The structure of a neutron-rich ${}^{25}\mathrm{F}$ nucleus is investigated by a quasifree ($p,2p$) knockout reaction at $270A\mathrm{MeV}$ in inverse kinematics. The sum of spectroscopic factors of $\pi 0{\mathrm{d}}_{5/2}$ orbital is found to be $1.0\pm 0.3$. However, the spectroscopic factor with residual ${}^{24}\mathrm{O}$ nucleus being in the ground state is found to be only $0.36\pm 0.13$, while those in the excited state is $0.65\pm 0.25$. The result shows that the ${}^{24}\mathrm{O}$ core of ${}^{25}\mathrm{F}$ nucleus significantly differs from a free ${}^{24}\mathrm{O}$ nucleus, and the core consists of $\sim 35\%$ ${{}^{24}\mathrm{O}}_{\mathrm{g}.\mathrm{s}.}$. and $\sim 65\%$ excited ${}^{24}\mathrm{O}$. The result may infer that the addition of the $0d_{5/2}$ proton considerably changes neutron structure in ${}^{25}\mathrm{F}$ from that in ${}^{24}\mathrm{O}$, which could be a possible mechanism responsible for the oxygen dripline anomaly.

• Received 25 October 2018
• Revised 15 December 2019
• Accepted 13 April 2020

DOI:https://doi.org/10.1103/PhysRevLett.124.212502