Narrow Results

It is discussed that the recently observed 0⁺ state at 13.5 MeV in ¹⁶O can be a good candidate of the 4α condensate. The 4α condensed state which is theoretically obtained in the vicinity of the 4α threshold is shown to have a relatively large α decay width, which is in well agreement with the value measured for the 0⁺ state at 13.5 MeV. The analysis of inelastic form factor of electron scattering to the 4α condensed state is made by artificially varying the size of the 4α condensate. Similarly to the case of Hoyle state, only the magnitude of the form factor is found to depend on its size, while the position of maximum and minimum are not changed.

In order to search for the α condensed state in ¹⁶O, we have performed the decay particle measurement via the reaction. Although the decay branching ratio of the α + ¹²C channel was predicted to be small, the branching ratio between the α + ¹²C and channels was obtained to be 7:3 by using the result of a simple simulation.

Cluster structures of 3/2^- and 1/2⁺ states in ¹¹B are investigated with a semi-microscopic α + α + t cluster model based on the Gaussian expansion method. The state, characterized by the monopole transition strength as large as the Hoyle state ( at E_x = 7.65 MeV in ¹²C), was found to have a loosely bound α+α+t cluster structure, although its features is not similar to those of the Hoyle state with the dilute 3α-condensate-like character (0S_α)³, in which the α particle is occupied in a single 0S-orbit with about 70% probability. We indicate that the 12.56-MeV state (J^π = 1/2⁺) around the α + α +t threshold observed in the ⁷Li(⁷Li,¹¹B*)t reaction etc. is a strong candidate for the Hoyle-analogue state with the main configuration of (0S_α)²(0S_t) from the analyses of the single-cluster motions in ¹¹B.

The condensed states of α-clusters or di-neutrons are studied by introducing a Monte Carlo technique for the description of the THSR (Tohsaki Horiuchi Schuck Röpke) wave function. The condensed state of five alpha particles is shown to be stable around the threshold energy even after taking into account the coupling effect between normal cluster states with ¹⁶O+α configurations. Next, the mixing of di-neutron components in ⁸He is studied. The ground state of ⁸He corresponds to the sub-closed-shell configuration of the spin-orbit favored orbits (p_3/2) for the neutrons, and the shell-model-like configuration is considered to be dominant. However, ⁸He is a drip-line nucleus with a two-neutron separation energy of 2.1 MeV, and the di-neutron configuration, which is a characteristic feature of the weakly bound systems, mixes in the ground state. Finally, characteristic mechanism for the binding of clusters in neutron-rich nuclei (s² bond) is discussed.