2007 International Nuclear Atlantic Conference - INAC 2007 Santos, SP, Brazil, September 30 to October 5, 2007 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN ISBN: 978-85-99141-02-1
THERMOLUMINESCENCE PROPERTIES OF EPIDOTE FROM BORBOREMA PROVINCE UNDER γ-IRRADIATION H. L. Sullasi1, E. F. Silva Jr.2, H. J. Khoury1, S. Watanabe3, P. L. Guzzo2 L. Thomaz Filho3 and S. B. Brito4 1
Departamento de Energia Nuclear (DEN / UFPE) Avenida prof. Luiz Freire 1000, 50740-540-Recife, PE, Brasil
[email protected] 2
Departamento de Engenharia de Minas, Universidade Federal de Pernambuco Avenida Acadêmico Hélio Ramos, s/n, 50740-530 Recife CDU, PE, Brasil 3
Instituto de Física, Universidade de São Paulo, SP, Brasil. Rua do Matão, Travessa R, 187. 05508-090, Cidade Universitária, SP, Brasil 4
Departamento de Geologia, Universidade Federal de Pernambuco Avenida Acadêmico Hélio Ramos, s/n, 50740-530 Recife CDU, PE, Brasil
ABSTRACT This work presents the thermoluminescence (TL) glow curve response of four samples obtained from a scheelita missing residues from Borborema province in Brazil. These samples with different concentrations of epidote minerals were identified as e-m, e-c, e-r and e-pm. The TL responses of these samples were analyzed and it was observed that the epidote mineral showed a TL curve of low intensity. The natural epidote showed one TL peak at 365oC, which corresponds to the natural irradiation due to the radioactive elements contained in the samples. After irradiation in laboratory, the pure epidote showed two TL peaks around 135oC and 270oC, which increase as a result of the additional dose of irradiation, but when this sample was annealed and irradiated, the peaks were not observed in TL glow curve. We believed that these peaks are related with the content of water in the sample. The analysis of X Ray Diffraction (XRD) showed that the procedure of magnetic separation used in this work to obtain the epidote (e-c) from the material resulted from the cominution of the rock (e-pm), was satisfactory.
1. INTRODUCTION Epidote of chemical formula Ca2(Fe3+,Al)Al2O.O(OH).Si2O7.SiO4 is a sorosilicate mineral, usually found in metamorphic rocks. It crystallizes in monoclinic system, and has structure containing chains of edge-sharing octahedral parallel to the Y axis and linked in the direction of the Z axis by single [SiO4] tetrahedra and double [Si2O7] tetrahedral groups. Within this framework there are relatively large cavities, usually accommodating large cations such Ca in nine- or ten-fold coordination [1]. The epidote with a Mohs hardness 6,5 to 7 [2] and a perfect cleavage in the {001}direction, can be found in nature with green, yellow and dark green colours. A clean mineral has relatively high gem value. Khalifa et. al. [3] analyzed TL glow curves of natural epidote as function of radiation dose. They observed several overlapping TL peaks at 107, 127 and 171oC. Guedes [4] and Green [5] have shown how heat treatments can restore crystalline structure that has been affected by fission tracks.
The purpose of the present work is to investigate the effects due γ-irradiation, as well as of heat treatments at different high temperatures in the TL response of epidote. The magnetic separation has been used to obtain concentrate of epidote (e-c) from the metamorphic rock. 2. MATERIALS AND METHODS In this work, four samples with different concentrations from epidote were used. The origin of the samples was a large fragment with 17.58kg in weight from a scheelite metamorphic rock was collected from the Province of Borborema in Brazil. This fragment was crushed further and separated into fragments seemingly single crystal of epidote, denominated (e-m) and other called (e-pm) mixture of epidote and others kinds of minerals. The (e-m) samples have been pulverized to select by a proper sieving grains with 0.074 to 0.149mm size for TL experiments. The (e-pm) samples, on the other hand, have been also pulverized and sieving to retain 0,149 to 0,360mm size grains for magnetic separation carried out in a Frantz isodynamic separator of model L1. This system operates with an angle of inclination of 15o and vibration level between 4 and 5. Electrical currents of 0,3 to 0,6A were used to increase the concentration of epidote in function the magnetic susceptibility. Thus, the sample (e-r) is obtained under 0,5A operation and (e-c) is obtained that under a current 0,6A. After this process of magnetic separation, all (e-pm), (e-c) and (e-r) samples have been submitted to further crushing to separated 0.074 to 0.149mm size grains for TL readings. The characteristics of the samples are presented in table 1. Table 1. Classification of powder samples according to epidote concentration. Powder Samples e-pm e-r e-c e-m
Origin Minerals mixture Magnetic fraction (I=0.5 A) Magnetic fraction (I =0.6A) Small epidote monocrystals
Epidote concentration Low concentration Very low concentration High concentration Pure epidote
Each sample shown in Table 1 was divided into five portions and each one of them were packed into small containers and then irradiated with 60Co using a dose rate of 10.04 kGy/h. with radiation doses of 50, 100, 250, 500 and 1000Gy. The TL reading of samples was carried out in the TL Harshaw model 4500, with a heating rate of 2oC/s. No optical filters have been used. The powders samples with grain size lower than 0.074 mm were used for the analysis of main composition and also of impurities content. The X-ray difractometry (XRD) was carried out by using a Bruker D4 Endeavor difractometer with Co-Kα radiation (35kV, 40mA), 0,02o step in 2θ and 1s counting/step. The X-ray fluorescence (XRF) analysis, the samples were further powdered in a vibratory mill and then pressed with 200kN in disk with boric acid. The semi quantitative analyses were carried out in a Bruker S4 Explorer spectrometer.
INAC 2007, Santos, SP, Brazil.
3. RESULTS AND DISCUSSION The XRF analyses shown in table 2 indicated that all samples possess the constituent oxides of epidote mineral. However, some differences are observed, as follows. The e-r sample has more quantity of CaO than e-m and e-pm indicating the presence of calcite in this sample. The sample e-pm possess high quantity of SiO2 suggesting high concentration of quartz phase. Comparing e-r with e-c one observed higher concentration of Fe2O3 in e-r which is in agreement with the magnetic concentration. The composition of e-c is similar as that found by the pure epidote (e-m). Table 2. Main oxides detected by X-ray fluorescence analysis. Semi quantity concentration (%) Oxide
e-pm
e-r
e-c
e-m
Al2O3
10,00
6,14
8,67
10,20
Fe2O3
13,72
22,79
16,65
16,27
SiO2
33,75
27,26
27,08
25,56
CaO
22,64
27,00
21,98
21,62
K2O
0,03
0,03
0,02
0,04
MgO
3,07
1,04
1,79
0,58
MnO
0,68
1,72
0,47
0,25
P2O5
0,14
0,30
0,28
0,21
SrO
0,07
0,02
0,12
0,15
TiO2
0,43
0,50
0,13
0,06
1600
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1400
e-m
XRD Intensity (arb. units)
1200
e-c 1000 800
e-r
600
e-pm
400
epidote
200 0
andradite
-200
calcite
-400
quartz
-600
10
20
30
40
50
60
70
o
2 theta ( )
Figure 1. XRD of samples compared to the epidote, andradite, calcite and quartz reference levels.
INAC 2007, Santos, SP, Brazil.
80
The XRD analyses shown in Figure 1 indicate that the e-pm sample is composed essentially by epidote, andradite, calcite and quartz. The e-r sample shows a large concentration of andradite (Ca3Fe3+2Si3O12). The diffratogram of e-c is similar to e-m with minor presence of andradite and calcite. The TL glow curves of the four samples are shown in Figure 2. The TL response of e-m is negligible compared to the other samples. The TL peaks recorded at 140oC and 275oC are not referred to epidote mineral. The XRF and XRD analyses suggest that these peaks are related to calcite and quartz.
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Figure 2. TL responses from e-pm, e-r, e-c and e-m samples annealed at 600oC for 1hr and irradiated at 500 Gy. The TL response of e-m and e-c samples was then analyzed in more detail. It was observed that e-m sample has TL peaks at 135oC, 270oC and 370oC. All these peaks increase with additional gamma dose as shown in Figure 3a while e-c sample presents TL peaks at 150oC and 270oC which do not linearly increase with additional gamma dose due to the low concentration of the mineral responsible for these peaks. This is shown in Figure 3b. The TL peaks at 135oC and 270oC are probably related to water content in the sample. These peaks are observed in samples with heat treatment and irradiation only after large time. N+0Gy N+50Gy N+100Gy N+250Gy N+500Gy
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Figure 3. TL curve without annealing in function of gamma dose, a) e-m sample, b) e-c sample.
INAC 2007, Santos, SP, Brazil.
o 300
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350
400
(b)
The Figure 4 shows the TL response of e-m and e-c samples after annealing at 600oC for one hour and later irradiated with gamma radiation of Co-60 with various doses ranging from 50 to 1000 Gy. It is observed (Figure 4a) that the sample e-m doesn't present the same TL response observed before. The intensity of the new TL peaks at 200oC and 260oC were low compared to those observed in the Figure 3a. The annealed e-c sample (Figure 4b) presents similar TL peaks intensity as in Figure3b. TT+50Gy TT+100Gy TT+250Gy TT+500Gy TT+1000Gy
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Figure 4. TL curves of annealed at 600oC(1h) samples in function of additional gamma dose, a) e-m sample, b) e-c sample. he effect of heat treatment at 600, 700, 800, 900 and 1000oC for one hour on the TL glow curve is shown in Figure 5 for the samples irradiated with gamma radiation of co-60 with the dose of 1 kGy. In Figure 5a, the TL peaks observed are of very low intensity similar with TL background. Thus, the analysis is complicated in this case. In Figure 5b, it can be shown that the e-c sample (TL peak 275oC) presents maximum sensitivity around 600-700oC TT-1000 TT-900 TT-800 TT-700 TT-600
Intensidade TL (u.a.)
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Temperatura ( C)
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-200000 0
100
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300 o
Temperatura ( C)
Figure 5. TL glow curve for annealed samples at different temperatures a) e-m sample b) e-c sample.
INAC 2007, Santos, SP, Brazil.
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(b)
3. CONCLUSIONS The procedure of magnetic separation used for epidote concentration was satisfactory. How is observed XRF and XRD analyzed. The TL curve response of the epidote is affected for the high sensibility of another mineral in very low concentrations. The epídoto presents TL peaks at 140oC, 270oC and 365oC these peaks are affected by irradiation dose. The TL peak in 365oC is related to the natural radiation absorbed due the presence of radioactive elements contained in the sample. ACKNOWLEDGMENTS The authors are grateful to Dr. Reiner Neumman from Centro de Tecnologia Mineral (CETEM/MCT, RJ) for XRD and XRF analyses. They also acknowledge Prof. Jose Carlos Silva (UFPE) for his assistance during the preliminary test for epidote concentration. H. S. L. Sullasi and E. F. Silva Jr. are grateful to CAPES and PIBIC/CNPq/CETEM for fellowship and scholarship grants, respectively. REFERENCES 1. Dollase W. A., Refinement of the crystal structures of epidote, allanite and hancockite, The American Mineralogist, Vol. 56, March-April, (1971). 2. Salah-Ud DIN, Munnawar AHMAD, Shahzed CHISHTI, Study of Slip Systems in Epidote Single Crystal, Turk J Phy. Vol-25, pp 229 - 235. (2001) 3. Khalifa M. S., AlKhalifa I. J. and Durrani S. A., Analyzed of thermoluminescence glow curve of mineral sphene and epidote for radiation damage studies. Vol 17, pp 407 – 410, (1986) 4. Guedes S., Hadler J. C., Oliveira K.M.G.,.Moreira P.A.F.P, Iunes P.J., Tello C.A., Kinetic model for the annealing of fission tracks in minerals and its application to apatite, Radiation Measurements, Vol 41, pp 392 – 398, (2006). 5. Green P. F. and Durrani S. A., Annealing studies of tracks in crystals, Nuclear Track Detection, Vol 1, Issue 1, pp 33-39, (1977).
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