  The proton drip-line nucleus 17Ne is investigated experimentally in order to
determine its two-proton halo character. A fully exclusive measurement of the
17Ne(p,2p)16F->15O+p quasi-free one-proton knockout reaction has been performed
at GSI at around 500 MeV/nucleon beam energy. All particles resulting from the
scattering process have been detected. The relevant reconstructed quantities
are the angles of the two protons scattered in quasi-elastic kinematics, the
decay of 16F into 15O (including gamma decays from excited states) and a
proton, as well as the 15O+p relative-energy spectrum and the 16F momentum
distributions. The latter two quantities allow an independent and consistent
determination of the ratio of l=0 and l=2 motion of the valence protons in
17Ne. With a resulting relatively small l=0 component of only around 35(3)%, it
is concluded that 17Ne exhibits a rather modest halo character only. The
quantitative agreement of the two values deduced from the energy spectrum and
the momentum distributions supports the theoretical treatment of the
calculation of momentum distributions after quasi-free knockout reactions at
high energies by taking into account distortions based on the Glauber theory.
Moreover, the experimental data allow the separation of valence-proton knockout
and knockout from the 15O core. The latter process contributes with 11.8(3.1)
mb around 40% to the total proton-knockout cross section of 30.3(2.3) mb, which
explains previously reported contradicting conclusions derived from inclusive
cross sections.
