er impurities C. Furettaa,*, F. Santopietroa, C. Sanipolia, G. Kitisb a
Department of Physics, Rome University ‘‘La Sapienza’’, Piazzale Aldo Moro 2, 00185, Rome, Italy b Nuclear Physics Laboratory, Aristotle University of Thessaloniki, 54006 Thessaloniki, Greece Received 26 July 2000; received in revised form 26 February 2001; accepted 2 April 2001
Abstract The perovskites are very interesting for their thermoluminescent properties and for their possible application as thermoluminescent dosimeters (TLD). In this paper we present results concerning the main dosimetric properties of KMgF3 activated by Ce and Er at various concentrations, i.e. annealing procedure, reproducibility of the TL signal, linearity and fading behaviour. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Thermoluminescence; Dosimetry; Kinetics; Perovskite
1. Introduction Thermoluminescence (TL) dosimetry has been developed to the stage that it now represents a key technique in absorbed dose determination. In particular, it has found significant use in clinical, personal and environmental dosimetry. Of the many materials that have been studied, several are now commonly used as thermoluminescent dosimeters (TLD): a number of reviews concerning the preparation and properties of commercially and home-made thermoluminescent materials have been published during the past several years (Azorin et al., 1993; McKeever et al., 1995). Interest in radiation dosimetry by the TL technique has resulted in numerous efforts seeking production of new, high performance TL materials. It is within such a framework that systematic investigation of the perovskite-like compound KMgF3 is currently in progress. The perovskite-like material KMgF3 in combination with various dopants has been extensively studied (Furetta et al., 1990, 1994; Bacci et al., 1992, 1993; Scacco et al., 1994; Kitis et al., 1996, 1999) and the reported thermoluminescence characteristics have *Corresponding author. Tel.: +39-06-6132550; fax: +39-064957697. E-mail addresses: [email protected] (C. Furetta).
shown this phosphor to be a very good candidate for ionising radiation dosimetry. The effective atomic number of KMgF3, Z eff, is 13, the material being intermediate between the tissue equivalence of soft biological tissues (Zeff=7.3) and high sensitivity materials such as CaSO4 and CaF2. The present paper concentrates on study of a new preparation of KMgF3 doped with rare earth impurities, i.e. Ce and Er. The first such results for KMgF3 : Ce have recently been published (Kitis et al., 1996, 1999). The present work concerns extention and completion of that work, also including characterisation of the TL properties of Er activated material. KMgF3 is a ternary compound belonging to the group of fluoroperovskites which have the general formula ABF3, where A and B have the respective meanings alkali metal and alkaline earth metal. The KMgF3 crystal has a cubic lattice. When activated with impurities, monovalent cations replace K+ ions and anions, i.e. OH@, substitute for F@ ions. In the case of divalent or trivalent cations, their location strongly depends on their size. Such substitutional ions can replace small Mg2+ ions only if the ionic radii are comparable, otherwise they are forced to occupy a K+ ion location. Detailed investigations have been carried out on the nature and structure of the defects induced in KMgF3 doped by Ce3+ (Francini et al., 1997; Martini et al.,
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1998). To the best of our knowledge no other reports exists that concern study of the luminescence by defects produced by Er.
tion is usually performed by systematic study of the variation of the TL sensitivity as a function of temperature, T, and time, t, at each given temperature. The values of (T, t), identified to provide an optimum annealing procedure, must satisfy the following basic requirements:
2. Experimental procedure KMgF3 doped phosphors have been obtained by melting KF and MgF2 in stoichiometric ratio, applying the Czochralski technique, using a platinum crucible under argon gas atmosphere. The furnace temperature was set at 13001C. A platinum crucible is needed because of the high reactivity of fluorides. The pulling rate was of the order of 1 cm h@1, and the typical dimensions of the crystal bolus were 4–5 cm in length and 2–3 cm in diameter. Prior to growth, doping has been obtained by adding a suitable amount of impurity. Owing to the tendency towards segregation during growth, the dopant concentration in the crystal is not constant along the ingot. Samples of good homogeneity were therefore carefully selected for thermoluminescent experiments. Samples were then cleaved or sawed from the ingots. Polishing was generally carried out in order to achieve good quality surfaces, the latter being necessary for good thermal contact during TL emission readout. Samples used in the measurements were thin slabs having typical dimensions of 2 2 1 mm3, cleaved from single crystals of KMgF3. The samples were then weighed and all data were normalised to the mass. Before each irradiation the samples were annealed appropriately, in accord with the thermal procedure to be described below. After irradiation, the samples were read out using a Toledo reader (supplied by Vinten, UK). The readout cycle consisted of a linear heating rate of 51 C s@1, over the temperature range 401C up to 4001C, followed by cooling to 401C within the reader itself. High purity nitrogen was allowed to flux into the reader during the heating cycle. Use of a PC allowed online glow curve analysis, obtaining TL intensity as a function of temperature. A 90Sr–90Y b-source was used for all of the sample irradiations. KMgF3 : Ce was prepared with four different concentrations of Ce: 0.24, 0.5, 1.0 and 1.5 mol% respectively. For KMgF3 : Er three different concentrations were prepared and tested: 1.0, 1.3 and 2.0 mol%. The following sections describe the experimental procedures carried out for thermoluminescence dosimetry characterisation of the two materials.
3. Experimental results 3.1. Annealing First an experiment was performed to determine the optimum annealing procedure. Annealing characterisa-
(a) the ability to completely erase all previously accumulated radiation information, (b) the ability to yield the highest sensitivity, and (c) the ability to retain stable sensitivity following many cycles of irradiation annealing. It is also very useful to avoid temperatures that may result in permanent loss of sensitivity (an example is LiF : Mg, Cu, P which, due to its high thermal sensitivity, suffers permanent loss of its original sensitivity at an annealing temperature just above 2401C). The experiment was carried out for a range of temperatures, between 4001C and 6001C, in steps of 501C, for 30 and 60 min periods for each selected temperature. The procedure was done for each selected concentration of Ce and Er. Fig. 1 shows the TL response obtained from irradiated samples doped by 0.5 mol% Ce after annealing at particular temperatures and time: the best annealing procedure was obtained at 5501C for 60 min. Similar results have been obtained for the other concentrations. The annealing procedure for KMgF3 : Er was investigated for the same temperature range used for KMgF3 : Ce, in steps of 501C for periods of 30 and 60 min respectively. The results obtained for samples having a 2% mol concentration of Er are shown in Fig. 2: annealing at 4501C for a period of one hour gives the highest TL emission. Higher temperatures provoke rapid decrease of the thermoluminescence emission. Similar results have been obtained for the remaining concentrations. The error bars in both Figs. 1 and 2 represent one standard deviation (S.D.) for five measurements made at each temperature. 3.2. Sensitivity Using appropriate annealing procedures as described above, comparison has been made of the sensitivity against LiF : Mg, Ti (TLD-100) (Harshaw, USA) for each of the materials under study. Sensitivity has been expressed as the ratio TL/(mD), with m the sample mass, in g, and D the dose, in Gy. Table 1 shows results, normalised to a given mass. The delivered dose was in every case 0.125 Gy. Comparing the sensitivity of KMgF3 to that for LiF is a reasonable procedure, the TL emissions for each being in the region 300–500 nm. In addition, the quantum efficiency of PM tubes used in commercial TL readers suffers only slight variation in the particular wavelength region. It is further of note that both Ce and Er doped KMgF3 provide sensitivities which are very much higher than that for LiF : Mg, Ti.
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Fig. 1. TL emission vs time and temperature of annealing for Ce activated KMgF3.
Fig. 2. Behaviour of the TL signal from KMgF3 : Er after various annealing treatments.
3.3. Glow-curve shape Fig. 3 shows the glow-curve shape for a Ce concentration of 0.5 mol%; the glow curve has been recorded at
a heating rate of 51 C s@1, following irradiation at a test dose of 0.25 Gy. Repetitive cycles of annealing and irradiation at the same dose reveal the same glow curve structure. Further, in changing the dose level the general
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structure of the TL curve is not observed to undergo alteration, while glow curves obtained at various concentrations also differ little between each other. The low temperature peak, due to the host material, is always seen to be present at about 1151C. The temperatures at which the dosimetric peak is observed ranges between 480 and 500 K. In the case of 0.5 mol% a small shoulder is observed on the high temperature side of the main peak (see Fig. 3); this shoulder disappears as the concentration of the activator increases. While fresh KMgF3 : Er has been observed to give rise to some initial instabilities in the glow curve structure, these instabilities disappear after the first few cycles of annealing-irradiation readout. Subsequently, the glow curves become highly similar to that for KMgF3 : Ce. The temperature at which the dosimetric peak appears
Table 1 Relative sensitivities (the sensitivities are compared to the one of LiF:Mg, Ti and normalised to the same mass). Test b dose of 0.125 Gy Ce (mol%)
Relative sensitivity
Er (mol%)
Relative sensitivity
0.24 0.5 1.0 1.5
33.8 41.0 73.8 92.0
1.0 1.3 2.0
9.8 14.0 16.8
decreases from 545 to 488 K as the Er concentration increases. 3.4. Reproducibility The reproducibility of the TL signal for Ce doping of KMgF3 (Fig. 4), has been tested for all four concentrations over ten repeated cycles of annealing-irradiation readout. Good reproducibility, to within less than 3%, is revealed for the 1.0 mol% concentration, and remains relatively good, being within less than 10%, for samples doped at 1.5 mol%; the other two preparations give a reproducibility of the signal between the two previous values. A slight increase, as a function of the cycles, can be observed for samples having concentrations of 0.5 and 1.5 mol%. The reproducibility of the TL emission of KMgF3 : Er is given in Fig. 5, ranging from between 2.5% (1 mol%) and 5.5% (1.3 mol%). 3.5. TL response versus dose The TL response, as a function of absorbed dose, D, from about 0.4 mGy up to 100 Gy, was investigated for all prepared concentrations of Ce. Fig. 6 shows plots of TL versus D for each concentration. Linearity of thermoluminescence response, obtained from integration of the main TL peak, is observed up to 10 Gy.
Fig. 3. Thermoluminescence glow curve of KMgF3 : Ce (0.5 mol%) after annealing at 5501C for 1 hour and irradiation at 0.25 Gy.
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Fig. 4. Reproducibility of the TL signal of KMgF3 : Ce over 10 repeated cycles of annealing-irradiation-readout. The TL responses have been normalised to the first cycle of the sample with activation concentration of 0.24 mol%.
Fig. 5. Reproducibility of the TL signal from KMgF3 : Er over subsequent cycles of annealing-irradiation readout. Normalisation is as in Fig. 3.
Above 10 Gy the TL response of all concentrations shows slight super-linearity. The TL-dose response plots for the three concentrations of KMgF3 : Er are given in Fig. 7. Again, the delivered doses ranged from 100 mGy to 100 Gy. A very slight super-linear effect appears at about 40 Gy for 1.3 mol% doping. Each experimental
point represents an average obtained over three repeated measurements; the maximum error was within 1 S.D. It is suggested that the slight super linear behaviour for both the materials can be overcome through obtaining calibration over a limited dose range for a particular dosimetric application.
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Fig. 6. TL-dose plot response for KMgF3 : Ce at various activator concentrations. The line at 451 indicates the linearity.
3.6. Fading For both Ce and Er doped materials, fading was studied over a period of one month subsequent to irradiation. Subsequent to irradiation the samples were stored in dark room conditions, at room temperature (B201C). The low temperature peak, corresponding to TL emission of the host material, was observed to fade rapidly within a period of about 6 h. As a result, ‘zero’ elapsed time from irradiation was set to be 10 h after irradiation. The fading behaviour for the four concentrations of KMgF3 : Ce each show a similar trend. At the end of a storage period of about 100 h, the TL emission was observed to decrease by less than 3–4% of the ‘zero’ time TL emission. Subsequent TL decrease was observed to be slight, remaining practically constant at a level of about 95% of the initial TL emission. Fig. 8 shows the fading behaviour for the four different Ce concentrations.
For KMgF3 : Er the low temperature peak of the host material was also allowed to fade to the initial ‘zero’ time readout. The behaviour of the TL signal as a function of storage time has been found to be quite similar to that obtained for the three Er concentrations, the TL signal decreasing slowly during the first 100 h, with loss of approximately 2% of the zero time TL emission. For further storage TL emission remains essentially constant and the maximum TL loss is about 5% of the initial emission intensity. Fig. 9 shows the fading behaviour for the different phosphor concentrations. 3.7. Visible light exposure To investigate the effect of visible light on the material, a number of samples were annealed. Several of these were irradiated to a b-dose of 0.125 Gy and then all of the samples were exposed to direct sunlight. After
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Fig. 7. TL-dose response of KMgF3 : Er for various Er concentrations.
10 h of light exposure, all of the samples were read out. No sensitivity to light was observed for either the b irradiated or annealed only samples.
4. Mean lifetime Any dosimetric characterisation of new thermoluminescent materials should include an assessment of the kinetic parameters, activation energy, E, and frequency factor, s, both of which characterise the trapping levels. Indeed, evaluation of the value of E and s are needed in order to provide an indication of the stability of the glow peak, i.e. the dosimetric peak, at environmental temperatures. The kinetic parameters have been determined using the peak shape method of Chen (1969a, b), based on the geometrical characteristics of an isolated peak. The averaged values determined in this way for all of the present materials are E ¼ 1:10 eV and s ¼ 8 1010 s@1. As mentioned above, these parameters can be used to
obtain an indication of the stability of the corresponding TL peak at a given temperature, i.e. for determining the mean lifetime of the trapped charges in the considered trap. The mean lifetime for a first order kinetics process, at temperature T, i.e. T ¼ 300 K, can be expressed as: E t ¼ s@1 exp ; ð1Þ kT where s is the frequency factor (s@1), E is the activation energy of the trapping level (eV) and k is Boltzman’s constant. In respect of experimentally evaluated fading data, it is possible to calculate the fading factor, l (d@1), again at T ¼ 300 K, as follows: 1 F l ¼ @ ln ; ð2Þ t F0 where t is elapsed time, F the TL emission at time t and F0 the TL emission at time zero. For the evaluated kinetics parameters and experimental data on fading, one obtains the value t ¼ 430 d, indicating a TL loss of
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Fig. 8. Fading of KMgF3 : Ce. Data obtained at room temperature and under dark-room conditions.
about 0.2% per day and l ¼ 0:0024 d@1, the latter corresponding to the same TL loss per day as determined by the kinetics parameters.
5. Discussion and conclusions The main property of KMgF3 is its very high sensitivity, particularly when doped by Ce activator. For the highest concentration of Ce used in this work, i.e. 1.5 mol%, the observed sensitivity is more than 90 times that for LiF : Mg, Ti and, in turn, about four time more sensitive than LiF : Mg, Cu, P which to-date has been considered to be one of the most sensitive TL phosphors available. The high sensitivity of KMgF3 : Ce makes it very useful for medical and environmental radiation dosimetry. The presence of 40K in any sample of KMgF3 produces a b self-irradiation as recently reported (Bos, 2000). A possible contribution of self-dose to the external dose can be accurately estimated in specific cases: e.g. during short monitoring periods in environmental dosimetry. The self-dose irradiation should not be a serious problem in other dosimetric applications
where high doses have to be monitored as in the case of clinical dosimetry. Indeed a quite linear range up to 100 Gy is very good for radiological examinations as well as for therapeutic treatments. Because of the high atomic number of the material, it is suggested that accurate calibration, in an appropriate beam of well known energy, may eliminate the possibility of overestimating the delivered dose. The good stability of the dosimetric information during the first 100 h of storage allows the use of this material in clinical environments, where time can be taken between irradiation and the readout. Another important characteristic of KMgF3 is its very simple glow curve, compared to other commercial thermoluminescent dosimeters. As an example LiF : Mg, Ti has a very complex glow curve structure which does not easily allow deconvolution. The very simple glow curve structure of KMgF3 allows use of simple settings of the TL reader, rendering it user friendly for applications say in a radiological department. Furthermore, the annealing procedure only needs high temperatures, 5501C for Ce doped material and 4501C for Er activated material, over a period of 1 hour, uncomplicated by preand post-irradiation annealing procedures.
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Fig. 9. Fading of the TL signal in KMgF3 : Er (room temperature and dark-room conditions).
Table 1 shows increase in sensitivity of Ce doped KMgF3 with increase in dopant concentration. Kitis et al. (1996, 1999) reported a TL sensitivity of more than 150, relative to that for LiF : Mg, Ti. This value is higher than the highest value reported in the present work. A possible reason for this discrepancy could be the presence of Ce3+ and Ce4+ ions as observed by Francini et al. (1997) and Martini et al. (1998). Ce3+ ions will be in competition with the Ce4+ ions and TL sensitivity depends on the ratio Ce3+/Ce4+ in the sense that TL increases with increase in the number of Ce3+. Conversely, TL decreases as the number of Ce4+ ions increase. Since the ratio depends on the crystal growing conditions and no special effort has been made to control Ce3+ and Ce4+ ions concentration, it is expected that the present batch of KMgF3 : Ce contains more Ce4+ ions than in the batch studied by Kitis et al. (1996, 1999). In addition, material doped by Er shows an increase in sensitivity as the dopant concentration increases. Thus said, its sensitivity remains lower than that for Ce doped KMgF3. Finally, from a practical point of view, it can be said that both of the materials, KMgF3 : Ce and KMgF3 : Er, can be considered promising dosemeters.
References Azorin, J., Furetta, C., Scacco, A., 1993. Preparation and properties of thermoluminescent materials. Phys. Stat. Solidi A 138, 9–46. Bacci, C., Fioravanti, S., Furetta, C., Missori, M., Ramogida, G., Rossetti, R., Sanipoli, C., Scacco, A., 1993. Photoluminescence and thermally stimulated luminescence in KMgF3: Eu+2. Rad. Prot. Dos. 47 (1–4), 277–280. Bacci, C., Furetta, C., Ramogida, G., Sanipoli, C., Scacco, A., 1992. Radiation dosimetry with Eu-doped crystals of KMgF3. Phys. Med. 9, 207–209. Bos, A.J.J., 2000. Delf University of Technology, Radiation Technology Group, private communication. Chen, R., 1969a. Glow curves with general order kinetics. J. Electrochem. Soc. 106, 1254–1259. Chen, R., 1969b. On the calculation of activation energies and frequency factors from glow curves. J. Appl. Phys. 46, 570–585. Francini, R., Grassano, U.M., Landi, L., Scacco, A., D’Elena, M., Nikl, M., Cechova, N., Zema, N., 1997. Ce3+ luminescent centers of different symmetries in KMgF3 single crystals. Phys. Rev. B 56 (23), 15109–15114. Furetta, C., Bacci, C., Rispoli, B., Sanipoli, C., Scacco, A., 1990. Luminescence and dosimetric performances of
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KMgF3 crystals doped with metal impurity ions. Rad. Prot. Dos. 33 (1–4), 107–110. Furetta, C., Ramogida, G., Scacco, A., Martini, M., Paravisi, S., 1994. Spectroscopy of complex defects in crystals of KMgF3: Tl+. J. Phys. Chem. Solids 55 (11), 1337–1345. Kitis, G., Furetta, C., Sanipoli, C., Scacco, A., 1996. Thermoluminescence properties of KMgF3 doped with Pb, Cr and Ag. Rad. Prot. Dos. 65 (1–4), 93–96. Kitis, G., Furetta, C., Sanipoli, C., Scacco, A., 1999. KMgF3: Ce, an ultra-high sensitivity thermoluminescent material. Rad. Prot. Dos. 82 (2), 151–152.
Martini, M., Meinardi, F., Scacco, A., 1998. Impurity-induced thermally stimulated luminescence of KMgF3: Ce3+ crystals. Chem. Phys. Lett. 293, 43–46. McKeever, S.W.S., Moscovitch, M., Townsed, P.D., 1995. Thermoluminescence Dosimetry Materials: Properties and Uses. Nuclear Technology Publishing, Ashford. Scacco, A., Furetta, C., Bacci, C., Ramogida, G., Sanipoli, C., 1994. Defects in g-irradiated KMgF3: Tl+. Nucl. Instr. Meth. B 91 (1–4), 223–226.
Thermoluminescent (TL) properties of the perovskite KMgF3 activated by Ce and Er impurities C. Furettaa,*, F. Santopietroa, C. Sanipolia, G. Kitisb a
Department of Physics, Rome University ‘‘La Sapienza’’, Piazzale Aldo Moro 2, 00185, Rome, Italy b Nuclear Physics Laboratory, Aristotle University of Thessaloniki, 54006 Thessaloniki, Greece Received 26 July 2000; received in revised form 26 February 2001; accepted 2 April 2001
Abstract The perovskites are very interesting for their thermoluminescent properties and for their possible application as thermoluminescent dosimeters (TLD). In this paper we present results concerning the main dosimetric properties of KMgF3 activated by Ce and Er at various concentrations, i.e. annealing procedure, reproducibility of the TL signal, linearity and fading behaviour. r 2001 Elsevier Science Ltd. All rights reserved. Keywords: Thermoluminescence; Dosimetry; Kinetics; Perovskite
1. Introduction Thermoluminescence (TL) dosimetry has been developed to the stage that it now represents a key technique in absorbed dose determination. In particular, it has found significant use in clinical, personal and environmental dosimetry. Of the many materials that have been studied, several are now commonly used as thermoluminescent dosimeters (TLD): a number of reviews concerning the preparation and properties of commercially and home-made thermoluminescent materials have been published during the past several years (Azorin et al., 1993; McKeever et al., 1995). Interest in radiation dosimetry by the TL technique has resulted in numerous efforts seeking production of new, high performance TL materials. It is within such a framework that systematic investigation of the perovskite-like compound KMgF3 is currently in progress. The perovskite-like material KMgF3 in combination with various dopants has been extensively studied (Furetta et al., 1990, 1994; Bacci et al., 1992, 1993; Scacco et al., 1994; Kitis et al., 1996, 1999) and the reported thermoluminescence characteristics have *Corresponding author. Tel.: +39-06-6132550; fax: +39-064957697. E-mail addresses: [email protected] (C. Furetta).
shown this phosphor to be a very good candidate for ionising radiation dosimetry. The effective atomic number of KMgF3, Z eff, is 13, the material being intermediate between the tissue equivalence of soft biological tissues (Zeff=7.3) and high sensitivity materials such as CaSO4 and CaF2. The present paper concentrates on study of a new preparation of KMgF3 doped with rare earth impurities, i.e. Ce and Er. The first such results for KMgF3 : Ce have recently been published (Kitis et al., 1996, 1999). The present work concerns extention and completion of that work, also including characterisation of the TL properties of Er activated material. KMgF3 is a ternary compound belonging to the group of fluoroperovskites which have the general formula ABF3, where A and B have the respective meanings alkali metal and alkaline earth metal. The KMgF3 crystal has a cubic lattice. When activated with impurities, monovalent cations replace K+ ions and anions, i.e. OH@, substitute for F@ ions. In the case of divalent or trivalent cations, their location strongly depends on their size. Such substitutional ions can replace small Mg2+ ions only if the ionic radii are comparable, otherwise they are forced to occupy a K+ ion location. Detailed investigations have been carried out on the nature and structure of the defects induced in KMgF3 doped by Ce3+ (Francini et al., 1997; Martini et al.,
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1998). To the best of our knowledge no other reports exists that concern study of the luminescence by defects produced by Er.
tion is usually performed by systematic study of the variation of the TL sensitivity as a function of temperature, T, and time, t, at each given temperature. The values of (T, t), identified to provide an optimum annealing procedure, must satisfy the following basic requirements:
2. Experimental procedure KMgF3 doped phosphors have been obtained by melting KF and MgF2 in stoichiometric ratio, applying the Czochralski technique, using a platinum crucible under argon gas atmosphere. The furnace temperature was set at 13001C. A platinum crucible is needed because of the high reactivity of fluorides. The pulling rate was of the order of 1 cm h@1, and the typical dimensions of the crystal bolus were 4–5 cm in length and 2–3 cm in diameter. Prior to growth, doping has been obtained by adding a suitable amount of impurity. Owing to the tendency towards segregation during growth, the dopant concentration in the crystal is not constant along the ingot. Samples of good homogeneity were therefore carefully selected for thermoluminescent experiments. Samples were then cleaved or sawed from the ingots. Polishing was generally carried out in order to achieve good quality surfaces, the latter being necessary for good thermal contact during TL emission readout. Samples used in the measurements were thin slabs having typical dimensions of 2 2 1 mm3, cleaved from single crystals of KMgF3. The samples were then weighed and all data were normalised to the mass. Before each irradiation the samples were annealed appropriately, in accord with the thermal procedure to be described below. After irradiation, the samples were read out using a Toledo reader (supplied by Vinten, UK). The readout cycle consisted of a linear heating rate of 51 C s@1, over the temperature range 401C up to 4001C, followed by cooling to 401C within the reader itself. High purity nitrogen was allowed to flux into the reader during the heating cycle. Use of a PC allowed online glow curve analysis, obtaining TL intensity as a function of temperature. A 90Sr–90Y b-source was used for all of the sample irradiations. KMgF3 : Ce was prepared with four different concentrations of Ce: 0.24, 0.5, 1.0 and 1.5 mol% respectively. For KMgF3 : Er three different concentrations were prepared and tested: 1.0, 1.3 and 2.0 mol%. The following sections describe the experimental procedures carried out for thermoluminescence dosimetry characterisation of the two materials.
3. Experimental results 3.1. Annealing First an experiment was performed to determine the optimum annealing procedure. Annealing characterisa-
(a) the ability to completely erase all previously accumulated radiation information, (b) the ability to yield the highest sensitivity, and (c) the ability to retain stable sensitivity following many cycles of irradiation annealing. It is also very useful to avoid temperatures that may result in permanent loss of sensitivity (an example is LiF : Mg, Cu, P which, due to its high thermal sensitivity, suffers permanent loss of its original sensitivity at an annealing temperature just above 2401C). The experiment was carried out for a range of temperatures, between 4001C and 6001C, in steps of 501C, for 30 and 60 min periods for each selected temperature. The procedure was done for each selected concentration of Ce and Er. Fig. 1 shows the TL response obtained from irradiated samples doped by 0.5 mol% Ce after annealing at particular temperatures and time: the best annealing procedure was obtained at 5501C for 60 min. Similar results have been obtained for the other concentrations. The annealing procedure for KMgF3 : Er was investigated for the same temperature range used for KMgF3 : Ce, in steps of 501C for periods of 30 and 60 min respectively. The results obtained for samples having a 2% mol concentration of Er are shown in Fig. 2: annealing at 4501C for a period of one hour gives the highest TL emission. Higher temperatures provoke rapid decrease of the thermoluminescence emission. Similar results have been obtained for the remaining concentrations. The error bars in both Figs. 1 and 2 represent one standard deviation (S.D.) for five measurements made at each temperature. 3.2. Sensitivity Using appropriate annealing procedures as described above, comparison has been made of the sensitivity against LiF : Mg, Ti (TLD-100) (Harshaw, USA) for each of the materials under study. Sensitivity has been expressed as the ratio TL/(mD), with m the sample mass, in g, and D the dose, in Gy. Table 1 shows results, normalised to a given mass. The delivered dose was in every case 0.125 Gy. Comparing the sensitivity of KMgF3 to that for LiF is a reasonable procedure, the TL emissions for each being in the region 300–500 nm. In addition, the quantum efficiency of PM tubes used in commercial TL readers suffers only slight variation in the particular wavelength region. It is further of note that both Ce and Er doped KMgF3 provide sensitivities which are very much higher than that for LiF : Mg, Ti.
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Fig. 1. TL emission vs time and temperature of annealing for Ce activated KMgF3.
Fig. 2. Behaviour of the TL signal from KMgF3 : Er after various annealing treatments.
3.3. Glow-curve shape Fig. 3 shows the glow-curve shape for a Ce concentration of 0.5 mol%; the glow curve has been recorded at
a heating rate of 51 C s@1, following irradiation at a test dose of 0.25 Gy. Repetitive cycles of annealing and irradiation at the same dose reveal the same glow curve structure. Further, in changing the dose level the general
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structure of the TL curve is not observed to undergo alteration, while glow curves obtained at various concentrations also differ little between each other. The low temperature peak, due to the host material, is always seen to be present at about 1151C. The temperatures at which the dosimetric peak is observed ranges between 480 and 500 K. In the case of 0.5 mol% a small shoulder is observed on the high temperature side of the main peak (see Fig. 3); this shoulder disappears as the concentration of the activator increases. While fresh KMgF3 : Er has been observed to give rise to some initial instabilities in the glow curve structure, these instabilities disappear after the first few cycles of annealing-irradiation readout. Subsequently, the glow curves become highly similar to that for KMgF3 : Ce. The temperature at which the dosimetric peak appears
Table 1 Relative sensitivities (the sensitivities are compared to the one of LiF:Mg, Ti and normalised to the same mass). Test b dose of 0.125 Gy Ce (mol%)
Relative sensitivity
Er (mol%)
Relative sensitivity
0.24 0.5 1.0 1.5
33.8 41.0 73.8 92.0
1.0 1.3 2.0
9.8 14.0 16.8
decreases from 545 to 488 K as the Er concentration increases. 3.4. Reproducibility The reproducibility of the TL signal for Ce doping of KMgF3 (Fig. 4), has been tested for all four concentrations over ten repeated cycles of annealing-irradiation readout. Good reproducibility, to within less than 3%, is revealed for the 1.0 mol% concentration, and remains relatively good, being within less than 10%, for samples doped at 1.5 mol%; the other two preparations give a reproducibility of the signal between the two previous values. A slight increase, as a function of the cycles, can be observed for samples having concentrations of 0.5 and 1.5 mol%. The reproducibility of the TL emission of KMgF3 : Er is given in Fig. 5, ranging from between 2.5% (1 mol%) and 5.5% (1.3 mol%). 3.5. TL response versus dose The TL response, as a function of absorbed dose, D, from about 0.4 mGy up to 100 Gy, was investigated for all prepared concentrations of Ce. Fig. 6 shows plots of TL versus D for each concentration. Linearity of thermoluminescence response, obtained from integration of the main TL peak, is observed up to 10 Gy.
Fig. 3. Thermoluminescence glow curve of KMgF3 : Ce (0.5 mol%) after annealing at 5501C for 1 hour and irradiation at 0.25 Gy.
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Fig. 4. Reproducibility of the TL signal of KMgF3 : Ce over 10 repeated cycles of annealing-irradiation-readout. The TL responses have been normalised to the first cycle of the sample with activation concentration of 0.24 mol%.
Fig. 5. Reproducibility of the TL signal from KMgF3 : Er over subsequent cycles of annealing-irradiation readout. Normalisation is as in Fig. 3.
Above 10 Gy the TL response of all concentrations shows slight super-linearity. The TL-dose response plots for the three concentrations of KMgF3 : Er are given in Fig. 7. Again, the delivered doses ranged from 100 mGy to 100 Gy. A very slight super-linear effect appears at about 40 Gy for 1.3 mol% doping. Each experimental
point represents an average obtained over three repeated measurements; the maximum error was within 1 S.D. It is suggested that the slight super linear behaviour for both the materials can be overcome through obtaining calibration over a limited dose range for a particular dosimetric application.
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Fig. 6. TL-dose plot response for KMgF3 : Ce at various activator concentrations. The line at 451 indicates the linearity.
3.6. Fading For both Ce and Er doped materials, fading was studied over a period of one month subsequent to irradiation. Subsequent to irradiation the samples were stored in dark room conditions, at room temperature (B201C). The low temperature peak, corresponding to TL emission of the host material, was observed to fade rapidly within a period of about 6 h. As a result, ‘zero’ elapsed time from irradiation was set to be 10 h after irradiation. The fading behaviour for the four concentrations of KMgF3 : Ce each show a similar trend. At the end of a storage period of about 100 h, the TL emission was observed to decrease by less than 3–4% of the ‘zero’ time TL emission. Subsequent TL decrease was observed to be slight, remaining practically constant at a level of about 95% of the initial TL emission. Fig. 8 shows the fading behaviour for the four different Ce concentrations.
For KMgF3 : Er the low temperature peak of the host material was also allowed to fade to the initial ‘zero’ time readout. The behaviour of the TL signal as a function of storage time has been found to be quite similar to that obtained for the three Er concentrations, the TL signal decreasing slowly during the first 100 h, with loss of approximately 2% of the zero time TL emission. For further storage TL emission remains essentially constant and the maximum TL loss is about 5% of the initial emission intensity. Fig. 9 shows the fading behaviour for the different phosphor concentrations. 3.7. Visible light exposure To investigate the effect of visible light on the material, a number of samples were annealed. Several of these were irradiated to a b-dose of 0.125 Gy and then all of the samples were exposed to direct sunlight. After
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Fig. 7. TL-dose response of KMgF3 : Er for various Er concentrations.
10 h of light exposure, all of the samples were read out. No sensitivity to light was observed for either the b irradiated or annealed only samples.
4. Mean lifetime Any dosimetric characterisation of new thermoluminescent materials should include an assessment of the kinetic parameters, activation energy, E, and frequency factor, s, both of which characterise the trapping levels. Indeed, evaluation of the value of E and s are needed in order to provide an indication of the stability of the glow peak, i.e. the dosimetric peak, at environmental temperatures. The kinetic parameters have been determined using the peak shape method of Chen (1969a, b), based on the geometrical characteristics of an isolated peak. The averaged values determined in this way for all of the present materials are E ¼ 1:10 eV and s ¼ 8 1010 s@1. As mentioned above, these parameters can be used to
obtain an indication of the stability of the corresponding TL peak at a given temperature, i.e. for determining the mean lifetime of the trapped charges in the considered trap. The mean lifetime for a first order kinetics process, at temperature T, i.e. T ¼ 300 K, can be expressed as: E t ¼ s@1 exp ; ð1Þ kT where s is the frequency factor (s@1), E is the activation energy of the trapping level (eV) and k is Boltzman’s constant. In respect of experimentally evaluated fading data, it is possible to calculate the fading factor, l (d@1), again at T ¼ 300 K, as follows: 1 F l ¼ @ ln ; ð2Þ t F0 where t is elapsed time, F the TL emission at time t and F0 the TL emission at time zero. For the evaluated kinetics parameters and experimental data on fading, one obtains the value t ¼ 430 d, indicating a TL loss of
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Fig. 8. Fading of KMgF3 : Ce. Data obtained at room temperature and under dark-room conditions.
about 0.2% per day and l ¼ 0:0024 d@1, the latter corresponding to the same TL loss per day as determined by the kinetics parameters.
5. Discussion and conclusions The main property of KMgF3 is its very high sensitivity, particularly when doped by Ce activator. For the highest concentration of Ce used in this work, i.e. 1.5 mol%, the observed sensitivity is more than 90 times that for LiF : Mg, Ti and, in turn, about four time more sensitive than LiF : Mg, Cu, P which to-date has been considered to be one of the most sensitive TL phosphors available. The high sensitivity of KMgF3 : Ce makes it very useful for medical and environmental radiation dosimetry. The presence of 40K in any sample of KMgF3 produces a b self-irradiation as recently reported (Bos, 2000). A possible contribution of self-dose to the external dose can be accurately estimated in specific cases: e.g. during short monitoring periods in environmental dosimetry. The self-dose irradiation should not be a serious problem in other dosimetric applications
where high doses have to be monitored as in the case of clinical dosimetry. Indeed a quite linear range up to 100 Gy is very good for radiological examinations as well as for therapeutic treatments. Because of the high atomic number of the material, it is suggested that accurate calibration, in an appropriate beam of well known energy, may eliminate the possibility of overestimating the delivered dose. The good stability of the dosimetric information during the first 100 h of storage allows the use of this material in clinical environments, where time can be taken between irradiation and the readout. Another important characteristic of KMgF3 is its very simple glow curve, compared to other commercial thermoluminescent dosimeters. As an example LiF : Mg, Ti has a very complex glow curve structure which does not easily allow deconvolution. The very simple glow curve structure of KMgF3 allows use of simple settings of the TL reader, rendering it user friendly for applications say in a radiological department. Furthermore, the annealing procedure only needs high temperatures, 5501C for Ce doped material and 4501C for Er activated material, over a period of 1 hour, uncomplicated by preand post-irradiation annealing procedures.
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Fig. 9. Fading of the TL signal in KMgF3 : Er (room temperature and dark-room conditions).
Table 1 shows increase in sensitivity of Ce doped KMgF3 with increase in dopant concentration. Kitis et al. (1996, 1999) reported a TL sensitivity of more than 150, relative to that for LiF : Mg, Ti. This value is higher than the highest value reported in the present work. A possible reason for this discrepancy could be the presence of Ce3+ and Ce4+ ions as observed by Francini et al. (1997) and Martini et al. (1998). Ce3+ ions will be in competition with the Ce4+ ions and TL sensitivity depends on the ratio Ce3+/Ce4+ in the sense that TL increases with increase in the number of Ce3+. Conversely, TL decreases as the number of Ce4+ ions increase. Since the ratio depends on the crystal growing conditions and no special effort has been made to control Ce3+ and Ce4+ ions concentration, it is expected that the present batch of KMgF3 : Ce contains more Ce4+ ions than in the batch studied by Kitis et al. (1996, 1999). In addition, material doped by Er shows an increase in sensitivity as the dopant concentration increases. Thus said, its sensitivity remains lower than that for Ce doped KMgF3. Finally, from a practical point of view, it can be said that both of the materials, KMgF3 : Ce and KMgF3 : Er, can be considered promising dosemeters.
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