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Nuclear Instruments and Methods in Physics Research A 582 (2007) 484–488 www.elsevier.com/locate/nima
Fricke xylenol gel characterization using a photoacustic technique A.M.F. Caldeiraa,, A. de Almeidaa, A.M. Netob, M.L. Baessob, A.C. Bentob, M.A. Silvac a
Departamento de Fı´sica e Matema´tica, Universidade de Sa˜o Paulo, Av. Bandeirantes 3900, 1040-91 Ribeira˜o Preto, SP, Brazil b Departamento de Fı´sica, Universidade Estadual de Maringa´, Av. Colombo 5790, 87020-90 Maringa´, PR, Brazil c Centro de Oncologia e Radioterapia Sant’ Ana, Av. Tiradentes 1377, 87013-260 Maringa´, PR, Brazil Received 13 July 2007; received in revised form 27 August 2007; accepted 1 September 2007 Available online 7 September 2007
Abstract Fricke chemical dosimetry measurements of the absorbed dose of ionizing radiation depend on the quality and characteristics of the system that reads each dosimeter. The final accuracy is significantly dependent on the technique used for measuring the chemical concentration changes in the dosimeters. We have used a photoacoustic technique to detect the Fricke xylenol gel (FXG) optical absorbance. The FXG, a derivation of the aqueous Fricke dosimeter, is made more sensitive and stable with addition of gelatin (300 Bloom) and xylenol orange. The light intensity transmitted through an FXG sample before and after irradiation was measured with an acoustic detector. The incremental optical absorbance is directly proportional to the ionizing radiation absorbed dose. We present the optical absorbance measurements as a function of absorbed dose and of post-irradiation time. We apply our photoacoustic technique to determine absorbed dose profiles. The results show that the photoacoustic technique applied to FXG provides a new dosimetric system, as good as those already established using spectrophotometric techniques. r 2007 Published by Elsevier B.V. PACS: 58.60.Gz; 87.58.Vr; 87.58.Sp Keywords: Photoacoustic technique; FXG; Optical absorbance; Absorbed dose
1. Introduction Chemical dosimetry has been used to obtain the spatial dose distribution for a selected beam and radiation geometry [1,2], for example for radiology and radiotherapy measurements. The accuracy of these dosimeters depends on the associated instrumentation. Several techniques can detect the changes in chemical dosimeters caused by ionizing radiation. Initially chemical dosimeters were investigated by potentiometric titration, later by UV–vis spectroscopy and, recently with nuclear magnetic resonance techniques [3–5]. We propose a photoacoustic technique to detect changes in the Fricke xylenol gel (FXG) optical absorbance values. Photoacoustic methods have a long history in analytical techniques, including those related to health [6,7]. In 1880, Bell discovered the photoacoustic effect [8] and later the Corresponding author. Tel.: +55 16 3602 4869; fax: +55 16 633 9949.
E-mail address:
[email protected] (A.M.F. Caldeira). 0168-9002/$ - see front matter r 2007 Published by Elsevier B.V. doi:10.1016/j.nima.2007.09.001
theoretical interpretation and applications to solid materials were given, by Parker [9] and Rosencwaig and Gersho [10], respectively. Photoacoustic spectroscopy (PAS) is a well-established experimental technique based on the measurement of the pressure increases, resulting from the non-radioative de-excitation processes that occur in a material in a closed volume, excited by a modulated electromagnetic radiation [11]. In conventional PAS, the sample to be investigated, solid or liquid, is placed inside a closed chamber (the photoacoustic cell, PAC) filled with air or a suitable gas. Modulated radiation energy is incident through a transparent window onto the sample surface. The pressure change, proportional to the energy absorbed in the sample, are detected by a sensitive microphone attached to the PAC [12]. The FXG is a system composed of ferrous ions that are induced to oxidize by ionizing radiation to ferric ions, forming a colored xylenol orange–ferric (XO–Fe3+) complex [13]. This complex has an absorption band centered at 585 nm, whose intensity increases linearly with the
ARTICLE IN PRESS A.M.F. Caldeira et al. / Nuclear Instruments and Methods in Physics Research A 582 (2007) 484–488
absorbed dose. Therefore, it is possible to determine the values of the absorbed dose indirectly by measuring the FXG optical absorbance [14]. We evaluated the accuracy and stability of the photoacoustic FXG system. Measurements were made to confirm the maximum in the optical absorbance spectrum and we determined the linear dependence of the optical absorbance on the absorbed dose. Also, we evaluated the absorbed dose limits, the fading and the spatial resolution for field size profiles. The dosimetric results obtained present linearity and a 2% reproducibility in the 0.4–10 Gy interval, comparable to those reported for other dosimeters. We can conclude that the photoacoustic technique can be used with the FXG radiation dosimeter. 2. Materials and methods The FXG dosimeter was prepared by mixing gelatin (300 Bloom powder from Aldrich), ferrous ammonium sulfate, xylenol orange, sulfuric acid and milli Q water [4,5,13,14]. This mixture was irradiated with 60Co g-beams to absorb dose in the range from 0.4 to 45 Gy. The incremental optical transmittance was measured at 585 nm by PAS. PAS measurements are seriously compromised by gas leaks into the PAC or by possible interference caused by volatile samples, both may affect the pressure on microphone membrane and consequently the signal [11,12]. Since the FXG is volatile, it is not convenient to use the normal PAS configuration. We have modified the PAS to operate in the transmission mode (TPAS) [15]. This mode employs the conventional gas-microphone type detection cell that contains a fine grain carbon black powder visible through a quartz window. The schematic representation of the TPAS experimental setup is shown in Fig. 1. The sample is placed on the PAC window and exposed to a monochromatic light source modulated by a chopper. The sample transmits part of the light into the PAC, where it is totally absorbed by the carbon black powder [15]. The sample transmittance is normalized with the response of the carbon black recorded with an identical sample that has not been irradiated. The result is the incremental optical absorption of the irradiated sample.
Fig. 1. (a) Photoacoustic experimental setup: Xe, xenon arc lamp; MC, monochromator; CH, chopper; E, mirror; L, lens system; PHC, photoacoustic cell; CB, carbon black; mic, microphone; LK, lock-in amplifier; PC, computer. (b) FXG sample position and photoacoustic cell.
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The measurements are recorded at room temperature using a chopper frequency of 30 Hz with an 800 W xenon (Xe) light source and a monochromator (Oriel Instruments/77250). A series of lenses (L) collimate the monochromatic beam through the FXG sample. The PAS microphone signal is detected by a lock-in amplifier (LK) (EG & G Instruments/5110) in synchrony with the incident light beam chopper. The results are stored in a computer (PC). We investigated the optical absorbance of the FXG dosimeters in the wavelength interval from 350 to 620 nm. For all the subsequent measurements, we selected the wavelength of 585 nm at which the FXG has its maximum sensitivity [16]. The linearity of the photoacoustic technique was verified by measuring low and high radiation doses. We confirmed the 60Co g-beam homogeneity and measured the fading as a function of temperature, postirradiation time, and absorbed dose. For comparison with conventional FXG dosimetry, the optical transmission spectra of FXG samples were also obtained with the spectrophotometers CVI Spectral Products/SM 240, Beckman/DU 640 and Varian/CaryVarian for absorbed dose from 0 to 10 Gy. The sensitivity was determined by the average slope of the results from the three spectrophotometers. 3. Results and discussion To analyze the experimental optical absorbance spectrum of an FXG sample irradiated with 6 Gy of absorbed dose, Lorentzian fittings were applied and are presented in Fig. 2. The bands centered at 460 and 585 nm correspond, respectively, to the contribution of the free xylenol orange concentration and concentration of the XO–Fe3+ complex, in accordance with previous reports [17,18]. Repeated measurements of separately irradiated FXG samples with
Fig. 2. Experimental curve of optical absorbance of FXG after irradiation with 6 Gy and a Lorentz fitting (S) in the wavelength range 450–610 nm.
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the system presented in this work revealed 2% reproducibility at 585 nm. Fig. 3 presents the incremental optical absorbance spectra of FXG samples exposed to different g-ray doses. It can be seen that the incremental absorption in each peak varies in opposite directions as the irradiation dose increases. This can be explained as follows: with increasing absorbed dose, the amount of complex XO–Fe3+ increases, while the concentration of free xylenol orange in the FXG decreases. One can see that the absorbance increase is more pronounced at 585 nm than the absorbance decrease at 460 nm, showing that the supply of ferrous ions is far from exhausted, a condition necessary for the linear response to radiation [19]. The behavior of the spectra obtained with PAS (Figs. 2 and 3), is in agreement with results reported by Bero et al. [13,19] and Gambarini et al. [20]. In Fig. 4, the FXG sensitivity values at 585 nm, in the dose range 0.4–10 Gy, were determined by comparing PAS and spectrophotometry measurements. The linear increase in optical absorbance with absorbed dose was confirmed. This result shows that both optical techniques are equivalent. The sensitivity discrepancy between the PAS (aa ¼ 0.056 Gy1) and spectrophotometer values (ab ¼ 0.057 Gy1) is within 1% experimental error. The behavior of the FXG-photoacoustic system versus absorbed dose is presented in Fig. 5. The PAS linear region obtained from FXG sample is from 0.4 to 32 Gy, which is comparable with those reported for FXG by other authors [21,22] using spectrophotometer measurements. The linearity of the dose–response curve is adequate to about 32 Gy. Above this limit, the complex XO–Fe3+ optical absorbance starts to saturate, owing to the lack of ferrous ions to oxidize and/or to the low concentration of free xylenol orange to form the complex with ferric ion.
Fig. 4. The dose–response curve slope (a and b) for the range up to10 Gy for (n) photoacoustics and for three spectrophotometers average (J)— CVI Spectral Products model SM 240, Beckman model DU 640 and Varian model Cary-Varian.
Fig. 5. Calibration curve for the photoacoustic FXG system. Optical absorbance curve versus radiation dose at 585 nm.
Fig. 3. Several optical absorbance spectra relative to those from a nonirradiated sample. The FXG samples were irradiated with g-ray doses of 2, 4, 6, 8 and 10 Gy.
In Fig. 6, the optical absorbance behavior versus time is compared for irradiated and non-irradiated samples that are maintained at 25 and 12 1C. The optical absorbance of the sample held at the lower temperature increases linearly with time by 4.1 103 h1 for the non-irradiated sample and by 1.9 103 h1 for the irradiated one. For the samples maintained at higher temperature, the natural oxidation rate is faster owing to a higher reaction rate and to a larger ionic mobility, because the ionizing radiation reduces the oxygen concentration in FXG [23–25] and because additional oxidation depends on the oxygen concentration, one should expect a slower oxidation for irradiated samples.
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Fig. 6. Optical absorbance dependence on irradiation time, storage temperature, and absorbed dose; (&) irradiated samples stored at 12 1C; (J) irradiated samples, stored at 25 1C; (’) non-irradiated samples stored at 12 1C; (K) non-irradiated samples stored at 25 1C. All were Gel FXG with 0.1 mM of xylenol orange and 0.5 mM of ferrous sulfate measured at wavelength 585 nm.
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Fig. 8. Optical absorbance values at 585 nm of FXG samples, after irradiation with 10 Gy, along the X and Y axes in the phantom shown in Fig. 7.
4. Conclusions This study proves that TPAS is an innovative alternative method to determine absorbed doses with an FXG dosimeter. The photoacoustic technique can characterize and determine optical changes in FXG and its sensitivity is comparable with the spectrophotometric FXG dosimetric detectors generally used for chemical dosimetry. Acknowledgments This work was supported by Coordenac- a˜o de Aperfeicoamento de Pessoal de Nı´ vel Superior (CAPES) and by Conselho Nacional de Desenvolvimento Cientı´ fico e Tecnolo´gico (CNPq). Thanks to Centro de Oncologia e Radioterapia Sant’ Ana, Av. Tiradentes 1377, 87013-260 Maringa´, PR, Brazil and to the Departamento de Fı´ sica da Universidade Estadual de Maringa´ (DFI-UEM). Fig. 7. Diagram FXG samples along the central axes of a 15 15 cm radiation field.
Finally, we demonstrated the PAS technique by measuring the 60Co g-beam homogeneity. The FXG samples were positioned along the X- and Y-axes in a 15 15 cm2 area, as shown in Fig. 7. The samples were irradiated with 10 Gy and their optical absorbances were recorded at 585 nm (Fig. 8). These optical absorbances presented percent variations of 2.3% along the X-axis and 2.0% along the Y-axis, representing 80% of the beam area. For the other 20% of the area, there are optical absorbance gradients owing to collimator penumbra. The profiles obtained agree with ICRU and AAPM’s protocols [26,27] that recommend a maximum uncertainty of 3% in 80% region of the field size.
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