A delta-function-shaped Sb doping spike in Si is prepared by deposition of Sb on a Si(001) surface followed by lowtemperature molecular beam epitaxy of Si. The depth profile of the Sb atoms is measured using high-resolution Rutherford backscattering spectroscopy, yielding a depth resolution of 0.3nm. The observed profile shows two peaks conesponding to the delta-doped layer (of width 0.5nm) and Sb atoms on the surface. The latter are due to surface segregation of Sb atoms during the growth of the Si cap layer. The surface segregation rate is derived from the observed results at temperatures 70-280°C. It is larger than the value extrapolated from high- temperature (>400°C) data by several orders of magnitude and shows a very weak temperature dependence as compared to the high-temperature data. These features indicate a new surface segregation mechanism at low temperature. A mechanism for this anomalous segregation is discussed.