@misc{oai:repo.qst.go.jp:00076807,
 author = {Izumoto, Yukie and Takamura, Kodai and Matsuyama, Tsugufumi and Nagai, Hiroki and Sakai, Yasuhiro and Yoshiyuki, Oguri and Hiroshi.Yoshii and Izumoto, Yukie and Takamura, Kodai and Matsuyama, Tsugufumi and Yoshiyuki, Oguri and Yoshii, Hiroshi},
 month = {Jun},
 note = {It is known that the peak of Uranium (U) Lα line (13.6 keV) overlaps with that of Rubidium (Rb) Kα line (13.4 keV) in XRF spectrum. Rb is, unfortunately, one of the abundant elements in natural environment. Our previous studies [1,2] showed that multiple Gaussian fitting could separate these lines when the sample contains comparable amounts of U and Rb. On the other hand, if the amount of Rb is much larger than that of U, the fitting may give incorrect results. In this case, U quantification using U Lβ lines (mainly U Lβ1 (17.2 keV) and Lβ2 (16.4 keV) lines) might be useful. 
In the present study, a new portable TXRF device was built to verify this idea. We prepared the new device by modifying a 200TX portable TXRF spectrometer (Ourstex Co., Ltd.), which was originally equipped with an X-ray tube with a tungsten (W) anode [1]. The anode material was changed to silver (Ag), whereas the other parts including the incident X-ray collimating system stayed as before. Since Ag has lower atomic number than W, background signal derived from bremsstrahlung X-ray is lower in our new device than that in an original 200TX. Moreover, strong Ag Kα line can excite L3 and L2 shell electrons of U more efficiently. 
Sample solutions were prepared by mixing a multi-element standard solution (XSTC-1407, Spex CertiPrep Inc.), which contains same concentrations of cobalt, cesium, cupper, thorium (Th), and U with dilution series of a Rb standard solution. In the measured TXRF spectra of the sample solution with low rubidium concentration, the U Lα peak can be clearly found and the intensity of the peak was much higher than that of U Lβ lines. Therefore, if the Rb concentration is low, quantification using the U Lα peak is favorable. On the other hand, for sample solutions with high Rb concentrations, the peak of U Lα was buried in that of Rb Kα. In this case, Gaussian fitting using the peaks of U Lβ lines along with that of the U Lα line was useful to U quantification. It should be noted that, however, the peak of U Lβ2 line overlapped with that of Th Lβ1 line (16.4 keV) originating from thorium contained in XSTC-1407. Since concentration of Th in environmental samples is usually similar to that of U, quantification based on U Lβ2 line can be less accurate than that using U Lβ1 line. 
This research has been conducted as regulatory supporting research funded by Secretariat of Nuclear Regulation Authority (S/NRA/R), Japan.
[1] T. Matsuyama et al., J. Nucl. Sci. Technol. 54, (2017), 940.
[2] T. Matsuyama et al., Spectrochim. Acta Part B 149, (2018), 35., 18th International Conference on Total Reflection x-ray Fluorescence Analysis and Related Methods (TXRF2019)},
 title = {TXRF analysis of uranium in the presence of competing elements},
 year = {2019}
}