Characteristics of fast beam switching for spot scanning

An irradiation method for spot scanning has been developed in order to provide accurate irradiation even for an irregular target shape. In spot scanning irradiation, the beam is turned off while the beam position is shifted to the next spot for precise dose management. Recently, the fast beam cut-off method, which can achieve a cut-off time of around 50 μs, was proposed and verified at the Heavy Ion Medical Accelerator in Chiba synchrotron. In order to utilize such a speciality of the technique for spot scanning, the characteristics of fast beam switching were investigated while turning on/off both the transverse and longitudinal RF fields many times during a single flattop. As a result, it was verified that this technique provides more than 50 spots during a single flattop in the synchrotron operation pattern with a cut-off time of 50 μs. With an increase in a switching number, the momentum spread and the size of the extracted beam increase within the allowable range for practical usage.