Study of isovector resonances with pion charge exchange

Studies with the pion charge exchange reactions (π^±, π⁰) at 164 MeV using the π⁰ spectrometer are yielding new results on the existence and systematic features of isovector resonances in nuclei. These experiments possess an unusually high signal/background ratio for isovector resonances of low-multipolarity. Results obtained to date are: (1) Observation and angular distribution measurement of the giant dipole resonances in nuclei ¹²C, ⁴⁰Ca, ⁹⁰Zr and ¹²⁰Sn; (2) Observation and angular distribution measurements in the (π^-, π⁰) reaction on ⁹⁰Zr and ¹²⁰Sn of large signals possessing the expected angular distribution shapes and magnitudes for the isovector monopole resonance. Excitation energies are near the hydrodynamical model values 170 A^{- 1 3}MeV. Differential cross sections are approximately 0.7 J²(qR) mb/sr. An overview of this experimental program, with emphasis on new results, and how they correlate with existing knowledge on the isovector resonances, is given.