Figure 2

Secondary neutron dose at proton eye snout. View of the Geant4 simulation for the NCC proton eye beam delivery system. Most secondary neutrons were generated at the range modulator, the eye snout, and the water phantom. In the figure, three EBT film measurements at three different points are shown for the proton beam: (a) exit position of the beam pipe, (b) interim position between the eye snout and ion chamber 2, (c) exit position of eye snout. The proton beam was spread out in the lateral direction by interaction with materials in the beam path. We measured the lateral diameters of the proton beam at each position by using EBT film. From the exit of beam pipe to the front of the eye snout, the diameter of the beam cross section increased from (a) 2.5 cm to (b) 19 cm and then decreased to (c) 4 cm by the eye snout.