Isospin structure of the giant dipole resonance inCa41

Differential 90° cross sections for ${}_{}{}^{40}{}_{}{}^{}\mathrm{K}$($p$,${\gamma }_0$)${}_{}{}^{41}{}_{}{}^{}\mathrm{Ca}$ were measured over the entire giant dipole resonance region. The data are compared with similar results for the ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$($n$,${\gamma }_0$)${}_{}{}^{41}{}_{}{}^{}\mathrm{Ca}$ reaction and with calculations based on the direct-semidirect capture model. An observed energy difference of about 1.5 MeV between the centroids of the ($p$,${\gamma }_0$) and ($n$,${\gamma }_0$) cross-section curves is interpreted as the result of isospin selection rules.