There is provided a cyclotron comprising a magnet yoke and a magnet assembly having a yoke body surrounding an acceleration chamber. The magnet assembly is configured to generate a magnetic field to direct charged particles along a desired path. The magnet assembly is located within the acceleration chamber. The magnetic field propagates through the acceleration chamber in the magnet yoke. A part of the magnetic field escapes to the outside of the magnet yoke as a stray magnetic field. Magnet yokes are dimensioned so that the stray field does not exceed 5 Gauss at a distance of 1 meter from the outer boundary.
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