Angular and energy dependence of $(e,e^{\prime})$ cross sections for orbital 1$^+$ excitations

Abstract

The main features of the $(e,e^{\prime})$ cross sections of low-lying orbital excitations with $K^{\pi} = 1^+$ in heavy deformed nuclei are studied in RPA on the example of $^{156}$Gd. The dependence of the DWBA E2 and M1 cross sections on the scattering angle $0^{\circ} < \theta < 180 ^{\circ}$ and incident electron energy $E_i < 210$ MeV is analyzed in PWBA. The cross section is larger for M1 than for E2 transitions at any angle if $E_i < 30$ MeV. The longitudinal (Coulomb) C2 excitation dominates the E2 response for $5^{\circ} < \theta < 170 ^{\circ}$. Only transverse M1 and E2 excitations compete for $\theta > 175 ^{\circ}$ and the former one is dominant for $q < 1.2$ fm$^{-1}$. The M1 response is almost purely orbital up to $q = 1.4$ fm$^{-1}$ even in backward scattering. Qualitative PWBA estimates based on the $q$-dependence of the form factors alone are not able to predict some important features of the $(e,e^{\prime})$ cross sections stemming from the strong magnetic and orbital character of the studied 1$^+$ excitations. The expectation for M1 over E2 dominance in backward scattering should not be extended to higher momentum transfers and incident energies.