α particles in stable nuclei: α Decay is a common mode of radioactivity in heavy elements such as uranium that entails the loss of particles comprising two protons and two neutrons. Despite more than a century of study, when and where these α particles form in stable and unstable nuclei alike remains an open question. Tanaka et al. bombarded a series of stable tin isotopes with high-energy protons and detected ejected α particles at an abundance inversely correlated with mass number (see the Perspective by Hen). This observation, relating α particle accumulation to the neutron skin thickness at the nuclear surface, bears on models spanning radioactive decay to neutron star dynamics. Science , this issue p. 260 ; see also p. 232

A kinematically complete quasifree $(p,pn)$ experiment in inverse kinematics was performed to study the structure of the Borromean nucleus ${}^{17}\mathrm{B}$, which had long been considered to have a neutron halo. By analyzing the momentum distributions and exclusive cross sections, we obtained the spectroscopic factors for $1s_{1/2}$ and $0d_{5/2}$ orbitals, and a surprisingly small percentage of 9(2)% was determined for $1s_{1/2}$. Our finding of such a small $1s_{1/2}$ component and the halo features reported in prior experiments can be explained by the deformed relativistic Hartree-Bogoliubov theory in continuum, revealing a definite but not dominant neutron halo in ${}^{17}\mathrm{B}$. The present work gives the smallest $s$- or $p$-orbital component among known nuclei exhibiting halo features and implies that the dominant occupation of $s$ or $p$ orbitals is not a prerequisite for the occurrence of a neutron halo.

Nucleus is essentially composed of protons and neutrons, which are commonly known as nucleons. Interestingly, some of nucleons may group together and exhibit collective behavior inside a nucleus. Such clustering effects have been known since the early stages of nuclear physics because of the observation and description of α-cluster decay from many heavy nuclei. Subsequent studies demonstrated that cluster structures exist in many nuclear systems, especially in weakly bound or excited states, and are complementary to the shell-like structures. In this review article, we provide a brief historical recall of the field, and follow it with a conceptual and logical description of the major theoretical models that have been frequently applied in the literature to describe nuclear clustering. Experimental methods and progress are outlined, recent outcomes are emphasized, and perspectives relevant to future studies of heavy neutron-rich systems are discussed.

We report here the first observation of the $0_2^{+}$ state of ${}^8\mathrm{He}$, which has been predicted to feature the condensatelike $\alpha +{}^{2}n+{}^{2}n$ cluster structure. We show that this state is characterized by a spin parity of $0^{+}$, a large isoscalar monopole transition strength, and the emission of a strongly correlated neutron pair, in line with theoretical predictions. Our finding is further supported by the state-of-the-art microscopic $\alpha +4n$ model calculations. The present results may lead to new insights into clustering in neutron-rich nuclear systems and the pair correlation and condensation in quantum many-body systems under strong interactions.

The shape and internal structure of an atomic nucleus can change significantly with increasing excitation energy, angular momentum, or isospin asymmetry. As an example of this structural evolution, linear-chain configurations in carbon or heavier isotopes have been predicted for decades. Recent studies have found non-stability of this structure in ¹²C while evidenced its appearance in ¹⁶C. It is then necessary to investigate the linear-chain molecular structures in ¹⁴C to clarify the exact location on the nuclear chart where this structure begins to emerge, and thus to benchmark theoretical models. Here we show a cluster-decay experiment for ¹⁴C with all final particles coincidentally detected, allowing a high Q-value resolution, and thus a clear decay-path selection. Unambiguous spin-parity analyses are conducted, strongly evidencing the emergence of the π-bond linear-chain molecular rotational band in ¹⁴C. The present results encourage further studies on even longer chain configurations in heavier neutron-rich nuclei.