This Letter reports on the ${}^1\mathrm{H}({}^{28}\mathrm{Ne},{}^{28}\mathrm{Ne})$ and ${}^1\mathrm{H}({}^{28}\mathrm{Ne},{}^{27}\mathrm{Ne})$ reactions studied at intermediate energy using a liquid hydrogen target. From the cross section populating the first $2^{+}$ excited state of ${}^{28}\mathrm{Ne}$, and using the previously determined $B(E2)$ value, the neutron quadrupole transition matrix element has been calculated to be $M_n=13.8\pm 3.7{\mathrm{fm}}^2$. In the neutron knockout reaction, two low-lying excited states were populated in ${}^{27}\mathrm{Ne}$. Only one of them can be interpreted by the $sd$ shell model while the additional state may intrude from the $fp$ shell. These experimental observations are consistent with the presence of $fp$ shell configurations at low excitation energy in ${}^{27,28}\mathrm{Ne}$ nuclei caused by a vanishing $N=20$ shell gap at $Z=10$.

• Received 13 July 2005

DOI:https://doi.org/10.1103/PhysRevLett.96.182501