The validity of the surrogate ratio method for determining ($n,f$) cross sections for actinide nuclei is examined. This method relates the ratio of two compound-nucleus reaction cross sections to a ratio of coincidence events from two measurements in which the same compound nuclei are formed via a direct reaction. With certain assumptions, the method allows one of the cross sections to be inferred if the other is known. We develop a nuclear reaction-model simulation to investigate whether the assumptions underlying the ratio approach are valid and employ these simulations to assess whether the cross sections obtained indirectly by applying a ratio analysis agree with the expected results. In particular, we simulate surrogate experiments that allow us to determine fission cross sections for selected actinide nuclei. The nuclei studied, ${}^{233}\mathrm{U}$ and ${}^{235}\mathrm{U}$, are very similar to those considered in recent surrogate experiments. We find that in favorable cases the ratio method provides useful estimates of the desired cross sections, and we discuss some of the limitations of the approach.

• Received 30 May 2006

DOI:https://doi.org/10.1103/PhysRevC.74.054601