Speaker
Description
Multiparticle emission, a novel decay mode discovered in proton-rich and neutron-rich nuclei at and beyond the dripline, represents a frontier in nuclear physics. Impressive experimental progress has been made for nuclei unbound by two or three protons [1]. However, multineutron emission remains largely unexplored on the neutron-rich side due to the very limited capability of multineutron detection. Recently, the 3$n$ emitter $^{27}$O and the first 4$n$ emitter $^{28}$O were observed [2]. The simultaneous detection of these constituent neutrons provides critical insights into the structural properties of these extremely neutron-rich nuclei. Moreover, studying multineutron correlations is crucial for understanding neutron clusters composed purely of neutrons. Low-lying resonance-like structures have been observed in tetraneutron ($^{4}n$) system [3,4], but no distinct peak was found for trineutron ($^{3}n$) system [5]. The existence of these neutron clusters remains controversial between experimental data and theoretical calculations.
The excited state of $^{7}$He is a prospective 3$n$ emitter decaying into $^{4}$He + 3$n$, as indicated by a missing-mass measurement [6]. However, due to the lack of three-neutron detection, the decay mode of $^{7}$He cannot be elucidated. Therefore, a new measurement with improved resolution, low background and 3$n$ detection is essential to solidly establish the structure of $^{7}$He.
We have carried out a new experimental study of $^{7}$He at RIKEN RIBF facility using the quasi-free one-neutron knock out reaction $^{8}$He($p$, $pn$)$^{7}$He. The momentum of the charged fragments was analyzed by the SAMURAI spectrometer and its associated detectors. Taking advantage of the large neutron detector array combining the NeuLAND demonstrator from GSI and the existing NEBULA array, multiple neutrons can be detected. This is the first invariant-mass measurement of the $^{7}$He excited state and its 3$n$ emission. The measured 3$n$ correlations also enable the search for a trineutron resonance.
In this talk, our results will be presented.
[1] M. Pfützner, I. Mukha, S.M. Wang, Two-proton emission and related phenomena, Prog. Part. Nucl. Phys. 132, 104050 (2023).
[2] Y. Kondo et al., First observation of $^{28}$O, Nature 620, 965. (2023).
[3] K. Kisamori et al., Candidate Resonant Tetraneutron State Populated by the $^{4}$He($^{8}$He, $^{8}$Be) Reaction, Phys. Rev. Lett. 116, 052501 (2016).
[4] M. Duer et al., Observation of a correlated free four-neutron system, Nature (London) 606, 678 (2022).
[5] K. Miki et al., Precise Spectroscopy of the 3$n$ and 3$p$ Systems via the $^{3}$H($t$, $^{3}$He)3$n$ and $^{3}$He($^{3}$He, $t$)3$p$ Reactions at Intermediate Energies. Phys. Rev. Lett. 133, 012501 (2024).
[6] A. A. Korsheninnikov et al., Observation of an Excited State in $^{7}$He with Unusual Structure. Phys. Rev. Lett. 82, 3581 (1999).
