A Monte Carlo simulation study of medical cyclotron accelerator in order to design a neutron shield reducing staff absorbed dose

Cyclotron is one of the accelerators that is widely used in the field of diagnostic and therapeutic radiopharmaceutical production. During nuclear reactions, occur in cyclotrons, in addition to the desired nuclide, neutron and gamma radiation are also produced. The resulting radioactivity depends on the type and energy of the emitted particles, as well as on the materials irradiated by the primary beam and the secondary radiation field. Secondary neutrons and components activated with secondary neutrons may affect the staff absorbed dose. Activated cyclotron components are a major source of potential radiation to staff who maintain and repair the device. The correct determination of neutron and gamma radiation is a basic condition for the design of protection and, as a result, the protection of employees. It is impossible to perform such a study in humans, and the only method to estimate the dose is the use of Monte Carlo simulation. In this research, by using GEANT4 Monte Carlo codes, the intensity and spectrum of neutrons and the dose of different organs inside, outside of the cyclotron room and also the in the operator's room were calculated. Three different materials of steel with special alloy, polyethylene, and cadmium boron were used for designing a shield. The neutron spectrum was calculated in different shield designs and compared with the design without shield. The optimum barrier for radiation protection against neutron depends on the energy of beam. In the case of cyclotrons that used for FDG (Fluorodeoxyglucose) production, the maximum neutron energy is 2 MeV. According to our simulation results, polyethylene (PE)is the best material for shielding.