We describe the development of a positron-based, non-destructive energy contamination monitor for use with low energy boron implantation. Using a simple model describing the effect of intrinsic electric fields on positron diffusion we show that for junction depths (X(j)) in the range of 50-500 nm, beam-based positron annihilation spectroscopy (PAS) has sensitivity to changes in X(j) of 10 nm. Corroborating experimental measurement of fully-formed p-n junctions fabricated by implantation of boron in n-type silicon at energies of 2, 10 and 25 keV, all to a dose of 1 x 10(14) cm(-2), are also described. Further, it is shown that PAS can distinguish B(+) implanted wafers which have been deliberately poisoned with higher energy implants, from those which received no contaminating implantation.