Charge distribution and recoil properties in the fission of 238U by 11·5 GeV protons

Abstract

Cross-sections and thick-target recoil properties have been determined for a number of products in the mass region 100–115 from the interaction of 238U with 11·5 GeV protons. The recoil ranges of the neutron deficient products are nearly a factor of two smaller than those of the neutron excessive products suggesting a difference in reaction mechanisms. On the basis of this finding a charge dispersion curve consisting of two separate gaussians, peaking at N Z = 1·26 and 1·40, was constructed to fit the measured cross-sections. The resulting curve is single-peaked but has a pronounced shoulder on the neutron deficient side. The isobaric yield at A = 111 is 35·0 ± 3·5 mb and the curve associated with the neutron excessive products contributes 80 per cent of the total yield. This curve is very similar to that obtained in the fission of 238U by 450 MeV protons except for a reduction in the fission cross-section by a factor of 2·4. The values of the average deposition energy, average total kinetic energy of the primary fragments, and average separation distance at scission obtained for the neutron excessive fragments from their recoil properties are nearly equal to the corresponding values at 450 MeV. Many of the properties of the neutron deficient products are consistent with either spallation or fission of a nucleus in the heavy rare earth region but neither mechanism can explain all the results.