Quest for Connecting Quarks to the Cosmos


          WCU Center for HIGH ENERGY PHYSICS


   RIKEN - RIBF : Nuclear Spectroscopy with Radioactive Isotope Beams

The RIKEN RI Beam Factory (RIBF) in Japan / Tokyo has started its regular operation in 2008 after ten years of construction. It is an accelerator complex consisting of five cyclotrons, AVF (K=78 MeV), RRC (K=540 MeV), fRC (K=570 MeV), IRC (K=980 MeV), and SRC (K=2500 MeV), and one linear accelerator RILAC. With these accelerators implemented, the facility can provide beams of various (stable) nuclear spices from deuterium to uranium with typical energies of up to 350 MeV/nucleon. From these primary beams a vast variety of unstable nuclear spices is produced as the secondary beam using a fragment separator called BigRIPS. Major experimental installations connected to BigRIPS are to be used to explore hitherto unknown properties of these exotic nuclei. Major goals of the facility are (1) to investigate exotic nuclear structures of nuclei far from stability and (2) to measure key cross sections relevant to important astrophysical phenomena.

Our group has repeatedly performed experiments in search for unbound excited states
in neutron rich nuclei using beams provided by RRC and RIPS. An experimental setup, which is sensitive to both charged fragments, projectile-rapidity neutrons and de-excitation gamma rays from the fragments, together with novel experimental technique called the invariant mass method, has been successfully exploited to identify new resonance states in light neutron rich nuclei. Reaction channels investigated so far are, for example, proton inelastic scattering on neutron-rich carbon and oxygen isotopes: the 19,17C(p,pí») and 24,23O(p,pí») reactions, the charge exchange reaction on the heaviest beryllium isotope: the 14Be(p,n) reaction, and one nucleon knock-out reactions on 14Be, 17B and 17C.

To perform equivalent measurements for secondary beams with higher energies provided by SRC and BigRIPS is a good idea: (1) we can carry out measurements under much more favorable conditions in terms of intensity (statistics) and energy loss (energy resolution). (2) We can expand the mass region of nuclear species to be investigated to heavier and more neutron-rich regions. The SAMURAI spectrometer presently under construction as one of the major experimental installations at RIBF will provide us with a good opportunity. The spectrometer features a large solid angle and a large momentum acceptance dedicated to particle-correlation studies. It consists of a superconducting H-type magnet with 7 Tm-rigidity and with a large pole gap of 80 cm, and associated particle as well as gamma-ray detectors. Our group is also involved in design studies of the spectrometer system, which include (a) an investigation of the response of a large-volume-and-highly-segmented plastic scintillator array to fast multiple neutrons and (b) optimization of various parameters associated with the branch beam line from BigRIPS to SAMURAI. The SAMURAI collaboration consists of Seoul National University, Tohoku University, RIKEN, Tokyo Institute of Technology, Kyoto University, and Osaka University.



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