We report on new observations of positron stopping in a series of Al foils mounted on glass backplates for incident energiesEin the range 10–50 keV. The measured median penetration depths forE≳30 keV are found to differ significantly from values derived from the empirical power law model of A. P. Mills, Jr. and R. J. Wilson [Phys. Rev. A26, 490 (1982)]. The associated disparity between the measured implantation profiles and their analytic representations, and the consequent implications for defect profiling analysis, are discussed.