Background: With the extensive application of industrial irradiation accelerator

more and more people get involved in this field. It is necessary to know the whole radiation distribution in accelerator room for the consideration of the human radiation safety. Purpose: In order to acquire qualitative knowledge for radiation distribution in machine hall

quantitatively calculation of the dose at points locating in the vertical and horizontal situation around the drift tube was carried out to achieve the distribution of radiation field and the best type of the hole when different types of holes were employed. Methods: The major source roots in the scattering radiation through the pore outside the drift tube

thus the MCNP5 was applied to the multi-step simulation of radiation field distribution on a 3-MeV Dynamitron. Results: The size and shape of hole have tiny effect on the dose rate in vertical direction; while the dose rates at different points from drift tube in the horizontal direction change with the type of holes apparently

and the best type of hole is circular hole with a diameter of 40?60 cm when the diameter of drift tube is about 10cm. Conclusion: Selecting the appropriate type of the hole is essential both for radiation room and machine hall. Adding stair-step platform in accelerating pipe hole not only help to save the axial height of accelerator

but also reduce the radiation level around drift tube effectively. Abstract