Reaction studies with neutron-rich light nuclei at the upgraded SEC Device: Exploring the excited structure of 11 Li

. The 11 Li nucleus is considered the best example of a two-neutron halo nucleus. While the structure of the ground state is well established there is still a significant debate regarding its excited states despite of the multiple experiments realised mainly from the excitation of the ground state of 11 Li that is rather complex. We propose a novel approach, to directly feed the different excited states of 11 Li by the (t,p) reaction as 9 Li has a simpler structure. We will employ a 9 Li beam produced at ISOLDE and post-accelerated by HIE-ISOLDE to energies of 7 MeV/u to impinge on a 3 H-target located at the upgraded SEC station, this station contains a new particle detection set up: composed of 5 Si-particle telescopes in a pentagon configuration with end caps consisting of 2 CD Si detectors downstream and 1 CD upstream has been prepared to offer the maximum angular coverage and resolution. Geant4 Monte Carlo simulations with the efficiency of the setup is presented in the following proceeding.


Introduction: Halo Nuclei
The term halo nucleus was coined to describe nuclei with a low biding energy for the last nucleons and an unusually large spatial extension, diverging from the standard r=rOA 1/3 .The first empirical observation of this behaviour came from scattering experiments of accelerated Lithium isotopes [1].The interaction crosssection for neutron-rich nuclei was measured, and it was found that the cross-section of 11 Li was unexpectedly larger than that of 9 Li.The drastic increase of crosssection pointed towards a nuclear radius larger than the theoretical prediction.This discovery was interpreted as a new type of nuclear structure [3], formed by a compact core and an external set of nucleons.A few years later [4], the 9 Li momentum distribution obtained from 11 Li break-up experiments confirmed this hypothesis.
The 11 Li nucleus is the archetype of a two-neutron halo: a three-body system formed by two somehow correlated neutrons loosely bound to the 9 Li ground state (g.s) [5].While the gs structure of 11 Li is known to be a mixture of p(59(1)%), s(35(4)%) and d(6(4)%) waves [6], knowledge of higher energy levels is not well settle as the different reaction studies give dispersed results. 11Li has no bound excited state.The low-lying continuum spectrum is dominated by broad dipole structures observed in several experiments, while narrower resonances have been proposed up to 6.2 MeV However, multiple experiments do not agree in the energies and width of the observed resonances.
Recent results on the low-lying continuum structure in 11 Li have been obtained from inelastic p and d scattering at TRIUMF [6,7].Both measurements gave consistent elastic cross sections.However, the inelastic scattering results indicated a resonant state at 0.80(4) MeV, Г=1.15(6) MeV for the proton inelastic scattering [7].While the determinate resonance was characterized to be at 1.03(4) MeV, Г= 0.51 (11) MeV for the deuteron case [6].Although the values are not far apart neither the energy nor the width coincide.It has been argued [8] that the difference could be due to low statistic in the (d,d') case, or problems with the angular determination that is particularly difficult in a cryogenic target.However, there is a more relevant question concerning the physics process involved: excitation to a resonance or excitation directly to the continuum.
Most of the discussed experiments explore the excited structure of 11 Li by promoting a 11 Li nucleus in the g.s to the excited levels.The only exception being the study of the (very complex) 14 C(π -,p+d) reaction [9].Whose low resolution did not allow a detailed characterization of the resonances.Instead, we propose [10] to use the two-neutron transfer reaction 9 Li(t,p) 11 Li to probe the resonant structure of 11 Li, see Fig. 1.The dominant reaction channel is a one-step neutron transfer process, the DWBA [11] calculations indicate a possible L=0 transmission to the 11 Li g.s or three L=1 excitations whose differential cross section peak for inverse kinematics at backward angles [12].Knowledge of the excited states will come from the energy and momentum distribution of the residual protons, detected using forward and backward angles.
This experiment would complement the 11 Li(p,t) 9 Li experiment carried at TRIUMF [13] and could also study other interesting reaction channels such as the elastic scattering channel which is essential to fix optical potentials in the theoretical models.

Experiment IS690
The experiment, IS690, will be carried at the HIE ISOLDE facility at CERN in 2023.A post-accelerated 9 Li beam at 7 MeV/u will impinge on a 3 H-target ( 3 H absorbed in a thin Ti-foil to a ~1/1 ratio).The energy of the incoming 9 Li beam is chosen, to facilitate the 2n transfer while reducing the number of additional open channels.Identifying the influence of background reactions, especially the 9 Li(p,d) 10 Li and elastic channels, requires a setup that can differentiate between very similar reaction products and offer an optimal angular coverage.
The upgraded detection set-up at SEC (Scattering Experimental Chamber) at ISOLDE will consist of three detector structures: a) the charged particle detectors, b) eight GAGG scintillators for gamma detection, and c) the SAND neutron time-of-flight detector.
The charged particle detector set up has been designed based on our previous experience from reaction experiments.Its objective is to offer a maximum angular coverage of the reaction target while allowing for an accurate differentiation of the multiple reaction products.The target will be placed between the pentagon and the backwards CD detector, the incoming 9 Li beam will enter via the central hole.The heavy reaction products will move in the forward direction, while the lighter ones will be detected by the surrounding detectors.Geant4 simulations of the Si-device, see Fig. 2, were used to fine tune the thickness and positions of the detector, optimizing angular resolution and coverage.These simulations where also employed to model the effect of the background channels identified by ∆E-E plots, see Fig 3 .The ratio of the (t,p) and (t,d) reaction was 0,3 and 0,7 respectively.Fig. 3. Simulated ΔE-E plot used for particle identification.Lower trace: proton from the main 9 Li(t,p) 11 Li reaction.upper deuterium ions from the 9 Li(t,d) 10 Li background reaction.

Fig. 2 .
Fig. 2. Geant4 simulation of the experimental set-up with the three parts shown in different colours.