A large discrepancy of about two orders of magnitude was observed between measured and conventional calculated results on the dose rate outside of the shielding wall of an 18 MeV electron linac for medical use. Measurements of radiation fields inside and outside of the wall have been performed. It was discovered that the secondary neutrons produced from the iron, which is placed inside the concrete shielding wall, and their related secondary gamma-rays contributed to the dose rate outside of the wall. It can be concluded that consideration of the (γ,n) reaction in the shielding wall is very necessary for the design calculation of high-energy electron linac facilities.
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