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Abstract:
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Background:
Recent measurements of fusion cross sections for the
28
Si
+
28
Si system revealed a rather
unsystematic behavior; i.e., they drop faster near the barrier than at lower energies. This was tentatively attributed
to the large oblate deformation of
28
Si because coupled-channels (CC) calculations largely underestimate the
28
Si
+
28
Si cross sections at low energies, unless a weak imaginary potential is applied, probably simulating
the deformation.
30
Si has no permanent deformation and its low-energy excitations are of a vibrational nature.
Previous measurements of this system reached only 4 mb, which is not sufficient to obtain information on effects
that should show up at lower energies.
Purpose:
The aim of the present experiment was twofold: (i) to clarify the underlying fusion dynamics by
measuring the symmetric case
30
Si
+
30
Si in an energy range from around the Coulomb barrier to deep sub-barrier
energies, and (ii) to compare the results with the behavior of
28
Si
+
28
Si involving two deformed nuclei.
Methods:
30
Si beams from the XTU tandem accelerator of the Laboratori Nazionali di Legnaro of the Istituto
Nazionale di Fisica Nucleare were used, bombarding thin metallic
30
Si targets (50
µ
g
/
cm
2
) enriched to 99
.
64%
in mass 30. An electrostatic beam deflector allowed the detection of fusion evaporation residues (ERs) at very
forward angles, and angular distributions of ERs were measured.
Results:
The excitation function of
30
Si
+
30
Si was measured down to the level of a few microbarns. It has a
regular shape, at variance with the unusual trend of
28
Si
+
28
Si. The extracted logarithmic derivative does not
reach the
L
CS
limit at low energies, so that no maximum of the
S
factor shows up. CC calculations were performed
including the low-lying 2
+
and 3
-
excitations.
Conclusions:
Using a Woods-Saxon potential the experimental cross sections at low energies are overpredicted,
and this is a clear sign of hindrance, while the calculations performed with a M3Y + repulsion potential nicely
fit the data at low energies, without the need of an imaginary potential. The comparison with the results for
28
Si
+
28
Si strengthens the explanation of the oblate shape of
28
Si being the reason for the irregular behavior of
that system. |
