Direct quantitative observation of a negative-parity intruder state in $^{28}$Ne

We present results for the neutron knockout from a 70 MeV beam of $^{28}$Ne interacting with a $^9$Be target. The projectile residues were detected in the high-resolution S-800 spectrograph and excited levels were identified via gamma-ray coincidences observed with the array of segmented germanium detectors SeGA. Events not coincident with gamma rays amounted to 38\%. These were predominantly associated with a momentum distribution with a clear $l$=3 shape corresponding to a spectroscopic factor of close to unity, so that, on the average, at least one of the 18 neutrons in the projectile must be in an $f_{7/2}$ orbital. This confirms for the first time the strong presence of $f$-shell intruders in the ground state predicted for this nucleus in the Monte-Carlo shell-model calculations by Utsuno et al., Phys. Rev. C 60, 054315 (1999).