  The production of leading neutrons, where the neutron carries a large
fraction x_L of the incoming proton's longitudinal momentum, is studied in
deep-inelastic positron-proton scattering at HERA. The data were taken with the
H1 detector in the years 2006 and 2007 and correspond to an integrated
luminosity of 122 pb^{-1}. The semi-inclusive cross section is measured in the
phase space defined by the photon virtuality 6 < Q^2 < 100 GeV^2, Bjorken
scaling variable 1.5x10^{-4} < x < 3x10^{-2}, longitudinal momentum fraction
0.32 < x_L < 0.95 and neutron transverse momentum p_T < 0.2 GeV. The leading
neutron structure function, F_2^{LN(3)}(Q^2,x,x_L), and the fraction of
deep-inelastic scattering events containing a leading neutron are studied as a
function of Q^2, x and x_L. Assuming that the pion exchange mechanism dominates
leading neutron production, the data provide constraints on the shape of the
pion structure function.
