Thermoluminescent (TL) characteristics of perovskite-like KMgF 3 activated by La ions to beta radiation dosimetry

Transcription

1 Thermoluminescent (TL) characteristics of perovskite-like KMgF 3 activated by La ions to beta radiation dosimetry F. Sepúlveda 1, J. Azorín 2, 3, T. Rivera 2,4, C. Furetta 2,5 y C. Sanipoli 5. sepulv@esfm.ipn.mx 1 Escuela Superior de Física y Matemáticas-IPN, México 2 Universidad Autónoma Metropolitana-Iztapalapa, México 3 Centro de Investigación en Ciencia Aplicada y Tecnología Avanzada-IPN, México 4 Escuela Superior de Ingeniería Mecánica y Eléctrica-IPN, México 5 Universita di Roma La Sapienza, Italy Abstract. New ICRP regulations about radiation protection push research groups to study new thermoluminiscent materials of high sensitivity for dosimetric applications (personal and environmental). This paper reports the experimental results concerning the thermoluminescent characteristics of a new preparation of the fluoroperovskite KMgF 3 :LaF 3 + PTFE. The main thermoluminescent properties investigated were: the TL response as the function of the absorbed dose, the precision obtainable in the dose measurement, the repeatability 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. 1. Introduction The large interest in thermoluminescence (TL) as a technique for radiation dosimetry in environmental, personal and clinical applications pushes the research for producing new and highperformance TL materials. Ternary compounds belonging to the group of the fluoroperovskites have the general form ABF 3, 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. KMgF 3, whose effective atomic number, Zeff, is about 13.4, was firstly proposed in 199 as a TL material showing good dosimetric performances [1]. After that, several studies have been carried out over the last years [2-1]. The aim of the present work is to report the dosimetric characteristics of KMgF 3 :LaF 3 +PTFE irradiated with β particles ( 9 Sr/ 9 Y). 1.1 Samples preparation KMgF 3 doped phosphor was obtained from the melt formed by KF and MgF 2 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 LaF 3, was achieved by adding, to the starting powder before the growth, a suitable amount of impurity. Owing to segregation phenomena during the growth, in general 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 KMgF 3 :LaF 3 + 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 1 Mpa. The pellets thus obtained, weighted approximately 2 mg, and 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. 1

2 After sinterization, the samples were left to cool down to the room temperature. They were then ready for TL evaluation. The samples of KMgF 3 :LaF 3 studied have three different dopant concentrations which were respectively equal to 1.2 mol % (336 mg LaF 3 ), 2.4 mol % (672 mg LaF 3 ) and 4.12 mol% (1358 mg LaF 3 ). The samples, having an average mass of (2 ± 3) mg, a diameter of 5 mm and a thickness of.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 KMgF 3 :LaF 3 +PTFE (2.4 mol%) is shown in Fig.1. Two well-resolved peaks can be observed: a first one at K (176.5ºC) and another at K(269.5ºC) K 14 TL INTENSITY (nc) K TEMPERATURE (K) FIG. 1. Typical glow curve from KM g F 3 :LaF 3 2. Experimental results 2.1 TL response versus dose Before using the samples, an annealing procedure at 673 K for 1 hour 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 KMgF 3 phosphor, as a function of the dose, was inspected in a wide range of β doses: from 1 mgy to 3 Gy. The heating rate used was 1 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. Fig.3 shows the contribution of each peak to the total TL response, in the range from.25 mgy from 2.5 mgy, in comparison to the total TL response in the same range of doses. 2

3 TL INTENSITY (nc) DOSE (Gy) 12 1 TL INTENSITY (nc) TOTAL 1ST PEAK 2ND PEA K Lineal (TOTAL) Lineal (1ST PEAK) Lineal (2ND PEA K) 2 EVALUATED DOSE (mgy) DOSE (mgy) FIG. 2. TL response of KM g F 3 :LaF 3 to β radiation. FIG. 3 Contribution of each peak to the total TL response Uniformity < 5 %.FIG 4. Uniformity test DOSIMETERS 3

4 2.2 Precision of the dose evaluation 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 12 mgy. According to the ANSI requirements for precision, the following expression has to be fulfilled D max D D min min % < 3% where D max and D min are the maximum and minimum TL readings, expressed in units of dose. The results are shown in Fig. 4. Using the values D min = 12.3 mgy and D max = mgy, one obtains a parameter of precision equal to 4.4%, which is much lower than the required 3%. 2.3 Repeatability of the TL measurements The repeatability is defined as the measure of the uniformity of the response of a dosimeter. To check this point, 1 pellets of KMgF 3 have been annealed, irradiated to 12 mgy and read out over 5 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 ten averages gave a standard deviation less than 1%, Fig 5. This value fulfils the ANSI requirement, which allows a standard deviation no bigger than 1% TL INTENSITY (nc) Repeatability < 3 % DOSIMETER READINGS.FIG. 5. Repeatability test. 4

5 2.4 Relative sensitivity The Harshaw LiF:Mg,Ti (TLD-1) dosimeters were used as reference materials. The lithium fluoride chips were annealed at 673 K during one hour, then rapidly cooled down to room temperature. After cooling a further annealing at 273 K for 2 hours was followed.table I shows the sensitivity of KMgF 3 :LaF 3 +PTFE (2.4 mol) relative to TLD-1. Also in this table a comparison with the CaSO 4 :Eu dosimeters results are shown. 2.5 Fading Table I. Sensitivity comparative of KMgF 3 :LaF 3 +PTFE (2.4 mol) TL Material Relative sensitivity with respect to: CaSO 4 :Eu TLD-1 KMgF 3 :LaF CaSO 4 :Eu TLD A fading experiment was performed over a period of about 8 hours. The samples after irradiation were stored in dark conditions at room temperature. The results of the experiment, given in Fig. 6, have shown a quite similar behaviour for all the samples: the TL lost is not bigger than 1% NORMALIZED TL T I M E ( H ) FIG. 6 Fading behaviour of a KMgF 3 :LaF 3 sample at room temperature and dark conditions. 2.6 Threshold dose The detection threshold, which can be defined as the smallest dose that can be distinguished significantly from the zero dose, can be taken as three times the standard deviation of the zero-dose reading, expressed in unit of absorbed dose. 5

6 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 II shows the results. The average value of the readings, expressed in absorbed dose units, is.467 mgy, with an associated standard deviation equal to.936 mgy. According to the previous definition, the threshold dose results to be equal to about.294 mgy, which is much lower of the limit of 1 mgy requested by the ANSI protocol. Table II. Threshold dose evaluation. Dosimeter Reading (nc) Evaluated dose (mgy) Discussion and conclusion The perovskite-like KMgF 3 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 1% 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 fluoroperovskite very promising for the purposes of TL dosimetry. References 1. Furetta C., Bacci C., Rispoli B., Sanipoli C. and Scacco A., Luminescence and dosimetric performances of KMgF 3 crystals doped with metal impurity ions. Rad. Prot. Dos. 33(1/4), Vol.1, 17-11, (199). 2. Bacci C., Fioravanti S., Furetta C., Missori M., Ramogida G., Rossetti R., Sanipoli C. and Scacco A., Photoluminescence and thermally stimulated luminescence in KMgF 3 :Eu 2+ crystals. Rad. Prot. Dos. 47(1/4), , (1993). 3. Scacco A., Furetta C., Bacci C., Ramogida G. and Sanipoli C. Defects in γ-irradiated KMgF 3 :Tl +. Nucl. Instr. Meth. Phys. Res. B91, , (1994). 6

7 4. Furetta C., Ramogida G., Scacco A., Martini M. and Paravisi S. Spectroscopy of complex defects in crystals of KMgF 3 :Tl +. J.Phys. Chem.Solids 55(11), , (1994). 5. Kitis G., Furetta C., Sanipoli C. and Scacco A., Thermoluminescence properties of KMgF 3 doped with Pb, Cr and Ag. Rad. Prot. Dos. 65(1/4), 93-96, (1996). 6. Gambarini G., Martini M., Scacco A., Rafflaglio C. and Sichirollo A.E., TL dosimetry in high fluxes of thermal neutrons using variously doped LiF and KMgF 3. Rad. Prot. Dos. 7(1/4), , (1997). 7. Kitis G., Furetta C., Sanipoli C. and Scacco A., KMgF 3 :Ce, an ultra-high sensitivity thermoluminescent material. Rad. Prot. Dos. 82(2), , (1999). 8. Furetta C., Sanipoli C. and Kitis G., Thermoluminescent kinetics of the Perovskite KMgF 3 activated by Ce and Er impurities. J. Phys. D: Appl. Phys. 34, , (21). 9. Furetta C., Santopietro F., Sanipoli C. and Kitis G., Thermoluminescent (TL) properties of the perovskite KMgF 3 activated by Ce and Er impurities. Appl. Rad. Isot. 55, , (21). 1. Le Masson N.J.M., Bos A.J.J., Van Eijk C.W.E., Furetta C. and Chaminade J.P., Optically and thermally stimulated luminescence of KMgF 3 :Ce 3+. Rad. Prot. Dos. 1, 229 (22). 11. American National Standards Institute. Performance, testing and procedural specifications for thermoluminescent dosimetry (Environmental applications). ANSI N545, New York, (1975). 7