Orateur
M.
Clément DELAFOSSE
(IPNO)
Description
Yrast (\textit{i.e.} the state with the lower energy for a given spin-partiy) and near-Yrast states were populated in the $^{78}$Ni region by fusion-fission reaction $^{238}$U($^9$Be,X$\gamma$) at GANIL. The prompt $\gamma$-rays were detected by the AGATA array\footnote{S. Akkoyun \textit{et al.}, \textit{AGATA - Advanced GAmma Tracking Array}, NIM A668 (2012) 26-58} and particle identification was achieved
using the VAMOS++ spectrometer\footnote{M. Rejmund \textit{et al.}, \textit{Performance of the improved larger acceptance spectrometer : VAMOS++}, NIM A646 (2011) 184-191}. Life time measurements were performed using the Recoil Distance Doppler Shift technique developed at Cologne\footnote{J. Litzinger \textit{et al.}, \textit{Transition probabilities in neutron-rich $^{84,86}$Se}, Phys. Rev. C 92, 064322 (2015)} with the Orsay plunger device OUPS\footnote{J. Ljungvall \textit{et al.}, \textit{The Orsay Universal Plunger System}, NIM A679 (2012) 61-66}.
The goal of the experiment was to populate Yrast states in N=51 neutron-rich odd-isotones from $^{89}$Sr ($Z=38$) down to $^{83}$Ge ($Z=32$) in order to study the high-$\ell$ single-particle states effective energy evolution above the $N=50$ shell gap and complement the scarce direct nucleon exchange data presently available\footnote{J.S. Thomas \textit{et al.}, \textit{Single-neutron excitations in neutron-rich $^{83}$Ge and $^{85}$Se}, Phys. Rev. c 76, 044302 (2007)}. These reactions are indeed difficult to exploit with presently available post-accelerated radioactive ion beams (especially for high-$\ell$ orbitals) in this exotic region. More specifically, we have focused our attention on the $\nu 1g_{7/2}$ monopole drift which is key to understanding the possible evolution of the spin-orbit splitting due to the action of the proton-neutron interaction terms in the $^{78}$Ni region. Our strategy was to measure low lying $7/2^+$ states life times as their relative change along the $N=51$ line towards $Z=28$ should reflect their possible $\nu 1g_{7/2}$ composition. The tensor mechanism\footnote{T. Otsuka \textit{et al.}, \textit{Evolution of nuclear shells due to the tensor force}, Phys. Rev. Lett. 95, 232502 (2005)} indeed predicts increasing low-lying $\nu 1g_{7/2}$ single particle components in the wavefunctions approaching $^{79}$Ni.
In this talk, the particle identification and the life time measurement method will be presented with some examples.
Auteurs principaux
M.
Clément DELAFOSSE
(IPNO)
Dr
David VERNEY
(Institut de Physique Nucléaire d'Orsay)
