Niobium is a reference dosimeter for the survey of neutron damage to nuclear reactor vessels. Its characterization is based on the activity of $^{93}$mNb resulting from the $^{93}$Nb(n,n') activation reaction. The decay of $^{93}$mNb results mainly in the emission of niobium K X-rays which are used to determine the activity of $^{93}$mNb. Direct measurement using X-ray spectrometry does not require any sample preparation, but fluorescence effects, due to impurities which are activated during irradiation, must be taken into account. Indeed, some of these radioactive impurities, in particular $^{182}$Ta, and other niobium isotopes remain present during the measurements and disturb the X-ray spectra. The fluorescence effect induces an additional X-ray emission from niobium to that due only to the decay of $^{93}$mNb, leading to an overestimation of the dosimeter activity. It is therefore necessary to assess the contribution of fluorescence effects to provide accurate values of $^{93}$mNb activity. Fluorescence correction factors were established by Monte Carlo simulation. The calculated fluorescence correction factors were validated by an experimental approach, using activated niobium dosimeters with different tantalum concentrations