Pb(n, n′x)at 65 MeV and the Isospin Structure of the Giant Quadrupole Resonance Region

The first $\mathrm{Pb}(n, n^{\prime }x)$ data (at 65 MeV) are compared to earlier (61-66 MeV) ($p, p^{\prime }x$) data. The isovector sensitivity of these nucleon probes is used to determine $\frac{M_n}{M_p}\simeq \frac{N}{Z}=1.54\mathrm{}$ for the giant quadrupole resonance (GQR). This result is consistent with other direct GQR measurements, $2_1^{+}$ data connected through core polarization, and several microscopic random-phase-approximation calculations, but in disagreement with a $\frac{{\pi }^{-}}{{\pi }^{+}}$ study which gives $\frac{M_n}{M_p}=3.8\mathrm{}\pm 1.2$.