Abstract
Purpose：To develop a technique to measure neutron dose in a high-energy photon field generated by a linear accelerator. Materials and Methods: Approximately 3.2g of phosphorus pentoxide P205 powder with purity over 98% was placed into a 5 mL. glass vial and capped. The vial was then placed in the radiation beam at 1m against the target and irradiated to 40 Gy for 10MV to 18MV x-ray under the field size of 40cm×40cm. Samples of P205 were exposed to a mixed photon-neutron field with the reactions of ^31P(n,y)^32P and ^31P(n,p)^31Si. The irradiated samples were then dissolved in distilled water and the activation products were counted using the liquid scintillation counting techique with a counting efficiency approaching 100%. The decay characteristics of the sample was then analyzed to determine the fast and thermal neutron absorbed dose. Results：Measurements for neutron contribution related to field size showed that the larger field size for irradiation, the more neutron contamination was generated. The total neutron fluence rate generated by five accelerators were in the range from 10^5 to 10^6ncm^-2 s^-1 for 10MV to 18MV x-ray machines with photon dose of 40Gy and field size 40cmX40cm. Conclusion：The total neutron absorbed dose was about 0.002% to 0.071% compared with treatment x-ray dose to the patient. It meets the criteria as set by the Atomic Energy Council. The requirement is that the neutron dose due to neutron contamination produced by a linear accelerator with voltage above 7MV should be less than 0.1% of radio-therapeutical dose to the patient. fTherapeut Radiol Oncol 1997; 4: 265-270]
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