Giant resonance spectroscopy of 40Ca with the (e,e′ x) reaction (III): Direct versus statistical decay

Abstract

The present article is the third out of three on a study of the 40Ca(e,e′ x) reaction discussing the role of direct and statistical contributions to the decay of the observed giant resonance strengths. The proton and α decay modes leading to low-lying final states in 36Ar and 39K were investigated. The branching ratios for the p 0, p 123, α 0 and α 1 channels are compared to statistical model calculations. In the excitation region of dominant isoscalar E2 strength ( E x=12–18 MeV) good agreement is observed. Model predictions of direct E2 decay for the ( α 0+ α 1)/(p 0+p 1) ratio describe the data poorly. In the isovector E1 excitation region large excess strength is found in the population of low-lying states in 39K. A fluctuation analysis shows the direct contributions to the p 0, p 1 channels to be ⩾85%. The presence of preequilibrium components is indicated by the significant nonstatistical decay to the p 3 level which has a dominant ‘phonon⊗hole’ structure. Cross correlations reveal no significant branching between the different channels. The correlations between different electron scattering angles in the p 0, p 1 and p 3 decay result in an interaction radius compatible with the whole nucleus acting as an emitting source.