[en] The energy spectra of Ne⁺ ions backscattered from a nickel surface were measured for chosen values of incidence angles and with two different target arrangements. It was found that the ratio of isotope components of the quasi-single-scattering peak reaches a maximum at some incidence angle. The maximum is attributed to the fact that ions doubly scattered at the atoms with smaller masses can have the same energy and exit angle as ions singly scattered at the atoms with larger masses. From geometrical considerations, the mean distance between two atoms of the topmost atomic layer can be found. This method may be used also for metallic alloys at low temperatures. (author)