Photoneutron radiation field of ducts in barrier of 15 MV medical electron accelerators

Shielding body of the high-energy medical electron accelerators is always penetrated by ducts, which would influence the shielding capability of local barrier. In order to quantitatively analyze the duct's impact on shielding of the photoneutron from 15 MV accelerators, the ambient dose equivalent rate and energy spectrum at the center of a typical duct and the external mouth of duct were calculated based on MCNP program for the first time. The results demonstrate that leakage neutrons at the external mouth of duct are mainly thermal neutron, and its dose rate is decreased with the increase of the intersection angle between duct and wall as well as the reduction of duct diameter. When a duct in a diameter no more than 30 cm penetrates the wall unidirectionally and the inclined Angle (θ) is 60°, neutron dose rate at the external mouth of duct could meet the requirements of protection. At last, according to the calculation results, some suggestions are proposed for the shielding design of ducts in walls. - Highlights: • A detailed geometrical model of a 15 MV linac was developed by MCNPX code. • Photoneutrons at the duct and the corresponding external wall were simulated. • Relationship between dose attenuation of duct and its diameter or angle was analyzed. • Suggestions are proposed for the shielding design of ducts in walls.