Alpha capture to the giant dipole resonances of 42Ca, 44Ca and 52Cr

Abstract

Differential cross sections for the 38 Ar(α, γ 0) 42 Ca, 40 Ar(α, γ 0, 1) 44 Ca and 48 Ti(α, γ 0, 1) 52 Cr reactions were measured at 90° to the beam direction in 50 or 100 keV steps over the bombarding energy ranges 6.0–15.0 MeV, 5.5–11.1 MeV and 6.0–12.0 MeV respectively. Gamma-ray angular distributions were measured at forty bombarding energies. These show that the ( α, γ 0) reaction proceeds through 1 − levels and to a lesser extent 2 + levels, whereas the ( α, γ 1) reaction most probably proceeds through 1 − and 3 − levels. It is deduced that 〈 Γ〉/〈 D〉 ≦ 1 for the 40Ar( α, γ) 44Ca. reaction whereas the fine structure observed in the 48Ti( α, γ) 52Cr reaction is probably due to fluctuations. From a comparison with other data it is shown that the (α, γ) reaction is most probably statistical in nature. Using Hauser-Feshbach theory it is deduced that the 36Ar( α, γ) 40Ca. reaction is inhibited by isospin selection rules and an estimate is made of isospin mixing in the 40Ca giant dipole resonance. The 38 Ar(α, γ) 2 42 Ca and 40 Ar(α, γ) 44 Ca data are considered with respect to theories of isosopin splitting of the giant dipole resonance.