The longitudinal momentum $\left(P_{\Vert }\right)$ distributions of one- and two-neutron removal fragments ( ${}^{21,22}\mathrm{O}$) of ${}^{23}\mathrm{O}$ 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 ${}^{21}\mathrm{O}$ ( $115\pm 34\mathrm{MeV}/c$ in FWHM) is consistent with two neutrons occupying the ${2s}_{1/2}$ orbital in ${}^{23}\mathrm{O}$. This indicates modification of core ( ${}^{22}\mathrm{O}$) structure for neutron halolike $\mathrm{sd}$ shell nuclei near the drip line. This also suggests the lowering of the s orbital providing a justification for the $N=16\mathrm{}$ magic number.

• Received 20 September 2001

DOI:https://doi.org/10.1103/PhysRevLett.88.142502