The accumulation of positrons in a two-dimensional layer of tin embedded in aluminum is examined by Doppler broadening spectroscopy (DBS). For this purpose samples are grown out of high purity materials consisting of a step-shaped layer (0.1–200nm) of tin on a substrate of aluminum and covered by an aluminum layer of constant thickness (200nm).The positron implantation profile is varied by different positron acceleration energies of up to 15keV. The pre-thermal implantation profile of the monoenergetic positron beam is examined since the effect of thermal positron diffusion is less significant at tin layers thicker than 50nm. At thin layers (<50nm), the positrons greatly accumulate either at the aluminum–tin interface or in the tin layer due to its higher positron affinity compared to aluminum. Thus a very high sensitivity of the measurement for low densities of tin is observed. Consequently from the experimental results, a sensitivity threshold for the detection of a low amount of tin in an aluminum matrix with DBS is determined. The DB results are compared to theory by an approximation for pre-thermal implantation in layered materials.