Tomohiro Uesaka

Universality in physics describes the emergence of common features in vastly different systems. One fascinating example is the Efimov phenomenon where three-body resonantly interacting systems display universal properties. Efimov states have been observed in ultra-cold-atom systems, but their manifestation in nuclei remains elusive due to the long-range repulsive Coulomb interaction and the stringent requirement for at least two s-wave resonances in its subsystems. Recent theories propose another universality in three-body halos with only one s-wave resonance. Here, we report the identification of a precursor of this phenomenon in a neutron-rich ¹⁰He nucleus. With higher statistics and better sensitivities than previous experiments, we identify two low-lying 0⁺ states of ¹⁰He at ~ 1 MeV and ~ 2 MeV above its two-neutron decay threshold, and determine an s-wave scattering length of ~ - 3.5 fm between ⁸He and neutron. It is revealed that the lower energy state, the ground state of ¹⁰He, is a three-body state with only s-wave interactions among its subsystems. This state manifests as a resonance structure, which is a direct consequence of a universal long-range three-body repulsion. Our work sheds new light on quantum halos with finite lifetimes, providing a path toward their unified understanding across various scales and fields.

The proton-induced deuteron knockout reaction (p, pd) provides a unique opportunity to exploit proton-neutron correlations and the deuteron cluster structure of nuclei. Direct deuteron knockout experiments on ¹²C and ¹⁶O under quasi-free scattering conditions have been carried out under the ONOKORO project using a 226 MeV proton beam at RCNP, Osaka, Japan. Deuterons from the knockout reaction, acting as clusters, were unambiguously detected at the focal plane of the Large Acceptance Spectrometer. Outgoing protons, following the knockout of clusters, were detected at the focal plane of the Grand Raiden spectrometer to correctly reconstruct the expected events. (p, pd) reactions are established successfully, and excitation energy spectra for the residual nuclei ¹⁰B and ¹⁴N are obtained. Results showed that a large difference in transition strength toward the low-lying energy level of residual nuclei, including the ground state, was found in comparison to other studies. Theoretical calculations using the PIKOE package based on the distorted-wave impulse approximation are conducted to deduce the triple differential cross section of (p, pd) reaction, and experimental spectroscopic factors of the deuteron cluster are obtained by normalizing the experimental cross section to the theoretical computation. Consistent spectroscopic factors in comparison to shell-model expectations are obtained for transitions involving the orbital angular momentum L = 0 of the cluster. The present work demonstrates that the (p, pd) reactions at 226 MeV as a spectroscopic tool have an advantage in examining spectroscopic aspects of deuteron clustering and the mechanism to form a proton-neutron pair in the spin triplet state with a negligible interference of final state interactions and refraction effects.

Polarized electron spins in photoexcited triplet states enable dynamic nuclear polarization (DNP) to enhance magnetic resonance imaging (MRI) sensitivity. For a practical nuclear polarization of 10%, single crystals must be precisely oriented in a magnetic field to generate narrow electron spin resonance lines, which is not appropriate for actual medical applications. Here, substituted fullerenes as triplet polarizing agents enable ¹H polarizations above 10%, even for random molecular orientations. While they have not been used as polarizing agents for triplet-DNP because of electron spin relaxation via pseudo-rotation, we overcome this by chemical modification of two sites on C₆₀ fullerenes. Symmetry considerations reveal fullerenes that avoided pseudo-rotations. Di-substituted fullerenes are ideal polarizing agents with sharp linewidths and long relaxation times that enabled 14.2% ¹H polarization in randomly oriented orientations. Optimized polarizing agents represent a promising approach for ultra-sensitive MRI medical diagnostics without the need for DNP under cryogenic temperatures and severe orientation control.

Polarized electron spins in photoexcited triplet states enable dynamic nuclear polarization (DNP) at high temperatures, leading to increased sensitivity in magnetic resonance imaging (MRI). For a practical nuclear polarization of 10%, single crystals must be precisely oriented in a magnetic field to generate electron spin resonances in a narrow field range that satisfies resonance conditions for polarization transfer. However, this is not realistic for medical applications. Substituted fullerenes as triplet polarizing agents have enabled 1H polarizations above 10%, even for random molecular orientations. They have not been used as polarizing agents for triplet-DNP because of electron spin relaxation via pseudo-rotation. Here, the spin-lattice relaxation time was significantly increased by chemical modification of two sites on C60 fullerenes. Symmetry considerations revealed fullerenes that avoided pseudo-rotations. Di-substituted fullerenes were ideal polarizing agents with sharp linewidths and long relaxation times that enabled 14.2% 1H polarization in randomly oriented orientations. Optimized polarizing agents would enable ultra-sensitive MRI medical diagnostics under mild preparation conditions.

Well-developed α clusters are known to exist in light N = Z nuclei, and their properties are reasonably well described with modern nuclear structure theories. However, ‘modestly’ developed clusters in medium to heavy nuclei remain little understood, both theoretically and experimentally. Extension of the focus to include modestly developed clusters leads us to a concept of ‘generalized clusters’ and ‘cluster ubiquitousness’. The former includes clusters more weakly bound than an α cluster, such as deuteron, triton and He 3 , and even clusters partially broken owing to nuclear medium effects. The latter means the existence of clusters in any nuclei, where cluster development was not previously discussed. Effects of the tensor and the spin–orbit interactions on the coexistence of clusters with independent nucleons are discussed using recent nuclear theoretical models. A mixture of the clusters with shell-like components plays an essential role in the synthesis of elements in the universe and the origin of life, together with an α decay. It is also pointed out that clustering in heavy nuclei may accelerate fission and fusion processes. Future experimental plans using cluster knockout reactions, which have the potential to extract information of ‘generalized clusters’ in a variety of nuclei including stable and unstable nuclei, are also discussed. This article is part of the theme issue ‘The liminal position of Nuclear Physics: from hadrons to neutron stars’.