The 36Ar(α, γ 0) 40Ca reaction at excitation energies in the region of the giant dipole resonance

Abstract

The differential cross section for the 36Ar(α, γ 0) 40Ca reaction was measured at 90° to the beam direction in 125 keV, 50 keV and 50 keV intervals over the bombarding energy ranges 6–17 MeV, 6.54–8.81 MeV and 11.67–13.65 MeV, respectively. Gamma-ray angular distributions were measured at eighteen bombarding energies. The results indicate that the reaction proceeds through isolated 1 − levels and to a lesser extent 2 + levels. Excitation energies and partial decay widths for the 1 − levels are presented. From a comparison of the 36Ar(α, γ o0 40Ca, 39K(p,γ 0) 40Ca and 40Ca(γ, n 0) 39Ca excitation functions, it is shown that the α-capture reaction most probably leads to exciting the predominantly T = 1 giant dipole resonance states through a T = 0 isospin impurity. The result presented here, that the α-reduced widths decrease by a factor of 40 over the excitation energy region considered, is discussed with regard to variations in the degree of isospin mixing with energy and a shell-model calculation.