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SUMMARY:Recent results of experiment IS690: Exploring the excited structur
 e of $^{11}$Li through (t\,p) reactions at CERN-ISOLDE
DTSTART;VALUE=DATE-TIME:20241119T155800Z
DTEND;VALUE=DATE-TIME:20241119T160500Z
DTSTAMP;VALUE=DATE-TIME:20260421T044435Z
UID:indico-contribution-25475@indico.ific.uv.es
DESCRIPTION:Speakers: Daniel Fernandez Ruiz (IEM-CSIC)\nHalo nuclei are a 
 group of nuclei characterized by a low binding energy for their last nucle
 ons\, situated in low orbital momentum states and\, as a consequence\, an 
 unusually large spatial extension that deviates from the standard $r=r_0 A
 ^{1/3}$ relation. The first empirical observation of this behaviour came f
 rom experimental measurements of the interaction cross-section for neutron
 -rich nuclei\, specifically the scattering cross-section of Lithium isotop
 es. As the number of neutrons approached the dripline\, the interaction ra
 dius deviated from theoretical predictions\, with $^{11}$Li being the most
  noticeable case [1]. This discovery was interpreted as a new type of nucl
 ear structure [2]\, formed by a compact core and an external set of nucleo
 ns. This hypothesis was confirmed a few years later in $^{11}$Li break-up 
 experiments [3].\n\n$^{11}$Li can be considered the archetype of a two-neu
 tron halo: a three-body system formed by two weakly correlated neutrons lo
 osely bound to the $^9$Li ground state (g.s.) [4]. Despite being intensive
 ly studied for a long time\, there are still open questions regarding the 
 structure of $^{11}$Li. While the g.s. is known to be a mixture of p ($59(
 1)\\%$)\, s ($35(4)\\%$)\, and d ($6(4)\\%$) waves [5]\, knowledge of high
 er-energy resonant states (no excited states are bound in $^{11}$Li) is no
 t well settled\, as different reaction studies give different results.\n\n
  The low-lying continuum spectrum of $^{11}$Li is dominated by broad dipol
 e structures observed in several experiments\, while narrower resonances h
 ave been proposed up to 6.2 MeV. Recent results on the low-lying continuum
  structure in $^{11}$Li have been obtained from inelastic p- and d-scatter
 ing at TRIUMF [6\,7]. The elastic cross-sections obtained from both experi
 ments are consistent\; however\, the inelastic scattering results indicate
 d a resonant state at 0.80(4) MeV\, $\\Gamma=1.15(6)$ MeV for proton inela
 stic scattering [7]\, and this same resonance was characterized to be at 1
 .03(4) MeV\, $\\Gamma=0.51(11)$ MeV with deuteron scattering [6]. However\
 , a more relevant question concerns the physical process involved: excitat
 ion to resonance or direct excitation to the continuum?\n\nMost experiment
 s that explore the excited structure of $^{11}$Li start from $^{11}$Li g.s
 \, which is promoted to excited levels. The only exception is the study of
  the (very complex) $^{14}$C($\\pi^-$\,p+d) reaction [8]\, whose results w
 ere limited by low resolution. The MAGISOL collaboration has performed an 
 experiment\, IS690 [9]\, intending to probe the excited structure of $^{11
 }$Li through an alternate approach: populate directly the excited states o
 f $^{11}$Li using a two-neutron transfer reaction $^9$Li(t\,p)$^{11}$Li\, 
 and obtain information of the excited states through the momentum distribu
 tion of the residual proton. This experiment complements 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 potential
 s in the theoretical models. \n\nIS690 took place at the Scattering Experi
 mental Chamber (SEC) in the HIE-ISOLDE facility at CERN between the 14th a
 nd 22nd of October 2024. A post-accelerated 7 MeV/u $^9$Li beam was imping
 ed on a $^3$H target ($^3$H absorbed in a thin Ti-foil at a ratio of $\\si
 m$0.4/1). The energy of the incoming $^9$Li beam\, 7 MeV/u\, was chosen to
  facilitate the 2n transfer while reducing the number of additional open c
 hannels. An upgraded detection setup was prepared to detect the emitted pr
 otons from the $^9$Li(t\,p)$^{11}$Li reaction and distinguish them from ba
 ckground reactions\, especially $^9$Li(p\,d)$^{10}$Li and elastic channels
 \, as well as protons from Ti(t\,p). The setup offered optimal angular cov
 erage and consisted of three detector structures: (a) five particle telesc
 opes (DSSD+PAD) forming a pentagon around the target\, (b) a frontal teles
 cope formed by two S3-CD detectors\, and (c) a backward S5 detector to det
 ect backward protons.\n\nIn this contribution\, we will give an overview o
 f the experiment and a summary of the (very recent) data obtained\, along 
 with our preliminary analysis.\n\n\n\n**References**\n  \n\n 1. I. Tanihat
 a et al.\, Phys. Rev. Lett. 55 (1985) 2676. \n 2. P.G. Hansen and B. Jonso
 n\, Europhys. Lett. 4 (1987) 409.\n 3. T. Kobayashi et al.\, Phys. Rev. Le
 tt. 60 (1988) 2599.\n 4. M.V. Zhukov et al.\, Phys. Rep. 231 (1995) 151.\n
  5. J. Tanaka et al.\, Phys. Lett. B 774 (2017) 268.\n 6. R. Kanungo et al
 .\, Phys. Rev. Lett. 114 (2015) 192502. \n 7. I. Tanihata and K. Ogata\, E
 ur. Phys. J. A 55 (2019) 239.\n 8. M.G. Gornov et al.\, Phys. Rev. Lett. 8
 1 (1998) 766.\n 9. M.J.G. Borge and J. Cederkäl\, Proposal to the ISOLDE 
 and Neutron\n        Time-of-Flight Committee (2021)\, European Organizati
 on for Nuclear\n        Research.\n 10. T. Roger et al.\, Phys. Rev. C 79 
 (2009) 031603(R).\n\nhttps://indico.ific.uv.es/event/7664/contributions/25
 475/
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URL:https://indico.ific.uv.es/event/7664/contributions/25475/
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