Speaker
Description
Double Gamow–Teller (DGT) transition is a nuclear process in which both the spin and isospin are flipped twice without a change in the orbital angular momentum. The high excitation energy region of the DGT transition is hardly known. Especially, the giant resonance in the DGT transition, double Gamow–Teller giant resonance (DGTGR), is expected to exist, but there has not been the established observation. The observation on the DGTGR will open up the new domain of the nuclear mode, as well as its observables will provide information on the nuclear matrix element of the neutrino-less double beta decay.
We have performed the experiment of the ($^{12}$C, $^{12}$Be(0$^{+}_{2})$) for 48Ca target at RIBF, aiming at the observation of the DGTGR. We used the intense primary beam of $^{12}$C with an energy of 250 MeV/nucleon provided from Superconducting Ring Cyclotron and used BigRIPS as a spectrometer. As $^{12}$Be(0$^{+}_{2}$) decays with emitting $e^{+}e^{-}$ pair, we identified the reaction channel by detecting the 511-keV gamma ray using DALI2. The cross section in the energy region of 0 to more than 30 MeV was measured with the energy resolution of 1.5 MeV and the angular resolution of 0.2$^{\circ}$, and we observed a forward peaking structure around 20 MeV in the excitation energy of $^{48}$Ti. We conclude that it is the candidate for the DGTGR. The DGT transition strength B(DGT) was also evaluated from the observed cross section by performing the multipole decomposed analysis. This first result of the B(DGT) in the high energy region has large uncertainty because of the lack of statistics, but it had established the way to obtain the information on the DGTGR experimentally.
The outline of the experiment and the prospects will be given in the talk.