Systematics of mass yield distributions for the nuclear fission of neptunium

Fission fragment energy correlation measurements were made for three isotopes of uranium. From these measurements, fission fragment mass yield distributions were determined. The proton-induced fission of the 236U and 235U targets used incident proton energies ranging in energy from 6.25 MeV to 30.0 MeV in energy steps of 0.25 MeV to 1.0 MeV. Protons ranging in energy from 6.5 MeV to 14.5 MeV were used on the 234U target. A total of ninety-nine fragment energy correlation measurements were made with the three targets. The uranium targets were of isotopic purity greater than ninety-nine percent. For incident proton energies greater than approximately 7 MeV, neutron emission competes favorably with fission for all of the uranium isotopes. Therefore, to obtain mass distributions due solely to the first-chance fission of 237Np, 236Np, 235Np, and 234Np from sharp excitation energy states ranging from below the fission barrier to 20 MeV, a method for separating experimental multichance fission mass yields was developed. This method utilizes a statistical formalism to describe the decay of the excited compound nucleus through fission, neutron emission, or gamma emission. First-chance mass yields from sharp states of excitation energy are presented for 237Np, 236Np, 235Np, and 234Np for excitation energies ranging from approximately 4 MeV to 20 MeV. Preliminary evidence for an enhanced fragment yield in the region of the doubly magic nucleus A = 78 (Z = 28, N = 50) is also shown. The effects of preequilibrium neutron emission for excitation energies between 12 MeV and 33 MeV are also discussed