Identification of the β+ Isovector Spin Monopole Resonance via the Pb208 and Zr90(t,He3) Reactions at 300MeV/u

The double-differential cross sections for the Pb208 and Zr90(t,He3) reactions at 300MeV/u have been measured at the RI Beam Factory at RIKEN. This was the first physics experiment with the SHARAQ magnetic spectrometer. The combined analysis of the present (t,³He) data and previous (n,p) data provides the clearest identification for the β⁺ isovector spin monopole resonance both in the Tl208 and Y90 nuclei, and puts the observations of this giant resonance on a firm foundation. The measured distributions of the (t,He3) monopole cross sections were well reproduced by the distorted-wave Born approximation calculation, where the target transition density was calculated with the self-consistent Hartree-Fock plus random-phase approximation using the T43 Skyrme interaction. A major part of the expected β⁺ isovector spin monopole strength was found in the measured cross section spectra.