Correlated Fragment Energy Measurements and Mass-Versus-Angle Correlations in Ra226 + p Fission

Correlated energies of fragment pairs from the proton-induced fission of Ra226 were measured using protons obtained from the Oak Ridge tandem Van de Graaff accelerator at energies Ep = 11.0, 12.0 and 13.5 MeV. Corresponding excitation energies in Ac227 are 16.3, 17.3 and 18.8 MeV respectively. Detailed kinematic calculations from.these data have provided mass distributions, kinetic energy distributions and mass versus energy correlations. At Ep = 13.5 MeV, such determinations were made at angles of 30°, 40°, 60° and 90° with respect to the incident proton beam; the resulting mass distributions and kinetics have been investigated in detail as a function of angle. The mass distribution for Ra226 + p fission has long been known to be triple-peaked; in the present experiment, for θ = 90°, the ratio p of fragment abundance at the symmetric peak to that at the asymmetric peak was found to increase from 1.11 for 11.0-MeV protons to 1.27 for 12.0-MeV protons and to 1.54 for 13.5-MeV protons. For all three proton energies, the average total fragment kinetic energy as a function of mass, EK(M), remains constant at ∼143 MeV through much of the symmetric peak. EK(M) then increases for increasing asymmetry and reaches a maximum of ∼152 MeV at MH ≅ 134amu (two mass units lower than the peak in the asymmetric mass yield). The maximum in Ek(M) is attributed to the closure of the Z = 50 and N = 82 shells in this range of heavy-fragment masses, as in the case of spontaneous and thermal-neutron- induced fission. The results of detailed analyses are given. Comparison of the ∼ 50 000-event mass distributions obtained at the four fragment angles indicated above, for Ep = 13.5 MeV, shows that the mass distribution does vary with angle. The effect is not large: preliminary analyses indicate a monotonie increase in the ratio p from ∼ 1.46 for θ = 30° to ∼ 1.54 for θ = 90°, which is well outside statistical and experimental uncertainties. The direction of the observed effect is in agreement with that expected from arguments based on the reduced excitation energy for p, n-fission and p, 2n-fission, and the consequent increase in the 0°/90° ratio for asymmetric fragments. A more complete analysis is in progress to determine the quantitative validity of such arguments. (author)