Isospin distribution of quadrupole strength in 118Sn: Comparison with pion, nucleon, and electron scattering.

The isospin structure of the quadrupole strength in /sup 118/Sn is examined by a variety of means in an attempt to understand the surprisingly large amount of isovector strength extracted from some recent ..pi../sup -/..pi../sup +/ experiments in the excitation-energy region (below 2(h2..pi..)..omega..) expected for the isoscalar giant-quadrupole resonance. The ratio of the giant-quadrupole resonance neutron and proton multipole matrix elements for /sup 118/Sn determined from ..pi../sup -/..pi../sup +/ data is M/sub n/M/sub p/ = 1.9 +- 0.4 compared to calculations which range from 1.1 to about NZ (1.36). It is demonstrated that this large ratio has unrealistic consequences in random-phase approximation mixing of the giant states into the first 2/sup +/ state transition (core polarization), which has been independently studied by other probes and is in rather good agreement with quasiparticle random-phase approximation calculations, which have M/sub n/M/sub p/less than or equal toNZ for the isoscalar giant-quadrupole resonance. Quasiparticle random-phase approximation transition densities are used to calculate pion, proton, and neutron cross sections to the isoscalar giant-quadrupole resonance and the first 2/sup +/ state using microscopic reaction models. A comparison of B(E2) from the same structure model is made to the various data for the first 2/sup +/ transitionmore » and to the /sup 116/Sn(e,e') data on the isoscalar giant-quadrupole resonance. Although not completely conclusive because of the lack of reliable (e,e') data, the evidence from all these comparisons is that the ..pi../sup -/..pi../sup +/ results are at odds with the results of other probes and with nuclear structure theory and that the problem seems to be with the ..pi../sup +/ scattering results« less