Abstract
The near- and sub-barrier fusion excitation function has been measured for the system 30 Si+30 Si at the Laboratori Nazionali di Legnaro of INFN, using the 30 Si beam of the XTU Tandem accelerator in the energy range 47 - 90 MeV. A set-up based on a beam electrostatic deflector was used for detecting fusion evaporation residues. The measured cross sections have been compared to previous data on 28 Si+28 Si and Coupled Channels (CC) calculations have been performed using M3Y+repulsion and Woods-Saxon potentials, where the lowlying 2+ and 3− excitations have been included. A weak imaginary potential was found to be necessary to reproduce the low energy 28 Si+28 Si data. This probably simulates the effect of the oblate deformation of this nucleus. On the contrary, 30 Si is a spherical nucleus, 30 Si+30 Si is nicely fit by CC calculations and no imaginary potential is needed. For this system, no maximum shows up for the astrophysical S-factor so that we have no evidence for hindrance, as confirmed by the comparison with CC calculations. The logarithmic derivative of the two symmetric systems highlights their different low energy trend. A difference can also be noted in the two barrier distributions, where the high-energy peak present in 28 Si+28 Si is not observed for 30 Si+30 Si, probably due to the weaker couplings in last case.
Highlights
Experimental set-upFusion cross sections for the 30Si+30Si system have been determined by direct detection of the evaporation residues (ER) at very forward angles by exploiting their different
Fusion cross sections for the 30Si+30Si system have been determined by direct detection of the evaporation residues (ER) at very forward angles by exploiting their different σfus(mb) 103 (a) 30Si +30Si 28Si + 30Si 28Si + 28Si 28+30 Ch30 W=5 28+28 Ch10 W=0 28+28 Ch10 W=5 Ec.m. (MeV) (b) 28Si+28Si
A fusion experiment has been recently carried out at Laboratori Nazionali di Legnaro (LNL) with the purpose to extend the data of 30Si+30Si down to energies deeply below the Coulomb barrier. In this contribution we present the results of this full measurement from well below to well above the Coulomb barrier, and we perform a comparison with the existing data for the 28Si+28Si system
Summary
Fusion cross sections for the 30Si+30Si system have been determined by direct detection of the evaporation residues (ER) at very forward angles by exploiting their different. Four silicon detectors were placed symmetrically at θlab=16◦ around the beam direction, in the sliding seal reaction chamber These detectors were used to monitor the beam and normalize to the Mott scattering cross section. From this first comparison it appears that the two excitation functions behave differently at low energies with a flatter slope for 28Si+28Si. A CC analysis has been performed to understand the origin of this difference. Electrical rigidity with respect to beam and beam-like particles, using an electrostatic deflector This experimental set-up is shown, it allows fast and reliable measurements of relative and absolute cross sections and has been employed in several sub- and near-barrier fusion experiments at LNL in recent years. Analogous twodimensional spectra of TOF vs. ∆E and ∆E vs. E were used in the data analysis
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