Anomalous behavior of the proton-induced fission cross sections ofU235and

Abstract

The proton-induced fission cross sections for 235 U and 238 U targets have been measured for £^ = 1-4 MeV for the first time. Compared with the measurements at higher energies, the present results indicate a pronounced change in the slope of the excitation function below 4 MeV such that standard opticalmodel estimates fall far short of the observed cross sections. No plausible explanation in terms of fission of either the target or the compound nucleus can be found.PACS numbers: 25.85.GeIn recent years considerable attention has been focused on the behavior of heavy-ion reaction cross sections in the sub-barrier region where significant enhancement over predictions which use a static barrier has been observed. x On the other hand, for very light projectiles on heavy targets, such an enhancement is not expected. Experimental measurements of the proton-induced fission cross sections of 235 U and 238 U in the sub-barrier region published so far do lie within the limits set by standard optical-model calculations. 2 " 5 To investigate this aspect at still lower proton energies, we have used a technique which enables measurement of very low (subnanobarn) fission cross sections.As shown schematically in Fig.