Experimental Evidence of Core Modification in the Near Drip-Line Nucleus O-23

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Copyright notice(c)2002 American Physical Society. All rights reserved. Publisher's version: http://link.aps.org/abstract/PRL/v88/e142502The longitudinal momentum (PII) distributions of one- and two-neutron removal fragments ( 21,22O) of 23O 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 unexpectedly narrow width of 21O ( 115±34 MeV/c in FWHM) is consistent with two neutrons occupying the 2s1/2 orbital in 23O. This indicates modification of core ( 22O) 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.