Calculations are made of spatial moments ⟨zn⟩ for particles from a plane source brought to rest in an infinite medium of randomly located target atoms of several kinds. The relationships obtained are illustrated by the determination of the mean penetration ⟨z⟩ and the straggling parameter &OHgr;= (⟨z2⟩ − ⟨z⟩2)1/2/⟨z⟩ for kilovolt125Xe ions in WO3. Results compare favorably with published experimental data.