A Broad-Beam Biology Facility for Proton Irradiation at the Oak Ridge Isochronous Cyclotron

The broad-beam facility of the Oak Ridge Isochronous Cyclotron was developed primarly for whole-body irradiation of mice with 60-MeV protons. The proton beam is defocused, with the use of three quadrupole lenses and thin scattering foils, to provide a uniform flux over a rectangular exposure field 10 by 12.5 cm. The beam, after leaving the vacuum system, passes through a thin-walled ionization chamber, and the beam charge is measured with a Faraday cup. Output of the ion chamber operates a current integrator that can be preset to stop irradiation at the desired absorbed dose, calculated from proton fluence and from range-energy tables. The beam profile is routinely surveyed with a pair of remotely controlled thimble chambers providing a readout of both dose rate and integral output. The chambers are positioned at the edges of the field to monitor operation during exposures. Small fluoroglass dosimeters also are used routinely to measure beam uniformity and range. Entrance-to-exit dose variation is minimized by exposing each mouse in a thin-walled plastic tube rotating broadside to the beam. Up to four mice can be irradiated simultaneously at absorbed dose rates of < 50 to > 10⁴ rads per minute.