The fraction of positrons annihilated in the Kapton support of a positron source has been measured as a function of the atomic number, Z, of the sample studied. The data have been filtered by both exponential and logarithmic laws obtained from a backscattering model. Two formulas are given to correct the source contribution in a positron lifetime experiment. The results shown that the exponential fit is satisfactory for low atomic numbers while a logarithmic fit is better for Z>30.