Giant resonance region ofN15studied by polarized and unpolarized proton capture measurements

Abstract

We have measured the cross sections and the analyzing powers for the ${}_{}{}^{14}{}_{}{}^{}\mathrm{C}(\overrightarrow{p}, \gamma ){}_{}{}^{15}{}_{}{}^{}\mathrm{N}$ reaction for proton energies from 4.0 to 16.2 MeV. The corresponding range in excitation energies, 13.94 to 25.33 MeV, covers the region of the giant dipole resonance in ${}_{}{}^{15}{}_{}{}^{}\mathrm{N}$. The five-angle angular distributions measured with unpolarized and with polarized beams at eight energies over this region were analyzed to determine the magnitudes and relative phases of the transition matrix elements. The analysis included $E1\mathrm{}$, $M1\mathrm{}$, and $E2\mathrm{}$ transitions. The observed $E1\mathrm{}$ strength exhausts only about 6% of the classical dipole sum rule. The dominant $E1\mathrm{}$ strength is compared to a shell-model calculation which includes 3p-3h excitations. A fairly uniform distribution of $E2\mathrm{}$ strength is observed which exhausts about 7% of the total energy-weighted sum rule. Although large uncertainties on this result could allow the existence of a giant $E2\mathrm{}$ resonance, the most probable values indicate that no appreciable concentration of $E2\mathrm{}$ strength is observed.