Properties of an energy degrader for light ions

Abstract

The Ion Beam Applications (IBA) Company works on the development of a superconducting cyclotron able to deliver various light ion beams with a maximal energy of 400 MeV per nucleon (MeV/u) [1]. This accelerator named C400 is designed to equip a hadrontherapy centre such as the ARCHADE project in France [2]. Fully stripped ions from He up to O can be accelerated inside the C400. They are extracted with an electromagnetic septum at fixed energy. To modulate the energy of the beams delivered to the patients, use is made of an energy selection system similar to the one developed by IBA for Proton Therapy (PT) systems. Figure 1 shows the layout of the Energy Selection System (ESS) for proton therapy system. After the beam extraction, a set of quadrupoles is used to focus the beam in order to obtain a minimal beam size at the entrance of the energy degrader. The later consists of a graphite wheel with variable thickness to degrade the energy beam from 230 MeV down to a minimal energy of 70 MeV. Behind the degrader, a collimator is used to limit the emittance of the transmitted beam. After another set of quadrupoles to refocus the beam, two sets of 2 dipole magnets are installed to bend the beam towards the irradiation rooms. The first set of dipoles act as an energy analyzer and a momentum slit is placed in the middle of the two sets of dipoles to limit the energy spread of the transported beam to a maximal value E/E = 0.6%. A similar ESS is under design to transport ion beams extracted from the C400. Compared to protons, light ions exhibit less angular divergence because of their higher masses but suffer from ion fragmentation due to nucleus-nucleus interactions inside the degrader, leading to a smaller transmission efficiency through the degrader. Charged ion fragments with the same magnetic rigidity as