Cryogenic aspects of the operation of a superconducting cyclotron-based neutron therapy facility.

The Harper Hospital superconducting cyclotron, which is used for neutron radiation therapy, is a unique device. It is the first superconducting cyclotron to be installed in a hospital. The novel magnet cryostat can be rotated through 360 degrees without spilling liquid and whilst remaining vented to a low pressure return line for collection of the boil-off gas. The mode of operation of the cryogenic magnet is described in detail. Some of the problems associated with the cryogenic nature of the cyclotron including those problems encountered in operating a helium liquefaction system in a hospital are discussed. At the present time the magnet is kept cold by filling the cryostat with approximately 75 L of liquid helium each day before patient treatments begin. This is a time-consuming process. The possibility of modifying the helium gas recovery and liquefaction system so that a continuous liquid helium supply could be delivered to the magnet cryostat is discussed.