Observation of enhanced monopole strength and clustering in (12)Be

Observation of the Exotic 0_{2}^{+} Cluster State in ^{8}He

We report here the first observation of the 0_{2}^{+} state of ^{8}He, which has been predicted to feature the condensatelike α+^{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 α+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.

New evidence of the Hoyle-like structure in 16O

An experiment of ¹²C(¹⁶O,¹⁶O → 4α)¹²C was performed at a beam energy of 96 MeV. A large number of 4-α events were recorded in coincidence and with full particle identification (PID). This was made possible by employing a series of silicon-strip-based telescopes that provided excellent position and energy resolutions. Four narrow resonances just above the 15.1 MeV state were firmly identified in the α + ¹²C(7.65 MeV; Hoyle state) decay channel. Combined with the theoretical predictions, these resonant states provide new evidence for the red predicted possible Hoyle-like structure in ¹⁶O above the 4-α separation threshold. Some very high-lying 4-α resonant states have also been observed and need to be further investigated.

Positive-Parity Linear-Chain Molecular Band in ^{16}C

An inelastic excitation and cluster-decay experiment ^{2}H(^{16}C,^{4}He+^{12}Be or ^{6}He+^{10}Be)^{2}H was carried out to investigate the linear-chain clustering structure in neutron-rich ^{16}C. For the first time, decay paths from the ^{16}C resonances to various states of the final nuclei were determined, thanks to the well-resolved Q-value spectra obtained from the threefold coincident measurement. The close-threshold resonance at 16.5 MeV is assigned as the J^{π}=0^{+} band head of the predicted positive-parity linear-chain molecular band with (3/2_{π}^{-})^{2}(1/2_{σ}^{-})^{2} configuration, according to the associated angular correlation and decay analysis. Other members of this band were found at 17.3, 19.4, and 21.6 MeV based on their selective decay properties, being consistent with the theoretical predictions. Another intriguing high-lying state was observed at 27.2 MeV which decays almost exclusively to ^{6}He+^{10}Be(∼6  MeV) final channel, corresponding well to another predicted linear-chain structure with the pure σ-bond configuration.

Unified studies of chemical bonding structures and resonant scattering in light neutron-excess systems, 10,12Be

The generalized two-center cluster model (GTCM), which can treat covalent, ionic and atomic configurations in general systems with two inert cores plus valence nucleons, is formulated in the basis of the microscopic cluster model. In this model, the covalent configurations constructed by the molecular orbital (MO) method and the atomic (or ionic) configuration obtained by the valence bonding (VB) method can be handled in a consistent manner. The GTCM is applied to the light neutron-rich system (10,12)Be = α + α + Xn (X = 2, 4). The continuous and smooth changes of the neutron orbits from the covalent MO states to the ionic VB states are clearly observed in the adiabatic energy surfaces (AESs), which are the energy curves obtained with a variation of the α-α distance. The energy levels obtained from the AESs nicely reproduce the recent observations over a wide energy region. The individual spectra are characterized in terms of chemical-bonding-like structures, such as the covalent MO or ionic VB structures, according to analysis of their intrinsic wave functions. From the analysis of AESs, the formation of the mysterious 0(2)(+) states in (10,12)Be, which have anomalously small excitation energies in comparison to a naive shell-model prediction, is investigated. A large enhancement in a monopole transition from a ground MO state to an ionic α + (6,8)He VB state is found, which seems to be consistent with a recent observation. In the unbound region, the structure problem, which handles the total system of α + α + Xn (X = 2, 4) as a bound or quasi-bound state, and the reaction problem, induced by the collision of an asymptotic VB state of α + (6,8)He, are combined by the GTCM. The properties of unbound resonant states are discussed in close connection to the reaction mechanism, and some enhancement factors originating from the properties of the intrinsic states are predicted in the reaction observables.