Orateur
Description
The study of nuclei around the neutron dripline allows to investigate surprising or unexpected phenomena that occur at the edges of the nuclear landscape. Neutron-rich nuclei provide access to regimes where the description of states becomes sensitive to nucleon-nucleon interactions, and the structure models to be challenged. In this context, the boron isotopic chain is an ideal case of study as it exhibits a wide variety of structures. Here, we focus on the most neutron-rich boron isotopes $^{18-21}$B. Although they consist of 18 to 21 nucleons, their core+neutron(s) structures allow a description using models with only two or three bodies.
First, $^{18}$B is unbound by one-neutron emission and its ground state was characterized as a virtual state [1]. In the low-energy scattering formalism within the effective range approximation, the $^{17}$B+n virtual state is described with two parameters : the scattering length $a_s$ and the effective range $r_e$ of the interaction [2]. At present, we only have an upper limit on the scattering length with $a_{s} < -50$ fm, and the effective range has not been investigated [1, 3]. For comparison, the largest scattering length at the nuclear scale is the neutron-neutron one with $a_{s} = -18,5$ fm and is of the order of several fm for most nuclear systems. If the $^{17}$B+n scattering length were large enough, of hundreds (or even thousands) of fm, $^{19}$B could exhibit universal behavior, including the possible emergence of a phenomenon never observed in nuclear physics : Efimov states [4, 5]. Finally, it is on this already very exotic system that $^{21}$B is built, the last known isotope in the chain. $^{21}$B was found to decay through direct two-neutron emission [6] but correlations between emitted neutrons were not investigated.
The structure of these boron isotopes is studied through two experiments conducted at RIKEN Nishina Center (Japan) as part of the SAMURAI collaboration using radioactive beams at $\sim230$ MeV/nucleon sent on a carbon target. $^{18}$B was populated by knockout reactions from $^{19}$C(-p) and $^{19}$B(-n) while $^{21}$B was populated from $^{22}$C(-p). The complete kinematics of the reactions were measured and the reaction products of interest, $^{17,19}$B and neutrons, were respectively detected using the SAMURAI spectrometer and the NEBULA and NeuLAND multi-neutron detectors. The relative energy spectra of the $^{17,19}B+xn$ systems were reconstructed by invariant mass method.
The ($^{19}$C, $^{17}$B+n) reaction populates only the virtual state, and the large acceptance and high resolution of the SAMURAI setup enabled its full characterization. The first measurement of the effective range of the interaction and the surprisingly large scattering length observed open the way for Efimov physics in $^{19}$B. The ($^{19}$B, $^{17}$B+n) reaction populates the virtual state but also two additional resonances. While the latter represents the first spectroscopy of $^{18}$B, the former is found to be very sensitive to the neutron separation energy of $^{19}$B. The population of a $^{21}$B resonant state via the ($^{22}$C, $^{19}$B+n+n) reaction and its decay through direct two-neutron emission was confirmed. The investigation of the correlations between emitted neutrons revealed strong neutron-neutron final-state interactions and a suppression of phase space during the two-neutron decay, a behavior that had never been reported in the literature.
[1] A. Spyrou et al. Phys. Lett. B 686 (2010) , 129-133
[2] F. Miguel Marqués and Emeline Oliveira, EPJ Web of Conf, 311,00006 (2024)
[3] S. Leblond, PhD Thesis, Université de Caen Normandie (2015)
[4] E. Hiyama et al. Phys. Rev. C 100,011603 (2019)
[5] S. Endo et al. Eur. Phys. J. A (2025)
[6] S. Leblond et al. Phys. Rev. Lett. 121,262502 (2018)