Estimation of LINAC X-ray Fluence Rate in a High-Energy Region Using Activation Detectors

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  • ライナックX線の高エネルギー領域におけるフルエンス率の放射化検出器による推定

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Abstract

Fluence rate of 10MV X-rays from the electron linear accelerator (LINAC) for radiation therapy at the Prefectural University of Hiroshima was estimated by measuring gamma rays from activities induced by X-rays in zinc, copper and indium metals. The gamma-ray measurements were performed using a large volume (160cm^3) Ge detector. The X-ray exposure was conducted so that the absorbed dose rate at a 5cm depth for radiation therapy was 2Gy/min. The measurements showed that ^<63>Zn, ^<62>Cu and ^<115m>In activities were induced in those metals. The estimated fluence rates were (1.43±0.15)×10^<13>m^<-2>s^<-1> in 5-10MeV, (3.15±0.20)×10^<11>m^<-2>s^<-1> in 10-12MeV and (1.51±0.16)×10^<10>m^<-2>s^<-1> in 12-15MeV, at the isocenter 20cm above the couch. The uncertainties of fluence rates do not include those due to the uncertainties of the cross sections. The fluence rates were estimated assuming that the fluence rate of the X-rays of energy larger than 15MeV was negligible. The X-rays in the energy region higher than 10MeV scarcely contributed to the X-ray doses for radiation therapy. However, X-rays of such high energies can induce many kinds of photonuclear reactions in the materials used in the LINAC. In our previous study, relatively large thermal neutron fluence was observed in the exposure room. The X-rays in the energy region beyond 10MeV presumably cause a large part of the photoneutron productions.

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