A Search for the ¹²Be Isomeric State

The 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 current experiments cannot determine the 1/2^- p-wave location. This p-wave resonance is expected to strongly decay to a long-lived 0₂⁺ state in ¹²Be with a mean lifetime of 331(12) ns. Its decay scheme has a 20% branch through the ¹²Be(2⁺ ) to the ground state (2.1 MeV) and 80% for an E0 transition giving two 511 keV gamma rays from positron annihilation.

A new experiment has been performed by the MoNA Collaboration at the NSCL to study the neutron decay of ¹³Be to this isomeric state in ¹²Be. This experiment features a sweeperless MoNA-LISA setup, with a new telescope detector for ¹²Be fragment, 96 modules of the MoNA-LISA detector for neutrons emitted, and the gamma ray detector CAESAR, placed around the telescope instead of the reaction target due to the long lifetime of the expected isomer.

A digital data acquisition system (DDAS) was applied to fragment, beam and gamma detectors, and synchronized with MoNA-LISA VME based electronics to get coincidence of all the decay products.

An overview and current data analysis of the experiment will be presented and discussed.