We report on the first measurement of the F2 structure function of the neutron from semi-inclusive scattering of electrons from deuterium, with low-momentum protons detected in the backward hemisphere. Restricting the momentum of the spectator protons to ≲ 100° MeV/c and their angles to ≳ 100° relative to the momentum transfer allows an interpretation of the process in terms of scattering from nearly on-shell neutrons. The Fn2 data collected cover the nucleon-resonance and deep-inelastic regions over a wide range of Bjorken x for 0.65 < Q2 < 4.52 GeV2, with uncertainties from nuclear corrections estimated to be less than a few percent. These measurements provide the first determination of the neutron to proton structure function ratio Fn2 /Fp2 at 0.2 ≲ x ≲ 0.8 with little uncertainty due to nuclear effects.

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Copyright © 2012 American Physical Society. This article first appeared in Physical Review Letters 108, no. 14 (April 6, 2012): 142001:1-6. doi:10.1103/PhysRevLett.108.142001.

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