Magnetic optics for proton radiography

High energy protons of 10 to 50 GeV can be used to radiograph dense objects. Because the transmitted beam particles undergo multiple Coulomb scattering (MCS) in the object, a magnetic lens system is used to focus the particles exiting each point of the object onto a distant image plane. Without the lens, the MCS would seriously blur the radiographic image. Correlations can be introduced in the illuminating beam to cancel a major part of the chromatic and geometric aberrations in the lens, while providing locations inside the lens where the rays are sorted by MCS angle. This allows the introduction of angle cut apertures to aid material identification. The requirement for a matched multistage lens system with successively smaller angle-cut apertures leads to the use of minus-identity (-I)lenses, in which the angle sorting is in the longitudinal mid plane of the lens, and the exit beam correlations are the same as the input correlations. A single stage-I lens has been successfully tested at Brookhaven with 10-GeV protons and another is being used in dynamic experiments with 0.8-GeV protons at Los Alamos. The resolution achievable at higher energies is briefly surveyed.