  Superconducting material parameters of the Nb film coating on the
Quarter-Wave Resonator (QWR) for the HIE-ISOLDE project were studied by fitting
experimental results with the Mattis-Bardeen theory. We pointed out a strong
correlation among fitted estimators of material parameters in the BCS theory,
and proposed a procedure to remove the correlation by simultaneously fitting
the surface resistance and effective penetration depth. Unlike previous
studies, no literature values were assumed in the fitting. As surface
resistance and penetration depth had a similar dependence on coherence length
and mean free path, the correlation between these two parameters could not be
eliminated by this fitting. The upper critical field measured by SQUID
magnetometry showed complementary constraint to the RF result, and this allowed
all the material parameters to be determined.
