3rd Joint Meeting of the APS Division of Nuclear Physics and the Physical Society of Japan
Volume 54, Number 10
Tuesday–Saturday, October 13–17, 2009;
Waikoloa, Hawaii
Session CA: Correlations in Structure Up To and Beyond Bound Limits
9:00 AM–12:00 PM,
Thursday, October 15, 2009
Room: Kona 5
Chair: Susumu Shimoura, CNS, University of Tokyo
Abstract ID: BAPS.2009.HAW.CA.4
Abstract: CA.00004 : Exotic clusters in an unbound region of light neutron-rich systems
11:15 AM–12:00 PM
Preview Abstract
Abstract
Author:
Makoto Ito
(Department of Pure and Applied Physics, Kansai University)
In light neutron-rich systems, many kinds of molecular
structures are discussed from the view point of the clustering
phenomena.In particular, much attention has been concentrated
on Be isotopes. The molecular orbital (MO), such as $\pi^-$ and
$\sigma^+$ associated with the covalent binding of atomic
molecules, have been shown
to give a good description for the low-lying states of these
isotopes. In their highly-excited states, furthermore,
recent experiments revealed the existence of the interesting
resonant states which dominantly decay to the $^{6,8}$He
fragments. In this report, we show the unified study of the
exotic structures of $^{12}$Be=$\alpha$+$\alpha$+4$N$ in an
unbound region and the $\alpha$+$^{6,8}$He resonant scattering.
We applied the generalized two-center cluster model in
which the covalent MO and the atomic orbital (AO)
configurations with $^x$He+$^y$He could be described in a
unified manner. First, we calculated the energy spectra
below an $\alpha$ decay-threshold. The ($\pi_{3\slash2}^-$)$^2
$($\sigma_{1\slash2}^+$)$^2$ configuration corresponding to
$\nu$(0p)$^4$(sd)$^2$ becomes the ground state, while ($\pi_{3
\slash2}^-$)$^2$($\pi_{1\slash2}^-$)$^2$ having a large overlap
with $\nu$(0p)$^6$ appears as the first excited state.
The rotational band of the ground state reaches to the maximum
spin of J$^\pi$ = 8$^+$. This result means that the magicity of
$N$=8 is broken in $^{12}$Be due to the formation of ($\pi_{3
\slash2}^-$)$^2$($\sigma_{1\slash2}^+$)$^2$.
Next, we solved the scattering problem of $\alpha$+$^8$He and
identified the several resonance poles. In the continuum
region, we found the rotational bands having the AO
configurations of $\alpha$+$^8$He, $^6$He+$^6$He, and $^5$He+$^7
$He. Furthermore, a much more exotic band appears in the same
energy region. In this band, two valence neutrons are localized
at individual $\alpha$-cores (the $^5$He+$^5$He cluster),
while the other two neutrons form the covalent $\sigma^+$-
bonding between two $^5$He clusters; hence, it has a ``hybrid
structure'' between the MO configuration and the AO one. In the
J$^\pi$=0$^+$ state, it is strongly excited by the two-neutron
transfer reaction, $\alpha$+$^8$He$\rightarrow$$^6$He+$^6$He.
We also calculated the matrix elements of isoscalar monopole
transition (MTR), $|<0_f^+|\sum_i^{12}r_i^2| 0_1^+>|^2$. The
MTR matrix element going to the AO state of $\alpha$+$^8$He is
the largest in all the excited states, although an certain
enhancement also occur in the transition to the first excited 0
$^+$, which seems to be consistent to the observed electric
MTR. Therefore, this result strongly suggests that the monopole
transition is enhanced when the final state have a developed
cluster (AO) structure.
In order to analyze the enhancement of MTR in the realistic
reaction process, we also performed the calculation of the
continuum discretized coupled-channels (CDCC) for the monopole
breakup of $^{10}$Be into $\alpha$+$^6$He by a $^{12}$C target.
This breakup reaction is mainly induced by a nuclear
interaction from the $^{12}$C target, and the multi-step of the
continuum-continuum coupling is quite strong. We confirmed the
strong enhancement in the transition of 0$_1^+$ $\rightarrow$ 0
$_3^+$, which have an AO configuration of $\alpha$+$^6$He(2
$_1^+$). Therefore, the cluster state is strongly excited in
the nuclear breakup reaction, which is consistent to the result
in the analysis of MTR for $^{12}$Be. Similar studies of even
Be isotopes and future perspectives will also be presented.
To cite this abstract, use the following reference: http://meetings.aps.org/link/BAPS.2009.HAW.CA.4