An interesting property of some neutron-unbound systems is true two-neutron emission where the neutrons are emitted simultaneously as opposed to a sequential decay through an intermediate state. Since neutrons are only affected by the angular momentum barrier, the timescale for this process is much shorter than for two proton emission which is dominated by the Coulomb barrier. One such case is ²2076O where a very low decay energy was measured and the two valence neutrons are expected to occupy d-wave orbitals. Also, the ground state of ²2075O is located 700 keV higher. Using the data from a previous measurement of the decay energy, the MoNA collaboration extracted a lifetime of 4:5+1:1-1:5(stat) ± 3(syst) ps with a confidence level of 82% (1). Results from a recent measurement give T1/2 = 5:0+2:0-1:6 (stat) ±1:7 (syst) ps and support the previous finding.Measurements of neutron-unbound systems using invariant mass spectroscopy are often performed using low-intensity radioactive ion beams. Low reaction yields can be countered by using a thicker target but at the expense of larger uncertainties in the reconstructed invariant mass. A new segmented target was designed to address this trade-off, and it was first used in this experiment to re-measure the ground state half-life of ²2076O.
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