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The thermoluminescent (TL) kinetics parameters of the perovskite-like KMgF3 activated by lutetium impurities
NIM B Beam Interactions with Materials & Atoms
Nuclear Instruments and Methods in Physics Research B 243 (2006) 349–353 www.elsevier.com/locate/nimb
The thermoluminescent (TL) kinetics parameters of the perovskite-like KMgF3 activated by lutetium impurities P.R. Gonzalez a
a,*
, C. Furetta b, E. Cruz Zaragoza
b
Departamento de Fı´sica, ININ, Instituto Nacional de Investigaciones Nucleares, Ap. Postal 18-1027, 11801 Mexico, DF, Mexico b Instituto de Ciencias Nucleares, UNAM, Ap. Postal 70-543, Me´xico, DF, Mexico Received 9 June 2005; received in revised form 11 August 2005 Available online 14 October 2005
Abstract The aim of this paper is to conclude a long experimental work carried out over many years on the thermoluminescence (TL) characterization of a complex fluoride, i.e. the perovskite-like KMgF3 activated by different dopants. In the present case, KMgF3 was doped by Lu impurities in three different concentrations. The kinetics parameters, i.e. the activation energy, E, the frequency factor, s and the kinetics order were determined using the computerized glow curve deconvolution (CGCD). Because the presence of low-temperature peaks in the TL glow curve, a post-irradiation annealing was used to erase these peaks. After that, a new deconvolution was carried out and also the initial rise method was used for determining the activation energy of the main peak. 2005 Elsevier B.V. All rights reserved. PACS: 87.53.Dq; 87.58.Sp; 87.66.Jj; 87.66.Pm; 87.66.Sq Keywords: Photon dosimetry measurements; Dosimetry; Ionization dosimetry; Solid state detectors; Thermoluminescence
1. Introduction Thermoluminescence (TL) dosimetry has been developed to the stage that it represents a key technique in absorbed dose determination. TL dosimetry has found a very important use in clinical, personal and environmental monitoring of ionizing radiation. Interest in radiation dosimetry by the TL technique has resulted in numerous efforts seeking production of new, high-performance TL materials. Of the many materials that have been produced and studied, several are now commonly used as thermoluminescent dosimeters (TLD). KMgF3 has been grown and extensively studied since 1990 in the Physics Department of Rome University ‘‘La Sapienza’’ and the reported thermoluminescence characteristics have shown this phosphor to be a very good candidate
*
Corresponding author. Tel.: +52 55 5329 7200; fax: +52 55 5329 7332. E-mail address: [email protected] (P.R. Gonzalez).
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.08.168
for ionizing radiation dosimetry [1–14]. 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 typical cubic symmetry. When the pure material is doped with impurities, monovalent cations replace K+ ions and anions, i.e. OH, substitute for F ions. If divalent or trivalent cations are used as dopants, they can replace Mg2+ or they are forced to occupy K+ ion locations. The interest in this particular material is caused by its very low hygroscopicity, which is of great importance to long-term use in different environmental conditions, its relatively high melting point, 1343 K, which allows high-temperature annealing procedures. Furthermore, its effective atomic number Zeff is 13 which is an intermediate value between tissue equivalent and high atomic number phosphors. The phosphors produced along the years were KMgF3 doped with Pb and Eu (1990), KMgF3:Tl (1994), KMgF3 doped with Pb, Cr or Ag (1996) and Ce (1997/1999) and finally, in 2001/2002, doped
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P.R. Gonzalez et al. / Nucl. Instr. and Meth. in Phys. Res. B 243 (2006) 349–353
with Er, La and Lu. For this work, three different dopant concentrations of Lu were used: 0.17, 0.34 and 0.66 mol% of LuF3, respectively. The aim of this paper is to present the TL kinetics parameters of the perovskite-like compound KMgF3 doped by Lu impurities at various concentrations. 2. Methods 2.1. Phosphor preparation
Several samples of the material were irradiated and the TL signal was recorded using a TLD reader Harshaw, Mod.4000. The resulting glow curves were stored in an online computer and then analyzed by the SQPGCD program. The goodness of the fitting was checked by the Figure Of Merit (FOM) [17] jf X 100jy j yðxj Þj ; ð4Þ FOM ¼ A j i
Samples of perovskite KMgF3 have been obtained from the melt using the Kyropoulos technique. The molten mass was formed by heating at 1335 K, in a platinum crucible and under nitrogen atmosphere, the starting powder consisting of a finely ground stoichiometric mixture (mole ratio 1:1) of pure and dried KF and MgF2. The growth of crystals was obtained with the pulling method from the above melt, starting the process with an air-cooled platinum finger or with the aid of a crystal seed. In the former case, polycrystalline ingots were obtained, in the latter single crystals were grown with typical dimensions of 2–3 cm in diameter and 2–3 cm in length. All samples were optically transparent. Doped crystals were obtained with the same technique by adding a proper amount of the desired impurity to the melt. Several samples having dimensions of 3 · 3 · 1 mm where obtained from the ingot and used for the experiments described here. 2.2. Glow curve deconvolution The kinetic parameters, i.e. the activation energy E of the traps involved in TL emission, the order of the kinetics b and, finally, the frequency factor s and the pre-exponential factor s* have been obtained using the sequential quadratic programming glow curve deconvolution (SQPGCD) developed at the Instituto Nacional de Investigaciones Nucleares [15]. The algorithm of the deconvolution program is based on the first, second and general order kinetics equations, respectively [16], i.e. The Randall–Wilkins equation for first order Z E s T E IðT Þ ¼ n0 s exp exp 0 dT 0 exp kT b T0 kT ð1Þ the Garlick–Gibson equation for the second order n0 s exp kTE IðT Þ ¼ h i2 RT 1 þ bs T 0 exp kTE 0 dT 0
ð2Þ
and the May–Partridge general order equation E IðT Þ ¼ s00 n0 exp kT b=ðb1Þ Z s00 ðb 1Þ T E exp 0 dT 0 ; exp 1 þ b kT T0
where b is the kinetics order, being 1 < b 6 2.
ð3Þ
where ji and jf are the first and the last channels, respectively, of the region of interest, yj and y(xj) are, respectively, the TL emission and the corresponding function in the channel j, A is the integral of the TL curve in the whole region of interest. A FOM equal or less than 5% means a very good fitting. 2.3. Initial rise method [16] In the low-temperature tail of a peak, the amount of trapped electrons can be assumed as a constant, the dependence on temperature being negligible. In fact, with increasing temperature up to TC 6 TM (the corresponding intensity IC should not be larger than 15% of IM for the full TL glow-peak under evaluation) the first exponential of Eq. (1) increases whereas the second term may still be unity. A further increase in temperature (T > TC) makes the second term decrease: the competition of both terms results in the maximum of I. In this assumption, as long as the second term is unity, the thermoluminescent emission can be described by E IðT Þ / exp . ð5Þ kT The ln(I) versus 1/T plot is then made and a straight line should be obtained. From the slope,E/k, E is evaluated without any knowledge of the frequency factor s as well as of the kinetics order. 2.4. Frequency and pre-exponential factors The frequency and the pre-exponential factors are given by the following expressions [16]. The first expression is for a first order kinetics and the second is used in the general case. Both are expressed in s1. bE E ; ð6Þ s ¼ 2 exp kT M kT M 2 31 kT 2M exp kTEM 2kT ðb 1Þ M 5 . 1þ ð7Þ s¼4 E bE The last expression can be also written as 2 31 kT 2M exp kTEM 1 2kT ðb 1Þ M 5 ; s ¼ b1 4 1þ E bE n0 which is expressed now in cm3(b1)s1.
ð8Þ
P.R. Gonzalez et al. / Nucl. Instr. and Meth. in Phys. Res. B 243 (2006) 349–353
351
rise method were then applied to the glow curves obtained after the post-irradiation annealing.
3. Experimental procedure 3.1. Glow curve shape
3.2. Peak parameters
TL Intensity (a.u.)
The irradiation of the samples was done using two levels of gamma dose: 0.1 and 10 Gy for checking any variation of the kinetics parameters as a function of the delivered dose. Before and after the irradiations, the samples were stored in dark condition and at room temperature. The glow curve shape of KMgF3:Lu, as obtained after irradiation, is shown in Fig. 1. The TL signals were recorded using the Harshaw TL reader model 4000, with a heating rate of 2 K/s under continuous nitrogen flux to reduce spurious TL signals. This system reader has an infrared–near visible optical filter between the sample and the phototube to obtain a better signal light measurement. The TL signal was integrated from 313 to 573 K and the deconvolution applied to the whole glow curve. Because in the low-temperature region, one or two peaks are visible, in a second experiment a post-irradiation annealing was carried out at a temperature of 403 K for 10 min for erasing those peaks. This procedure was necessary for the application of the initial rise method to the main peak. The results of the thermal procedure is shown in Fig. 2 where the main peak appears now without any satellites in the low-temperature region. A second deconvolution as well as the initial
4.5E+05 4.0E+05 3.5E+05 3.0E+05 2.5E+05 2.0E+05 1.5E+05 1.0E+05 5.0E+04 0.0E+00 200
300
400 Temperature (K)
500
600
The perovskite samples have been irradiated at two different gamma doses, i.e. 0.1 and 10 Gy, respectively, for testing any change in the kinetic parameters as a function of the delivered dose. After irradiation, as a second experiment, the post-irradiation annealing procedure has been used, as explained so far. 4. Results and discussion At first, for checking the effect of the dopant concentration on the TL emission, the TL sensitivity (TL signal/dose · mass) as a function of the Lu concentration was studied. Fig. 3 shows the results: the sensitivity increases as the Lu concentration increases. Fig. 1 shows the deconvolution for the whole glow curve of provskite as obtained after irradiation. Three peaks were usually used for obtaining the best fitting. Indeed, the FOM was always better than 5%. Table 1 lists the kinetics parameters for each concentration as well as for the two irradiation doses. Fig. 2 shows the deconvolution applied to the glow curve as obtained after the post-irradiation annealing procedure and Table 2 shows the results obtained. Also in this case, the use of three peaks was necessary, although the first and the third peaks are just used for obtaining the best FOM and then they cannot be considered as peaks derived by real trapping levels. Comparing the values of the peak temperature at the maximum obtained before and after the post-irradiation annealing procedure, it can be noted a shift of the temperature towards high values after the thermal treatment. This shift also influences the values of E as well as the kinetic order. These results are inconsistent with the assumption of a single trapping level. However, this behavior may be qualitatively explained considering a complex structure of
Fig. 1. Glow curve of KMgF3:Lu (0.34 mol%) as measured after irradiation. In the same figure the deconvolution is also given. 0.3 0.25 0.2
2.0E+06
Sensitivity
TL Intensity (a.u.)
2.5E+06
1.5E+06 1.0E+06 5.0E+05 0.0E+00 200
0.15 0.1 0.05
300
400 500 Temperature (K)
600
Fig. 2. Glow curve of KMgF3:Lu (0.34 mol%) as measured after the postirradiation treatment. In the same figure the deconvolution is also given.
0 0
0.2 0.4 0.6 Concentration Lu (mol %)
Fig. 3. TL sensitivity versus Lu concentration.
0.8
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P.R. Gonzalez et al. / Nucl. Instr. and Meth. in Phys. Res. B 243 (2006) 349–353
Table 1 Kinetics parameters as obtained by deconvolution of the glow curves after irradiation Lu (mol%)
Peak no.a
TM (K)
IM (a.u.)
b
E (eV)
s (s1)
n0 (cm3)
s* (cm3(b1) s1)
Irradiation dose: 0.1 Gy 0.17 1 2 3 0.34 1 2 3 0.66 1 2
385 467 503 365 430 492 354 456
1248 18,525 2060 3546 14,670 355 320 7641
1.05 1.08 2.00 2.00 2.00 2.00 1.54 1.32
0.75 0.95 1.39 0.88 1.08 1.94 0.84 1.04
7.51E+08 1.68E+09 0.41E+12 2.00E+11 5.68E+11 1.21E+19 1.40E+11 3.43E+10
2.71E+04 4.77E+05 6.10E+04 8.64E+04 4.06E+05 7.33E+03 6.56E+03 1.92E+05
4.51E+08 6.10E+08 1.54E+08 2.32E+06 1.40E+06 1.65E+15 1.21E+15 7.15E+08
Irradiation dose: 10 Gy 0.17 1 2 3 0.34 1 2 3 4 0.66 1 2 3
382 461 508 339 372 424 473 363 447 513
223,390 1,866,910 173,433 31,445 68,711 378,232 17,345 25,275 414,566 5874
1.76 1.37 2.00 1.05 2.00 1.76 2.00 2.00 1.44 2.00
0.86 1.06 2.40 0.82 1.02 1.22 1.42 0.65 1.04 2.24
2.69E+10 4.19E+10 1.49E+23 2.17E+11 8.92E+12 4.24E+13 1.67E+14 1.20E+08 6.73E+10 1.81E+21
5.61E+06 4.81E+07 3.10E+06 4.96E+05 1.52E+06 8.42E+06 4.47E+05 8.03E+05 1.05E+07 1.15E+05
2.15E+05 5.80E+07 4.81E+16 1.13E+11 5.86E+06 2.27E+08 3.75E+08 1.49E+02 5.42E+07 1.58E+16
a
The numbers in bold correspond to the main peak.
Table 2 Kinetics parameters as obtained by deconvolution of the glow curves received after the post-irradiation annealing at 403 K for 10 min Peak temperature (K)
0.17 0.34 0.66
470 458 467
0.17 0.34 0.66
457 462 463
b
E (eV)
s (s1)
n0 (cm3)
s* (cm3(b1) s1)
18,014 3808 1880
1.83 2.00 1.86
1.52 1.36 1.48
3.29E+15 1.27E+14 1.33E+15
4.04E+05 9.57E+04 4.04E+04
7.73E+10 1.32E+09 1.42E+11
73,836 1,882,700 391,698
2.00 1.97 1.79
1.50 1.48 1.49
6.02E+15 1.86E+15 2.26E+15
1.68E+06 4.42E+07 8.64E+06
3.58E+09 7.07E+07 7.74E+09
IM (a.u.)
the trapping level, i.e. a continuous distribution of traps. As the samples are immediately readout after irradiation, all the trapping levels are emptied. When the irradiated samples are annealed before reading, i.e. post-irradiation annealing at 403 K for 10 min, this thermal procedure empties the shallow trapping levels and a subsequent readout empties all the remaining deep traps. The activation energy values should correspond, according to the model so far suggested, to an average value over the values of each single level. In this sense, we can indicate with E1 the average activation energy value measured immediately after the irradiation; with E2 the average activation energy evaluated after the post-irradiation annealing. According to the proposed model, we have E1 < E2, in agreement to the experimental results. The same considerations can be applied to the temperature at the maximum, TM, which also increases after the post-irradiation annealing procedure. Furthermore, the kinetics order b also increases. The present experimental results are in perfect agreement to the theoretical observations of Hornyak and Chen [18]. In their study, they initially assumed a first-order kinetics and a continuous distribution of trapping levels
uniformly distributed over a finite energy range DE = E2 E1; the resulting glow-peak shape depends strongly on the value of DE. As DE increases, both the values of E and kinetics order increase too. Fig. 4 shows, as an example, the initial rise plot: the ln(I) is given as a function of 1000/T. From the slope the values of the activation energy are obtained for each concentra-
0.17 mol % 0.1 Gy 8.5 8 ln(I)
Lu (mol%)
7.5 7 y = -14.392x + 41.439 R2 = 0.9997
6.5 6 2.3
2.35
2.4
2.45
1000/T
Fig. 4. Example of the plot of ln(I) as a function of 1000/T for determining the activation energy, E, of the main peak using the initial rise method. The plot concerns to the perovskite sample having a Lu concentration equal to 0.17 mol% irradiated at 0.1 Gy.
P.R. Gonzalez et al. / Nucl. Instr. and Meth. in Phys. Res. B 243 (2006) 349–353 Table 3 Activation energy of the main peak determined by the initial rise (IR) method after the post-irradiation annealing procedure Lu (mol%)
Peak temperature, TM (K)
E (eV) 1.17 1.22 1.25
Irradiation dose: 10 Gy 0.17 457 0.34 462 0.66 463
1.22 1.24 1.26
Activation Energy (eV)
Irradiation dose: 0.1 Gy 0.17 470 0.34 458 0.66 467
353
plex trap structure for the main peak: i.e. a continuous trap distribution. It is evident that the post-annealing procedure only empties the shallow traps of the distribution, so that the evaluated activation energy results to be bigger than the value evaluated just after irradiation. According to this hypothesis, it seems more physically corrected to speak about average values of both activation energy and temperature at the maximum. Acknowledgement The authors acknowledge L. Martı´nez (Fuente de Gammas, Instituto Nacional de Investigaciones Nucleares) her technical assistance in irradiating the KMgF3:Lu. References
1.27 1.26 1.25 1.24 1.23 1.22 1.21 1.2 1.19 1.18 1.17 1.16
0.1 Gy
0
10
20 30 40 50 Concentration (mol %)
10 Gy
60
70
Fig. 5. Behavior of the activation energy, determined by the initial rise method, as a function of the Lu concentration.
tion as well as for each level of dose. Table 3 reports the data obtained using this method. The values of the activation energy are in this case smaller than the values obtained by deconvolution. Indeed, an eventual limitation of the initial rise method is given by the risk to underestimate the actual E value. This might be caused by non-radiative events, which could lead to a computation of an apparent energy, differing from the real one by an amount W connected to the characteristic nonradiative contribution depth [19]. Fig. 5 shows the behavior of the activation energy, determined by the initial rise method, as a function of the Lu concentration and of the given dose. It is clear from this figure that the Lu concentration is strongly related to the position of the traps in the band gap: more higher is the Lu concentration, more are the numbers of the trapping levels and more deeper they are located in the band gap between the conduction and the valence bands: as a consequence, more stable during time should be the stored TL information. 5. Conclusions The experimental results concerning the kinetics parameters of the perovskite-like KMgF3 seem to suggest a com-
[1] C. Furetta, C. Bacci, B. Rispoli, C. Sanipoli, A. Scacco, Radiat. Prot. Dosim. 33 (1/4) (1990) 107. [2] C. Bacci, C. Furetta, G. Ramogida, C. Sanipoli, A. Scacco, Physica, Medica IX (Suppl. 1) (1993) 207. [3] C. Furetta, A. Scacco, L.M. Rabkin, V.N. Rudkovsky, Stimolazione termica di luminescenza ed emissione elettronica in cristalli di KMgF3 contenenti impurezze. XXVIII Congresso Nazionale IRPA, Taormina, Italy, 1993. [4] C. Bacci, S. Fioravanti, C. Furetta, M. Missori, G. Ramogida, R. Rossetti, C. Sanipoli, A. Scacco, Radiat. Prot. Dosim. 47 (1993) 277. [5] A. Scacco, C. Furetta, C. Bacci, G. Ramogida, C. Sanipoli, Nucl. Instr. and Meth. B 91 (1994) 223. [6] C. Furetta, G. Ramogida, A. Scacco, M. Martini, S. Paravisi, J. Phys. Chem. Solids 55 (11) (1994) 1337. [7] G. Kitis, C. Furetta, C. Sanipoli, A. Scacco, Radiat. Prot. Dosim. 65 (1/4) (1996) 93. [8] G. Kitis, C. Furetta, C. Sanipoli, A. Scacco, Radiat. Prot. Dosim. 82 (2) (1999) 151. [9] C. Furetta, F. Santopietro, C. Sanipoli, G. Kitis, Appl. Rad. Isot. 55 (2001) 857. [10] C. Furetta, C. Sanipoli, G. Kitis, J. Phys. D: Appl. Phys. 34 (2001) 857. [11] N.J.M. Le Masson, J.J. Bos, C.W.E. Van Eijk, C. Furetta, J.P. Chaminade, Radiat. Prot. Dosim. 100 (2002) 229. [12] C. Furetta, J. Azorin, F. Sepulveda, T. Rivera, P.R. Gonzalez, J. Mater. Sci. Lett. 21 (2002) 1727. [13] P.R. Gonzalez, C. Furetta, J. Azorin, T. Rivera, G. Kitis, F. Sepulveda, C. Sanipoli, J. Mater. Sci. 39 (2004) 1601. [14] F. Sepulveda, J. Azorin, T. Rivera, C. Furetta, C. Sanipoli, Nucl. Instr. and Meth. B 213 (2004) 329. [15] J. Lo´pez, Reporte Te´cnico-Cientı´fico ININ, No. DSICGI-10-94, 1994. [16] C. Furetta, Handbook of Thermoluminescence, World Scientific, Singapore, 2003. [17] Y.S. Horowitz, Radiat. Prot. Dosim. 60 (1) (1995). [18] W.F. Hornyak, R. Chen, J. Lumin. 44 (1–2) (1989) 73. [19] C. Furetta, P.S. Weng, Operational Thermoluminescence Dosimetry, World Scientific, Singapore, 1998.
Nuclear Instruments and Methods in Physics Research B 243 (2006) 349–353 www.elsevier.com/locate/nimb
The thermoluminescent (TL) kinetics parameters of the perovskite-like KMgF3 activated by lutetium impurities P.R. Gonzalez a
a,*
, C. Furetta b, E. Cruz Zaragoza
b
Departamento de Fı´sica, ININ, Instituto Nacional de Investigaciones Nucleares, Ap. Postal 18-1027, 11801 Mexico, DF, Mexico b Instituto de Ciencias Nucleares, UNAM, Ap. Postal 70-543, Me´xico, DF, Mexico Received 9 June 2005; received in revised form 11 August 2005 Available online 14 October 2005
Abstract The aim of this paper is to conclude a long experimental work carried out over many years on the thermoluminescence (TL) characterization of a complex fluoride, i.e. the perovskite-like KMgF3 activated by different dopants. In the present case, KMgF3 was doped by Lu impurities in three different concentrations. The kinetics parameters, i.e. the activation energy, E, the frequency factor, s and the kinetics order were determined using the computerized glow curve deconvolution (CGCD). Because the presence of low-temperature peaks in the TL glow curve, a post-irradiation annealing was used to erase these peaks. After that, a new deconvolution was carried out and also the initial rise method was used for determining the activation energy of the main peak. 2005 Elsevier B.V. All rights reserved. PACS: 87.53.Dq; 87.58.Sp; 87.66.Jj; 87.66.Pm; 87.66.Sq Keywords: Photon dosimetry measurements; Dosimetry; Ionization dosimetry; Solid state detectors; Thermoluminescence
1. Introduction Thermoluminescence (TL) dosimetry has been developed to the stage that it represents a key technique in absorbed dose determination. TL dosimetry has found a very important use in clinical, personal and environmental monitoring of ionizing radiation. Interest in radiation dosimetry by the TL technique has resulted in numerous efforts seeking production of new, high-performance TL materials. Of the many materials that have been produced and studied, several are now commonly used as thermoluminescent dosimeters (TLD). KMgF3 has been grown and extensively studied since 1990 in the Physics Department of Rome University ‘‘La Sapienza’’ and the reported thermoluminescence characteristics have shown this phosphor to be a very good candidate
*
Corresponding author. Tel.: +52 55 5329 7200; fax: +52 55 5329 7332. E-mail address: [email protected] (P.R. Gonzalez).
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.08.168
for ionizing radiation dosimetry [1–14]. 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 typical cubic symmetry. When the pure material is doped with impurities, monovalent cations replace K+ ions and anions, i.e. OH, substitute for F ions. If divalent or trivalent cations are used as dopants, they can replace Mg2+ or they are forced to occupy K+ ion locations. The interest in this particular material is caused by its very low hygroscopicity, which is of great importance to long-term use in different environmental conditions, its relatively high melting point, 1343 K, which allows high-temperature annealing procedures. Furthermore, its effective atomic number Zeff is 13 which is an intermediate value between tissue equivalent and high atomic number phosphors. The phosphors produced along the years were KMgF3 doped with Pb and Eu (1990), KMgF3:Tl (1994), KMgF3 doped with Pb, Cr or Ag (1996) and Ce (1997/1999) and finally, in 2001/2002, doped
350
P.R. Gonzalez et al. / Nucl. Instr. and Meth. in Phys. Res. B 243 (2006) 349–353
with Er, La and Lu. For this work, three different dopant concentrations of Lu were used: 0.17, 0.34 and 0.66 mol% of LuF3, respectively. The aim of this paper is to present the TL kinetics parameters of the perovskite-like compound KMgF3 doped by Lu impurities at various concentrations. 2. Methods 2.1. Phosphor preparation
Several samples of the material were irradiated and the TL signal was recorded using a TLD reader Harshaw, Mod.4000. The resulting glow curves were stored in an online computer and then analyzed by the SQPGCD program. The goodness of the fitting was checked by the Figure Of Merit (FOM) [17] jf X 100jy j yðxj Þj ; ð4Þ FOM ¼ A j i
Samples of perovskite KMgF3 have been obtained from the melt using the Kyropoulos technique. The molten mass was formed by heating at 1335 K, in a platinum crucible and under nitrogen atmosphere, the starting powder consisting of a finely ground stoichiometric mixture (mole ratio 1:1) of pure and dried KF and MgF2. The growth of crystals was obtained with the pulling method from the above melt, starting the process with an air-cooled platinum finger or with the aid of a crystal seed. In the former case, polycrystalline ingots were obtained, in the latter single crystals were grown with typical dimensions of 2–3 cm in diameter and 2–3 cm in length. All samples were optically transparent. Doped crystals were obtained with the same technique by adding a proper amount of the desired impurity to the melt. Several samples having dimensions of 3 · 3 · 1 mm where obtained from the ingot and used for the experiments described here. 2.2. Glow curve deconvolution The kinetic parameters, i.e. the activation energy E of the traps involved in TL emission, the order of the kinetics b and, finally, the frequency factor s and the pre-exponential factor s* have been obtained using the sequential quadratic programming glow curve deconvolution (SQPGCD) developed at the Instituto Nacional de Investigaciones Nucleares [15]. The algorithm of the deconvolution program is based on the first, second and general order kinetics equations, respectively [16], i.e. The Randall–Wilkins equation for first order Z E s T E IðT Þ ¼ n0 s exp exp 0 dT 0 exp kT b T0 kT ð1Þ the Garlick–Gibson equation for the second order n0 s exp kTE IðT Þ ¼ h i2 RT 1 þ bs T 0 exp kTE 0 dT 0
ð2Þ
and the May–Partridge general order equation E IðT Þ ¼ s00 n0 exp kT b=ðb1Þ Z s00 ðb 1Þ T E exp 0 dT 0 ; exp 1 þ b kT T0
where b is the kinetics order, being 1 < b 6 2.
ð3Þ
where ji and jf are the first and the last channels, respectively, of the region of interest, yj and y(xj) are, respectively, the TL emission and the corresponding function in the channel j, A is the integral of the TL curve in the whole region of interest. A FOM equal or less than 5% means a very good fitting. 2.3. Initial rise method [16] In the low-temperature tail of a peak, the amount of trapped electrons can be assumed as a constant, the dependence on temperature being negligible. In fact, with increasing temperature up to TC 6 TM (the corresponding intensity IC should not be larger than 15% of IM for the full TL glow-peak under evaluation) the first exponential of Eq. (1) increases whereas the second term may still be unity. A further increase in temperature (T > TC) makes the second term decrease: the competition of both terms results in the maximum of I. In this assumption, as long as the second term is unity, the thermoluminescent emission can be described by E IðT Þ / exp . ð5Þ kT The ln(I) versus 1/T plot is then made and a straight line should be obtained. From the slope,E/k, E is evaluated without any knowledge of the frequency factor s as well as of the kinetics order. 2.4. Frequency and pre-exponential factors The frequency and the pre-exponential factors are given by the following expressions [16]. The first expression is for a first order kinetics and the second is used in the general case. Both are expressed in s1. bE E ; ð6Þ s ¼ 2 exp kT M kT M 2 31 kT 2M exp kTEM 2kT ðb 1Þ M 5 . 1þ ð7Þ s¼4 E bE The last expression can be also written as 2 31 kT 2M exp kTEM 1 2kT ðb 1Þ M 5 ; s ¼ b1 4 1þ E bE n0 which is expressed now in cm3(b1)s1.
ð8Þ
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rise method were then applied to the glow curves obtained after the post-irradiation annealing.
3. Experimental procedure 3.1. Glow curve shape
3.2. Peak parameters
TL Intensity (a.u.)
The irradiation of the samples was done using two levels of gamma dose: 0.1 and 10 Gy for checking any variation of the kinetics parameters as a function of the delivered dose. Before and after the irradiations, the samples were stored in dark condition and at room temperature. The glow curve shape of KMgF3:Lu, as obtained after irradiation, is shown in Fig. 1. The TL signals were recorded using the Harshaw TL reader model 4000, with a heating rate of 2 K/s under continuous nitrogen flux to reduce spurious TL signals. This system reader has an infrared–near visible optical filter between the sample and the phototube to obtain a better signal light measurement. The TL signal was integrated from 313 to 573 K and the deconvolution applied to the whole glow curve. Because in the low-temperature region, one or two peaks are visible, in a second experiment a post-irradiation annealing was carried out at a temperature of 403 K for 10 min for erasing those peaks. This procedure was necessary for the application of the initial rise method to the main peak. The results of the thermal procedure is shown in Fig. 2 where the main peak appears now without any satellites in the low-temperature region. A second deconvolution as well as the initial
4.5E+05 4.0E+05 3.5E+05 3.0E+05 2.5E+05 2.0E+05 1.5E+05 1.0E+05 5.0E+04 0.0E+00 200
300
400 Temperature (K)
500
600
The perovskite samples have been irradiated at two different gamma doses, i.e. 0.1 and 10 Gy, respectively, for testing any change in the kinetic parameters as a function of the delivered dose. After irradiation, as a second experiment, the post-irradiation annealing procedure has been used, as explained so far. 4. Results and discussion At first, for checking the effect of the dopant concentration on the TL emission, the TL sensitivity (TL signal/dose · mass) as a function of the Lu concentration was studied. Fig. 3 shows the results: the sensitivity increases as the Lu concentration increases. Fig. 1 shows the deconvolution for the whole glow curve of provskite as obtained after irradiation. Three peaks were usually used for obtaining the best fitting. Indeed, the FOM was always better than 5%. Table 1 lists the kinetics parameters for each concentration as well as for the two irradiation doses. Fig. 2 shows the deconvolution applied to the glow curve as obtained after the post-irradiation annealing procedure and Table 2 shows the results obtained. Also in this case, the use of three peaks was necessary, although the first and the third peaks are just used for obtaining the best FOM and then they cannot be considered as peaks derived by real trapping levels. Comparing the values of the peak temperature at the maximum obtained before and after the post-irradiation annealing procedure, it can be noted a shift of the temperature towards high values after the thermal treatment. This shift also influences the values of E as well as the kinetic order. These results are inconsistent with the assumption of a single trapping level. However, this behavior may be qualitatively explained considering a complex structure of
Fig. 1. Glow curve of KMgF3:Lu (0.34 mol%) as measured after irradiation. In the same figure the deconvolution is also given. 0.3 0.25 0.2
2.0E+06
Sensitivity
TL Intensity (a.u.)
2.5E+06
1.5E+06 1.0E+06 5.0E+05 0.0E+00 200
0.15 0.1 0.05
300
400 500 Temperature (K)
600
Fig. 2. Glow curve of KMgF3:Lu (0.34 mol%) as measured after the postirradiation treatment. In the same figure the deconvolution is also given.
0 0
0.2 0.4 0.6 Concentration Lu (mol %)
Fig. 3. TL sensitivity versus Lu concentration.
0.8
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P.R. Gonzalez et al. / Nucl. Instr. and Meth. in Phys. Res. B 243 (2006) 349–353
Table 1 Kinetics parameters as obtained by deconvolution of the glow curves after irradiation Lu (mol%)
Peak no.a
TM (K)
IM (a.u.)
b
E (eV)
s (s1)
n0 (cm3)
s* (cm3(b1) s1)
Irradiation dose: 0.1 Gy 0.17 1 2 3 0.34 1 2 3 0.66 1 2
385 467 503 365 430 492 354 456
1248 18,525 2060 3546 14,670 355 320 7641
1.05 1.08 2.00 2.00 2.00 2.00 1.54 1.32
0.75 0.95 1.39 0.88 1.08 1.94 0.84 1.04
7.51E+08 1.68E+09 0.41E+12 2.00E+11 5.68E+11 1.21E+19 1.40E+11 3.43E+10
2.71E+04 4.77E+05 6.10E+04 8.64E+04 4.06E+05 7.33E+03 6.56E+03 1.92E+05
4.51E+08 6.10E+08 1.54E+08 2.32E+06 1.40E+06 1.65E+15 1.21E+15 7.15E+08
Irradiation dose: 10 Gy 0.17 1 2 3 0.34 1 2 3 4 0.66 1 2 3
382 461 508 339 372 424 473 363 447 513
223,390 1,866,910 173,433 31,445 68,711 378,232 17,345 25,275 414,566 5874
1.76 1.37 2.00 1.05 2.00 1.76 2.00 2.00 1.44 2.00
0.86 1.06 2.40 0.82 1.02 1.22 1.42 0.65 1.04 2.24
2.69E+10 4.19E+10 1.49E+23 2.17E+11 8.92E+12 4.24E+13 1.67E+14 1.20E+08 6.73E+10 1.81E+21
5.61E+06 4.81E+07 3.10E+06 4.96E+05 1.52E+06 8.42E+06 4.47E+05 8.03E+05 1.05E+07 1.15E+05
2.15E+05 5.80E+07 4.81E+16 1.13E+11 5.86E+06 2.27E+08 3.75E+08 1.49E+02 5.42E+07 1.58E+16
a
The numbers in bold correspond to the main peak.
Table 2 Kinetics parameters as obtained by deconvolution of the glow curves received after the post-irradiation annealing at 403 K for 10 min Peak temperature (K)
0.17 0.34 0.66
470 458 467
0.17 0.34 0.66
457 462 463
b
E (eV)
s (s1)
n0 (cm3)
s* (cm3(b1) s1)
18,014 3808 1880
1.83 2.00 1.86
1.52 1.36 1.48
3.29E+15 1.27E+14 1.33E+15
4.04E+05 9.57E+04 4.04E+04
7.73E+10 1.32E+09 1.42E+11
73,836 1,882,700 391,698
2.00 1.97 1.79
1.50 1.48 1.49
6.02E+15 1.86E+15 2.26E+15
1.68E+06 4.42E+07 8.64E+06
3.58E+09 7.07E+07 7.74E+09
IM (a.u.)
the trapping level, i.e. a continuous distribution of traps. As the samples are immediately readout after irradiation, all the trapping levels are emptied. When the irradiated samples are annealed before reading, i.e. post-irradiation annealing at 403 K for 10 min, this thermal procedure empties the shallow trapping levels and a subsequent readout empties all the remaining deep traps. The activation energy values should correspond, according to the model so far suggested, to an average value over the values of each single level. In this sense, we can indicate with E1 the average activation energy value measured immediately after the irradiation; with E2 the average activation energy evaluated after the post-irradiation annealing. According to the proposed model, we have E1 < E2, in agreement to the experimental results. The same considerations can be applied to the temperature at the maximum, TM, which also increases after the post-irradiation annealing procedure. Furthermore, the kinetics order b also increases. The present experimental results are in perfect agreement to the theoretical observations of Hornyak and Chen [18]. In their study, they initially assumed a first-order kinetics and a continuous distribution of trapping levels
uniformly distributed over a finite energy range DE = E2 E1; the resulting glow-peak shape depends strongly on the value of DE. As DE increases, both the values of E and kinetics order increase too. Fig. 4 shows, as an example, the initial rise plot: the ln(I) is given as a function of 1000/T. From the slope the values of the activation energy are obtained for each concentra-
0.17 mol % 0.1 Gy 8.5 8 ln(I)
Lu (mol%)
7.5 7 y = -14.392x + 41.439 R2 = 0.9997
6.5 6 2.3
2.35
2.4
2.45
1000/T
Fig. 4. Example of the plot of ln(I) as a function of 1000/T for determining the activation energy, E, of the main peak using the initial rise method. The plot concerns to the perovskite sample having a Lu concentration equal to 0.17 mol% irradiated at 0.1 Gy.
P.R. Gonzalez et al. / Nucl. Instr. and Meth. in Phys. Res. B 243 (2006) 349–353 Table 3 Activation energy of the main peak determined by the initial rise (IR) method after the post-irradiation annealing procedure Lu (mol%)
Peak temperature, TM (K)
E (eV) 1.17 1.22 1.25
Irradiation dose: 10 Gy 0.17 457 0.34 462 0.66 463
1.22 1.24 1.26
Activation Energy (eV)
Irradiation dose: 0.1 Gy 0.17 470 0.34 458 0.66 467
353
plex trap structure for the main peak: i.e. a continuous trap distribution. It is evident that the post-annealing procedure only empties the shallow traps of the distribution, so that the evaluated activation energy results to be bigger than the value evaluated just after irradiation. According to this hypothesis, it seems more physically corrected to speak about average values of both activation energy and temperature at the maximum. Acknowledgement The authors acknowledge L. Martı´nez (Fuente de Gammas, Instituto Nacional de Investigaciones Nucleares) her technical assistance in irradiating the KMgF3:Lu. References
1.27 1.26 1.25 1.24 1.23 1.22 1.21 1.2 1.19 1.18 1.17 1.16
0.1 Gy
0
10
20 30 40 50 Concentration (mol %)
10 Gy
60
70
Fig. 5. Behavior of the activation energy, determined by the initial rise method, as a function of the Lu concentration.
tion as well as for each level of dose. Table 3 reports the data obtained using this method. The values of the activation energy are in this case smaller than the values obtained by deconvolution. Indeed, an eventual limitation of the initial rise method is given by the risk to underestimate the actual E value. This might be caused by non-radiative events, which could lead to a computation of an apparent energy, differing from the real one by an amount W connected to the characteristic nonradiative contribution depth [19]. Fig. 5 shows the behavior of the activation energy, determined by the initial rise method, as a function of the Lu concentration and of the given dose. It is clear from this figure that the Lu concentration is strongly related to the position of the traps in the band gap: more higher is the Lu concentration, more are the numbers of the trapping levels and more deeper they are located in the band gap between the conduction and the valence bands: as a consequence, more stable during time should be the stored TL information. 5. Conclusions The experimental results concerning the kinetics parameters of the perovskite-like KMgF3 seem to suggest a com-
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