Abstract
The emission of neutron pairs from the neutron-rich N = 12 isotones C-18
and O-20 has been studied by high-energy nucleon knockout from N-19 and
O-21 secondary beams, populating unbound states of the two isotones up
to 15 MeV above their two-neutron emission thresholds. The analysis of
triple fragment-n-n correlations shows that the decay N-19(-1p)18C*->
C-16 + n + n is clearly dominated by direct pair emission. The two-
neutron correlation strength, the largest ever observed, suggests the
predominance of a C-14 core surrounded by four valence neutrons arranged
in strongly correlated pairs. On the other hand, a significant
competition of a sequential branch is found in the decay
O-21(-1n)O-20*-> O-18 + n + n, attributed to its formation through the
knockout of a deeply bound neutron that breaks the O-16 core and reduces
the number of pairs.
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