Dependence of theKx-ray energy upon the mode of excitation

The energy of the Ta $K\alpha$ x rays emitted after electron capture of ${}_{}{}^{181}{}_{}{}^{}\mathrm{W}$ has been observed to be lower by 0.79±0.06 eV compared to the fluorescent Ta $K\alpha$ x rays. Similarly, it was found that the Ta $K\alpha$ x rays emitted after internal conversion following $\beta$ decay of ${}_{}{}^{181}{}_{}{}^{}\mathrm{Hf}$ were higher by 0.7±0.1 eV in energy compared to the same fluorescent Ta x rays. The observed effects cannot be explained by the known mechanisms of chemical shift, hyperfine effects, isomer shifts, and shake-off effects alone. Similar observations of the $K$-x-ray energy shifts in the electron-capture decays of ${}_{}{}^{153}{}_{}{}^{}\mathrm{Gd}$ and ${}_{}{}^{175}{}_{}{}^{}\mathrm{Hf}$ with the corresponding fluorescent x rays in Eu and Lu, respectively, can be accounted for by the hyperfine effects, isotope shifts, and chemical shifts.