RIKEN - RIBF : Nuclear Spectroscopy with Radioactive Isotope Beams
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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.
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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,p¡¯) and 24,23O(p,p¡¯)
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.
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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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