Study of multi-neutron emission in the ${\hat{I}}^2$-decay of ${}^{11}$Li

Abstract

The kinematics of two-neutron emission following the ${\hat{I}}^2$-decay of ${}^{11}$Li was investigated for the first time by detecting the two neutrons in coincidence and by measuring their angle and energy. An array of liquid-scintillator neutron detectors was used to reject cosmic-ray and ${\hat{I}}^3$-ray backgrounds by pulse-shape discrimination. Cross-talk events in which two detectors are fired by a single neutron were rejected using a filter tested on the ${\hat{I}}^2$-1n emitter ${}^9$Li. A large cross-talk rejection rate is obtained ($>95\%$) over most of the energy range of interest. Application to ${}^{11}$Li data leads to a significant number of events interpreted as ${\hat{I}}^2$-2n decay. A discrete neutron line at $\approx$ 2 MeV indicates sequential two-neutron emission, possibly from the unbound state at 10.6 MeV excitation energy in ${}^{11}$Be.

4 pages, Proceedings of the EuNPC2018 Conference