Measurement and analysis of a 13 MeV cyclotron magnetic field

A four-sector 13-MeV cyclotron magnet, which does not employ the usual trim coils, was designed for positron-emission-tomography (PET) diagnostics. Instead, isochronism was achieved by placing a few side shims, as well as by slightly varying the vertical gap, along the radius of the cyclotron. The magnet was built based on the design, and its magnetic field was measured using a Hall probe. Equilibrium orbits were computed using the measured data. Then, a series of systematic optimizations of the magnetic field structure was performed. The final shape of the magnetic field was shown to satisfy the simultaneous requirements for isochronism and vertical focusing of the beam. Based on measured magnetic field, we designed a central region. The cyclotron has been routinely operationed at the Korea Cancer Center Hospital with a nominal beam current of around 50 μA.