Isospin Splitting of the Giant Dipole Resonance inNi60

Abstract

The radiative-capture reaction ${}_{}{}^{59}{}_{}{}^{}\mathrm{Co}(p,\gamma ){}_{}{}^{60}{}_{}{}^{}\mathrm{Ni}$ has been studied for proton energies from 4.40 to 13.60 MeV. Cross-section and angular-distribution data were measured with a large anticoincidence-shielded NaI(T1) detector. The yield curve for ground-state capture shows two broad peaks, at ${}_{}{}^{60}{}_{}{}^{}\mathrm{Ni}$ excitations of 16.6 and 19.6 MeV; these are interpreted as the $T_{<} \mathrm{and} T_{>}$ components of the giant dipole resonance. The results of a detailed calculation ignoring isospin effects do not adequately explain all of the observations; a simple 1p-1h calculation using eigenfunctions of $T$ describes the results quite well. The first-excited-state-capture yield curve is similar to the ground-state curve, shifted in energy by ∼300 keV. The data suggest that most of the observed strength in the giant dipole resonance based on the first excited state of ${}_{}{}^{60}{}_{}{}^{}\mathrm{Ni}$ is concentrated in $2^{-}$ states.