Study of multi-neutron emission in the $β$-decay of $^{11}$Li
nucl-ex
/ Authors
F. Delaunay, N. L. Achouri, A. Algora, M. Assié, J. Balibrea, K. Banerjee, C. Bhattacharya, M. J. G. Borge, D. Cano-Ott, B. Fernández-Domínguez
and 17 more authors
L. M. Fraile, J. Gibelin, M. V. Lund, M. Madurga, F. M. Marqués, I. Marroquin, T. Martínez, E. Mendoza, N. A. Orr, M. Pârlog, X. Pereira-López, V. Pestel, K. Riisager, C. Santos, M. Sénoville, O. Tengblad, V. Vedia
/ Abstract
The kinematics of two-neutron emission following the $β$-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 $γ$-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 $^9$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 $β$-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.