The GANIL campaign around the first 4n signal was very peculiar. The beginning and end were both dictated by unexpected events that, unfortunately, do not fit within the streamlined format of standard scientific publications. However, they illustrate many aspects of how basic research should work, or at least does work. Therefore, I take this opportunity to share them with those not involved in the campaign, hoping that they will offer a better perspective of that research in particular and of basic research in general. As a disclaimer, this is only a personal recollection of those events.

Spectroscopy of an unbound nucleus $$^{17}$$ C was performed using the SAMURAI spectrometer at RIBF of RIKEN. Six resonances were observed for the $$^{16}$$ C+n system with relative energies of 0.52, 0.77, 1.36, 1.91, 2.22 and 3.20 MeV. The excitation energies ( $$E_x$$ ) of the observed resonances were deduced, by taking into account the states of the $$^{16}$$ C fragments identified by coincident $$\gamma $$ rays, as $$E_x$$ =(3.02), 1.51, (3.86), 2.65, (4.72) and 3.94 MeV. The orbital angular momenta of the two observed states in $$^{17}$$ C at $$E_x$$ =2.65 and 3.94 MeV were determined as 1 by comparing parallel momentum distributions with theoretical predictions.

A neutron star is pictured as a gigantic nucleus overwhelmed by the number of neutrons, unlike real atomic nuclei, that have a similar number of neutrons and protons. Is this true? What if we could find or create nuclei without protons? How far can we go in neutron richness? Our common sense tells us that these neutral nuclei should not exist, but if they do they would change our knowledge on neutron stars, on the properties of nuclei in general and ultimately on the nucleon–nucleon interaction itself, the building block of matter. This huge potential impact has pushed some ambitious nuclear physicists to search for them since the 1960s. The first positive hints appeared only in the XXI century, and nowadays, several collaborations are trying to corner these weird objects and give a definite answer to this crucial question. In this review, we will go through this fascinating quest, that started with humble experiments and has now reached a stage of ambitious and sophisticated projects, both in experiment and theory.

The long history of the research concerning the possible existence of bound or resonant states in light multineutron systems, essentially (3)n and (4)n, is reviewed. Both the experimental and the theoretical points of view have been considered, with the aim of showing a clear picture of all the different detection and calculation techniques that have been used, with particular emphasis in the issues that have been found. Finally, some aspects of the present and future research in this field are discussed.

We review an extensive study of the beryllium isotopic chain carried out at RIKEN. In particular, we discuss the results on $$^{13,15}\hbox {Be}$$ , two key isotopes for the understanding of dineutron configurations and decays in $$^{14,16}\hbox {Be}$$ . In the $$^{13}$$ Be case, a detailed analysis of its spectroscopy, including for the first time a well-founded reaction framework and a realistic three-body model of $$^{14}\hbox {Be}$$ that incorporates core excitations, confirms the dominant $$\ell =1$$ content of the low-lying spectrum. In the $$^{15}$$ Be case, the fragmentation of $$^{18}$$ C, a priori free of any selection rules that may have precluded the observation of states in previous experiments, has confirmed the only known state at about 1.8 MeV, assigned according to shell-model calculations to a spin-parity $$5/2^+$$ . Some perspectives of these studies are also given.

We consider the evolution of the neutron-nucleus scattering length for the lightest nuclei. We show that, when increasing the number of neutrons in the target nucleus, the strong Pauli repulsion is weakened and the balance with the attractive nucleon-nucleon interaction results into a resonant virtual state in $^{18}$B. We describe $^{19}$B in terms of a $^{17}$B-$n$-$n$ three-body system where the two-body subsystems $^{17}$B-$n$ and $n$-$n$ are unbound (virtual) states close to the unitary limit. The energy of $^{19}$B ground state is well reproduced and two low-lying resonances are predicted. Their eventual link with the Efimov physics is discussed. This model can be extended to describe the recently discovered resonant states in $^{20,21}$B.

We present a model description of the bound B-19 isotope in terms of a B-17-n-n three-body system where the two-body subsystems B-17-n and n-n are unbound (virtual) states close to the unitary limit. The B-19 ground state is well described in terms of two-body potentials only, and two low-lying resonances are predicted. Their eventual link with the Efimov physics is discussed. This model can be naturally used to describe the recently discovered resonant states in B-20,B-21.

We consider the evolution of the neutron-nucleus scattering length for the lightest nuclei. We show that, when increasing the number of neutrons in the target nucleus, the strong Pauli repulsion is weakened and the balance with the attractive nucleon-nucleon interaction results into a resonant virtual state in 18B. We describe 19B in terms of a 17B-n-n three-body system where the two-body subsystems 17B-n and n-n are unbound (virtual) states close to the unitary limit. The energy of 19B ground state is well reproduced and two low-lying resonances are predicted. Their eventual link with the Efimov physics is discussed. This model can be extended to describe the recently discovered resonant states in 20,21B.

The spectroscopic structure of 19C, a prominent one-neutron halo nucleus, has been studied with a 20C secondary beam at 290 MeV/nucleon and a carbon target. Neutron-unbound states populated by the one-neutron knockout reaction were investigated by means of the invariant mass method. The preliminary relative energy spectrum and parallel momentum distribution of the knockout residue, 19C*, were reconstructed from the measured four momenta of the 18C fragment, neutron, and beam. Three resonances were observed in the spectrum, which correspond to the states at Ex = 0.62(9), 1.42(10), and 2.89(10) MeV. The parallel momentum distributions for the 0.62-MeV and 2.89-MeV states suggest spin-parity assignments of 5/2+ and 1/2−, respectively. The 1.42-MeV state is in line with the reported 5/22+ state.

A spectroscopic study of 17C was performed via the one-neutron knockout reaction of 18C on a carbon target at RIKEN-RIBF. Three unbound states at excitation energies of 2.66(2), 3.16(5), and 3.97(3) MeV (preliminary) were observed. The energies are compared with shell-model calculations and existing measurements to deduce their spin-parities. From the comparison, the states at 2.66(2) and 3.97(3) MeV are suggested to be 1/2− and 3/2−, respectively. From its decay property, the state at 3.16(5) MeV is indicated to be 9/2+.

The spectroscopic structure of 19C, a prominent one-neutron halo nucleus, has been studied with a 20C secondary beam at 290 MeV/nucleon and a carbon target. Neutron-unbound states populated by the one-neutron knockout reaction were investigated by means of the invariant mass method. The preliminary relative energy spectrum and parallel momentum distribution of the knockout residue, 19C*, were reconstructed from the measured four momenta of the 18C fragment, neutron, and beam. Three resonances were observed in the spectrum, which correspond to the states at Ex = 0.62(9), 1.42(10), and 2.89(10) MeV. The parallel momentum distributions for the 0.62-MeV and 2.89-MeV states suggest spin-parity assignments of 5/2+ and 1/2−, respectively. The 1.42-MeV state is in line with the reported 5/22+ state.

Unbound states in 17C were investigated via one-neutron knockout of 18C. The experiment was performed using SAMURAI spectrometer in RIBF at RIKEN. By invariant mass spectroscopy, three resonances were measured at excitation energies of 3.03(12), 2.74(3), and 4.03(6) MeV as preliminary results. For the excited state at 2.74(3) MeV, the parallel momentum distribution was satisfactorily described by the distribution calculated for p-wave knockout from 18C.

The present work aims at exploring neutron-unbound states of 19C via the one-neutron knockout reaction. The invariant mass measurement in inverse kinematics was carried out with a carbon target and a 20C secondary beam at 290 MeV/nucleon. The preliminary relative energy spectrum and the parallel momentum distribution of the system of 18C + n were reconstructed from the measured momenta of the 18C fragment and decayed neutron. A new resonance was observed at Erel = 2.3 MeV, which corresponds to the unbound state of 19C at Ex = 2.9 MeV. The parallel momentum distribution for this resonance suggests a spin-parity assignment of 1/2−. Additionally, two known states were seen at Ex = 0.6 and 1.4 MeV, consistent with the 5/2+1 and 5/2+2 states, respectively.

A new approach to the production and detection of multineutrons, based on breakup reactions of beams of very neutron-rich nuclei, is presented. The first application of this technique to the breakup of Be-14 into Be-10 and 4n revealed 6 events consistent with the formation of a bound tetraneutron. The description of these data by means of an unbound tetraneutron resonance is also discussed. The experiments that have been undertaken at GANIL in order to confirm this observation with Be-12,Be-14 and He-8 beams are presented. Special attention is paid to the angular correlations of some candidate events observed in the channel (He-8,He-4).