Neutron reactions that produce $\alpha$ particles have been investigated experimentally and analyzed by reaction model calculations for incident neutron energies from threshold to 150 MeV on elemental chromium and iron. The cross sections were measured at the Los Alamos Neutron Science Center by direct observation of $\alpha$ particles. Previous data on isotopes ${}^{59}$Co and ${}^{58,60}$Ni were also included in the analysis. The model calculations are made for both statistical decay and pre-equilibrium processes. This study particularly focuses on the pre-equilibrium cluster emission, which is described by the clustering exciton model of Iwamoto and Harada. We calculate the $\alpha$-particle formation factors numerically without any of the approximations that appeared in the original model. The model parameter $\Delta R$, the nuclear surface area where the pickup reaction may occur, is determined by fitting the calculated $\alpha$-particle energy spectra to experimental data. The calculated $\alpha$-particle-production cross sections agree well with the measured data, except for the Cr case. With a simple sensitivity study for the level density parameters, it is reported that relatively small changes in the level density parameters improve the reproduction of experimental data significantly. Our realistic model calculations for the pre-equilibrium process shed light on uncertainties in the nuclear level densities in statistical decay calculation.

• Received 13 November 2011

DOI:https://doi.org/10.1103/PhysRevC.85.054602