RI-beam induced Charge-Exchange reactions as new probes to nucle by
Prof. Tomohiro Uesaka (RIKEN)
Seminars Friday 13 May 2011
Study of spin-isospin responses in nuclei forms one of cores in nuclear physics. A variety of collective states, for example isovector giant dipole resonances, isobaric analogue states, Gamow-Teller resonances, have been extensively studies by use of electromagnetic and hadronic reactions. Despite decades of efforts, several states of fundamental interests are left to be discovered. They include double Gamow-Teller resonances (DGTR), β+-type isovector spin monopole resonances, and 0- resonances. Their absences are mostly due to lack of proper experimental ways to approach the states.
RI-beam induced charge exchange reactions (RICE) have unique properties which are missing in stable-beam induced reactions and can be used to reach the yet-to-be-discovered states. We have constructed the high-resolution SHARAQ spectrometer  at the RI Beam Factory to pursue the capabilities of RICE as new probes to nuclei.
A series of experiments were conducted in 2009--2010 to investigate isovector monopole resonances (IVMR) in 90Zr and 208Pb. IVMRs are isovector analogue of isoscalar monopole resonances which have strong relevances to incompressibility of nuclei, and thus properties of IVMR are expected to give impacts on nuclear matter theories. The first experiments with the SHARAQ spectrometer was performed to search for β+- type isovector spin monopole resonances (IVSMR). The (t,3He) reaction at 300 MeV/nucleon was used to extract β+ strengths selectively. An intense triton beam of 107 sec-1 was produced by the projectile fragmentation of a primary 320-MeV/nucleon 4He beam and the scattered 3He ions were momentum-analyzed by the SHARAQ spectrometer. Clear signatures of the IVSMR were observed, for the first time, at excitation energies of 20 MeV and 12 MeV in 208Pb and 90Zr, respectively. The values of excitation energies are in good agreement with HF+Tamm-Dancoff calculations. In 2010, two experiments were carried out to reveal different aspects of IVMR: one is the 90Zr(10C,10B(0+)) experiment to search for isovector non-spin-flip monopole resonances. The reaction selectively populates isovector (T=1) non-spin-flip (ΔS=0) modes in nuclei, while any stable-beam induced charge exchange reaction never has the selectivity. In the other experiment, we explored a new method based on the exothermic charge exchange (12N,12C) reaction. The reaction has a large positive Q-value of about +17 MeV, which enables us to populate the highly-excited β--IVSMR in a recoil-less manner. Since β- -IVSMR is most strongly excited at around q=0, prominent strength is expected to be observed by the exothermic reaction.
We are planning in near future to apply the RICE method to studies of DGTR. Compared with the reactions used so far, the RI-beam induced (10C,10Be) reaction has clear advantages and would lead us to the discovery of DGTR.
In the talk, I will start with a brief overview of spin-isospin studies with stable beams and present new experimental results from SHARAQ experiments. The last 10--15 minutes will be devoted to future prospects of RICE studies to search for DGTR and other yet-to-be-discovered states.
 T. Uesaka et al., Nucl. Instrum. Methods B 266, 4218--4222 (2008).