Experimental evidence of core modification in the near drip-line nucleus ²³O

First measurements of momentum distribution were carried out using a new technique which allows several fragmentation reactions to be studied in the same condition. Two-neutron and one-neutron removal P∥ distributions of ²³O were measured simultaneously for the first time. The ²³O → ²²O P∥ distribution in a core-plus neutron model was consistent with mixed occupancy of the neutron in 2s_1/2 and 1d_5/2 orbitals, under the assumption of ²³O having J^π=1/2⁺ or 5/2⁺.