Abstract
NCRP 151 provides very detailed examples demonstrating the necessary concerns for shielding a conventional radiotherapy vault with a maze where the useful beam is parallel to the maze. However, it provides little guidance on how to properly shield a vault with the maze-wall acting as part of a compound primary barrier. We have modeled a new radiotherapy vault with this configuration and assessed the additional photon shielding burden at the door with MCNP5. MCNP simulations demonstrated an increase in overall photon shielding burden at the door relative to calculations that only consider photon workloads presented in NCRP 151. Two additional components of scattered radiation are considered and methods for calculation are presented.
Highlights
NCRP Report 151 lays out the broad considerations for shielding a megavoltage linear accelerator to achieve reasonably low levels of radiation [1]
The example for a conventional linear accelerator vault presented in NCRP 151 only applies to a layout where the useful beam does not intersect the maze barrier
Allowing adjacent vaults to share barriers minimizes the amount of shielding material for a facility, which can lead to having the primary beam incident on the maze depending on the facility geometry
Summary
NCRP Report 151 lays out the broad considerations for shielding a megavoltage linear accelerator to achieve reasonably low levels of radiation [1]. The example for a conventional linear accelerator vault presented in NCRP 151 only applies to a layout where the useful beam does not intersect the maze barrier. A direct shielded door or shortened maze design for a high-energy accelerator (>10 MV) requires considerable shielding material to provide adequate attenuation from direct photo-neutrons and gamma capture photons. Allowing adjacent vaults to share barriers minimizes the amount of shielding material for a facility, which can lead to having the primary beam incident on the maze depending on the facility geometry. With a beam incident on the maze, it is desirable to split the primary barrier shielding between the maze barrier and the maze wall, making the maze barrier as thin as possible to minimize dose at the door and the necessary volume of concrete required for the facility. Splitting the primary barrier into a compound barrier introduces new considerations, namely primary beam that is scattered from the maze barrier to the door, primary transmission through the maze barrier incident on the door, and primary transmission through the maze barrier that is scattered to the door by the far maze wall
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