Radionuclides as Radiation Sources in X-ray Fluorescence

The application of radionuclides as radiation sources in X-ray fluorescence analysis is illustrated in Fig.17.5. The X rays or y rays emitted by a radionuclide are absorbed in the sample and the X rays emitted by the sample are measured by means of a semiconductor in combination with a multichannel analyser. Quantitative evaluation of the spectra is possible by use of suitable standards. In comparison with excitation by means of X-ray tubes, the main advantages of radionuclides are 

The detection limits obtained by excitation with radionuclides are of the order of several pg/g. Appreciably higher intensities and lower detection limits are, in general, achieved by X-ray tubes. Monoenergetic radiation may also be obtained with X-ray tubes in combination with a secondary target, and the intensity of the secondary radiation is of the same order as that obtained with radionuclide sources. 

Radionuclides suitable for use as X-ray sources for XFA are listed in Table 17.6. Cd is applied most frequently. The use of y emitters such as Co, Ba or Ir offers the possibility of exciting heavy elements, for example Au, Pb, Th and U, to emit K radiation with high yield. Furthermore, heavy elements can also be measured in thick samples or within tubes or vessels, due to the high energy of their K radiation. These are distinct advantages in comparison with X-ray tubes. 

The independence from high-voltage supply and the small amounts of technical equipment required allow construction of mobile units for multielement analysis of mineralogical or geological samples in the field. 

Radionuclide Half-life Decay mode Energy of the emission lines used [keV] 

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