Evidence for the 15Be ground state from 12Be+3n events
Abstract
Background: 15Be is an unbound nuclide that has been observed to decay by one-neutron emission. Shell model calculations predict two low-lying states in its energy spectrum, however, only a single resonance has been observed from coincident measurements of 14Be+n. It has been suggested that the yet unobserved state may decay sequentially through the first excited state in 14Be followed by a two-neutron emission to 12Be. Purpose: The ground state of 15Be has yet to be confirmed. A search for this predicted 15Be state by reconstructing 12Be+3n events allows a possible determination of its ground state properties. Methods: A neutron-pickup reaction was performed with a 14Be beam on a CD2 target to populate unbound 15Be states. Decay energies were reconstructed using invariant mass spectroscopy by detecting 12Be daughter nuclei in coincidence with up to three neutrons. Results: Evidence for at least one resonance in 15Be is presented based on the reconstruction of 12Be+3n events. Through comparison with simulations, the energy of the strongest resonance in the analyzed reaction and decay channel is determined to be E12Be+3n=330(20) keV. Conclusions: The inclusion of a new 15Be state among the 12Be+3n events lower in relative decay energy than the previous 14Be+n observations provides the best fit to the data. Because this suggested new state would be lower in energy than the previously observed state, it is a candidate for the ground state of 15Be.
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