High Resolution Photofission Measuremfnts Using Proton Capture Gamma Rays

High intensity proton beams from the Dynamitron at Brooklyn College are used to generate monochromatic gamma rays from known (p, r) resonances in various nuclei. The gamma rays, whose energy varies continuously as a function of the angle with the proton beam, are used in turn to measure photofission cross sections in 238U and 232Th. The number of suitable (p, r) resonances is sufficient to cover the 5 - 13 MeV range of excitation energy with an average step of ~ 100 keV. For a resonance of average strength the yield is 103 photons per μA of protons on target. The energy spread is typically 20 - 40 keV and a gamma ray energy resolution of 200 eV can easily be reached. This resolution is adequate for investigating structure in the photofission cross sections resulting from class II states in the second well of the fission barrier.