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Thermoluminescence (TL) characteristics of perovskite-like KMgF3 activated by La ions
Nuclear Instruments and Methods in Physics Research B 213 (2004) 329–332 www.elsevier.com/locate/nimb
Thermoluminescence (TL) characteristics of perovskite-like KMgF3 activated by La ions F. Sepulveda a, J. Azorin b, T. Rivera a b
b,*
, C. Furetta b, C. Sanipoli
c
Escuela Superior de Fisica y Matem aticas, Instituto Politecnico Nacional, 07738 Mexico D.F, Mexico Departamento de Fisica, Universidad Autonoma Metropolitana-Iztapalapa, 09340 Mexico D.F, Mexico c Dip. Di Fisica, Universita’ di Roma, La Sapienza, Piazzale A.Moro 2, 00185 Roma, Italy
Abstract This paper reports the experimental results concerning the thermoluminescent characteristics of a new preparation of the fluoroperovskite KMgF3 :La + PTFE.The main thermoluminescent properties investigated were: the thermoluminescence (TL) response as a function of the absorbed dose, the precision obtainable in the dose measurement, the reproducibility of the TL readings and the threshold dose. The results obtained were then compared to the requirements of the ANSI protocol for thermoluminescent environmental dosimetry, resulting in a very good agreement with the required performances. Ó 2003 Elsevier B.V. All rights reserved. PACS: 78.45+h; 78.60 kn; 29.40.)n
1. Introduction Ternary compounds belonging to the group of the fluoroperovskites have the general form AB where A and B stand for an alkali metal and an alkaline earth metal, respectively. Such compounds recently received full attention in view of their possible use as thermoluminescent dosimeter (TLD), if doped with suitable activators, in radiation dosimetry. KMgF3 , whose effective atomic number, Zeff , is about 13.4, was first proposed in 1990 as a thermoluminescence (TL) material showing good dosimetric performances [1]. After that, several
*
Corresponding author. E-mail address: [email protected] (T. Rivera).
studies have been carried out over the last years [2– 10]. The aim of the present work is to report the dosimetric characteristics of KMgF3 :LaF3 + PTFE irradiated with b particles (90 Sr/90 Y).
2. Samples preparation KMgF3 -doped phosphor was obtained from the melt formed by KF and MgF2 in the stoichiometric ratio, by the Czochralski technique, using a platinum crucible and under argon gas atmosphere. Typical dimensions of the crystal boules are 4–5 cm of length and 2–3 cm in diameter. Doping, in the form of LaF3 , was achieved by adding, to the starting powder before the growth, a suitable amount of impurity (2% mol).
0168-583X/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0168-583X(03)01638-0
330
F. Sepulveda et al. / Nucl. Instr. and Meth. in Phys. Res. B 213 (2004) 329–332 180
14000 TL INTENSITY (nC) .
418 K
TL INTENSITY (nC)
160 140 120
544 K
100 80
12000 10000 8000 6000 4000
60
2000
40
0 0
20 0
0
50
100
150
200
CHANNEL NUMBER
0.5
1
1.5 2 DOSE (Gy)
2.5
3
3.5
Fig. 2. TL response of KMgF3 :LaF3 to b radiation.
Fig. 1. Typical glow curve from KMgF3 :LaF3 .
Owing to segregation phenomena during the growth, the dopant concentration in the crystal does not coincide with that in the melt and, moreover, it is not constant along the ingot. Therefore, the crystal was reduced in powder. In order to obtain KMgF3 :LaF3 + PTFE pellets, a mixture 2:1 of the phosphor powder and PTFE resin powder was placed in a stainless steel die to be pressed at room temperature, at about 0.1 Gpa. The pellets obtained, weighted approximately 20 mg, were thermally treated to be sintered by a period longer than five hours in an oven, with nitrogen atmosphere, increasing the temperature up to 673 K, slightly lower than that of PTFE fusion. After sinterization, the samples were left to cool down to the room temperature.They were then ready for TL evaluation. The samples, having an average mass of (20 ± 3) mg, a diameter of 5 mm and a thickness of 0.6 mm, were carefully selected in order to obtain good homogeneity both in mass (±3%) and in TL response (±5%). A typical glow curve of KMgF3 :LaF3 + PTFE is shown in Fig. 1. Two well-resolved peaks can be observed: a first one at 418 K and another at 544 K.
3. Experimental results 3.1. TL response versus dose Before using the samples, an annealing procedure at 673 K for 1 h was used for erasing any
previous possible irradiation effect and for stabilising the trap structure [9]. The annealed samples were allowed to cool down to room temperature outside of the furnace. After that, the TL response of KMgF3 phosphor, as a function of the dose, was inspected in a wide range of b doses: from 1 mGy to 3 Gy. The heating rate used was 283 K/s, with a maximum readout temperature of 673 K. Fig. 2 shows the results. Each experimental point represents the average response over five irradiated pellets. The TL response is evaluated as the total area under the glow curve, after background subtraction. It can be observed a very good linearity over all the range of the delivered doses. 3.2. Precision If several dosimeters are irradiated at the same dose, some variations in sensitivity may be observed. These variations determine the precision with which a certain dose can be measured. According to the procedure given by the American National Standards Institute [11], 12 dosimeters received the annealing at 673 K during one hour and then were irradiated to a dose of 120 mGy. According to the ANSI requirements for precision, the following expression has to be filled: Dmax Dmin % < 30%; Dmin where Dmax and Dmin are the maximum and minimum TL readings, expressed in units of dose. The results are shown in Fig. 3. Using the values Dmin ¼ 120:03 mGy and Dmax ¼ 125:74 mGy, one obtains a parameter of precision equal to 4.4%, which is much lower than the required 30%.
EVALUATED DOSE (mGy)
F. Sepulveda et al. / Nucl. Instr. and Meth. in Phys. Res. B 213 (2004) 329–332 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0
331
Uniformity < 5 %
0
1
2
3
4
5
6
7
8
9
10
11
12
13
DOSIMETERS
Fig. 3. Uniformity test.
3.3. Reproducibility The reproducibility is defined as the measure of the uniformity of the response of a dosimeter. To check this point, 10 pellets of KMgF3 have been annealed, irradiated to 120 mGy and read out over five repeated cycles. The standard deviation associated to the average calculated over five repeated cycles carried out on the same dosimeter was less than 5%. Furthermore, the average carried out over the 10 averages gave a standard deviation less than 3%, Fig. 4. This value fulfils the ANSI requirement, which allows a standard deviation no bigger than 10%.
nificantly from the zero dose, can be taken as three times the standard deviation of the zero-dose reading, expressed in units of absorbed dose. The determination of the threshold dose has been obtained using 12 pellets. The pellets received the standard annealing and, after the cooling down to the room temperature, they were read-out. Table 1 shows the results. The average value of the readings, expressed in absorbed dose units, is 0.467 mGy, with an associated standard deviation equal to 0.0936 mGy. According to the previous definition, the threshold dose results to be equal to about 0.294 mGy, which is much lower of the limit of 1 mGy requested by the ANSI protocol.
3.4. Threshold dose
3.5. Fading
The detection threshold, which can be defined as the smallest dose that can be distinguished sig-
The stability of the TL signal has been studied over a period of one month. Several samples have
700
TL INTENSITY (nC)
600 500 400 300
Reproducibility < 3 %
200 100 0 0
2
4
6 DOSIMETER READINGS
Fig. 4. Reproducibility test.
8
10
332
F. Sepulveda et al. / Nucl. Instr. and Meth. in Phys. Res. B 213 (2004) 329–332
Table 1 Threshold dose evaluation Dosimeter 1 2 3 4 5 6 7 8 9 10 11 12
Threshold dose Reading (nC)
Evaluated dose (mGy)
1.65 1.89 1.74 1.06 1.18 1.74 1.93 1.04 1.33 1.66 1.82 1.84
0.5 0.6 0.5 0.3 0.4 0.5 0.6 0.3 0.4 0.5 0.5 0.5
been irradiated at the same dose and then stored in a black box at room temperature. From time to time some samples were readout. At the end of the storage time the residual TL emission was 90% of the TL emission at zero elapsed time from irradiation.
4. Discussion and conclusion The perovskite-like KMgF3 activated by La ions shows a glow curve composed by two well resolved peaks. The TL response, as a function of the absorbed dose, is linear from 1 mGy to about 3 Gy. Other characteristics, like precision and reproducibility of the TL response, are very good and meet the requirements of the ANSI protocol. Furthermore, the fading over one month is only 10% of the zero time reading.
Its atomic number, in the middle between tissue equivalent and high atomic number phosphors, the possibility to obtain solid chips and the above described results show that this material, doped with La ions, represents a real success in the search for efficient dosimetric materials. The sensitivity of such phosphor, the noticeable reliability of the response together to the other properties make fluoroperveskite very promising for the purposes of TL dosimetry.
References [1] C. Furetta, C. Bacci, B. Rispoli, C. Sanipoli, A. Scacco, Rad. Prot. Dos. 33 (1/4) (1990) 107. [2] C. Bacci, S. Fioravanti, C. Furetta, M. Missori, G. Ramogida, R. Rossetti, C. Sanipoli, A. Scacco, Radiat. Prot. Dos. 47 (1/4) (1993) 277. [3] A. Scacco, C. Furetta, C. Bacci, G. Ramogida, C. Sanipoli, Nucl. Instr. and Meth. B 91 (1994) 223. [4] C. Furetta, G. Ramogida, A. Scacco, M. Martini, S. Paravisi, J. Phys. Chem. Solids 55 (11) (1994) 1337. [5] G. Kitis, C. Furetta, C. Sanipoli, A. Scacco, Radiat. Prot. Dos. 65 (1/4) (1996) 93. [6] G. Gambarini, M. Martini, A. Scacco, C. Rafflaglio, A.E. Sichirollo, Radiat. Prot. Dos. 70 (1/4) (1994) 175. [7] G. Kitis, C. Furetta, C. Sanipoli, A. Scacco, Radiat. Prot. Dos. 82 (2) (1999) 151. [8] C. Furetta, C. Sanipoli, G. Kitis, J. Phys. D 34 (2001) 857. [9] C. Furetta, F. Santopietro, C. Sanipoli, G. Kitis, Appl. Rad. Isot. 55 (2001) 533. [10] N.J.M. Le Masson, A.J.J. Bos, C.W.E. Van Eijk, C. Furetta, J.P. Chaminade, Rad. Prot. Dos. 100 (2002) 229. [11] American National Standards Institute, Performance, testing and procedural specifications for thermoluminescent dosimetry (Environmental applications), New York, NY, American Standards Institute Inc., ANSI N545, 1975.
Thermoluminescence (TL) characteristics of perovskite-like KMgF3 activated by La ions F. Sepulveda a, J. Azorin b, T. Rivera a b
b,*
, C. Furetta b, C. Sanipoli
c
Escuela Superior de Fisica y Matem aticas, Instituto Politecnico Nacional, 07738 Mexico D.F, Mexico Departamento de Fisica, Universidad Autonoma Metropolitana-Iztapalapa, 09340 Mexico D.F, Mexico c Dip. Di Fisica, Universita’ di Roma, La Sapienza, Piazzale A.Moro 2, 00185 Roma, Italy
Abstract This paper reports the experimental results concerning the thermoluminescent characteristics of a new preparation of the fluoroperovskite KMgF3 :La + PTFE.The main thermoluminescent properties investigated were: the thermoluminescence (TL) response as a function of the absorbed dose, the precision obtainable in the dose measurement, the reproducibility of the TL readings and the threshold dose. The results obtained were then compared to the requirements of the ANSI protocol for thermoluminescent environmental dosimetry, resulting in a very good agreement with the required performances. Ó 2003 Elsevier B.V. All rights reserved. PACS: 78.45+h; 78.60 kn; 29.40.)n
1. Introduction Ternary compounds belonging to the group of the fluoroperovskites have the general form AB where A and B stand for an alkali metal and an alkaline earth metal, respectively. Such compounds recently received full attention in view of their possible use as thermoluminescent dosimeter (TLD), if doped with suitable activators, in radiation dosimetry. KMgF3 , whose effective atomic number, Zeff , is about 13.4, was first proposed in 1990 as a thermoluminescence (TL) material showing good dosimetric performances [1]. After that, several
*
Corresponding author. E-mail address: [email protected] (T. Rivera).
studies have been carried out over the last years [2– 10]. The aim of the present work is to report the dosimetric characteristics of KMgF3 :LaF3 + PTFE irradiated with b particles (90 Sr/90 Y).
2. Samples preparation KMgF3 -doped phosphor was obtained from the melt formed by KF and MgF2 in the stoichiometric ratio, by the Czochralski technique, using a platinum crucible and under argon gas atmosphere. Typical dimensions of the crystal boules are 4–5 cm of length and 2–3 cm in diameter. Doping, in the form of LaF3 , was achieved by adding, to the starting powder before the growth, a suitable amount of impurity (2% mol).
0168-583X/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0168-583X(03)01638-0
330
F. Sepulveda et al. / Nucl. Instr. and Meth. in Phys. Res. B 213 (2004) 329–332 180
14000 TL INTENSITY (nC) .
418 K
TL INTENSITY (nC)
160 140 120
544 K
100 80
12000 10000 8000 6000 4000
60
2000
40
0 0
20 0
0
50
100
150
200
CHANNEL NUMBER
0.5
1
1.5 2 DOSE (Gy)
2.5
3
3.5
Fig. 2. TL response of KMgF3 :LaF3 to b radiation.
Fig. 1. Typical glow curve from KMgF3 :LaF3 .
Owing to segregation phenomena during the growth, the dopant concentration in the crystal does not coincide with that in the melt and, moreover, it is not constant along the ingot. Therefore, the crystal was reduced in powder. In order to obtain KMgF3 :LaF3 + PTFE pellets, a mixture 2:1 of the phosphor powder and PTFE resin powder was placed in a stainless steel die to be pressed at room temperature, at about 0.1 Gpa. The pellets obtained, weighted approximately 20 mg, were thermally treated to be sintered by a period longer than five hours in an oven, with nitrogen atmosphere, increasing the temperature up to 673 K, slightly lower than that of PTFE fusion. After sinterization, the samples were left to cool down to the room temperature.They were then ready for TL evaluation. The samples, having an average mass of (20 ± 3) mg, a diameter of 5 mm and a thickness of 0.6 mm, were carefully selected in order to obtain good homogeneity both in mass (±3%) and in TL response (±5%). A typical glow curve of KMgF3 :LaF3 + PTFE is shown in Fig. 1. Two well-resolved peaks can be observed: a first one at 418 K and another at 544 K.
3. Experimental results 3.1. TL response versus dose Before using the samples, an annealing procedure at 673 K for 1 h was used for erasing any
previous possible irradiation effect and for stabilising the trap structure [9]. The annealed samples were allowed to cool down to room temperature outside of the furnace. After that, the TL response of KMgF3 phosphor, as a function of the dose, was inspected in a wide range of b doses: from 1 mGy to 3 Gy. The heating rate used was 283 K/s, with a maximum readout temperature of 673 K. Fig. 2 shows the results. Each experimental point represents the average response over five irradiated pellets. The TL response is evaluated as the total area under the glow curve, after background subtraction. It can be observed a very good linearity over all the range of the delivered doses. 3.2. Precision If several dosimeters are irradiated at the same dose, some variations in sensitivity may be observed. These variations determine the precision with which a certain dose can be measured. According to the procedure given by the American National Standards Institute [11], 12 dosimeters received the annealing at 673 K during one hour and then were irradiated to a dose of 120 mGy. According to the ANSI requirements for precision, the following expression has to be filled: Dmax Dmin % < 30%; Dmin where Dmax and Dmin are the maximum and minimum TL readings, expressed in units of dose. The results are shown in Fig. 3. Using the values Dmin ¼ 120:03 mGy and Dmax ¼ 125:74 mGy, one obtains a parameter of precision equal to 4.4%, which is much lower than the required 30%.
EVALUATED DOSE (mGy)
F. Sepulveda et al. / Nucl. Instr. and Meth. in Phys. Res. B 213 (2004) 329–332 150 140 130 120 110 100 90 80 70 60 50 40 30 20 10 0
331
Uniformity < 5 %
0
1
2
3
4
5
6
7
8
9
10
11
12
13
DOSIMETERS
Fig. 3. Uniformity test.
3.3. Reproducibility The reproducibility is defined as the measure of the uniformity of the response of a dosimeter. To check this point, 10 pellets of KMgF3 have been annealed, irradiated to 120 mGy and read out over five repeated cycles. The standard deviation associated to the average calculated over five repeated cycles carried out on the same dosimeter was less than 5%. Furthermore, the average carried out over the 10 averages gave a standard deviation less than 3%, Fig. 4. This value fulfils the ANSI requirement, which allows a standard deviation no bigger than 10%.
nificantly from the zero dose, can be taken as three times the standard deviation of the zero-dose reading, expressed in units of absorbed dose. The determination of the threshold dose has been obtained using 12 pellets. The pellets received the standard annealing and, after the cooling down to the room temperature, they were read-out. Table 1 shows the results. The average value of the readings, expressed in absorbed dose units, is 0.467 mGy, with an associated standard deviation equal to 0.0936 mGy. According to the previous definition, the threshold dose results to be equal to about 0.294 mGy, which is much lower of the limit of 1 mGy requested by the ANSI protocol.
3.4. Threshold dose
3.5. Fading
The detection threshold, which can be defined as the smallest dose that can be distinguished sig-
The stability of the TL signal has been studied over a period of one month. Several samples have
700
TL INTENSITY (nC)
600 500 400 300
Reproducibility < 3 %
200 100 0 0
2
4
6 DOSIMETER READINGS
Fig. 4. Reproducibility test.
8
10
332
F. Sepulveda et al. / Nucl. Instr. and Meth. in Phys. Res. B 213 (2004) 329–332
Table 1 Threshold dose evaluation Dosimeter 1 2 3 4 5 6 7 8 9 10 11 12
Threshold dose Reading (nC)
Evaluated dose (mGy)
1.65 1.89 1.74 1.06 1.18 1.74 1.93 1.04 1.33 1.66 1.82 1.84
0.5 0.6 0.5 0.3 0.4 0.5 0.6 0.3 0.4 0.5 0.5 0.5
been irradiated at the same dose and then stored in a black box at room temperature. From time to time some samples were readout. At the end of the storage time the residual TL emission was 90% of the TL emission at zero elapsed time from irradiation.
4. Discussion and conclusion The perovskite-like KMgF3 activated by La ions shows a glow curve composed by two well resolved peaks. The TL response, as a function of the absorbed dose, is linear from 1 mGy to about 3 Gy. Other characteristics, like precision and reproducibility of the TL response, are very good and meet the requirements of the ANSI protocol. Furthermore, the fading over one month is only 10% of the zero time reading.
Its atomic number, in the middle between tissue equivalent and high atomic number phosphors, the possibility to obtain solid chips and the above described results show that this material, doped with La ions, represents a real success in the search for efficient dosimetric materials. The sensitivity of such phosphor, the noticeable reliability of the response together to the other properties make fluoroperveskite very promising for the purposes of TL dosimetry.
References [1] C. Furetta, C. Bacci, B. Rispoli, C. Sanipoli, A. Scacco, Rad. Prot. Dos. 33 (1/4) (1990) 107. [2] C. Bacci, S. Fioravanti, C. Furetta, M. Missori, G. Ramogida, R. Rossetti, C. Sanipoli, A. Scacco, Radiat. Prot. Dos. 47 (1/4) (1993) 277. [3] A. Scacco, C. Furetta, C. Bacci, G. Ramogida, C. Sanipoli, Nucl. Instr. and Meth. B 91 (1994) 223. [4] C. Furetta, G. Ramogida, A. Scacco, M. Martini, S. Paravisi, J. Phys. Chem. Solids 55 (11) (1994) 1337. [5] G. Kitis, C. Furetta, C. Sanipoli, A. Scacco, Radiat. Prot. Dos. 65 (1/4) (1996) 93. [6] G. Gambarini, M. Martini, A. Scacco, C. Rafflaglio, A.E. Sichirollo, Radiat. Prot. Dos. 70 (1/4) (1994) 175. [7] G. Kitis, C. Furetta, C. Sanipoli, A. Scacco, Radiat. Prot. Dos. 82 (2) (1999) 151. [8] C. Furetta, C. Sanipoli, G. Kitis, J. Phys. D 34 (2001) 857. [9] C. Furetta, F. Santopietro, C. Sanipoli, G. Kitis, Appl. Rad. Isot. 55 (2001) 533. [10] N.J.M. Le Masson, A.J.J. Bos, C.W.E. Van Eijk, C. Furetta, J.P. Chaminade, Rad. Prot. Dos. 100 (2002) 229. [11] American National Standards Institute, Performance, testing and procedural specifications for thermoluminescent dosimetry (Environmental applications), New York, NY, American Standards Institute Inc., ANSI N545, 1975.