# Study of multi-neutron emission in the $\beta$-decay of $^{11}$Li

## TL;DR

This study investigates the two-neutron emission following the beta decay of lithium-11 by detecting neutrons in coincidence, revealing sequential emission possibly from an unbound state in beryllium-11.

## Contribution

First experimental measurement of two-neutron emission kinematics in the beta decay of $^{11}$Li using coincidence detection and background rejection techniques.

## Key findings

- Detection of a neutron line at approximately 2 MeV.
- Evidence for sequential two-neutron emission from an unbound state.
- High cross-talk rejection rate (>95%) in neutron detection.

## Abstract

The kinematics of two-neutron emission following the $\beta$-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 $\gamma$-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 $\beta$-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 $\beta$-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.