Charge Exchange Reaction: a powerful tool to study the ``Weak Response'' of nuclei

Weak-interaction processes, for example, Gamow-Teller (GT) transitions, play important roles in the stellar evolution and nucleosynthesis. $\beta$-decay studies provide the most direct information on the transition strengths caused by these processes, but the accessible $E_x$ range is limited by the decay $Q$ values. On the other hand, Charge-Exchange (CE) reactions can access the transitions to higher excited states. Pioneering ($p,n$) reactions at intermediate incident energies ($E_{\rm beam} > 100$ MeV/nucleon) found the main part of the GT transition strength expected by the GT sum rule in GT resonances (GTRs) situated in the $E_x = 10-15$ MeV region, and detailed studies in combination with ($n,p$) studies provide further knowledge on the GT sum rule. Recent development of CE reactions toward higher resolution and sensitivity is impressive. The ($p,n$)-type ($^3$He,$t$) reaction achieved a $\Delta E \sim 30$ keV and GTRs were resolved into individual states. The ($d,^2$He) reaction also provides much better resolution in the ($n,p$)-type study and recent ($t,^3$He) reaction in coincidence with $\gamma$ decay could detect weakly excited states. We can also see future in the CE reactions using unstable incident beams.