Experimental evidence of core modi cation in the near drip-line nucleus 23O -: art. no. 142502

The longitudinal momentum (P-parallel to) distributions of one- and two-neutron removal fragments (O-21,O-22) of O-23 from the reaction with a C target at 72 A 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 unexpectedlynarrow width of O-21(115 +/- 34 MeV/c in FWHM) is consistent with two neutrons occupying the 2s(1/2) orbital in O-23. This indicates modi cation of core (O-22) structure for neutron halolike sd shell nuclei near the drip line. This also suggests the lowering of the s orbital providing a justification for the N = 16 magic number.