Locating the first p1/2^- state in ¹³Be

Spectroscopy of the neutron unbound ¹³Be is key to understanding the unexpected structural changes of neutron-rich nuclei around N=8. Invariant mass spectra from previous experiments have not been able to determine the 1/2^- p-wave location. This resonance is expected to strongly decay into a long-lived 0₂⁺ state in ¹²Be with a mean lifetime of 331(12) ns. Its decay scheme has a 20% branch through ¹²Be(2⁺) into the ground state by emitting a 2.1 MeV gamma ray, and 80% for an E0 transition emitting two 511 keV gamma rays from positron annihilation. An experiment was performed by the MoNA Collaboration at the National Superconducting Cyclotron Laboratory of Michigan State University to study the decay of ¹³Be by one neutron emission into this isomeric state in ¹²Be. It consisted of the first usage of a dedicated new telescope made of a stack of 5 silicon detectors and a CsI stopping detector to identify the ¹²Be fragment. 96 modules of the MoNA-LISA neutron array were used as well as the gamma ray detector CAESAR that was placed around the telescope to detect the decay of the expected isomeric state with long lifetime in the daughter nucleus. This experiment also tested the possibility for sweeperless experiments for the MoNA Collaboration. An overview and preliminary results of the experiment will be presented and discussed.