Di-triton molecular structure in
6
He
H. Akimune,
1,
* T. Yamagata,
1
S. Nakayama,
2
, Y. Arimoto,
3
M. Fujiwara,
4,5
K. Fushimi,
2
K. Hara,
4
M. Ohta,
1
A. Shiokawa,
1
M. Tanaka,
6
H. Utsunomiya,
1
K. Y. Hara,
1
H. P. Yoshida,
4
and M. Yosoi
7
1
Department of Physics, Konan University, Kobe 658-8501, Japan
2
Department of Physics, University of Tokushima, Tokushima 770-8502, Japan
3
Japan Synchrotron Radiation Research Institute (JASRI), Hyogo 679-5198, Japan
4
Research Center for Nuclear Physics, Osaka University, Osaka 567-0047, Japan
5
Advanced Science Research Center, JAERI, Ibaragi 319-1195, Japan
6
Kobe Tokiwa College, Kobe 654-0838, Japan
7
Department of Physics, Kyoto University, Kyoto 606-8502, Japan
Received 28 July 2002; published 28 May 2003
Resonances above the t+t threshold in
6
He have been studied via the
6
Li(
7
Li,
7
Be t )
3
H reaction at 0°
taken with a
7
Li beam of 65 MeV/nucleon. By observing the binary triton decay, a new prominent resonance
is found at E
x
=18.00.5 MeV with a width of 7.71.0 MeV. The branching ratio for the binary triton decay
from this resonance is deduced to be 9010 %, suggesting that the resonance at 18 MeV has a di-triton
molecular structure.
DOI: 10.1103/PhysRevC.67.051302 PACS numbers: 24.30.Gd, 23.70.+j, 25.70.Hi
Over many years, an enormous amount of effort has been
devoted to understand excitation energy spectra of light nu-
clei. Low-lying discrete states in light nuclei are often dis-
cussed in terms of the independent-particle shell model, as
well as of the cluster model 1–3. For high-lying reso-
nance states, the most successful model is the cluster
model 3. Some of the excited states in light nuclei are well
explained as an oscillation of a few-nucleon cluster. For ex-
ample, in
6
He, one of the neutron-rich light nuclei, the giant
dipole resonance, and the giant spin-dipole resonance are
understood as a vibration of a two-proton cluster against a
four-neutron cluster. In addition, the existence of the soft-
dipole resonance becomes evident 4. This resonance is un-
derstood as an oscillation of an cluster in the neutron halo.
One naive question naturally arises; are there any reso-
nances including
3
He, or
3
H particles acting as a cluster?
Such resonant states are known to exist at low excitation
energies 5,6. Trinucleon cluster states at high excitation
energies were predicted by Thompson and Tang 7, who
claimed that the ‘‘molecular’’ resonance with two trinucleon
clusters should exist in the A =6 triad,
6
He,
6
Li, and
6
Be.
In their model, the triton and
3
He clusters behave like a
neutron and a proton, respectively, in the two-nucleon sys-
tem, respectively. These two-trinucleon systems are expected
to have resonant states similar to those generated in the two-
nucleon systems; the lowest state is classified as
13
S
1
and
other multiplets as
31
S
0
,
33
P , which are expected to be lo-
cated at higher excitation energies. Here, the symbols denote
(2 T +1)(2 S +1)
L
J
.
In the past, trinucleon resonances were experimentally re-
ported in
6
Li and
6
Be on the basis of radiative capture reac-
tions 8–10, and of the phase shift analysis on the
3
He
+
3
H and
3
He+
3
He elastic scattering data 11,12. In the
case of
6
Li, Ventura et al. 9 found evidence for the
33
P
2
resonance at E
x
=18.3 MeV. On the other hand, Vlastou
et al. 11 reported that the
33
P
2
and
33
P
0
resonances exist at
21.0 and 21.5 MeV, respectively. Concerning the
33
P
2
reso-
nance in
6
Li, there was a serious discrepancy by about 3
MeV in the excitation energy. In order to understand the
reason for this discrepancy, Mondrago
´
n and Herna
´
ndez 13
reanalyzed simultaneously both the data from the
3
He+
3
H
elastic scattering 11 and from the radiative capture reaction
9. They concluded that the
33
P
2
resonance in
6
Li should
exist at E
x
=17.984 MeV and that the resonance energy for
other multiplets in
6
Li should be lower by 3 MeV than
those reported in Ref. 11. Recently, a similar conclusion
was theoretically reported by Ohkura et al. 14. However,
the precision of the extracted level parameters stated in Ref.
13 is still a subject of some objection 15.
In the case of
6
Be, contradictory results were also re-
ported about the trinucleon cluster resonance. Ventura et al.
assigned a broad resonance at E
x
=23 MeV in
6
Be to be the
33
F
3
resonance from the radiative capture reaction of
3
He on
3
He 10. However, Vlastou et al. did not observe this state
in the phase shift analysis of elastic scattering of polarized
3
He on
3
He, but they observed the
33
F
4
,
33
F
2
, and
33
F
3
resonances located at E
x
=23.4, 26.2, and 26.7 MeV, respec-
tively 12. Thus, the issue of trinucleon clustering in A =6
nuclei appears highly contentious and seems to be presently
unproven.
In order to solve the long standing controversy over the
trinucleon clustering in A =6 nuclei, we employed a differ-
ent experimental approach: we searched for the di-triton mo-
lecular state in
6
He by means of a coincidence measurement
of decay tritons in the
6
Li(
7
Li,
7
Be) reaction at 0° and
at an incident energy of 65 MeV/nucleon. If the two-triton-
cluster molecular state in
6
He exists above the t +t threshold
and is excited, it is natural to expect that a branching ratio of
triton emission from the resonance would be much larger
than that expected by the statistical model calculations.
The experiment was performed at the RCNP cyclotron
facility of Osaka University with a
7
Li
3 +
beam of 65 MeV/ *Email address: akimune@konan-u.ac.jp
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PHYSICAL REVIEW C 67, 051302R2003
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