當醫用直線加速器使用之當公稱電壓大於6 MV時，其所發射之光子會與加速器機頭中的物質發生作用，經由(γ,n)、(γ,2n)與(γ,pn)等光核反應產生中子。這些反應的作用截面會隨光子能量和機頭元件之材料質量數而變化。
本研究利用多球體搭配金箔量測18 MV及15 MV醫用直線加速器誘發光核反應產生之中子能譜，討論在不同的光子能量、照野大小及經過不同假體深度等條件後的中子能譜變化情形。加速器產生的高能光子誘發中子能譜主要由快中子及中低能量中子所構成，當光子能量愈高，誘發產生之光中子通量愈大。依據不同光照野大小的評估結果，光中子通量會隨照野大小增加而先增後減，當照野在10x10 cm2時產生的光中子通量為最大。中子經過不同深度假體時，部分快中子會緩速為熱中子，隨著中子經過的假體愈厚，快中子所佔比例會逐漸減少，而熱中子比例則會逐漸增加。
傳統不加鎘片方法評估之中子劑量為熱中子與高能中子劑量貢獻之加總，依鎘差法修正後可區分出熱中子與高能中子各別之劑量貢獻，若將傳統方法評估出之中子劑量視為熱中子劑量將過於高估。在假體表面(深度0 cm)之情況下，因高能量中子佔多數，因此傳統方法之中子劑量與鎘差法評估之熱中子劑量有較大之差異；而在假體厚度較大(大於15 cm)之情況下，傳統方法評估出之總中子劑量結果與鎘差法評估之熱中子劑量結果逐漸相符，此乃因經較厚之假體厚度時高能中子通量大量減少、熱中子通量大量增加所致。比較傳統不加鎘片方法評估之總中子劑量與鎘差法修正後之熱中子劑量差異，18 MV加速器之結果較15 MV加速器差異大。因此欲評估Linac使用愈高能量光子進行治療時所誘發之中子劑量，了解中子能譜是非常重要的。

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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