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

The present article is the third out of three on a study of the Ca-40(e, e'x) reaction discussing the role of direct and statistical contributions to the decay of the observed giant resonance strengths. The proton and alpha decay modes leading to low-lying final states in Ar-36 and K-39 were investigated. The branching ratios for the p(0), p(123), alpha (0) and alpha (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 (alpha (0) + alpha (1))/(p(0) + p(1)) ratio describe the data poorly. In the isovector El excitation region large excess strength is found in the population of low-lying states in K-39. A fluctuation analysis shows the direct contributions to the po, pl channels to be greater than or equal to 85%. The presence of preequilibrium components is indicated by the significant nonstatistical decay to the p(3) level which has a dominant 'phonon circle times 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. (C) 2001 Elsevier Science B.V. All rights reserved.