Objective To discuss the method how to identify wall correction factor (K_ wall ). Methods The paper introduce how to use extrapolation of the ionization current measured for different wall thickness by the spherical ...Objective To discuss the method how to identify wall correction factor (K_ wall ). Methods The paper introduce how to use extrapolation of the ionization current measured for different wall thickness by the spherical graphite cavity chamber whose volume is 10 cm3 and 30 cm3 for 60 Co γ-rays. The wall correction factors were determined by this method and Monte Carlo(MC) calculations. Results The results of the present experiment and calculation indicated that the wall correction factor determined by traditional geometry thickness extrapolation method was 1.0% less than that by MC calculation.The result of applying the equivalent mean thickness to calculate with extrapolation method and that of MC calculation were accordable within 0.45%.The relative dose and wall correction factors for the spherical graphite cavity chamber whose volume is 30 cm3 and 50 cm3 of the standards of air kerma of the NIST for 60 Co γ-rays were calculated by MC calculation,and the calculation results were in accordance with the published values of NIST within 0.06%. Conclusion The spherical graphite cavity chamber wall correction factor determined by traditional extrapolation method and MC calculation is accordable with the results of foreign labs (PSDL).;展开更多
Purpose: All present dosimetry protocols recommend well-guarded parallelplate ion chambers for electron dosimetry. For the guard-less Markus chamber, an energy dependent fluence perturbation correction pcav is given. ...Purpose: All present dosimetry protocols recommend well-guarded parallelplate ion chambers for electron dosimetry. For the guard-less Markus chamber, an energy dependent fluence perturbation correction pcav is given. This perturbation correction was experimentally determined by van der Plaetsen by comparison of the read-out of a Markus and a NACP chamber, which was assumed to be “perturbation-free”. Aim of the present study is a Monte Carlo based reiteration of this experiment. Methods: Detailed models of four parallel-plate chambers (Roos, Markus, NACP and Advanced Markus) were designed using the Monte Carlo code EGSnrc and placed in a water phantom. For all chambers, the dose to the active volume filled with low density water was calculated for 13 clinical electron spectra (E0 = 6 - 21 MeV) and three energies of an Electra linear accelerator at the depth of maximum and at the reference depth under reference conditions. In all cases, the chamber’s reference point was positioned at the depth of measurement. Moreover, the dose to water DW was calculated in a small water voxel positioned at the same depth. Results: The calculated dose ratio DNACP/DMarkus, which according to van der Plaetsen reflects the fluence perturbation correction of the Markus chamber, deviates less from unity than the values given by van der Plaetsen, but exhibits similar energy dependence. The same holds for the dose ratios of the other well-guarded chambers. But, in comparison to water, the Markus chamber reveals the smallest overall perturbation correction which is nearly energy independent at both investigated depths. Conclusion: The simulations principally confirm the energy dependence of the dose ratio DNACP/DMarkus as published by van der Plaetsen. But, as shown by our simulations of the ratio DW/DMarkus, the conclusion drawn in all dosimetry protocols is questionable: in contrast to all well-guarded chambers, the guard-less Markus chamber reveals the smallest overall perturbation correction and also the smallest energy dependenc展开更多
文摘Objective To discuss the method how to identify wall correction factor (K_ wall ). Methods The paper introduce how to use extrapolation of the ionization current measured for different wall thickness by the spherical graphite cavity chamber whose volume is 10 cm3 and 30 cm3 for 60 Co γ-rays. The wall correction factors were determined by this method and Monte Carlo(MC) calculations. Results The results of the present experiment and calculation indicated that the wall correction factor determined by traditional geometry thickness extrapolation method was 1.0% less than that by MC calculation.The result of applying the equivalent mean thickness to calculate with extrapolation method and that of MC calculation were accordable within 0.45%.The relative dose and wall correction factors for the spherical graphite cavity chamber whose volume is 30 cm3 and 50 cm3 of the standards of air kerma of the NIST for 60 Co γ-rays were calculated by MC calculation,and the calculation results were in accordance with the published values of NIST within 0.06%. Conclusion The spherical graphite cavity chamber wall correction factor determined by traditional extrapolation method and MC calculation is accordable with the results of foreign labs (PSDL).;
文摘Purpose: All present dosimetry protocols recommend well-guarded parallelplate ion chambers for electron dosimetry. For the guard-less Markus chamber, an energy dependent fluence perturbation correction pcav is given. This perturbation correction was experimentally determined by van der Plaetsen by comparison of the read-out of a Markus and a NACP chamber, which was assumed to be “perturbation-free”. Aim of the present study is a Monte Carlo based reiteration of this experiment. Methods: Detailed models of four parallel-plate chambers (Roos, Markus, NACP and Advanced Markus) were designed using the Monte Carlo code EGSnrc and placed in a water phantom. For all chambers, the dose to the active volume filled with low density water was calculated for 13 clinical electron spectra (E0 = 6 - 21 MeV) and three energies of an Electra linear accelerator at the depth of maximum and at the reference depth under reference conditions. In all cases, the chamber’s reference point was positioned at the depth of measurement. Moreover, the dose to water DW was calculated in a small water voxel positioned at the same depth. Results: The calculated dose ratio DNACP/DMarkus, which according to van der Plaetsen reflects the fluence perturbation correction of the Markus chamber, deviates less from unity than the values given by van der Plaetsen, but exhibits similar energy dependence. The same holds for the dose ratios of the other well-guarded chambers. But, in comparison to water, the Markus chamber reveals the smallest overall perturbation correction which is nearly energy independent at both investigated depths. Conclusion: The simulations principally confirm the energy dependence of the dose ratio DNACP/DMarkus as published by van der Plaetsen. But, as shown by our simulations of the ratio DW/DMarkus, the conclusion drawn in all dosimetry protocols is questionable: in contrast to all well-guarded chambers, the guard-less Markus chamber reveals the smallest overall perturbation correction and also the smallest energy dependenc
