Comparison of Amptek X-ray tube with Fe55 radionuclide source

The figures below show spectra obtained with the same electronics (in this case the X-123-FastSDD) and the same settings, using the isotope 55 Fe and an X-ray tube to excite a pure manganese (Mn) target.

In all three cases the energy resolution was the same and was 131.5 eV. But many other characteristics were different. The figures show that the noise level is much worse for the X-ray tube, also when analyzing the target with the tube, several additional spectral lines appear and a difference in the ratios of the manganese lines (K α and K β ) is observed.

The K α /K β ratios are different due to mutual absorption. The X-rays from the tube penetrate much deeper into the manganese target. The K α rays have a shorter absorption length, so some of them are absorbed by the target, reducing the K α /K β ratio .

The noise level varies because the tube also produces bremsstrahlung, and the intensity of this radiation is much greater than the detector noise level. It also depends on the value of the voltage applied to the tube, the focusing geometry, etc. In addition, collimators, filters, and shielding are required to use the tube. All this contributes to the recorded spectrum and noise level.

Thus, for pure X-ray fluorescence analysis (especially for low element concentrations), the use of radioactive sources is preferable. In addition, very often the parts of the X-ray tubes vibrate, producing acoustic and ultrasonic signals, to which many detectors are very sensitive - hence another form of unnecessary interference.