Relative line intensities of L1, L2 and L3 sub-shell X-rays were measured for Er, Tm, Yb, Hf, Ta, W, Pt and Au. The L-shell X-ray spectra were recorded by exciting pure element samples (eight cases) and oxide samples (two cases) with approximately 17-keV exciting radiation from a filtered X-ray tube source, and measuring the fluorescence spectra with a silicon drift detector. The spectra were carefully fitted to determine line energies and intensities, accounting for Lorentzian line broadening, incomplete charge collection and escape effects. A Monte Carlo approach was used to calculate attenuation and detector efficiency corrections. We report up to 15 line intensity ratios for each element and compare these to Scofield's theoretical predictions and Elam's extrapolated experimental database. Our measured relative line intensities agree best with Elam's data, but overall we find significant discrepancies with previously reported results. For the element Ta, we also find significant errors in the accepted L-shell line energies in the widely used National Institute of Standard and Technology (NIST) database. Our results highlight the need for an experimental and theoretical re-evaluation of L-shell intensity databases to support high-accuracy X-ray analysis methods such as X-ray fluorescence and particle-induced X-ray emission.

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