|Budget Amount *help
¥3,700,000 (Direct Cost : ¥3,700,000)
Fiscal Year 2004 : ¥700,000 (Direct Cost : ¥700,000)
Fiscal Year 2003 : ¥3,000,000 (Direct Cost : ¥3,000,000)
We have studied inelastic scatterings of drip-line nuclei, ^<11>Be, ^<14>Be, ^<15>B, ^<17>B, ^<18>C and ^<19>C on a proton target as well as on a carbon target. In this study we aimed at exciting primarily the first excited state, which has information of deformation of these nuclei. The deformation of such nuclei have attracted much attention since the extreme neutron to proton ratio may cause the change of nuclear shell structures represented by the magicity loss, or even the decoupling effect on the valence neutrons.
In this study, not only the bound excited states which can be measured by in-beam gamma spectroscopy, but also the unbound states which can be reached by the invariant mass spectroscopy. For the latter, we have developed a new drift chamber which can measure angles of an ejectile with very high accuracy (less than 0.1 deg). We have also developed a new scheme of analyzing two-neutron events, which was essential to specify the excited state in two-neutron halo nucleus ^<1
A series of experiments were performed at RIKEN accelerator Research facility, where the intense radioactive beam is available. The drip-line nuclei were produced by the projectile, fragmentation and purified through the fragment separator RIPS. The excitation energy was obtained either by the method mentioned above.
We have observed for the first time the 1^<st> excited state of ^<17>B in the bound excited state and that of ^<14>Be just above the particle decay threshold at Ex=1.56 MeV. The cross sections for producing the first excited states in ^<17>B and ^<15>B suggest that the effective charges for these nuclei are extremely small compared to the conventional value. This may be related to the decoupling effect which was found for the neighboring nucleus ^<16>C. As for ^<14>Be, the first excited states located at 1.56 MeV, lower than that of ^<12>Be at 2.1 MeV. This suggests that the ^<14>Be has even more melted shell structure than the case of 12Be, which was recently found as a magicity-lost nucleus. However, the deformation parameter extracted for ^<14>Be was smaller than that of 12Be. This may be understood as a restoration of shell stability. This interesting findings are to be further studied with theories including the shell melting or cluster nature of these nuclei. For other nuclei, we have also observed a new state as well.
We are now planning to use the different probe of alpha target, an ideal isoscalar probe. This may clarify the shape difference between the neutron distribution and the proton distribution. For the very extreme neutron/proton ratios, we expect more decoupled situation where neutrons moves independently of protons. Studies of such very exotic excitation is important to understand the behavior of very neutron rich nuclei. Less