Abstract: A cyclotron suitable for use in neutron therapy, and comprising a pair of opposed, spaced pole shoes having their adjacent inner surfaces defining an accelerator zone, an electromagnetic coil system, at least one hollow accelerating dee electrode positioned in the accelerator zone, and having a radio-frequency resonator associated therewith, a magnet yoke shaped to substantially enclose the accelerator zone and constitute a neutron attenuation shield for neutrons produced in the cyclotron, a vacuum chamber enclosing the accelerator zone and each dee electrode, means for providing charged particles for acceleration within the accelerator zone, a target zone for a target device, and a neutron beam outlet in the magnet yoke for emission of a neutron beam produced in the cyclotron. The cyclotron includes auxiliary neutron shield means in the forward peak zone of a neutron beam produced in the cyclotron to attenuate neutron and gamma radiation in the forward peak zone to a patient tolerable level.