  Nitrogen doping study on niobium (Nb) samples used for the fabrication of the
superconducting radio frequency (SRF) cavities has been carried out. The
samples' surface treatment was attempted to replicate that of the SRF cavities,
which includes heavy electropolishing (EP), nitrogen doping and the subsequent
EP with different amounts of material removal. The magnetization curves of both
the doped and the un-doped samples have been measured, from which the field of
the first flux penetration and the upper critical field were extracted. The
thermodynamic critical field, the the Ginzburg-Landau parameter, the
penetration depth, the coherence length and the superheating field were
calculated from the determined reversible magnetization curves. The
superheating field of the doped samples is obviously smaller than that of the
un-doped samples. Direct observation of hydrides precipitation on both the
doped and the un-doped samples has been carried out using Scanning Electron
Microscope (SEM) with a cold stand at 80K, which showed that the amount of
hydrides precipitation on the doped samples was reduced to varying degrees with
different amounts of material removal. Under our doping recipe, the doped
sample with subsequent EP material removal of 7-9 microns corresponds to the
minimum nitrogen concentration that can effectively reduce the amounts of
hydrides precipitation.
