Comparison of different shielding materials used at proton accelerators and cost-benefit analysis

During last decades physicians' and physicist's experience and confidence in proton beam radiotherapy has grown significantly. Construction of a number of new proton therapy facilities is already underway, and several are in planning stages for the near future. Cost-effective shielding design of these facilities is important. We present comparative analysis of different shielding materials that are typically used at proton accelerators and in proton radiotherapy facilities. We have used Geant4 tool-kit for simulation of the passage of particles through matter. We have analyzed shielding properties of iron, borated concrete, a water tank with borated polyethylene walls, borated polyethylene, and borated fire retardant plywood. We have simulated 240 MeV protons incident on a thin copper target that generated radiation fields of primary protons as well as secondaries produced in the target incident on the shielding block. We found that iron is most effective per unit length. It may be the most cost-effective option if one considers using the so called SEG blocks in combination with 1 to 2 ft of concrete layer. The material of the SEG block consists primarily of iron from recycled government facility metals, and is slightly radioactive. The slight inherent radioactivity as well as low energy (< 847 keV) secondary neutrons from iron will be shielded by a thin concrete layer. We also find that borated fire retardant plywood can be a cost-effective alternative for borated polyethylene in many shielding applications where borated polyethylene sheets are used that are arguably not less fire hazardous. (authors)