Study of multi-neutron emission in the β-decay of 11Li

Abstract

The kinematics of two-neutron emission following the β-decay of 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 γ-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 β-1n emitter Li. A large cross-talk rejection rate is obtained (> 95%) over most of the energy range of interest. Application to Li data leads to a significant number of events interpreted as β-2n decay. A discrete neutron line at ≈ 2 MeV indicates sequential two-neutron emission, possibly from the unbound state at 10.6 MeV excitation energy in Be.