Single-particle states in neutron-rich ^69,71Cu by means of the (d,³He) transfer reaction

In two (d,³He) transfer reactions with MUST2 at GANIL and the split-pole at Orsay, we have determined the position of the proton-hole states in the neutron-rich ⁷¹Cu (N=42) and ⁶⁹Cu (N=40) isotopes. We have found that in ⁷¹Cu the hole strength of the f7/2 orbital lies at higher excitation energies than expected. From beta-decay and laser spectroscopy, the f5/2 first excited particle state in these isotopes was known to come down rapidly in energy when passing N=40 and even become the ground state in ⁷⁵Cu. This sudden energy shift has been explained in a number of theoretical works. The prediction for the f7/2 spin-orbit partner was that it would change in energy too through a related effect. Experimentally, the f7/2 proton-hole state is not known for N≥40. In ⁷¹Cu two 7/2- states around 1 MeV are candidates to be a proton-hole. The experiment at GANIL took place in March 2011. A secondary beam of ⁷²Zn at 38 AMeV was produced by fragmentation and purified through the LISE spectrometer. The transfer reaction in inverse kinematics was studied with the MUST2 detectors plus four 20 micrometer silicon detector to identified the ³He of low kinetic energy. The excitation spectrum of ⁷¹Cu was reconstruct thanks to the missing mass method and the angular distributions were extracted and compared with a reaction model using the DWUCK4 and DWUCK5 code. From this work no states have been populated around 1 MeV concluding that the centroid of the f7/2 lies at higher excitation energy. We then remeasured the single-particle strength in ⁶⁹Cu in the corresponding (d,³He) reaction at Orsay in March 2013 in order to extend the existing data where 60% of the f7/2 strength is missing and make sure that there is a consistent analysis of spectroscopic factors between both isotopes in order to well understood and well quantify the evolution of the f7/2 orbital when we start filling the g9/2 orbital. In this second experiment we have performed the reaction in direct kinematics using a deuteron beam at 27 MeV provided by the tandem and a target of ⁷⁰Zn. In this work we were able to extract three new angular distributions and we have measured a new part of the f7/2 strength. Finally in order to interpret the results we have obtained from those two experiments, state-of-the-art shell-model calculations have been carried out in collaboration with the Strasbourg group using the Antoine code. The valence space consists in a core of ⁴⁸Ca with the valence orbitals for protons f7/2, p3/2, f5/2, p1/2 and the orbitals p3/2, f5/2, p1/2, g9/2, d5/2 for neutrons. The calculations have been done allowing 8p-8h and show that the strength is indeed at high energy and no f7/2 proton-hole state lies around 1 MeV in ⁷¹Cu. (author)