DISTRIBUTION OF E-2 STRENGTH IN SI-28 BELOW 50-MEV EXCITATION-ENERGY

Inelastic electron scattering in Si28 between 4 and 50 MeV excitation energy reveals two concentrations of E2 strength in the continuum. One is between 15 and 20 MeV, with a peak at 17 MeV, and can be identified with the giant quadrupole resonance in the ground state oblate well. A broad distribution of E2 strength between 22 and 42 MeV is predominatly isovector in nature. In addition, a small but persistent E2 peak at 24 MeV was found, which may be interpreted as being the corresponding state in the prolate well to the 17 MeV resonance. It is shown that 50% or more of the photon cross section in excess of the classical dipole sum rule between 10 MeV and the pion threshold may be due to E2 absorption. NUCLEAR REACTIONS Si28(e,e′), E0=92 MeV. Measured d2σdΩdEx, bound and continuum states. Deduced multipolarity λ, reduced matrix element B(Eλ), sum rule exhaustion. © 1979 The American Physical Society.