Speaker
Description
Halo nuclei are a group of nuclei characterized by the combination of a low binding energy for their last nucleons and an unusually large spatial extension that deviates from the standard r=r$_oA^{1/3}$ relation. The first empirical observation of this behaviour came from experimental measurements of the interaction cross-section for neutron-rich nuclei, to be more precise, when the scattering cross-section of Lithium isotopes is measured as the number of neutrons gets closer to the dripline the interaction radius deviates from the theoretical predictions $^{11}$Li being the most noticeable case [1]. This discovery was interpreted as a new type of nuclear structure [2], formed by a compact core and an external set of nucleons, this hypothesis was confirmed a few years later in $^{11}$Li break-up experiments [3].
$^{11}$Li can be considered the archetype of a two-neutron type of halo: a three-body system formed by two somehow correlated neutrons loosely bound to the $^{9}$Li ground state (g.s) [4]. Despite having wildly been studied for a long time there are still some questions regarding the structure of $^{11}$Li, while the gs is known to be a mixture of p(59(1)%), s(35(4)%) and d(6(4)%) waves [5], knowledge of higher energy levels is not well established since different reaction studies give different results.$^{11}$Li 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 between 3.2 and 6.2 MeV. Recent results on the low-lying continuum structure in $^{11}$Li have been obtained from inelastic p and d scattering at TRIUMF [6,7]. The elastic cross sections obtained from both experiments are consistent, however, the inelastic scattering results indicated a resonant state at 0.80(4) MeV, Г=1.15(6) MeV for the proton inelastic scattering channel [7] while this same resonance was characterized to be at 1.03(4) MeV, Г= 0.51(11) MeV in inelastic deuteron scattering [6]. In addition, there is a more relevant question concerning the physics process involved: excitation to resonance or directly to the continuum?
Most experiments exploring the excited structure of $^{11}$Li start from the $^{11}$Li gs nucleus that is promoted to the excited levels. The only exception is the study of the (very complex) $^{14}$C(π$^-$,p+d) reaction [8], whose results were limited by low resolution. The MAGISOL collaboration has performed the IS690 experiment [9] intending to probe the excited structure of $^{11}$Li through an alternate approach: populate directly the excited state of $^{11}$Li using a two-neutron transfer reaction $^{9}$Li(t,p)$^{11}$Li and obtain information of the excited states through the momentum distribution of the residual proton. This experiment acts as a complement to the $^{11}$Li(p,t)$^{9}$Li experiment carried out at TRIUMF [10], additionally, knowledge of the elastic scattering channel can be employed to fix optical potentials in the theoretical models.
IS690 took place at the Scattering Experimental Chamber (SEC) in the HIE-ISOLDE facility at CERN between the 14th and 22nd of October 2024. A post-accelerated 7 MeV/u $^{9}$Li impinged on upon a $^{3}$H-target (3H absorbed in a thin Ti-foil to a ~0.4/1 ratio). The energy of the incoming $^{9}$Li beam was chosen, to facilitate the 2n transfer while reducing the number of additional open channels. An upgraded detection set-up was prepared to detect the emitted protons from the $^{9}$Li(t,p)$^{11}$Li reaction and distinguish it from background reactions, (especially the $^{9}$Li(p,d)$^{10}$Li and elastic channels) while offering optimal angular coverage. This set-up consists of three detector structures: a) five particle telescopes (DSSD+PAD) forming a pentagon around the target (covering 32$^{o}$-83$^{o}$), b) a frontal telescope formed by two S3-CD detectors (covering 6.7$^{o}$-29.4$^{o}$), and c) a backwards S5 detector to detect the backward protons (covering 111$^{o}$-143$^{o}$).
In this contribution, we will provide an overview of the experiment, a summary of the (very recent) data, and our preliminary analysis.
References
1. I. Tanihata et al., Phys. Rev. Lett 55 (1985) 2676.
2. P.G. Hansen and B. Jonson, Europhys. Lett 4 (1987) 409.
3. T.Kobayashi et al., Phys. Rev. Lett 60 (1988) 2599.
4. M.V. Zhukov et al., Phys Rep 231 (1995) 151.
5. J. Tanaka et al., Phys. Lett. B 774 (2017) 268.
6. R. Kanungo et al., Phys. Rev. Lett. 114 (2015) 192502.
7. I. Tanihata and K. Ogata, Eur. Phys. J. A 55 (2019) 239.
8. M.G. Gornov et al., Phys. Rev. Lett. 81 (1998) 766.
9. M.J.G. Borge and J. Cederkäl, Proposal to the ISOLDE and Neutron Time-of-Flight Committee (2021) European organization for nuclear research
10. T. Roger et al., Phys. Rev. C 79 (2009) 031603(R).
