High Dose Dosimetry Using Antigorite-Teflon Composite

2011 International Nuclear Atlantic Conference - INAC 2011 Belo Horizonte,MG, Brazil, October 24-28, 2011 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN ISBN: 978-85-99141-04-5 HIGH DOSE DOSIMETRY USING ANTIGORITE-TEFLON COMPOSITE René R. Rocca 1, Sonia H. Tatumi 2 and Shigueo Watanabe 1 1 Instituto de Física Universidade de São Paulo Rua do Matão 187 – Travessa R. 05508-090 Butantã, SP [email protected] – [email protected] 2 Faculdade de Tecnologia de São Paulo CEETESP Pça. Cel. Fernando Prestes - Bom retiro, 30 01124-060 São Paulo, SP [email protected] ABSTRACT Pellets of antigorite crystals (Mg 3-x [Si 2O5] (OH) 4-2x ) and teflon powder were obtained in order to investigate the thermoluminescence (TL) dosimetric characteristics. ICP analysis was performed showing the presence of 0.25 mol% of Fe 2O3, 0.07 mol% of Al 2O3 and 0.006 mol% of MnO impurities. These pellets show two prominent TL peaks at 150 oC and another broad one at 250 oC observed in samples previously irradiated with gamma-rays from 60 Co source. The 150 oC peak increased up to 2 kGy and after this dose the intensity reaches a maximum then decreases gradually. However the 250 oC peak increased up even with a dose of 172 kGy. A good reproducibility of TL results was obtained showing that these pellets can be used for high dose measurements. An excellent theoretical fit using second order kinects model [10] was obtained for all the peaks; however the theoretical deconvolution [4] showed the presence of two additional peaks at 206 and 316 oC. Frequency factor (s) and activation energies ( E) values, of electron traps related to TL emission, were evaluated using three o methods: the peak shape, initial rise and Tm-Tstop [10]; for 150 C peak we found the following mean values: E = (1.003±0.064) eV and s=0.79x10 12 s-1 and for 250 oC peak E = (1.26±0.52)eV and s=1.16x10 12 s-1. Lifetime of these meta-stable centers can be calculated using the equation t=s -1exp(E/kT) , where k is the Boltzmann’s constant and T is the absolute temperature, we obtained t = 2 days for the first peak and 270 years for second one, for samples held at a constant T=27 oC. Therefore, the 250 oC TL peak has an enough thermal stability which allows its use in dosimetry. 1. INTRODUCTION Thermoluminescent crystals are extensively used in experimental determination of ionizing radiation dose; some TL centers have a high thermal stability offering in this way very good results for dating, natural silicates are frequently used in dating of Quaternary geological formations. In the present work we are going to analyze the TL response of antigorite crystal, a natural silicate belonging to serpentine group. The majority of the papers about this crystal are related to geosciences area but works related to TL properties of this mineral was not found. Hunt et al, studied reflectance spectra in visible and near infrared of 87 silicate, among them, they observed two samples named serpentine 318B and asbestos 323B-Arizona, which have shown ferric ion bands at 0.65, 0.45 and 0.8 µm and also bands related to hydroxyl ion at 1.4 and 2.34 µm [9]. An antigorite sample from India was investigated by Reddy et al [8], using Optical absorption technique. They found bands related to Fe 3+ ion at 1542, 17852, 19602, 22773, 24869 and 26448 cm -1 and the others at 9122, 10635, 11679, 15380 and 22929 cm -1 due to Fe 2+ . Using EPR they verified the presence of Fe 3+ , with g-values ranging from 5.4 to 3.1, and a sextet lines of the Mn 2+ ion, which substitute Mg 2+ in the crystalline lattice, with g-values near to 2.0. Götze et al, investigated clay minerals using cathodoluminescence (CL), EPR, X-ray diffraction, scanning electron microscopy and trace element analysis; the selected mineral groups are the sepertine-kaolin, talc-pyrophyllite, smectite and illite [5]. They concluded that the minerals kaolinite, dickite, nacrite, halloysite and pyrophyllite are characterized by a blue CL emission (400 nm), and EPR results indicated that this blue emission can be related to radiation induced defects centres, as electron holes trapped on apical oxygen atoms ( Si-O centre) or located at the Al-O-Al group. They also verified that serpentine does not show visible CL. 2. MATERIALS AND METHODOLOGY The specimen was collected from the state of Goiás - Brazil, this one shows a green coloration. Structure of the minerals was verified by X-rays diffraction method certifying that this sample is actually antigorite ( monoclinic, Mg 3-x [Si 2O5] (OH) 4-2x ) belonging to serpentine group. Antigorite is commercially also named “New Jade”. It is known in the literature that theses crystals, when heated at about 350 oC, present a phase change to forsterite emitting light [11]. TL measurements have been carried out in an oxygen-free nitrogen atmosphere using a Daybreak Nuclear and Medical Systems Inc, Model 1100-series, with a photomultiplier EMI 9235QA coupled to optical filters Corning 7-59 and Schott BG–39, resulting in a transmittance at 335-470 nm and heating rate was 10 °C/s. Specimens was pulverized with a mortar and a pestle, and sieved separating grains between 75 and 150 µm. The selected grains were heated at 480 oC by 5 minutes in order to erase the natural TL, subsequently they were pressed with teflon powder producing pellets of 5 mm diameter and 1 mm thickness. Samples were irradiated with γ-rays in a 60 Co source; doses from 20 to 172 kGy were applied with a rate of 0.3906 kGy/h. 3. RESULTS AND DISCUSSION By using ICP technique the major elements concentrations and impurities were identified Figure 1 . The silicon in the SiO 4 tetrahedron can be replaced by aluminium and the magnesium by aluminium ferric and ferrous ions in different environments. The presence of Mn was also detected and can contribute to blue luminescence emission . Usually this emission band is related to the characteristic line of the Mn 2+ emissions due to the transitions 4 T1 to fundamental state, which is in the blue region, considering an octahedral symmetry. Two prominent TL peaks at 148 and 248 oC were observed in sample previously irradiated with γ-rays, as is shown in Figure 2 . TL glow curves can be fitted by using second order kinetics model [2][9] and the equation for TL intensity ( I) is: −2 T = ()()− β − + . (1) TI )( sn o exp E / kT s / ∫ exp( E / kT ´) dT 1´ INAC 2011, Belo Horizonte, MG, Brazil. where s is the frequency factor, no is the initial concentration of trapped electron, E is the activation energy, k is the Boltzmann constant, T is the absolute temperature, β is the linear heating rate. 50 1.0 0.10 SiO 2 MgO K2O 40 0.8 0.08 Al 2O3 30 0.6 0.06 20 0.4 0.04 % mol Fe O 2 3 10 0.2 0.02 P2O5 MnO CaO Na 2O 0 0.0 0.00 Compound Figure 1. ICP analyses, divided in three groups for better visualization, for Antigorite crystal. In the present work s and E were evaluated through peak shape, initial rise ( Figure 2 ) and Glow Curve Deconvolution methods [8][4]. The results are summarized in Table 1 . By thermal cleaning was possible to find the initial rise of the 250 oC peak, trying to eliminate this peak was found one more peak at approximately 300 oC as we can see on Figure 2. The individual mean life of the peaks were calculated following τ = exp ( E/kT)/s , with T = 27 oC, these results are also listed in Table 2 . 6000 a) 20 Gy 3x10 5 1 kGy b) 50 Gy 2 kGy 5000 100 Gy 5 kGy 200 Gy 10 kGy 4000 5 500 Gy 2x10 20 kGy 1000 Gy 3000 42 kGy 172 kGy 2000 1x10 5 TLIntensity(a.u.) TLIntensity(a.u.) 1000 0 0 0 100 200 300 0 50 100 150 200 250 300 350 o Temperature ( oC) Temperature ( C) Figure 1. TL glow curves for Antigorite-Teflon pellets submitted to gamma-rays irradiation, a) for doses until 1000 Gy, b) for higher doses. An excellent theoretical fit using second order kinects model [10] was obtained for all the peaks, however the theoretical deconvolution [4] showed the presence of two additional peaks at 206 and 316 oC, as is shown in Figure 3 . The presence of 4 peaks is confirmed with Tm-Tstop method [7] as we can see on Figure 4 . INAC 2011, Belo Horizonte, MG, Brazil. ant 300 oC 5 ant 250 oC 4 ant 150 oC Ln(I)(a.u.) 3 2.0x10 -3 2.4x10 -3 2.8x10 -3 -1 1/T (K ) Figure 2. Fit linear for initial rise method for the three peaks. 148 oC 12000 206 oC 248 oC 316 oC 8000 GCD o exp. 4000 TLu.) Intensity (a. 0 0 100 200 300 400 o Temperature ( C) Figure 3. Theoretical (line) and experimental (open circle) TL glow curves of Antigorite-teflon pellet. 300 250 C) o Tm ( Tm 200 150 100 150 200 250 o Tstop ( C) Figure 4. The Tm-Tstop method shows the presence of one more peak at 206 oC approximately, for antigorite crystal. INAC 2011, Belo Horizonte, MG, Brazil. Table 1. Activation energy and frequency factor of the TL peaks found in Antigorite- Teflon pellets.

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