Evidence of a new state in $^{11}$Be observed in the $^{11}$Li $\beta$-decay

TL;DR

This study observed a new excited state at 16.3 MeV in $^{11}$Be through detailed analysis of $^{11}$Li beta decay, revealing insights into nuclear structure and decay channels.

Contribution

The paper reports the first identification of a previously unobserved 16.3 MeV state in $^{11}$Be using beta decay of $^{11}$Li with full kinematic reconstruction.

Findings

Discovery of a 16.3 MeV state in $^{11}$Be.

Assignment of spin and parity 3/2$^-$ to the new state.

Confirmation of the 18 MeV state in $^{11}$Be.

Abstract

Coincidences between charged particles emitted in the $\beta$-decay of $^{11}$Li were observed using highly segmented detectors. The breakup channels involving three particles were studied in full kinematics allowing for the reconstruction of the excitation energy of the $^{11}$Be states participating in the decay. In particular, the contribution of a previously unobserved state at 16.3 MeV in $^{11}$Be has been identified selecting the $\alpha$ + $^7$He$\to\alpha$ + $^6$He+n channel. The angular correlations between the $\alpha$ particle and the center of mass of the $^6$He+n system favors spin and parity assignment of 3/2$^-$ for this state as well as for the previously known state at 18 MeV.