Mass, spectroscopy and two-neutron decay of $^{16}$Be

B. Monteagudo, F. M. Marqués, J. Gibelin, N. A. Orr, A. Corsi, Y. Kubota, J. Casal, J. Gómez-Camacho, G. Authelet, H. Baba, C. Caesar, D. Calvet, A. Delbart, M. Dozono, J. Feng, F. Flavigny, J. -M. Gheller, A. Giganon, A. Gillibert, K. Hasegawa, T. Isobe, Y. Kanaya, S. Kawakami, D. Kim, Y. Kiyokawa, M. Kobayashi, N. Kobayashi, T. Kobayashi, Y. Kondo, Z. Korkulu, S. Koyama, V. Lapoux, Y. Maeda, T. Motobayashi, T. Miyazaki, T. Nakamura, N. Nakatsuka, Y. Nishio, A. Obertelli, A. Ohkura, S. Ota, H. Otsu, T. Ozaki, V. Panin, S. Paschalis, E. C. Pollacco, S. Reichert, J. -Y. Rousse, A. T. Saito, S. Sakaguchi, M. Sako, C. Santamaria, M. Sasano, H. Sato, M. Shikata, Y. Shimizu, Y. Shindo, L. Stuhl, T. Sumikama, Y. L. Sun, M. Tabata, Y. Togano, J. Tsubota, T. Uesaka, Z. H. Yang, J. Yasuda, K. Yoneda, J. Zenihiro
Nuclear Experiment, Nuclear Experiment (nucl-ex), Nuclear Theory (nucl-th)
2024-01-30 00:00:00
The structure and decay of the most neutron-rich beryllium isotope, $^{16}$Be, has been investigated following proton knockout from a high-energy $^{17}$B beam. Two relatively narrow resonances were observed for the first time, with energies of $0.84(3)$ and $2.15(5)$ MeV above the two-neutron decay threshold and widths of $0.32(8)$ and $0.95(15)$ MeV respectively. These were assigned to be the ground ($J^{\pi}=0^+$) and first excited ($2^+$) state, with $E_x=1.31(6)$ MeV. The mass excess of $^{16}$Be was thus deduced to be $56.93(13)$ MeV, some $0.5$ MeV more bound than the only previous measurement. Both states were observed to decay by direct two-neutron emission. Calculations incorporating the evolution of the wavefunction during the decay as a genuine three-body process reproduced the principal characteristics of the neutron-neutron energy spectra for both levels, indicating that the ground state exhibits a strong spatially compact dineutron component, while the 2$^+$ level presents a far more diffuse neutron-neutron distribution.
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