Single-particle effects in precompound reactions: Influence of thef7/2shell closure

We present angle integrated spectra for the (p,n) reaction with 25 MeV protons on targets of ${}_{{\mathrm{}}^{50}}{}^{}{}_{}{}^{}$,52,${\mathrm{}}_{24}^{53}$Cr, and on ${}_{26}{}^{58}{}_{}{}^{}\mathrm{Fe}$, ${}_{27}{}^{59}{}_{}{}^{}\mathrm{Co}$, ${}_{28}{}^{60}{}_{}{}^{}\mathrm{Ni}$, and ${}_{29}{}^{63}{}_{}{}^{}\mathrm{Cu}$. These spectra are compared with two-quasiparticle state densities for proton-particle–neutron-hole configurations, calculated using Nilsson-model single-particle energies. The influence of shell structure on the precompound spectra, previously shown for the $g_{9/2}$ neutron shell closure, is shown here for the $f_{7/2}$ neutron and proton shells. Semiquantitative agreement is shown between experimental results and calculated two-quasiparticle densities. The parameter space of pairing energy, deformation, and oscillator stiffness is explored. Precompound spectra near shell closures are in better agreement with partial state densities from Nilsson-model considerations than with results from more commonly used equidistant spacing models.