Isospin effects in the giant dipole resonance of 42Ca

The giant dipole resonance (GDR) of ⁴²Ca was investigated through the ⁴¹K(p,γ₀)⁴²Ca reaction. The gross structure indicates two peaks at 17.4 ± 0.1 MeV (Γ=3.3±0.5 MeV) and 20.4 ± 0.1 MeV (Γ=4.4±0.1 MeV) which contain 13 and 87%, respectively, of the E1 strength in the (γ,p₀) channel. From a comparison of structure and excitation energies of the GDR in ⁴⁰Ca and ⁴²Ca, the two peaks can be identified with the T=1 (at 17.4 MeV) and the T=2 (at 20.4 MeV) isospin components of the GDR. This energy separation gives an effective symmetry potential of V=63 MeV in agreement with the general predictions and observation in other nuclei. However, identification of the peaks with the K=0 and K=1 dipole oscillations in the deformed ⁴²Ca ground state is not ruled out. A shell-model calculation of the dipole states in ⁴²Ca with good isospin was done by diagonalizing large matrices and using the Kuo-Brown G matrix elements. This gives a concentration of T=1 strength at 17.1 MeV and T=2 strength at 19.6 MeV, in good agreement with the experimental results, and supports the isospin identification of the two components.