Transfer couplings and hindrance far below the barrier for ⁴⁰Ca + ⁹⁶Zr

¹ INFN, Laboratori Nazionali di Legnaro, I-35020 Legnaro (Padova), Italy
² Dipartimento di Fisica e Astronomia, Università di Padova, and INFN, Sezione di Padova, I-35131 Padova, Italy
³ Physics Division, Argonne National Laboratory, Argonne, IL 60439, USA
⁴ IPHC, CNRS-IN2P3, Université de Strasbourg, F-67037 Strasbourg Cedex 2, France
⁵ CSNSM, CNRS/IN2P3 and Université Paris-Sud, F-91405 Orsay Campus, France
⁶ Institute of Nuclear Physics, Polish Academy of Sciences, PL 31-342 Cracow, Poland
⁷ Ruđer Bošković Institute, HR-10002 Zagreb, Croatia

^a e-mail: alberto.stefanini@lnl.infn.it

Published online: 29 January 2015

Abstract

The sub-barrier fusion excitation function of ⁴⁰Ca + ⁹⁶Zr has been measured down to cross sections ≃2.4µb, i.e. two orders of magnitude smaller than obtained in the previous experiment, where the sub-barrier fusion of this system was found to be greatly enhanced with respect to ⁴⁰Ca + ⁹⁰Zr, and the need of coupling to transfer channels was suggested. The purpose of this work was to investigate the behavior of ⁴⁰Ca + ⁹⁶Zr fusion far below the barrier. The smooth trend of the excitation function has been found to continue, and the logarithmic slope increases very slowly. No indication of hindrance shows up, and a comparison with ⁴⁸Ca + ⁹⁶Zr is very useful in this respect. A new CC analysis of the complete excitation function has been performed, including explicitly one- and two-nucleon Q >0 transfer channels. Such transfer couplings bring significant cross section enhancements, even at the level of a few µb. Locating the hindrance threshold, if any, in ⁴⁰Ca + ⁹⁶Zr would require challenging measurements of cross sections in the sub-µb range.