Q-Value Systematics for Isovector Giant Resonances Excited by (p,n) Reactions on Zr, Nb, Mo, Sn, and Pb Isotopes

The ($p$,$n$) reaction at 45 MeV is used to study two broad peaks found previously with the target ${}_{}{}^{90}{}_{}{}^{}\mathrm{Zr}$. They have now been observed with all but one of seventeen targets from ${}_{}{}^{90}{}_{}{}^{}\mathrm{Zr}$ to ${}_{}{}^{208}{}_{}{}^{}\mathrm{Pb}$. Energy systematics favor the conclusion that these peaks are antianalogs of the giant $M1\mathrm{}$ and $E1$ resonances in the target nucleus. The first experimental determinations of $T$, $T-1\mathrm{}$ splittings of the giant $E1$ resonance are reported. Their low values in comparison to $T$, $T+1\mathrm{}$ splittings observed previously can be interpreted as due to a tensor part of the effective isospin potential.