目的:建立中子劑量量測技術以評估醫用直線加速器產生之X光中之中子劑量。材料與方法:將3.2公克純度98%以上的五氧化二磷(P□O□)裝入小玻璃瓶內，並置於射柱內距離X射線靶1公尺處，照野面積40公分x40公分，照射劑量為40戈雷(10-18MV)。五氧化二磷經過光子與中子混合場照射後而產生活化反應，如□P(n, □)□及□P(n, p)□Si。活化物用蒸餾水溶解後，以計測效率趨近100%之液體閃爍器計測。最後利用活化物飽和活度及劑量轉換因子之乘積，則可決定快中子與熱中子的吸收劑量。結果:加速器產生中子輻射的多寡與加速器的照野有關，當照野面積愈大時產生的中子輻射也愈多。照野40公分x40公分，照射劑量40戈雷的條件下，對10MV至18MV之X射線所產生的中子通量率範圍在10□至10□n cm□s□之間。結論:醫用直線加速器產生之中子與病人治療劑量的比值在0.002%至0.071%之間。此結果符合原子能委員會規定中子劑量與病人治療劑量的比值必須小於0.1%之要求。

　　　　　Purpose: To develop a technique to measure neutron dose in a high-energyphoton field generated by a linear accelerator.Materials and Methods: Approximately 3.2 g of phosphorus pentoxide P□O□ powder withpurity over 98% was placed into a 5 mL glass vial and capped. The vial was then placedin the radiation beam at 1 m against the target and irradiated to 40 Gy for 10 MVto 18 MV x-ray under the field size of 40 cm X 40 cm. Samples of P□O□ were exposedto a mixed photon-neutron field with the reactions of □P(n, □) □P and □P(n, p)□Si. The irradiated samples were then dissolved in distilled water and the activationproducts were counted using the liquid scintillation counting techique with acounting efficiency approaching 100%. The decay characteristics of the sample wasthen analyzed to determine the fast and thermal neutron absorbed dose.Results: Measurements for neutron contribution related to field size showed that thelarger field size for irradiation, the more neutron contamination was generated. Thetotal neutron fluence rate generated by five accelerators were in the range from 10□ to 10□n□□MV to 18 MV x-ray machines with photon dose of 40 Gy and field size40 cm X40cm.Conclusion: The total neutron absorbed dose was about 0.002% to 0.071 % compared withtreatment x-ray dose to the patient. It meets the criteria as set by the Atomic EnergyCouncil. The requirement is that the neutron dose due to neutron contaminationproduced by a linear accelerator with voltage above 7 MV should be less than 0.1%of radio-therapeutical dose to the patient.