Electron scattering and nuclear structure at the S-DALINAC

Abstract

Results on ongoing research of elementary electric and magnetic nuclear excitations at the superconducting Darmstadt electron linear accelerator (S-DALINAC) are presented. In the first part of this lecture electric excitation modes in the continuum are discussed by way of two examples employing the coincident detection of inelastically scattered electrons in an (e,e′x) reaction. The first example is the reaction 48Ca(e,e′n) where the excitation and decay of isoscalar electric monopole and quadrupole giant resonances has been studied. The results on the strength distribution and the partial exhaustion of energy weighted sum rules are compared to those from RPA and second-RPA (SRPA) predictions as well as to those from hadronic reactions of the type (p,p′) and (p,p′n). The second example, a discussion of the 90Zr(e,e′p) reaction and its results, can be viewed as a direct proof that in coincidence reactions of the form (e,e′x) one is able to isolate and identify narrow weakly excited levels — here isobaric analog resonances — buried in the continuum and determine their excitation and decay properties and thus subtle nuclear structure properties. The second part of the lecture is concerned with high-resolution inelastic electron scattering under 180° which is selective with respect to magnetic excitations. Of those, first systematic studies on the hitherto scarcely explored magnetic quadrupole giant resonance which is mainly a spin-isospin excitation are presented. As an example, results for 48Ca and 90Zr, in which a strongly fragmented and quenched M2 strength has been detected, are compared to SRPA and sum rules approaches. Evidence is presented for a new excitation mode, the so called nuclear twist mode, and its parameters are determined. In the third part of the lecture, the possible use of transverse electron scattering form factors at low momentum transfer for an advocated test of in-medium modifications of vector mesons is critically examined, and finally an outlook is given on the nuclear physics research program at the S-DALINAC.