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N. Kazuchits et al. / Medical Physics in the Baltic States 7 (2009) 76 - 78 3. Results and discussion Temporal dependence of current for several γ-rays 137 Cs dose rates for the synthetic diamond detector is shown in figure 1a. For comparison, the similar characteristic for IIa-type natural diamond detector is shown in figure 1b. For both detectors maximum irradiation dose rate is equal 6,02 сGy/s and bias voltage is 30 V. As shown in figure 1, both detectors require preirradiation dose about 10 Gy for current stabilization. Requirement of pre-irradiation is character feature of all types of diamond detectors and related to trapping of charge carriers on the local states in the band gap of material. It is necessary to note that sensitivity of synthetic diamond is about 20 times higher than that of natural diamond. Nevertheless, the synthetic diamond detector current fluctuations are higher and reach the value about 3%. Fig. 1. Temporal dependence of current for several γ-rays 137 Cs dose rates for synthetic (a) and natural (b) diamond detectors Knowledge of the detector dose rate response is of importance for accurate dosimetry. Figure 2 shows such dependences for both types of detectors under the γ-rays irradiation of 137 Cs source. For natural and synthetic diamond detectors this dependence is close to linear and can be approximated by the equation [5]: i = i0 + RD ∆ , (1), where i - detector current, i0 - detector current without irradiation (dark current), D – the dose rate, R – sensitivity, and ∆ - linearity index. Fit of the dependences in figure 2 with expression (1) with fitting parameters R and ∆ gives ∆ = 1,011, R = 0,1 μC/Gy, and ∆ = 0,978, R = 1,7 μC/Gy for natural and synthetic diamond respectively. 77 Fig. 2. Dose rate dependence of current for natural and synthetic diamond detectors under γ-rays 137 Cs source exposure The suitability of the synthetic diamond detector for γrays registration was investigated by measuring depthdose curves and profiles in 6 and 18 MeV scanning photon beam of “Varian” linear accelerator. Relative dose rate distribution measurements by both PTW natural diamond and synthetic diamond detectors were carried out in water phantom. Detectors were placed perpendicular to the beam axis. Depth-dose curve of a 6 MeV photon beam measured by the PTW natural diamond detector is presented in figure 3. Fig. 3. Depth-dose curve of a 6 MeV photon beam measured by PTW natural diamond detector It was not possible to obtain such dependence for synthetic diamond detector because of its current exceeds maximum threshold of used PTW registration equipment. For this reason further dose rate distribution measurements were carried out while detectors were submerged in water phantom on depth of 150 mm. Radiation field distribution is shown in figures 4 and 5 for natural and synthetic diamond detectors respectively. Parameters of registered radiation field are presented in table 1. As shown in figure 5, synthetic diamond detector measurements with integration time of 0,5 s are characterized by higher signal fluctuations in comparison with natural diamond detector. Such
N. Kazuchits et al. / Medical Physics in the Baltic States 7 (2009) 76 - 78 results are in good agreement with mentioned above signal fluctuations. The same measurements carried out with integration time of 4,5 s correspond to the reference measurements with natural diamond detector. Fig. 4. Profile of a narrow 6 MeV photon beam measured by the PTW natural diamond detector. Integration time is 0,5 s. Field size is 10 cm×10 cm. Depth is 150 mm. Fig. 5. Profile of a narrow 6 MeV photon beam measured by synthetic diamond detector with integration time of 0,5 s and 4,5 s. Field size is 10 cm×10 cm. Depth is 150 mm. 78 Table 1. Parameters of a 6 and 18 MeV photon beam measured by both type diamond detectors with various integration time. Field size is 10 cm×10 cm. Depth is 150 mm. Parameter γ-rays 6 МeV PTW Synth γ-rays 18 МeV PTW Synth Integr. time, s 0,5 0,5 4,5 0,5 0,5 4,5 Left pen, mm 6,96 6,63 6,82 8,06 8,36 7,71 Right pen, mm 6,83 6,81 6,48 8,37 8,22 8,09 Field width 11,49 11,53 11,54 11,5 11,53 11,55 (on 50%), cm Field heterogeneity, % 3,89 4,51 3,14 2,07 2,76 2,53 Other parameters of a radiating field, such as the field size and penumbras sizes (i.e. the distance between relative dose levels of 80 % and 20 %) have close values for both tested diamond detectors. 4. Conclusion The presented results indicate that the synthetic diamond detector is comparable with PTW natural diamond detector and can be used for accurate measurements of dose rate distributions with a high dose gradient in high-energy photon beam for a wide variety of applications, especially when high response and high spatial resolution are required. 5. References 1. http://www.ptw.de. 2. Marczewska B., Nowak T., Olko p., Gajewski W., Pal'yanov Yu., Kupriyanov I., and Waligorski M.. Synthetic diamonds as active detectors of ionizing radiation. Diamond and Related Materials, V. 13, 2004. p. 918-922. 3. Descamps C., Tromson D., Mer C., Nesladek M., Bergonzo P., and Benabdesselam M.. Synthetic diamond devices for radio-oncology applications. Phys. Stat. Sol. (a), V. 203, 2006. p. 3161-3166. 4. The properties of natural and synthetic diamond. Edited by J.E. Field, Academic press, 1992. p. 710. 5. Laub W., Kaulich T. and Nusslin F.. A diamond detector in the dosimetry of high-energy electron and photon beams. Phys. Med. Biol., V. 44, 1999. p. 2183-2192.
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2009 PROCEEDINGS OF THE 7 th INTERN
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Executive editor: D.Adlienė CONFER
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11.15-11.30 Daiva Leščiute, Valda
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MEDICAL PHYSICS IN THE BALTIC STATE
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