Beam monitor system for high-energy beam transportation at HIMAC

Heavy-Ion Medical Accelerator in Chiba (HIMAC) provides ion beams for radiotherapy of cancers and for other basic researches. High-energy beam transport lines deliver the beams to three treatment rooms and two experiment rooms with the aid of 41 beam monitor units. Each monitor unit consists of a wire grid as a profile monitor, or a combined unit in which the wire grid and a parallel plate ionization chamber are united for an additional measurement of a beam intensity. They are operated in a mixed gas of 80% Ar and 20% CO2. The gas gain of the wire grid achieves about 8000 at an applied voltage of -2700 V. Dynamic ranges of the wire gird and the parallel plate ionization chamber were measured to be 8x105 and 1x106 in test using ion beams, respectively. A control system of these monitor units offers easy operation, so that operators are almost free from miss-operations. The monitor units are interlocked with a system which protects patients from the undesired irradiation. Five wire grids are used for continuously monitoring the beam during treatments, which play a role in keeping qualities of the treatments. The monitor system has been designed to meet requirements for medical uses, and works in a stable and reliable manner and satisfies the requirements. (author)