The influence of the 2-neutron elastic transfer on the fusion of ⁴²Ca + ⁴⁰Ca

¹ INFN, Laboratori Nazionali di Legnaro, I-35020 Legnaro, Padova, Italy
² Dip. di Fisica e Astron., Univ. di Padova and INFN, Sezione di Padova, Padova Italy
³ CSNSM, CNRS/IN2P3 and Univ. Paris-Sud, F-91405 Orsay, France
⁴ Institute of Nuclear Physics, PL 31-342 Cracow, Poland
⁵ IPHC, CNRS-IN2P3, Univ. de Strasbourg, F-67037 Strasbourg, France

Published online: 11 May 2016

Abstract

Strong coupling to a single channel with zero Q-value is predicted to produce a characteristic fusion barrier distribution with two peaks, one on each side of the original uncoupled Coulomb barrier. In practical cases, only coupling to an elastic transfer channel may produce such a distribution which, however, has never been observed sofar, probably because low-lying surface vibrations usually have a dominant role, and this may obscure the two-peak structure. The case of the two-neutron (2n) elastic transfer in ⁴²Ca + ⁴⁰Ca is particularly attractive, because of the relatively rigid nature of the two nuclei.

We have measured the fusion excitation function of this system using the ⁴²Ca beam of the XTU Tandem of LNL on a thin ⁴⁰Ca target enriched to 99.96% in mass 40. Cross sections have been measured down to ≤1 mb. The extracted barrier distribution shows clearly two main peaks. We have performed preliminary CC calculations where the 2⁺ coupling strengths have been taken from the literature and the schematic 2n pair transfer form factor has been used, with a deformation length σ_t= 0.39 fm. The excitation function is well reproduced by the calculation including the 2n transfer channel. However, including the octupole excitations destroys the agreement.