When the channeling-effect technique is used to determine the lattice location of an impurity which is not completely substitutional, quantitative interpretation of the results requires knowledge of the interaction yield between a channeled beam and an interstitial atom. We have investigated this problem for Yb implanted into silicon. Along the <110> direction, a peak of almost a factor of two is observed in backscattering yield from the Yb atoms, using a 1-MeV He beam. The height and angular width of the peak is satisfactorily interpreted in terms of flux-peaking of the channeled beam in the central region of the <110> channels.