A synchrotron injector system for injecting ions into a synchrotron, comprising: - a first ion source for generating first ions; - a second ion source for generating second ions having a charge-to-mass ratio ( q2 / A2) lower than the charge-to-mass ratio ( q1 / A1) of the first ions; - a drift tube linear accelerator including: - a cylindrical resonator; and - a plurality of drift tubes (T, T1 to T7) arranged linearly along the center axis of the cylindrical resonator, for accelerating ions in an accelerating half cycle that is a radio frequency half cycle containing an accelerating phase of radio frequency electric fields produced in a plurality of drift tube gaps (G1 to G7) formed between the plurality of drift tubes (T1 to T7); - a radio frequency generator for feeding radio frequency power to the drift tube linear accelerator; and - a low-energy beam delivery line for injecting either the first ions or the second ions into the drift tube linear accelerator, wherein the radio frequency generator is configured to feed the radio frequency power to the drift tube linear accelerator such that, when the first ions are injected from the low energy beam delivery line, the phase difference between an accelerating half cycle for accelerating the first ions in one drift tube gap (Gi) of the plurality of drift tube gaps (G1 to G7) and an accelerating half cycle for accelerating the accelerated first ions reaching the next drift tube gap (Gi+1) is set to a first accelerating cycle phase difference, characterised in that the radio frequency generator is further configured to feed the radio frequency power to the drift tube linear accelerator such that, when the second ions are injected from the low energy beam delivery line, the phase difference between an accelerating half cycle for accelerating the second ions in the one drift tube gap (Gi) and an accelerating half cycle for the accelerated second ions reaching the next drift tube gap (Gi+1) is set to a second accelerating cycle phase difference that is larger than the first accelerating cycle phase difference. - a first ion source for generating first ions; - a second ion source for generating second ions having a charge-to-mass ratio ( q2 / A2) lower than the charge-to-mass ratio ( q1 / A1) of the first ions; - a drift tube linear accelerator including: - a cylindrical resonator; and - a plurality of drift tubes (T, T1 to T7) arranged linearly along the center axis of the cylindrical resonator, for accelerating ions in an accelerating half cycle that is a radio frequency half cycle containing an accelerating phase of radio frequency electric fields produced in a plurality of drift tube gaps (G1 to G7) formed between the plurality of drift tubes (T1 to T7); - a radio frequency generator for feeding radio frequency power to the drift tube linear accelerator; and - a low-energy beam delivery line for injecting either the first ions or the second ions into the drift tube linear accelerator, wherein - a cylindrical resonator; and - a plurality of drift tubes (T, T1 to T7) arranged linearly along the center axis of the cylindrical resonator, - a radio frequency generator for feeding radio frequency power to the drift tube linear accelerator; and - a low-energy beam delivery line for injecting either the first ions or the second ions into the drift tube linear accelerator, wherein