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Isotopic effects in sub-barrier fusion of Si + Si systems

Colucci, G.; Montagnoli, G.; Stefanini, A. M.; Esbensen, H.; Bourgin, D.; Čolovic, Petra; Corradi, L.; Faggian, M.; Fioretto, E.; Galtarossa, F.; Goasduff, A.; Grebosz, J.; Haas, F.; Mazzocco, M.; Scarlassara, F.; Stefanini, C.; Strano, E.; Szilner, Suzana; Urbani, M.; Zhang, G. L. (2018) Isotopic effects in sub-barrier fusion of Si + Si systems. Physical Review C, 97 (4). ISSN 2469-9993

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Abstract

Background: Recent measurements of fusion cross sections for the 28Si+28Si 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 28Si because coupled-channels (CC) calculations largely underestimate the 28Si+28Si cross sections at low energies, unless a weak imaginary potential is applied, probably simulating the deformation. 30Si 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 30Si+30Si in an energy range from around the Coulomb barrier to deep sub-barrier energies, and (ii) to compare the results with the behavior of 28Si+28Si involving two deformed nuclei. Methods: 30Si beams from the XTU tandem accelerator of the Laboratori Nazionali di Legnaro of the Istituto Nazionale di Fisica Nucleare were used, bombarding thin metallic 30Si targets (50 μg/cm2) 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 30Si+30Si was measured down to the level of a few microbarns. It has a regular shape, at variance with the unusual trend of 28Si+28Si. The extracted logarithmic derivative does not reach the LCS 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 28Si+28Si strengthens the explanation of the oblate shape of 28Si being the reason for the irregular behavior of that system.

Item Type: Article
Uncontrolled Keywords: nuclear reaction ; nuclear fusion
Subjects: NATURAL SCIENCES > Physics
Divisions: Division of Experimental Physics
Projects:
Project titleProject leaderProject codeProject type
Nuklearna struktura i reakcije: eksperimentima prema neutronskoj liniji kapanja-SR-ETNoSuzana SzilnerIP-2013-11-7194HRZZ
Depositing User: Suzana Szilner
Date Deposited: 18 Jul 2018 09:39
Last Modified: 18 Jul 2018 09:40
URI: http://fulir.irb.hr/id/eprint/4101
DOI: 10.1103/PhysRevC.97.044613

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