Nuclear structure physics are undergoing a renaissance with a new type of beam, radioactive isotope beams (RIB). Many new discoveries have been made that changed the view of nuclear structure using RIB. New structures such as neutron halo, neutron skin, new magic numbers appears only off the stability line, to name a few. The new experimental field of nuclear astrophysics, experimental studies of nuclear reactions relevant to nucleosynthesis and other phenomena in the universe, has been opened. ISAC-II is a new facility that gives high-quality RIBs at energies higher than the Coulomb barrier and thus provides new opportunity to promote new structure physics of nuclei far from the stability line.
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The first experiment ISAC-II was to study the pairing correlation between two neutrons in the most pronounced halo nucleus, 11Li. Many of the two-neutron halo nuclei have borromean[1] structure and thus the correlation between two neutrons plays important role in its binding and the structure. Although several break-up experiments have been done, the correlation between the two halo neutrons is not well understood yet. A so called two-neutron transfer reaction 11Li +p → 9Li +t, that is believed to be the best tool for studying two-nucleon correlation in nuclei, has been studied at beam energy of 3.6MeV/A immediately after the commissioning of ISAC-II. An accelerated beam of 11Li with intensity about 2000 /s was delivered stably to the MAYA active target detector. MAYA is operated with isobutane gas of 150 mbar thus providing the proton target. A silicon-detector array and a CsI array were also equipped in MAYA for detection of forward going high-energy particles that leave gas detection area of MAYA. The active target provides advantages for efficient detection of the reaction: almost 4π detection of the reaction, effectively thickest usable target, and efficient detection of low-energy recoil particles. The experiment has been performed under the collaboration between GANIL(France), ANL(US) and TRIUMF group and some amount of data has been accumulated during the commissioning run. The analysis of the data is in progress. An example of the 11Li +p → 9Li + t reaction event in MAYA is presented in the figure, together with the observed kinematic loci of (p,t) reaction.
The success of the first delivery and the first experiment at ISAC-II presents a promise for the future studies of nuclear physics and nuclear astrophysics.•
[1] borromean nucleus: a bound nucleus with three-body character. Any of two constituents among three does not bind the nucleus, but the nucleus does bind with the three constituents together.•
