Isotopic effects in sub-barrier fusion of Si plus Si systems
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Cita com:
hdl:2117/118346
Tipus de documentArticle
Data publicació2018-04-23
EditorAmerican Physical Society (APS)
Condicions d'accésAccés obert
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Abstract
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.
CitacióColucci, R., Montagnoli, G., Stefanini, A. M., Urbani, M. Isotopic effects in sub-barrier fusion of Si plus Si systems. "Physical review C", 23 Abril 2018, vol. 97, núm. 4, p. 44613-1-44613-8.
ISSN2469-9985
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