Background: The $0_2^{+}$ state of ${}^8\mathrm{He}$ has been discovered by a recent experiment, which suggested a developed cluster structure of spatially correlated neutron pairs, called “dineutrons” (${}^{2}n$).

Purpose: We aim to investigate the structure of ${}^8\mathrm{He}\left(0_1^{+}\right)$ and ${}^8\mathrm{He}\left(0_2^{+}\right)$ to clarify the monopole excitation mode in ${}^8\mathrm{He}$ system while focusing on the $\alpha +{}^{2}n+{}^{2}n$ cluster structures and the breaking of ${}^{2}n$ clusters.

Methods: We apply a microscopic cluster model with the generator coordinate method for the $\alpha +{}^{2}n+{}^{2}n$ cluster and ${}^6\mathrm{He}+{}^{2}n$ cluster dynamics. The $p_{3/2}$-closure component, which is induced by the spin-orbit force, is also incorporated.

Results: The present calculation reasonably reproduces the experimental data of the properties of ${}^8\mathrm{He}$, such as $2n$ and $4n$ separation energies, energy spectra, and radii. The spatially developed cluster structure of the $0_2^{+}$ state is obtained. The $0_1^{+}$ and $0_2^{+}$ states have dominant components of $\alpha +{}^{2}n+{}^{2}n$ 3-body cluster, but contain significant ${}^{2}n$ breaking components which contribute to the energy gain and size shrinking of the ${}^8\mathrm{He}$ system. The monopole excitation in ${}^8\mathrm{He}$ is regarded as a radial excitation, which is similar to that in ${}^{12}\mathrm{C}$.

Conclusions: The $\alpha +{}^{2}n+{}^{2}n$ cluster structures play a dominant role in ${}^8\mathrm{He}\left(0_{1,2}^{+}\right)$, and the mixing of the dineutron breaking contributes to significant structural effects.

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• Binding energy & masses
• Cluster models
• Nuclear radii
• Nuclear structure & decays
• 6 ≤ A ≤ 19