Patent Number: 
Section: claims

1. A charged particle beam system comprising:an ion source generating a plurality of different kinds of ions differing in weight from each other;an accelerator accelerating the ions generated in the ion source and that includes a plurality of magnets;a beam transport system transporting an ion beam extracted from the accelerator; andan irradiation nozzle irradiating the ion beam to an irradiation target;a rotating gantry that rotates the irradiation nozzle around the irradiation target; anda control apparatus configured to control the rotating gantry, the irradiation nozzle, and the accelerator to select which of the different kinds of ions are irradiated from the irradiation nozzle from a plurality of different irradiation directions based on a water equivalent depth of the irradiation target at each of the irradiation directions, andwherein the control apparatus is configured to control respective magnetic field strengths of the magnets when accelerating the different kinds of ions at a radius along a circular track, and the accelerator accelerates a first kind of the ions to a first maximum energy and accelerates a second kind of the ions to a second maximum energy, andwherein the control apparatus is further configured to set, the first maximum energy and the second maximum energy such that a first magnetic rigidity of the accelerator for accelerating the first kind of ions to the first maximum energy and a second magnetic rigidity of the accelerator for accelerating the second kind of ions to the second maximum energy are approximately equal. 2. The charged particle beam system according to claim 1,wherein the irradiation nozzle is installed in the rotating gantry,wherein the control apparatus is configured to control the accelerator to accelerate the plurality of different kinds of ions so that an underwater range at a highest energy after acceleration is different in each species of the ions, andwherein the water equivalent depth of the irradiation target in a first one of the irradiation directions is equal to or less than the underwater range at the highest energy after the acceleration of the first kind of ions, andwherein the selected ions are transported to the irradiation nozzle using the ion source, the accelerator, the beam transport system, and the rotating gantry, thereby irradiating the irradiation target with the selected ions from the irradiation nozzle. 3. The charged particle beam system according to claim 2,wherein the control apparatus is configured to compare the water equivalent depth of each of a plurality of layers of the irradiation target divided in a depth direction in the irradiation target with a longest underwater range of each of the different kinds of ions, andwherein the control apparatus is configured to, for each of the layers and for each of the different irradiation directions, select the first kind of ions to be irradiated when the water equivalent depth of the respective layer is greater than 4 cm from a surface in the respective irradiation direction and select the second kind of ions to be irradiated when the water equivalent depth of the respective layer is equal to or less than 4 cm from the surface in the respective irradiation direction. 4. The charged particle beam system according to claim 2,wherein the irradiation nozzle includes a scanning magnet,wherein the control apparatus is configured to control an irradiation position and irradiation range of the ions from the irradiation nozzle in a lateral direction by controlling the scanning magnet based on a position and a range in the lateral direction of each of a plurality of volume elements of the irradiation target, andwherein the control apparatus is configured to, for each of the volume elements and for each of the different irradiation directions, select the first kind of ions to be irradiated when the water equivalent depth of the respective volume element is greater than 4 cm from a surface in the respective irradiation direction and select the second kind of ions to be irradiated when the water equivalent depth of the respective volume element is equal to or less than 4 cm from the surface in the respective irradiation direction. 5. The charged particle beam system according to claim 1,wherein the irradiation nozzle is installed in the rotating gantry,wherein the control apparatus is configured to control the accelerator to accelerate the plurality of different kinds of ions so that an underwater range after acceleration of a heaviest kind of the ions to a highest energy is shorter than an underwater range after acceleration of ions other than the heaviest kind of the ions to a highest energy. 6. The charged particle beam system according to claim 1,wherein the irradiation nozzle is installed in the rotating gantry,wherein the control apparatus is configured to control the accelerator to accelerate the plurality of different kinds of ions so that an underwater range of a heaviest kind of the ions after acceleration to a highest energy is shorter than an underwater range after acceleration of ions lighter than said heaviest kind of the ions after acceleration to a highest energy. 7. The charged particle beam system according to claim 1,wherein the irradiation nozzle is installed in the rotating gantry,wherein the control apparatus is configured to control the accelerator to accelerate each of the plurality of different kinds of ions so that an underwater range after accelerating the different kinds of ions to a highest energy decreases in correspondence with an increase in ion weight thereof,wherein the selected ions are transported to the irradiation nozzle using the ion source, accelerator, beam transport system, and rotating gantry, thereby irradiating the irradiation target with the selected ions from the irradiation nozzle. 8. The charged particle beam system according to claim 1,wherein the first kind of ions are selected and irradiated from a first one of the irradiation directions when the water equivalent depth of the irradiation target is greater than 4 cm, and the second kind of ions are selected and irradiated from a second one of the irradiation directions when the water equivalent depth of the irradiation target is equal to or less than 4 cm. 9. The charged particle beam system according to claim 8,wherein the first kind of ions are hydrogen ions and the second kind of ions are helium ions. 10. The charged particle beam system according to claim 1,wherein each of the first magnetic rigidity and the second magnetic rigidity is 4.5 Tm. 11. The charged particle beam system according to claim 1,wherein the control apparatus is configured to control the rotating gantry, the irradiation nozzle and the accelerator to select the first kind of ions to be irradiated from a second one of the irradiation directions for the water equivalent depth of the irradiation target that is greater than 4 cm, and thereafter select the second kind of ions to be irradiated from the second one of the irradiation directions for the water equivalent depth of the irradiation target that is equal to or less than 4 cm. 12. The charged particle beam system according to claim 1,wherein the first kind ions are selected and irradiated from a first one of the irradiation directions when the water equivalent depth of the irradiation target is greater than or equal to 10 cm, and the second kind of ions are selected and irradiated from a second one of the irradiation directions when the water equivalent depth of the irradiation target is less than 10 cm. 13. The charged particle beam system according to claim 12,wherein the first kind of ions are helium ions and the second kind of ions are carbon ions. 14. The charged particle beam system according to claim 12,wherein each of the first magnetic rigidity and the second magnetic rigidity is 4.5 Tm. 15. The charged particle beam system according to claim 12,wherein the control apparatus is configured to control the rotating gantry, the irradiation nozzle and the accelerator to select the first kind of ions to be irradiated from the second one of the irradiation directions for the water equivalent depth of the irradiation target that is greater than or equal to 10 cm, and thereafter select the second kind of ions to be irradiated from the second one of the irradiation directions for the water equivalent depth of the irradiation target that is less than 10 cm. 16. The charged particle beam system according to claim 1, wherein the first magnetic rigidity of the accelerator for accelerating the first kind of ions to the first maximum energy of 220 MeV and the second magnetic rigidity of the accelerator for accelerating the second kind of ions to the second maximum energy of 69 MeV are approximately equal. 17. The charged particle beam system according to claim 1, wherein the first magnetic rigidity of the accelerator for accelerating the first kind of ions to the first maximum energy and the second magnetic rigidity of the accelerator for accelerating the second kind of ions to the second maximum energy are approximately ½ of a third magnetic rigidity for obtaining a water equivalent depth of 30 cm for the second kind of ions.