The recent need for measuring depth profiles for ultralow-energy B ion implants in Si has pushed the technique of secondary ion mass spectrometry (SIMS) into unprecedented degrees of high depth resolution. For such shallow implant distributions, it remained to be seen if the quantification procedures which have been used for determining deeper B in-depth distributions are accurate for these very shallow profiles. What is more, the B concentrations at the surface can be in the percentage range for implants of 1E15/cm(2) at energies below 1 keV. It has not been demonstrated that SIMS can be accurate in this high-concentration regime. In this article, we use the nuclear reaction B-11(p, alpha)Be-8 to confirm the accuracy of the implanted doses in low-energy B implants in Si. Our results indicate that the doses measured by SIMS are within 5% of those measured using nuclear reaction analysis.