Proton-induced fission at ultra sub-barrier energies

Cross sections for proton-induced fission of {sup 238}U have been measured at seven proton energies ranging from 3.0 to 4.45 MeV using two position sensitive parallel-grid avalanche counters in a kinematic coincidence. In addition, an upper limit for the fission cross section was established at extreme sub-barrier energies down to a level of {approx equal}20 pb at 3 MeV. This result is in contradiction to recent findings of Ajitanand et al., who found that the fission excitation function exhibited a plateaux at about 1000 pb in the energy range from 1.0 to 3.5 MeV. 7 refs., 3 figs., 1 tab.