Thermoluminescence of Sodium Borate Compounds Containing Copper. Santiago, M.; Lester, M.; Caselli, E.; Lavat, A.; Ges, A.; Spano, F. and Kessler, C.

Transcription

1 Thermoluminescence of Sodium Borate Compounds Containing Copper Santiago, M.; Lester, M.; Caselli, E.; Lavat, A.; Ges, A.; Spano, F. and Kessler, C. Publicado en el Journal of Material Science Letters; Vol. 17, Nº 15, p , 1998

2 THERMOLUMINESCENCE OF SODIUM BORATE COMPOUNDS CONTAINING COPPER Santiago; M.*, Lester; M.*, Caselli, E.*; Lavat, A.**; Ges, A.***; Spano, F. **** and Kessler, C.***** * IFAS, Universidad Nacional del Centro de la Provincia de Buenos Aires ** Facultad de Ingeniería, Universidad Nacional del Centro de la Provincia de Buenos Aires *** IFIMAT, Universidad Nacional del Centro de la Provincia de Buenos Aires **** Autoridad Regulatoria Nuclear ***** Comisión Nacional de Energía Atómica Argentina The thermoluminescence (TL) of borates containing dopants, such as Li 2 B 4 O 7 or MgB 4 O 7, has been extensively investigated because they have tissue-like characteristics and high sensitivity, and therefore they are worth considering for personal dosimetry. The sensitivity and thermal stability of borates vary widely and depend not only on the starting materials, but also on the preparation method. This situation led to investigations into the mechanisms involved in the thermoluminescence [1, 2], and into how they depend on the synthesis methods [3-5]. So far, Li 2 B 4 O 7 containing copper is the most promising material. From TL, photoluminescence (PL), electron spin resonance (ESR) and X-ray diffraction (XRD) measurements it has been concluded that samples of lithium tetraborate having high sensitivity are those prepared either by sintering or the Czochralski method, and that copper is in the monovalent state Cu + [6, 7]. lt has been observed that in samples having low efficiency a significant amount of copper is present as Cu ++ [6]. Upon irradiation an ESR signal shows up in crystalline samples having only monovalent copper indicating that some Cu + ions have become Cu 0 [7]. It has been suggested that the peaks showing up in the glow curves of lithium tetraborate are related to the relaxation of excited copper Cu + ions, i.e., (Cu + )*, according to either of the following mechanisms: Cu 0 + hole (Cu + )* Cu + + hv, or Cu 2+ + electron (Cu + )* Cu + + hv. In both cases the relaxation has been ascribed to a 3d 9 4s 3d 10 transition [1,7,8]. This transition gives rise to a peak in the emission spectrum of lithium tetraborate at nearly 360 nm. This value agrees satisfactorily with the 381 nm peak corresponding to one of the four wavelengths related to the transition 3d 9 4s 3d 10 [9]. In single crystals of lithium tetraborate grown by the Czochralski method, ESR measurements show that the first mechanism is involved in the TL [7]. As to other borates, investigations into the thermoluminescence of both borate and calcium borate containing copper have been reported [10-12]. Contrary to what is observed in lithium tetraborate, glassy calcium tetraborate has a better efficiency than sintered samples [12]. The thermoluminescence of sintered calcium tetraborate containing Dy and Eu has been reported [13]. To our knowiedge, no attempt has been made to dope other borates like, sodium borate and potassium borate in order to investigate the changes brought about by dopants in their TL properties. In this letter we report the findings of our investigations into the therinoluminescence of sodium borate compounds containing copper with a dopant concentration of 0.1%mol. This dopant concentration has been chosen because it is equal to that yielding the highest efficiency in lithium tetraborate. The doped compounds have been prepared from amorphous sodium tetraborate supplied by Merck (Merck 6306) and following a procedure similar to that described in [14] for the synthesis of doped lithium tetraborate. This procedure was chosen because lithium tetraborate with high 363

3 efficiency is obtained. The copper was introduced by adding CuCl 2 (Merck 2733) to pure compounds. The diffusion of the doping ions into the lattice of the compounds has been carried out by baking the batches in a Pt crucible at temperatures near the melting point (741 C). Every batch was crushed with a pestle in an agate mortar and later held in a furnace at 400 C for 1 h. The grain size of the powders ranged from 20 to 150 µm. X-ray experiments showed that the compounds baked at temperatures higher than the melting point are glassy, while those prepared at lower temperatures are crystalline. Glow curves have been recorded employing a phototube EMI 6097B. Before irradiation, reutilized samples were annealed at 400 C for 15 min. The spectral distribution of the light emitted at different temperatures was found by employing long-wave pass filters as described in [15]. By fitting the data with the Kringing method included in the Origin 4.1, isometric plots have been computed. Glow curves have been obtained employing 15 mg of powder. The samples were given doses ranging from 0.1 to 20 Gy at room temperarture employing a 60 Co radiation source. The heating rate was 1 C s -1. Amorphous powders containing copper have been prepared by baking the compounds at 756 C They have a much higher efficiency than that of the pure compound. The glow curve of doped compounds is shown in Figure 1 along with the glow curve of the TLD-100 dosimeter. They were recorded 4 days after irradiation. The low temperature part of the glow curve of sodium borate (below 120 C) shows a strong fading similar to that observed in pure compounds [16]. Figure 1. Glow curves of amorphous and crystalline doped sodium borate powders. The glow curve of TLD-100 powder is shown for comparison. 364

4 Figure 2. Isometric plots of (a) amorphous and (b) crystalline sodiurn borate. lt disappears after 30 days. There is a high temperature peak whose maximum lies at 400 C. Only the initial rise of the peak is shown because its intensity varies randomly. lf mechanical energy is given to a sample that has not been previously irradiated a glow curve is obtained that consists only of the high temperature peak. This finding explains why the intensity of the peak varies randomly for a given dose since it can be caused by both ionizing radiation and mechanical stimulation. The isometric plot of doped compounds is shown in Figure 2. The light covers the whole range of wavelengths shown in the figure. This result contrasts sharply with the change in the spectrum of lithium tetraborate caused by the addition of copper. Pure lithium tetraborate emits light whose wavelengths cover the whole visible region [16]. The addition of copper brings about a significant change in the wavelength composition of the light, i.e. the spectral distribution shows a strong peak around 360 nm resulting from transitions of excited monovalent copper ions to their ground states, as described above [8, 17]. Therefore the emission spectrum of glassy sodium borate cannot be assumed to be caused by monovalent copper. lt should be noted that McKeever [18] reported that the emission spectrum of LiF containing Cu ++ consists of a broad peak ranging from 350 to 600 nm and having its maximum at approximately 425 nm. A similar emission spectrum is observed in borate glasses containing copper [10]. The suggested mechanism is: Cu + + hole (Cu ++ )* Cu ++ + hv Since the emission spectrum of glassy sodium tetraborate extends from 400 to 600 nm it is possible that Cu ++ ions are involved in the TL processes. To determine the valence of copper in glassy sodium borate, ESR measurements have been performed. The spectrum of an irradiated sample is shown in Figure 3. lt exhibits the hyperfine structure typical for isolated Cu ++ and is similar to the spectrum reported by Hosono et al. [19] for glassy sodium tetraborate containing copper. Ramana et al. [20] reported a similar signal for glassy L 2 0-Na 2 0-B compounds, and Gundurao et al. [6] for glassy Li 2 B Thus, this result lends support to Cu ++ ions being related to the emission spectrum of glassy compounds. No change of the ESR signal is observed by annealing an irradiated sample at temperatures ranging between 100 and 450 C. It follows that the amount of Cu + involved in the TL processes is negligible compared to the amount of divalent copper. 365

5 Figure 3. Electron spin resonance spectra at room temperature of (a) a glassy sample and (b) a crystalline sample. Microwave frequency: 9.58 GHz. Modulation frequency: 100 khz. The modulation amplitude to obtair the spectrum shown in (a) is 0.5 G and 1.25 G for that shown in (b). Compounds prepared via diffusion of the dopant at 715 C have a higher response to gamma radiation than those baked at temperatures above the melting point, as can be seen in Figure 1. The glow curve is made of three peaks: a peak below 100 C, which has a strong fading, a broad peak at 250 C, which is thermally stable, and a peak similar to that shown at high temperatures in amorphous compounds, whose intensity varies randomly. The spectral composition of the light is quite different from the spectra of both pure and copper doped glassy compounds. It has a broad peak having its maximum at nearly 500 nm, as shown in the isometric plot (Figure 2). Other copper doped borates reported in the literature also exhibit a peak in their emission spectrum. As mentioned above, the emission spectrum of lithium tetraborate is made up of a peak at 360 nm. Copper doped borate glass has a peak at about 448 nm [10], and for copper doped calcium tetraborate the peak shows up at 540 nm [11]. Although the last wavelength differs significantly from those corresponding to the 3d 9 4s 3d 10 transition of excited Cu + to its ground state (wavelengths between 381 and 456 nm) the peak at 540 nm has been attributed to monovalent copper [11]. As with the mentioned borates containing copper the peak at nearly 500 nm could be ascribed to 3d 9 4s 3d 10 transitions of excited Cu + ions. lf so, monovalent copper must be present in the crystalline samples. ESR measurements reveal the presence of Cu + ions. The Cu ++ 1 signal of crystalline samples is several times lower than that observed in glasses (see Figure 3b). Since the concentration of copper is the same in crystalline and glassy compounds it follows that the majority of copper is in monovalent form. A possible explanation for the observed emission spectrum is that it is made of the superposition of both the emission of monovalent copper ions and that of divalent copper ions. The latter accounts for the width of the spectrum and the former for the appearance of a peak. Upon irradiation some Cu + ions should change to either Cu ++ or Cu 0. No difference has been observed between the intensity of the ESR signal of irradiated samples and that of the same samples heated at 400 C for 15 min. This result may be consistent with the emission of light by monovalent copper if the amount of the Cu + ions that change to Cu 0 or Cu ++ upon irradiation for doses lower than 20 Gy is negligible compared to the amount of divalent copper, so that the conversion may not be reflected in the intensity of the Cu + signal. In order to identify the borate compounds present in the crystalline samples we employed a Philips PW3710 difractometer. The main finding is that although the samples have been prepared from amorphous sodium tetraborate free of water different borates containing water are present. The following compounds have been identified: Na 2 B 4 O 7.5H 2 O, Na 2 B 4 O 5 (OH) 4.8H 2 O, Na 2 B 4 O 6 (OH) 2.3H 2 O, Na 2 B 5 O 8.5H 2 O and Na 2 B 5 O 8 (OH).H 2 O. The infrared spectra of crystalline samples have been recorded with a FTIR Nicolet Magna 500. They exhibit absorption bands characteristic of the H 2 O and HO - groups. Since lithium borate prepared following the same procedure employed for the synthesis of sodium borate has a much better TL efficieney, as 366

6 mentioned above, its X-ray diffraction pattern was recorded to determine the compounds present in the samples. The analysis of the spectra indicates that the only compound present is anhydrous lithium tetraborate. Summarizing, the findings reported in this letter indicate that the aggregation state of sodium borate compounds, crystalline or amorphous, has a strong influence on the valence state of the copper ions. As for both lithium tetraborate and lithium fluoride compounds containing copper, the highest TL efficiency is related to the presence of monovalent copper. The broad peak at 500 nm shown in the emission spectrum of crystalline compounds partially accounts for the low TL sensitivity since the wavelengths corresponding to the higher intensities do not match the optimal spectral range for photomultiplier tubes. Since ESR measurements show that both Cu + and Cu ++ ions are present in the crystalline samples it is suggested that the emission spectrum could possibly result from the addition of the emissions of both ions. Finally X-ray diffraction measurements reveal that: the samples are made of different borate compounds. Since lithium tetraborate having high efficiency is anhydrous, it is interesting to search for a synthesis method that allows production of anhydrous sodium tetraborate and then to investigate its TL properties. This work is on progress in our laboratories. ACKNOWLEDGMENT Grateful acknowledgment is made to Dr. Julio Sanchez and Jorge Danazo from Sanatorio Tandil for irradiating the samples. 367

7 REFERENCES 1. J.K. SRIVASTAVA and S.J. SUPE, J Phys. D, Appl. Phys. 22 (1989) M. MARTINI, C. FURETTA, C. SANIPOLI, A. SCACCO and K. SOMAIAH, Radiat. Eff. Defects Solids 135 (1995) D.I. SHAHARE, B.T. DESHMUKH, S.V. MOHARIL, S.M. DHOPTE, P.L. MUTHAL and V.K. KONDAWAR, Phys. Status Solidi A 141 (1994) M. TAKENAGA, O. YAMAMOTO and T. YAMASHITA, Health Phys. 44 (1983) A.R. LAKSHMANAN, B. CHANDRA and R.C. B H AT T, Radiat. Prot. Dosim. 2 (1982) T.K. GUNDURAO, J.R. SAMADE and S.V. MOHARIL, Radiat. Eff. Defects Solids 138 (1996) Y. KUTOMI and A. TOMITA, Radiat. Prot. Dosim. 33 (1990) Y. KUTOMI, A. TOMITA and N. TAKEUCHI, ibid. 17 (1986) C.E. MOORE, in Atomic Energy Levels, Circular Natl. Bur. Stand. 467 (11) (1952) Y. FUKUDA, T. OKUNO and N. TAKEUCHI, Radiat. Prot. Dosim. 6 (1984) Y. FUKUDA and N. TAKEUCHI, J. Mater Sci. Lett. 4 (1985) Y. FUKUDA, A. TOMITA and N. TAKEUCHI, Phys. Status Solidi A 85 (1984) K Y. FUKUDA, K. MIZUGUCHI and N. TAKEUCHI, Radiat. Prot. Dosim. 17 (1986) S. LORRAIN, J.P. DAVID, R. VISOCEKAS and G. MARINELLO, ibid. 17 (1986) F. SPANO, E. CASELLI, R. WAINSCHENKER and G. SANCHEZ, J. Phys. Condens. Matter 3 (1991) G. SANCHEZ, F. SPANO, E. CASELLI, E. MEURISSE, M. CUTTELA and M. MANSANTI, ibid. 1 (1989) M. MARTINI, F. MEINARDI, L. KOVACS and K. POLGART, Radiat, Prot. Dosim. 65 (1996) S.W.S. McKEEVER, J Phys. D., Appl. Phys. 24 (1991) H. HOSONO, H. KAWASOWE and T. KANAZAWA, J. Non-Cryst. Solids 33 (1979) M.V. RAMANA, K. SIVA KUMAR, S. RAHMAN, D SURESH BABU, S.G. SATHYANARAYAN and G. S. SASTRY, J. Mater Sci. Lett. 8 (1989) Volver a Cotenido 368