Experimental Evidence of Core Modification in the Near Drip-Line Nucleus [Formula presented]

The longitudinal momentum [Formula presented] distributions of one- and two-neutron removal fragments ([Formula presented]) of [Formula presented] from the reaction with a C target at 72 [Formula presented] MeV have been measured for the first time using a new direct time-of-flight method with nearly full acceptance for the breakup fragments. The unexpectedly narrow width of [Formula presented] ([Formula presented] in FWHM) is consistent with two neutrons occupying the [Formula presented] orbital in [Formula presented]. This indicates modification of core ([Formula presented]) structure for neutron halolike [Formula presented] shell nuclei near the drip line. This also suggests the lowering of the s orbital providing a justification for the [Formula presented] magic number.