Competing electric and magnetic excitations in backward electron scattering from heavy deformed nuclei

Abstract

Important $E2$ contributions to the $(e,e^{\prime })$ cross sections of low-lying orbital $M1$ excitations are found in heavy deformed nuclei, arising from the small energy separation between the two excitations with $I^{\ddot{I}}€K=2^{+}1$ and 1${}^{+}1$, respectively. They are studied microscopically in QRPA using DWBA. The accompanying $E2$ response is negligible at small momentum transfer $q$ but contributes substantially to the cross sections measured at $\hat{I}¸=165^{\circ }$ for $0.6<q_{\mathrm{eff}}<0.9$ fm${}^{-1}$ ($40\le E_i\le 70$ MeV) and leads to a very good agreement with experiment. The electric response is of longitudinal $C2$ type for $\hat{I}¸\le 175^{\circ }$ but becomes almost purely transverse $E2$ for larger backward angles. The transverse $E2$ response remains comparable with the $M1$ response for $q_{\mathrm{eff}}>1.2$ fm${}^{-1}$ ($E_i>100$ MeV) and even dominant for $E_i>200$ MeV. This happens even at large backward angles $\hat{I}¸>175^{\circ }$, where the $M1$ dominance is limited to the lower $q$ region.

RevTeX, 19 pages, 8 figures included Accepted for publication in Phys Rev C