Unusual near-threshold potential behavior for the weakly bound nucleus
9
Be
in elastic scattering from
209
Bi
C. Signorini,
1
A. Andrighetto,
1
M. Ruan,
2
J. Y. Guo,
1
L. Stroe,
1
F. Soramel,
3
K. E. G. Lo
¨
bner,
4
L. Mu
¨
ller,
1
D. Pierroutsakou,
5
M. Romoli,
5
K. Rudolph,
4
I. J. Thompson,
7
M. Trotta,
5
A. Vitturi,
1
R. Gernha
¨
user,
6
and A. Kastenmu
¨
ller
6
1
Physics Department of University and INFN, via Marzolo 8, I-35131 Padova, Italy
2
INFN, Laboratori Nazionali di Legnaro, I-35020 Legnaro, Padova, Italy
3
Physics Department of University and INFN, via delle Scienze 208, I-33100 Udine, Italy
4
Sektion Physik, Ludwig-Maximilians-University Mu ¨nchen, D-85748 Garching, Germany
5
Physics Department of University and INFN, via Cintia, Monte S. Angelo, I-80125 Napoli, Italy
6
Physics Department, Technical University Mu ¨nchen, D-85748 Garching, Germany
7
Physics Department, University of Surrey, Guildford GU2 5XH, United Kingdom
Received 20 December 1999; published 18 May 2000
The cross sections for elastic scattering of the weakly bound
9
Be on
209
Bi around the Coulomb barrier have
been measured with 5 % absolute accuracy from 40 to 48 MeV. The potential obtained from an optical model
analysis has an unusual behavior. At the strong absorption radius the imaginary absorptive potential is
increasing rather than decreasing with decreasing energy, as would be consistent with a long range polariza-
tion potential arising mainly from couplings to breakup channels. The real part, on the other hand, displays a
strong attractive polarization contribution with the maximum at the barrier, as would be normally expected
from a polarization contribution arising from strong couplings. The inelastic multiplet in
209
Bi of collective
nature around 2.6 MeV, originating from the coupling
208
Pb(3
-
) h
9/2
-
J
, was seen only at 48 MeV. The
total multiplet cross section is well reproduced by coupled channel calculations with the potential obtained
from the optical analysis and the experimental B ( E 3) strengths of the
209
Bi multiplet levels.
PACS numbers: 25.70.Bc, 24.10.Eq
Due to the increasing interest in physics with radioactive
nuclear beams, the interaction of weakly bound or halo nu-
clei at colliding energies around the Coulomb barrier is a
very lively topic. In fact, among radioactive nuclei there are
most of the best candidates for such investigations, and they
will be more numerous in the future. The most relevant ques-
tion is whether there is any signature of the expected influ-
ence of the weak binding, plus the eventual halo structure, on
the various processes going on at the barrier energies,
namely scattering elastic and inelastic, transfer, breakup,
and fusion. All of these nonelastic processes influence, to
some extent, the entrance channel optical potential, so mea-
surement of elastic scattering is a necessary first step.
Within this research framework, systematic investigations
are going on in the systems
9,11
Be+
209
Bi,
208
Pb for the fol-
lowing reasons: i
11
Be is a weakly bound unstable nucleus,
S
n
=0.50 MeV, with a well established halo structure 1,
and a low energy radioactive beam has already been devel-
oped 2,3 for this isotope. ii
208
Pb and
209
Bi have very
well established shell model structures, constitute two
‘‘easy’’ low cost targets from an experimental view point,
and are easy to be treated theoretically. iii
9
Be represents a
reference stable nucleus with which high precision measure-
ments are possible due to the much higher beam intensity
achievable. Moreover, the
9
Be nucleus is quite interesting by
itself, since it is one of the two less bound stable nuclei with
S
n
=1.67 MeV, the other one being
6
Li with S
=1.47 MeV.
The fusion process was extensively studied in the systems
9,10,11
Be+
209
Bi 2,4,5 and
9
Be+
208
Pb 6. One important
finding in all these experiments is that the breakup process of
both
9
Be and
11
Be projectiles has a significant influence on
the fusion one. Our systematic work therefore continued with
precision measurements of the elastic scattering cross section
of the
9
Be+
209
Bi system around the Coulomb barrier. The
goal was to get the interaction potential from a consistent
optical model analysis, and then to look for possible signa-
tures of the breakup process.
Some theoretical work done for a similar system
11
Be+
197
Au 7,8 predicts a hindrance, i.e., ‘‘stronger’’ ab-
sorption, of d / d
R
at the barrier, E (
11
Be) =40 MeV, and
no hindrance below, E (
11
Be) =30 MeV. The optical model
analysis of the
6,7
Li+
208
Pb elastic scattering 9 shows, for
the
6
Li projectile, that the imaginary absorptive potential at
the distance corresponding to the strong interaction radius
increases with decreasing energy, but not for
7
Li. This
should reflect the fact that
6
Li with S
=1.47 MeV can break
more easily than
7
Li with S
=2.47 MeV as discussed also
in Ref. 10. The behavior of
9
Be optical potential near the
barrier is predicted 10 to be close to that of
6
Li since both
nuclei have similar breakup thresholds. Indeed the analysis
of elastic scattering of
6
Li,
9
Be, as well as
7
Li by light
targets shows that the real potentials, calculated with the
double folding procedure, have to be all renormalized by a
factor of 0.5 in order to reproduce the data 11–13.
The elastic scattering data were taken using the Tandem
Van de Graaff accelerator of the Munich Universities. Angu-
lar distributions were measured at
9
Be bombarding energies
of 40, 42, 44, 46, and 48 MeV. The
9
Be 4
+
beam currents
were ranging from 2 nA to 25 nA electrical on target de-
pending on bombarding energies and scattering angles. The
incoming beam was well defined in direction by means of a
4.5 mm diameter collimator located at 18 cm from the
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