Proton Capture into the Giant Dipole Resonance ofCa40

The ${}_{}{}^{39}{}_{}{}^{}\mathrm{K}(p,\gamma ){}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$ reaction has been studied from 6.5- to 17-MeV bombarding energy covering the electric giant dipole resonance (GDR) in ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$. The GDR displays fine, intermediate, and gross structure. A fluctuation analysis of the fine structure gives a coherence width ∼25 keV and indicates that compound levels contribute <3% to the total dipole strength. The intermediate structure has widths clustered between 150 and 450 keV. Despite the large yield fluctuations angular distributions have a uniform shape throughout the GDR with $A_2=-0.33\mathrm{}$, characteristic of dominating $f$-wave capture. A broad coherent state observed at 19.3 MeV contains 90% of the capture strength with the remainder contained in a weak broad peak at 22 MeV. These positions and relative strengths agree well with calculations of the coherent dipole states based on 1p-1h excitations. However, continuum calculations fail to predict the total widths and angular distributions.