Giant resonance spectroscopy of 40Ca with the (e,e′x) reaction (II): Multipole decomposition of 4π-integrated spectra and angular correlations Original Research Article

The present article is the second out of three on a study of the 40Ca(e,e′x) reaction discussing the multipole decomposition of the measured cross sections and the analysis of angular correlations. The decomposition of the strongly overlapping E0, E1 and E2 giant resonance strengths using the (e,e′x; x=p,α) reaction in 40Ca is discussed for excitation energies between 10 and about 21 MeV. Two extraction methods are presented based on the variation of the form factors for the different multipoles. The resulting B(E1) strength distribution is in good agreement with (γ,x) photoabsorption data. The summed B(E2) and B(E0) strength is highly fragmented and spread out over the energy region investigated. Microscopic continuum RPA calculations including the coupling of the basic particle–hole states to the low-lying surface vibrations are capable of reproducing the strength distributions quite accurately. Exhaustion of the energy-weighted sum rules (EWSR) for the various decay channels is presented. A complete decomposition of E0, E1 and E2 contributions in 40Ca is possible for (e,e′α) angular correlations populating the 36Ar ground state. Contrary to expectations, the form factors of isoscalar E0 and E2 strengths in the 40Ca(e,e′α0) reaction exhibit increasing differences towards smaller momentum transfers. Angular correlations for proton decay into low-lying states of 39K are compared to a self-consistent continuum RPA calculation which allows a systematic description of the strong variations observed as a function of 40Ca excitation energy and momentum transfer. The success implies that direct knock-out models of the 40Ca(e,e′p) reaction are too simple. Furthermore, the shapes of the angular correlations seem to be determined largely by the final-state interaction, in particular by charge exchange reactions in the nuclear medium.