We study the two-neutron decay of the unbound 26O nucleus with a three-body model assuming an inert 24O core and two valence neutrons. We first discuss the importance of the neutron-neutron final state interaction in the observed decay energy spectrum. We then show that the energy and and angular distributions for the two emitted neutrons manifest a clear evidence for the strong neutron-neutron correlation in the three-body resonance state. In particular, we find an enhancement of two-neutron emission in back-to-back directions. This is interpreted as a consequence of dineutron correlation, with which the two neutrons are spatially localized before the emission.
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