Probing core polarization around78Ni: intermediate energy Coulomb excitation of74Ni

T Marchi,A Covello,E Farnea,A Gargano,A Gottardo,G De Angelis,L Coraggio,R Stroberg,D Bazin,F Recchia,T Glasmacher,S M Lenzi,F Gramegna,A Gadea,C Michelagnoli,A Bonaccorso ,J J Valiente-Dobón ,H L Crawford ,B Quintana ,R M Clark ,S Mcdaniel ,D R Napoli ,N Itaco ,T Baugher ,K Wimmer ,D Weißhaar ,E Šahin ,A Ratkiewicz ,M Doncel ,A Gade ,Ravinder Kumar ,J S Berryman ,R Winkler

doi:10.1051/epjconf/20136301021

Abstract

The study of the evolution of nuclear shells far from stability provides fundamental information about the shape and symmetry of the nuclear mean field. Nuclei with large neutron/proton ratio allow to probe the density dependence of the effective interaction. Indeed, it was recently shown that tensor and three-body forces play an important role in breaking and creating magic numbers. Of particular interest is the region of 78 Ni where the large neutron excess coincides with a double shell closure.We have recently measured the B(E 2; 0+ → 2+ ) of the 74 Ni nucleus in an intermediate-energy Coulomb excitation experiment performed at the National Superconducting Cyclotron Laboratory of the Michigan State University. The 74 Ni secondary beam has been produced by fragmentation of 86 Kr at 140 AMeV on a thick Be target. Selected radioactive fragments impinged on a secondary 197 Au target where the measurement of the emitted γ -rays allows to extract the Coulomb excitation cross section and related structure information. Preliminary B(E2) values do not point towards an enhancement of the transition matrix element and the comparison to what was already measured by Aoi and co-workers in [1] opens new scenarios in the interpretation of the shell evolution of the Z=28 isotopes.

Highlights

The experiment was performed at the National Superconducting Cyclotron Laboratory (NSCL) of the Michigan State University (MSU)
The A1900 momentum acceptance was set to 3% and the selected cocktail-beam contained 74Ni with an energy of 67 AMeV and an intensity of 0.7 pps
Given the extremely low production cross section of the 74Ni nucleus, the A1900 was used with a higher momentum acceptance with respect to what is normally done at NSCL

Summary

Introduction

After a proper correction of the time of flight, accounting for effects like the different path of the particles through the S800 spectrograph, the 74Ni blob is nicely separated This is not the case for the incoming beam identification where the A1900 momentum acceptance plays an important role. A clean separation over the whole range of the particles Because of this and due to the need of selecting only 74Ni ions, a small subset of events could be considered safe for the data analysis and a very strict gate was applied to the incoming beam particle identification (i.e. only the lowestleft part of the correlation shown in figure 2 was selected).

Result

Findings

Discussion

Summary