Journal of Non-Crystalline Solids 352 (2006) 3663–3667 www.elsevier.com/locate/jnoncrysol
ESR dosimeter based on P2O5–CaO–Na2O glass system Z.M. Da Costa *, W.M. Pontuschka, J.M. Giehl, C.R. Da Costa Instituto de Fı´sica da Universidade de Sa˜o Paulo/IFUSP, Departamento de Fı´sica Geral-Ala I, Cidade Universita´ria, R. do Mata˜o, Travessa R. 187, 05508-900 Sa˜o Paulo, Brazil Available online 4 August 2006
Abstract This paper presents the results of a study of thermal properties, solubility and response to 60Co c-rays by electron spin resonance of the P2O5–CaO–Na2O glass system. The sample compositions were selected by fixing the P2O5 mol% content at 50 mol%, and varying the CaO mol% at 30 and 40 mol%. The spectrum is characterized by hyperfine doublet from 31P isotope (nuclear spin = 1/2), and its stability and response to the c-ray dose were studied to establish the suitability of this glass as a c-ray dosimeter. 2006 Elsevier B.V. All rights reserved. Keyword: Bioglass
1. Introduction Phosphate-based glass materials have potential use as biomaterials, because of their chemical composition similarity to that of natural bone. Simple phosphate glasses do not have enough chemical durability for applications. Since the discovery of bioglassTM (BG) by Hench [1], much research has focused on glasses and biomedical glasses as promising materials for different diversified applications. This paper presents a systematic study of the CaO– Na2O–P2O5 glass system, which consists of components that are also present in the human body, with the purpose to develop a suitable material for dosimetric purposes. Dosimetric evaluation is important especially at highdose levels, so that many devices have been developed and applied in various work areas. Glass dosimeters are important means for the various different high-dose level applications, e.g., for medical, industrial and food irradiation purposes. Soda-lime silica glasses have been used as radiation dosimeters in medical areas by using their color change property as a function of the absorbed dose and that glass system has been proven suitable for c-radiation
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[email protected] (Z.M. Da Costa).
0022-3093/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2006.03.113
dosimetry for up to 8 kGy [2–4], where 1 gray (Gy) is equivalent to 1 J/kg = 100 rad. Several other glass systems have been evaluated for dosimetric purpose in order to determine integrated high doses by measuring the optical density as a function of radiation dose [5]. A number of high-level exposure applications, employing mainly spectrophotometric or densitometric analysis of color-center formation have been reported [2,6]. In addition, different borate, silicate and phosphate glasses using optical and ESR techniques have shown to be considerably promising for high dose gamma measurements [7]. When a glass is exposed to ionizing radiation numerous changes in the physical properties can take place. The interaction between radiation and glasses causes a variety of changes depending on the composition and structure of the materials, as well as the irradiation parameters, energy dose and dose rate. One of the essential processes occurring during the irradiation of the samples by c-rays is the formation of electron–hole pairs. These free carriers can move and recombine, so that the photoelectrons are trapped at structural defects or impurities such as oxygen vacancies and multivalent impurities, while the holes are self-trapped at bridging or non-bridging oxygens. These new electronic configurations give rise to some preferential high absorption levels called ‘color centers’ [8].
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Optical absorption (AO) and electron spin resonance (ESR) studies of different types of irradiated glasses proved that many questions concerned not only with the nature of the defect centers but also with the glass structure can be explained. ESR is a non-destructive technique that consists in the application of physical principles based on the number of unpaired spins created on irradiation (holes and electrons trapped at different sites in the glass structure). ESR spectroscopy is currently employed in dosimetry and several materials are suitable for this purpose. Among these, some types of glasses based on P2O5–CaO–Na2O were chosen because its composition is similar to that of bones [9–13]. As their constituent atoms are found in the mineral phase of bone, they have a chemical affinity with natural bone and the ease of manufacture of these phosphate glasses makes them useful for many applications [10]. Radiation induced defects in phosphate glasses of different compositions are widely studied. The radiation induced hole centers in alkali phosphate glasses are characterized by the ESR hyperfine doublet of 31P isotope (nuclear spin = 1/2) were studied by different authors [14–21], but irradiated bioglasses of different compositions were previously investigated by Padlyak [9]. They found that the efficiency of the generation of electron and hole centers depends on the basic bioglass composition and is almost independent of the type of ionizing radiation source. The phosphate glass network is dominated by a pattern of the linkages between the PO4 tetrahedral structural units. In the case of vitreous P2O5 (one of the four Zachariasen glass-forming oxides: SiO2, GeO, B2O3 and P2O5) the three- and four-coordinated glassforming elements are linked by the vertices and not by the edges [22–25]. The basic building blocks of crystalline and amorphous phosphates, the PO4 tetrahedra result from the formation of four sp3 hybrid orbitals linking the P with the four neighboring oxygens. The fifth electron of the p-orbital is mixed with a 3d orbital where strong p-bonding molecular orbitals are formed with oxygen 2p electrons [26–30]. The P2O5–CaO–Na2O glasses consist of a polymer like structure of regular tetrahedrons based on PO4 groups linked by the vertices and interrupted by the glass modifier ions Ca2+ and Na+ that enter into the glass network modifier positions acting locally as charge compensators (Fig. 1(a) and (b)). There is a need for the standardization of high absorbed-dose measurements in the radiation processing industry in order to provide measurement, assurance and traceability of standards. Some physical parameters such as dose response, thermal stability of the centers, dissolution properties and signal reproducibility were determined aiming to standardize the sample preparation method and measurement conditions.
Fig. 1. Schematic representation chain structure of phosphate glass structure: (a) cross-link formation by Na+ ion, (b) cross-link formation by Ca2+ ion.
2. Experimental details 2.1. Preparation of the glass Two glass compositions have been prepared using NaH2PO4, CaCO3 and P2O5 as starting materials: P50C30N20 (P2O5 50 mol%; CaO 30 mol%; Na2O 20 mol%) and P50C40N10 (P2O5 50 mol%; CaO 40 mol%; Na2O 10 mol%). The oxides were weighed, homogenized and placed into an alumina crucible. The batch was heated in air at a furnace temperature between 1000 and 1100 C for 1 h. The glass was poured into a Pt or stainless steel mould, preheated at 350 C. The mould was then placed back into this annealing furnace for 1 h and left to cool slowly to room temperature in order to remove residual stress. 2.2. Dosimeter calibration The most important requirement in dosimetry radiation processing is the provision for a careful calibration of the routine response dosimeters as a function of absorbed dose over the exposure energy interval and absorbed dose-rate range of interest. The samples used as dosimeters were calibrated by using the following procedure. • Initial visual inspection was performed to detect any flaws that might render the batch unsuitable. • The samples were protected from sunlight, fluorescent lamps, and other light sources and stored in the dark. • Random samples were taken from the batch for being calibrated. These samples were irradiated to specific
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absorbed dose levels and their responses were fitted in function of the peak to peak signal amplitude of the low-field component of the ESR doublet. The validity of this assumption has been verified experimentally for periods of 6–12 months. The glasses were marked at a corner with a permanent ink pen so that consistent orientation could be maintained in all subsequent analysis procedures. Before the calibration irradiations were carried out, the glasses were stored at room temperature (24 C) inside a controlled humidity environment (within the range of 40–50% R.H.) for at least 24 h. After this period, the samples were packaged in an appropriate equilibrium build-up material. This is an important requirement for performing proper calibration measurements. Such condition exists when the number and energy of secondary electrons (generated by the primary photons) entering the volume of interest are equal to the number and energy of the secondary electrons leaving this place [31]. In our case the electronic equilibrium was established surrounding the glass samples with 3 mm of luciteTM. Calibration irradiation was performed in the Gammacell 60Co source, a unit commercially supplied (manufactured by AECL of Canada), with 24 60Co rods of about 4697 Ci total activity in a stationary annular array. The absorbed dose rate in water in January 2005 was 3.88 kGy h1. This rate was calibrated by means of reference transfer dosimeters. The estimated overall uncertainty in this rate is about ±3% at 95% confidence. Each 200 mg of sample was irradiated with dose ranging from 100 Gy to 5 kGy by c-rays from a 60C source, at room temperature, under electronic equilibrium conditions and placed in a fused silica ESR sample tubes. The dosimetric calibrations were carried out in the radiation of the Radiation Technology Center – CTR of IPEN. After irradiation, the dosimeters were stored for 24 h to eliminate transient signal. For each sample the initial ESR signal (before irradiation), S1, and final (after irradiation), S2, were recorded. The dosimeter response was defined as R = S2 S1. The mean R for each group of five dosimeters irradiated in the same conditions was plotted against the absorbed dose. A curve was fitted by means a least squares regression method. Glass samples taken from the same batch were irradiated to some unknown absorbed dose and the reading procedure was performed as described above, yielding the resulting absorbed dose obtained from the calibration curve.
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(model ER 4102 ST) of the Physics Institute, University of Sa˜o Paulo. The spectra were obtained using the field modulation of 100 kHz with peak to peak amplitude of 8 G and the microwave power was 10 mW with frequency of 9.77 GHz. As the first derivative of absorption spectrum is not saturated, the peak to peak amplitude of the signal has linear dose dependence. This work is based on the dose response curve method for dose reconstruction, a non-destructive fast method used to measure the absorbed radiation by a material similar to human tissue. The parameters of calibration curve are shown to be linear, being determined by linear regression analysis. Standard sample of DPPH (a,a-diphenyl, b-picryl hydrazyl) was used to calibrate the ESR g-factor of the signal.
3. Results The ESR spectrum of a typical gamma irradiated glass powder sample, exposed and measured at room temperature, is shown in Fig. 2, characterized by a doublet centered at g = 2.0203 ± 0.0002. The ESR intensity was defined from the optimized spectrometer parameters such as the amplifier gain, 100 kHz field modulation amplitude and microwave power sample amount introduced into the fused quartz tube was always the same (1 cm height). Fig. 3 shows the ESR signal relative intensity in function of the time the samples of P50C40N10 and P50C30N20 were stored at room temperature after c-irradiation. Fig. 4 shows the dose response of ESR intensity for the P50C40N10 glass sample. It is observed a linear relationship between absorbed dose and the peak-to-peak height of the first line.
2.3. ESR measurements The ESR measurements were performed using a conventional X-band spectrometer Bruker–EMX (Bruker Instruments, Billerica, MA) with a standard rectangular cavity
Fig. 2. ESR spectrum, taken at the frequency of 9.77 GHz of c-irradiated P2O5–CaO–Na2O glass system sample (P50C40N10) at room temperature, after exposure to a dose of 1 kGy.
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Fig. 3. The decay of signal intensity for P2O5–CaO-Na2O glass system samples (P50C40N10) and (P50C30N20) stored at room temperature (25 C).
Fig. 4. A linear dose response relationship for P2O5–CaO–Na2O glass system samples (P50C40N10) as a function of the dose.
4. Discussion 4.1. ESR analysis The ESR spectrum of a typical gamma irradiated glass powder sample measured at room temperature is characterized by a doublet centered at g = 2.0203 ± 0.0002 ascribed to phosphorus oxygen hole center (POHC). The term POHC designates the defect responsible for a wellknown ESR spectrum in irradiated phosphate glasses characterized by an approximately axial g tensor with g? 2.009 ± 0.0002 and a small, nearly isotropic 31P hyperfine interaction (40 G) [32]. Among the components of the glass only phosphorus atom has a nuclear spin 1/2, so that this doublet can be ascribed on phosphorus atom. The g value of the doublet is larger than that of the free spin, consistent with the nature of hole center. In view of the occurrence of similar paramagnetic defects in crystalline and glassy SiO2 and GeO2 it would be expected that phosphate glasses and crystalline compounds of same composition might have similar defects,
e.g., the PO 3 radical. There is evidence from NMR spectra that the local order in a glass is similar to the local order in crystalline compound of the same composition [17]. Weeks and Bray [16] demonstrated the existence of at least three 31P hyperfine doublets of large splitting (70– 140 G) in various phosphate glasses subjected to cradiation. Crystalline phosphorus pentoxide P2O5 has a structure in which one of the four oxygen ions is only bound to a phosphorus ion while the remaining three oxygen ions may either be bonded to another phosphorus ion as bridging oxygens (BO) or they can be at the end positions of the chain, as non-bridging oxygens (NBO). In crystalline materials two types of oxygen vacancies can be formed, one type by the removal of an oxygen from the non-bridging site and one type by removal of an oxygen from de bridging site. These two types of oxygen vacancies might be expected to occur in glassy P2O5 since, on average, the same number of BO and NBO per phosphorus ion are expected to be present in the glass as in the crystalline material. The fading in relative ESR intensity for each glass sample was monitored at room temperature for a period of 6 months. As shown in Fig. 3, the radical concentration has stabilized after about two months of storage. We choose the P50C40N10 samples for dosimetric purpose because they have shown a small decrease from the signal intensity after this period. This fading is explained as the electron–hole recombinations based on isothermal decay kinetics of the remaining mestable (PEC’s) at room temperature. There is a continuous distribution of depths of the PEC’s, ranging from very shallow levels to those sufficiently deep to be stable at room temperature. The POHC’s are much deeper and are thus stable at room temperature, but their decrease is noticed because of the reactions with the electrons thermally released from the PEC’s. The observed relationship between absorbed dose and the peak-to-peak height of the ESR first derivative line suggests a linear function in the dose range of 100–103 Gy. The slope of this line is sufficiently high in order to these glass samples be elected as suitable for dosimetric purpose. 5. Conclusions In this work, we have undertaken to obtain some information that could contribute to the development of a method for estimating the absorbed dose using a P2O5– CaO–Na2O glass system samples (P50C40N10) as a dosimeter up to 5 kGy gamma dose level. We have chosen this composition because of its stability and low solubility. The color changes may also be used as another useful parameter for the determination of the absorbed dose, but the linear relationship between absorbed dose and ESR signal intensity in the range of 100 Gy to 5 kGy provide a liable method for dose measurements. If the glass dosimeter is compared with other high-dose dosimeters, e.g., chemical dosimeter, thermoluminescent
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dosimeter, the glass system has some advantages: the system is not expensive and presents stability over a wide dose range. The glass dosimeter for use in industry and medical dosimetric purpose and also radiation sterilization projects can be preferred since they are small, easy to transport, easily available and economic. Acknowledgements The authors gratefully acknowledge the support given to this work by: Instituto de Pesquisas Energe´ticas e Nucleares, IPEN, CNEN/SP, Instituto de Fı´sica da USP/IFUSP, Instituto de Fı´sica da UNICAMP in Brazil. The authors wish to express thanks to Professor Valdemir Ludwig, Professor Luis Carlos Barbosa and Professor Rosaˆngela Itri. References [1] L.L. Hench, R.J. Splinter, W.C. Allen, T.K. Greenlee, J. Biomed. Mater. Res. Symp. 2 (1971) 117. [2] N. Dogan, A.B. Tugrul, Radiat. Meas. 33 (2001) 211. [3] F.M. Ezz-Eldin, F. Abdel-Rehim, A.A. Abdel-Azin, A.A. Ahmed, Med. Phys. 21 (7) (1994) 1085. [4] G.M. Hassan, M.A. Sharaf, O.S. Desouky, Radiat. Meas. 38 (2004) 31. [5] W.A. Hedden, J.F. Kircher, B.W. King, J. Am. Ceram. Soc. 43 (8) (1960) 413. [6] W.L. McLaughlin, Dosimetry for radiation processing, Taylor and Francis Inc, London, 1989. [7] A. Bishay, J. Non-Cryst. Solids 3 (1970) 54. [8] D.L. Griscom, J. Non-Cryst. Solids 40 (1980) 211. [9] B. Padlyak, S. Szaiska, H. Jungner, Bioglass. Opt. Appl. 4 (2000) 709. [10] M. Uo, M. Mizuno, Y. Kuboki, A. Makishima, F. Watari, Biomaterials 19 (1998) 2277.
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