Ch 5 Mcginley pg 69
Most medical accelerators operating above 10 MeV use a maze with a door shielded for neutrons and photons at the outer maze entrance. A typical door consists of a steel case 0.635 cm thick containing 10.2 cm of borated polyethylene.
The polyethylene is used to moderate the fast and intermediate energy neutrons, which then react with the boron and produce a 0.473 MeV photon.
The lead is placed after the polyethylene where it will attenuate the photons where it will attenuate the photons produced in the boron and any capture gamma rays generated in the maze by neutron capture in the concrete in the concrete wall, ceiling and floor.
Recently McCall (1997) has indicated a more efficient door shield is produced by placing the lead before the polyethylene. With this arrangement, the high energy neutron component is reduced in energy by interactions in the lead before entering the polyethylene layer.
The overall result is an increased attenuation of the neutrons with approximately the same penetration for the capture gamma rays that originate in the maze. The low energy photons aristing from the (n, alpha) reaction with the boron in the polyethylene are attenuated sufficiently by the steel case of the door.
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