Effect of particle size in the TL response of natural quartz sensitized with high gamma doses

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1 Effect of particle size in the TL response of natural quartz sensitized with high gamma doses A B Carvalho Jr 1 ; P L Guzzo ; H L Sullasi 3 and H J Khoury 3 1 Departament of Mechanical Engineering, Federal University of Pernambuco, 57-53, Recife, PE, Brazil Departament of Mining Engeneering, Federal University of Pernambuco, 57-53, Recife, PE, Brazil 3 Departament of Nuclear Energy, Federal University of Pernambuco, 57-5, Recife, PE, Brazil alvarobcjr@yahoo.com.br Abstract. The aim of this study is to investigate the effect of particle size in the thermoluminescence (TL) response of natural quartz sensitized with high gamma doses. For this, fragments of a single crystal taken from the Solonópole (CE, Brazil) were ground and classified into several fractions. The particle size distribution and the mean particle sizes were measured. Aliquots of each size range were sensitized with 5 kgy of gamma dose of 6 Co. Non-sensitized and sensitized samples were heat-treated in a muffle furnace at o C for one hour. The non-sensitized samples were exposed to test doses between.5 and 5 kgy and the sensitized samples were exposed to a test dose of 5 mgy. For non-sensitized samples, the TL peak at 35 C increases as the particle size decreases. The sensitized samples showed that the TL output near 8 C increases with increasing particle size up to mean sizes equal to 38 µm. This behavior was discussed in relation to the formation of E centers by high gamma doses. For particle size larger than 38 µm, an abrupt reduction in the TL intensity was observed. 1. Introduction Many natural crystals exhibit thermoluminescent (TL) properties that are suitable for TL dating amongst which quartz is one of the most important because it is very abundant in ancient ceramic fragments and geological sediments [1,]. Recently, natural quartz has been investigated for TL dosimetry applications [3]. Even though there are a large number of studies performed on the TL properties of quartz and silica, it is believed that quartz has not been used as a dosimeter for ionizing radiation. The reason for this is that its glow curve are very dependent on the geological conditions in which quartz was grown and that the defect centers acting as electron traps and recombination centers, are still not properly characterized [1,3]. An important effect that can be observed in quartz is the increase in the sensibility of a given glow curve peak following the absorption of high radiation doses. This sensitization process, also know as pre-dose effect, is an important feature in radiation dosimetry, where the enhancement in sensitivity is highly desirable. Until now, studies on the sensitization in quartz have been restricted to its first glow peak, which appears around 11 o C after the absorption of ionising radiation at room temperature, followed by subsequent heating. This process has been explained using a phenomenological model,

2 which involves an increase in the radiative recombination probability at the luminescence sites with irradiated samples [,5]. On the other hand, recent studies have reported the sensitization of the peak in the to 3 o C region [6]. These studies showed that after the sensitization it was possible to observe an increase of TL output of this peak with doses in range of mgy. Thus, sensitization process for TL peaks appearing above 11 o C is a very promising technique for dating and dosimetry applications due to the lower effect of the thermal fading. There are several materials produced for dosimetry applications where one of its forms is powder [7]. However, it is known that TL sensitivity is influenced by the particle size [8,9]. Studies have been performed with the objective to understand the behavior of TL materials of different grain sizes [8,1,11]. In case of silica powders, it was reported that the intensity of the TL peaks at 18 C grows with decreasing grain size for gamma doses like 3 Gy of 6 Co [11]. In addition, efforts have been done to produce TL dosimeters using quartz powders prepared from natural single crystals [1]. Therefore, it seems important to better understand the behavior of TL glow curves as a function of quartz powders with different grain sizes. Thus, the aim of this study is to investigate the effect of particle size in the TL response of non-sensitized and sensitized quartz powders with high gamma dose.. Experimental procedure For this study, it was used a quartz single crystal, extracted from Solonópole deposit located in the Ceará State of Brazil. Fragments of plates from the crystals were cleaned using an ultrasonic bath with acetone for 15 minutes and then were manually crushed using an agate mortar and pestle. After grinding, the material was classified into fine fractions (3x5 µm, 15x315 µm, 15x3 µm, 75x15 µm, 38x75 µm and < 38 µm) and coarse fractions (8x76 µm, x8 µm, 1x µm and 85x17 µm) by using Tyler standard sieves. The particle size distributions of fine fractions were measured with a laser particle size analyzer, model Malvern Mastersize, using the wet procedure. For these fractions, the mean particle size (D m ) corresponds to the aperture associated with 5% passing of material. For the coarse fractions, D m corresponds to the average of size measurements determined with a digital microscope along three directions. Part of the fine and the coarse grains were sensitized with a dose 5 kgy of 6 Co in a gamma cell irradiator with a dose rate of 1 kgyh -1. Three heat treatments were then successively performed in order to guarantee the release of change carriers from the trap levels. The heat-treatments were performed in a muffle furnace as follows: continuous heating up to C, annealing for one hour at C, cooling, annealing for two hours at 1 C, and cooling. This thermal cycle was adopted as the standard procedure throughout this work. For the non-sensitized samples this heat-treatment was performed only once. For the study of TL response as a function of particle size, batches of non-sensitized and sensitized samples were produced containing three aliquots. The mass of each aliquot (~ 5 mg) was determined with an analytical balance accurate to.1 g. The non-sensitized samples were exposed to gamma test doses of.5,.5, and 5 kgy of 6 Co. The sensitized samples were exposed to a test dose of 5 mgy. The TL glow curves were record from 5 to o C using a Harshaw 35 reader with a heating rate equal to Cs -1. The averaged TL signal was normalized into respect weight and dose. The TL intensities were obtained integrating the mean TL glow curves between 175 to 39 C. To analyze the effect of grinding on the crystalline structure of quartz particles, samples of three fine fractions (75x15 µm, 38x75 µm and < 38 µm) were examined by X-ray diffraction (XRD). XRD patterns were obtained with a θ-θ diffractometer between 1 to 6 using Cu-Kα radiation. Afterwards, the diffractions were investigated with the full width at half maximum (FWHM) method. Using a JEOL-JSM-66 microscope, scanning electron microscopy (SEM) was used to observe the shape and the fracture pattern of the crushed particles.

3 3. Results The TL curves of the non-sensitized samples with particle sizes ranging from 38 to 5 µm show glow peaks near 9, 15 and 35 C. An intense TL peak was observed at 9 o C which corresponds to the well documented 11 o C peak of quartz. In the present study, the peak at 9 C was not considered due to its unstable behavior at room temperature. Figure 1 shows the characteristic TL glow curves for samples with different particle size irradiated with.5 kgy. The results show that the TL intensity of the peak at 35 C increases with decreasing particle size. The TL glow curves of samples irradiated with.5 kgy are shown in Figure. Besides the 11 and 35 o C TL peaks, a peak at 15 o C is observed in this figure. In this case, an inverse effect is observed with respect to particle size, i.e., the TL intensity at 15 o C decreases with the particle size decreasing. Relative TL Intensity *1 5 (a.u.) x5 µm 75x15 µm 38x75 µm < 38 µm Relative TL Intensity *1 5 (a.u.) x5 µm 75x15 µm 38x75 µm < 38 µm Figure 1. TL glow curve of quartz powder with various particle sizes exposed to.5 kgy Figure. TL glow curve of quartz powder with various particle sizes exposed to.5 kgy. Figure 3 summarizes the behavior of the TL signal integrated from 175 to 39 o C as a function of the mean particle size. In this figure, the TL intensity was normalized in relation to the intensity read for the smaller particle size (D m < 38 µm). Relative TL Intensity (a.u.) kgy.5 kgy kgy 5 kgy D m (µm) Figure 3. TL signal integrated from 175 to 39 o C for non-sensitized samples versus the mean particle sizes (D m ) of quartz.

4 The samples irradiated with.5 and.5 kgy show a similar behavior in relation to the mean particle size (D m ), i.e., the increase in TL intensity is observed for samples with D m < 15 µm. For the samples irradiated with.5 kgy, the decrease observed in the TL intensity of the 35 o C peak was compensated by the increase of the 15 o C peak. In case of samples irradiated with and 5 kgy, one observes that TL intensity integrated between 175 and 39 o C becomes to be more affected by the increase of the peak at 15 o C. In order to better observe the effect of particle size in the TL response, the sensitized samples with 5 kgy and exposed to a test dose of 5 mgy, were separated in two groups. Figure presents the TL glow curves for samples with fine fractions, between 38 and 3 µm. Figure 5 presents the TL glow curves of samples with coarse fractions, between 85 and 8 µm. In Figure, an increase in the TL intensity of the 8 o C TL peak is observed for larger particle sizes. This behavior is similar to the 15 o C peak observed in non-sensitized samples (Figure ). It is observed that the TL peak at 8 o C shifts to higher temperatures for the particles of finer sizes. This is probably due to the fact that finer grain sizes have a higher homogeneity over the heating element of the TL reader. In this way a larger amount of the material reaches the set temperature in a smaller amount of time, which results in a slight shift of the TL peak to lower temperatures. Therefore, the shifts to higher temperatures for the particles of finer sizes, is probably associated to increasing of the specific surface area. Relative TL Intensity *1 (a.u.) x3 µm 75x15 µm 38x75 µm < 38 µm Relative TL Intensity *1 3 (a.u.) x8 µm 11x µm 85x11 µm Figure. TL glow curve of quartz fine grains sensitized with 5 kgy (test dose of 5 mgy) Figure 5. TL glow curve of quartz coarse grains sensitized with 5 kgy (test dose of 5 mgy). 1 Relative TL Intensity *1 6 (a.u.) D m (µm) Figure 6. TL signals integrated from 175 to 39 o C for quartz grains sensitized with 5 kgy (test dose of 5 mgy) versus mean particle size.

5 Figure 6 shows the TL signals integrated from 175 to 39 o C as a function of the mean particle size. It can be seen that TL intensity increase with mean particle size from 17 to 38 µm. For grain sizes larger than 38 µm, an abrupt decrease in the TL intensity is noticed. Above µm, no significant change in TL intensity is observed.. Discussion Studies have shown that the manual crushing of quartz grains with mortar and pestle can induce the amorphization of the crystal structure when the particles are submitted to very high pressures [13,1]. However, the FWHM analysis of the diffraction peaks related to (1 1 ) and (11 ) planes did not show any signal of amorphization in the fine fractions between 38 and 15 µm. For this reason the hypothesis of the bulk amorphization of the quartz grains was ruled out. In parallel, several aliquots of particles have their external morphology observed by SEM. For instance, Figures 9 and 1 show typical images obtained with particles sizes of 38 and 3 µm. Besides the particle size itself and fine grains deposited on the surfaces of the 38 µm particles, no difference was observed in the fracture pattern of these particles that can be associated with the TL intensity dependence shown in Figure 6. Under this crushing condition, brittle micro cracking is the exclusive mechanism associated with size reduction and no evidence of plastic deformation was observed. Figure 9. SEM micrograph of quartz particles with 38 µm mean size. Figure 1. SEM micrograph of quartz particles with 3 µm mean size. The effect of the particle size is much less pronounced for quartz samples in non sensitized condition. For the test dose of.5 and.5 kgy, it was observed that the TL intensity integrated from 175 to 39 o C increase with the decreasing in the particle size. A similar behavior was shown for particles of potassium iodate (KI) and amorphous silica exposed to similar doses [1,11]. As stated before, the increase in TL intensity can be attributed to the increase in the specific surface area. Figure 11 shows the behavior of the integrated TL signal as a function of the specific surface area for the nonsensitized samples. The TL intensities demonstrate an increase with the specific surface area, for test doses of.5 and.5 kgy. This behavior was not observed for and 5 kgy test doses due to the sensitization process. The increase of the sensitized peak, initially appearing near 15 o C and then moving to higher temperatures near 8 o C, is associated with the beginning of the sensitization with high doses.

6 Relative TL Intensity *1 7 (a.u.) kgy.5 kgy Specific surface area (cm /g) Figure 11. Relationship between TL intensity and the specific surface area of non sensitized quartz samples with different particles sizes. In case of the sensitized samples, when the integrated TL intensity was plotted against the specific surface area no additional information as that already shown in Figure 6 is observed, i.e., the TL intensity slightly increase with the increase of the specific surface area up to 3 µm particles; an abrupt increase occurs at 38 µm and then it decrease with the increasing of the surface area. In the present time, the TL vs. particle size dependence for sensitized samples can be explained as follows. For coarse particles, the increase in the TL intensity noticed from coarse to medium (38 µm) particles can be explained by the formation of the paramagnetic centers induced by mechanical fracturing as it was previously observed in amorphous silica [15] According to Munekuni et al [15], the population of E' and non bridging oxygen hole centers (NBOHC), which probably act as electron traps during irradiation, increased in those samples that was ground in a ball mill. On the other hand, Takeuchi et al [16] suggested that the decrease in the TL intensity noticed in quartz grains with diameter lower than µm can be attributed to the mechanical damage introduced by the grinding action. According to these authors, the excessive mechanical damage in the surface structure of the fine quartz particles would be responsible to the destruction of the recombination centers associated to the glow peaks appearing above o C. One imagine that the hypothesis of the reduction in the population of recombination centers can be preliminary used to explain the decrease of the TL intensity in quartz particles with small dimensions. Further studies are required to clarify the complex dependence of the TL response with quartz particles sensitized with high gamma doses. 5. Conclusions The increase in TL intensity for non-sensitized samples is related to the increase of the specific surface area. In this study, this behavior was observed more clearly for a dose of.5 kgy. The samples exposed to.5 and kgy present a peak at 15 o C, which is related to the beginning of the sensitization process. After sensitization process, it was observed that the TL intensity increase with the increasing of the specific surface area from coarse to particle sizes with 38 µm. The decrease in TL intensity observed for fine particles was preliminary explained by the mechanical damage related to the crushing procedure. Thus, quartz particles with diameter near 38 µm are recommended for its future use in dosimetry applications.

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