| Summary: | 碩士 === 國立清華大學 === 生醫工程與環境科學系 === 100 === When the nominal voltage greater than 6 MV, photons emitting from the medical linear accelerators used in radiotherapy would interact with the high atomic number materials in the accelerator head and induce neutrons via the (γ, n), (γ, 2n), (γ, pn) photonuclear reactions. Cross sections of these reactions vary with the photon energy and the nucleus mass number of target and shielding.
In this study, a system of Bonner sphere spectrometer with gold foil was used to measure the neutron spectra produced from photonuclear reactions induced by the 18 MV and 15 MV Linacs. We also discussed the variations of the neutron spectrum with different photon energies, field sizes and depths of phantom. The neutron spectrum produced by the accelerator is composed of fast neutrons, intermediate neutrons and low energy neutrons. The larger the photon energy, the more amount of photon neutron induced. According to the results of the evaluation of the field size effect, induced photon neutron flux will change with the field size. As the field size increased, the induced photon neutron flux would increase firstly and then decreased. The maximum neutron flux occurred at the field size of 10x10 cm2 in this study. When the neutrons passed through the different depths of the phantom, the high energy neutrons would slow down into a low energy region. The more thick the depths of the phantom, the portion of high energy neutrons decreased, and the portion of thermal neutron increased.
The neutron doses assessed by the traditional method, no cadmium covered, were contributed from thermal neutrons and higher energy neutrons. The doses contributed from thermal neutrons and higher energy neutrons could be distinguished separately by means of the cadmium difference method. It would be overestimated the thermal neutron dose, if treated the total neutron dose assessed by traditional method as thermal neutron dose. In the surface case, depth = 0 cm, due to the largest proportion of the high energy neutron, the difference between the total neutron dose assessed by traditional method and the thermal neutron dose assessed by cadmium difference method would become larger. In the cases of deeper depths (>15 cm), the difference between the total neutron dose assessed by traditional method and the thermal neutron dose assessed by cadmium difference method would become smaller. This is due to the thicker the phantom, the less flux the high energy neutron, and the more flux the thermal neutron. For the comparison of the total neutron dose assessed by traditional method and thermal neutron dose assessed by cadmium difference method, the difference is larger in the case of 18 MV photons than 15 MV photons. Thereby, realization of neutron spectra is useful and important for estimating the induced neutron dose exactly in using higher photon energy Linac in radiotherapy.
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