Mean Energy of Neutrons from an Electron Linear Accelerator for Radiation Therapy

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  • 医療用電子線線形加速器で発生する中性子線の平均エネルギー

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Mean energy of neutrons from an electron linear accelerator (LINAC) was experimentally estimated. The LINAC is used for radiation treatments at the Hiroshima Prefectural College of Health Sciences. The thermal neutron fluence rate was determined measuring the gamma rays from ^<116ml>In (half-life of 54.29 min) induced in indium foil exposed to 10 MV X-rays. The mean energy of the neutrons was estimated from the attenuation coefficient of the thermal neutron fluence rates in a polyethylene block, the surface of which was located at the isocenter. The block was surrounded by 10 cm of boric acid layers, except for the upper surface. In the estimations, it was assumed that all thermal neutrons in the block came from the head of the LINAC. Gamma rays from ^<116ml>In were measured by a Ge semiconductor detector, which was shielded by lead blocks thicker than 15 cm. The thermal neutron fluence rate was (7.1±0.4)×10^8 m^<-2>s^<-1> at a 2 cm depth of the polyethylene block, when the X-ray dose rate was 2 Gy/min at the reference depth for radiation therapy and the field size was 10cm×10cm. Under the same condition, the mean energy of the neutrons from the head of the LINAC was estimated to be 1.22±0.02 MeV. The mean energy increased to 1.34±0.03 MeV through the 5 cm boric acid filter, and decreased to 1.00±0.03 MeV through a 3 cm copper filter. This can be explained by the facts that the boric acids absorbed the incident thermal neutrons, and that the copper plate moderated the incident fast neutrons. The mean neutron energies in 5cm×5cm, 10cm×10cm and 15cm×15cm field sizes were approximately coincident to each other.

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