An analysis of the I x-ray spectra, induced by -50 keV electron impact and recorded with a curved crystal spectrometer, has been performed based on a calculation model that takes into account all physical events taking place during the initial ionization and inner-vacancy processes. The L-subshell fluorescence yields co&, co&, and co3 for ten elements with atomic number Z =73-83 were obtained. They are all in good agreement with recent measurements but display an appreciable disagreement with the semiempirically compiled data for the elements around Z=80. These experimental values of co& are approximately 20% larger than the compiled data and 55% larger than the latest theoretical calculation for the elements with Z =80.

The L x-ray spectra induced by -50-keV electron impact on metallic targets of uranium and thorium have been analyzed and the relative fluorescence yields of the L&, L2, and L3 subshell of the two elements, cu2/~3 and co&/~"have been derived. The present values of co2/~3 and co2 are notably different