There is more to proton shielding than meets the eye

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There is more to proton shielding than meets the eye

by Philip F. Jacobus, CEO | October 12, 2015
From the October 2015 issue of HealthCare Business News magazine

Routing of the cables is important, their exposure to the radiation (primary, at least) should be minimized. In certain cases a decontamination layer is necessary. It is the part of the inner wall which can be relatively easily dismantled and utilized as radioactive waste. So care should be taken about the materials used in this decontamination layer, because the materials should be minimally activated by radiation. As for the protons and high energy cyclotrons, special mathematical modeling algorithms are used (Monte Carlo method) which allow us to create a statistically- proven picture of the dose distribution during operation of this equipment. Further, if we have a dose distribution, we can calculate the necessary shielding geometry and most effective materials. 

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The radiation oncology center
being constructed in Russia by DTC;
the facility features concrete
shielding walls for both
proton therapy suites
and linac suites.

PJ: Is the thickness of the wall the only difference between the proton site and a linear accelerator or Gamma Knife site?

AS: No, absolutely not. In the case of a linear accelerator, only the room with the linear accelerator needs to be shielded. But in the case of a proton center, the room with the cyclotron must be shielded as well. Also, when the beam travels out of the cyclotron and travels down the beam line it has to turn at least twice. Once to get behind the proton gantries and again to get from the beam line into the proton gantries.
In these areas where the turning takes place there is a considerable amount of neutron scatter, and in these areas the concrete walls must be very thick. As I said, about 15 feet. For effective deceleration and further shielding from neutrons, additional materials like plastic or boronated polyethylene can be used. Penetration capacity of the neutrons is very high, since they are uncharged particles, and such additional shielding gives very good results when installed in the direction of high neutron flux.
Along the walls, on both sides of the beam line and around the gantry rooms, concrete walls are required. Also, in the case of a linear accelerator, the walls might be 15 feet high but in the case of the proton gantry room, the concrete walls must be almost 60 feet high. So between the cyclotron, the beam line, where the beam lines turn and the three story tall gantry room, there is a lot more concrete involved in a proton system than in a linear accelerator or Gamma Knife.

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