Neutron capture cross sections from Surrogate measurements

Abstract

The prospects for determining cross sections for compound-nuclear neutron-capture reactions from Surrogate measurements are investigated. Calculations as well as experimental results are presented that test the Weisskopf-Ewing approximation, which is employed in most analyses of Surrogate data. It is concluded that, in general, one has to go beyond this approximation in order to obtain (n, ) cross sections of su cient accuracy for most astrophysical and nuclear-energy applications. Cross sections for compound-nuclear (n, ) reactions are needed for a variety of applications, including astrophysics and nuclear energy. Modeling astrophysical processes that produce the heavy isotopes beyond iron, simulating nu- clear reactor operations, exploring alternative fuel cycles for energy generation, and studying transmutation options for radioactive waste, requires cross sections for neutron- induced reactions on isotopes from di erent regions of the nuclear chart. As many short-lived species cannot be made into targets for direct cross-section measurements, one has to rely on calculations or explore indirect approaches. The accuracies for the cross sections of inter- est, often in the range of 10% or less, can be much smaller than the theoretical uncertainties that exist when the model parameters are insu ciently constrained by data. For in- stance, standard evaluations for the (n, ) reaction on the s-process branch point nucleus 95 Zr (t1=2 = 64 d) vary from each other roughly by a factor of four 1 . Exploiting regional systematics, whenever cross sections or relevant structural data (level densities, -ray strength functions, etc.) for near- by nuclei are known, can provide valuable constraints for the calculations. In this contribution we explore the prospects for de- termining or constraining (n, ) cross sections through Sur- rogate measurements. The Surrogate nuclear reaction tech- nique combines experiment with theory to obtain cross sec- tions for compound-nuclear (CN) reactions, a+A! B ! c+C, involving targets (A) that are di cult or impossible to obtain (1-3). In the Hauser-Feshbach formalism, the cross section for this desired reaction takes the form: (Ea) = X