We report direct, simultaneous measurements of the London penetration depth (${\lambda }_L$) and Bardeen-Cooper-Schrieffer coherence length (${\xi }_0$) in oxygen-doped niobium, with impurity concentrations spanning the “clean” to “dirty” limits. Two depth-resolved techniques—low-energy muon spin spectroscopy and secondary-ion mass spectrometry—were used to quantify the element's Meissner screening profiles, analyzed within a framework that accounts for nonlocal electrodynamics. The analysis indicates intrinsic length scales of ${\lambda }_L=29.1\left(10\right)\mathrm{nm}$ and ${\xi }_0=39.9\left(25\right)\mathrm{nm}$, corresponding to a Ginzburg-Landau parameter $\kappa =0.70\left(5\right)$. The obtained ${\lambda }_L$ and $\kappa$ values, accurately quantified at the nanoscale, are smaller than those commonly used in applications and modeling, and indicate that clean niobium lies at the boundary between type-I and type-II superconductivity, supporting the contemporary view that its intrinsic state may be type I.

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• Coherence length
• Impurities in superconductors
• Meissner effect
• Penetration depth
• Superconducting phase transition
• Elemental superconductors
• Type-I superconductors
• Type-II superconductors
• Muon spin relaxation & rotation
• Secondary ion mass spectrometry