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

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 p0, p123, α0 and α1 channels are compared to statistical model calculations. In the excitation region of dominant isoscalar E2 strength (Ex=12–18 MeV) good agreement is observed. Model predictions of direct E2 decay for the (α0+α1)/(p0+p1) 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 p0, p1 channels to be ⩾85%. The presence of preequilibrium components is indicated by the significant nonstatistical decay to the p3 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 p0, p1 and p3 decay result in an interaction radius compatible with the whole nucleus acting as an emitting source.