Standard Energy Dispersive XRay Fluorescence Analysis

The XRF analysis system consists of the excitation source and the X-ray spectrometer. In standard XRF, the elemental analysis is based on measurements of the XRF spectrum from the excited sample. XRF systems are classified according to the types of excitation sources and the types of detectors installed. The XRF mode in which the count rate and the energy (distribution) of characteristic X-rays are measured using an energy-dispersive X-ray spectrometer comprising the X-ray detector, electronic components and a multichannel analyzer (MCA) is known as EDXRF.

Excitation can be performed using almost monochromatic radioisotope sources, such as Fe-55 (activity, 20 mCi), Cd-109 (25 mCi) and Am-241 (25 mCi) or polychromatic radiation from X-ray tubes. X-ray tubes can be air cooled (low power, 50 W) or water cooled (high power, 2 kW), with different anodes, such as Ag, Mo and Cr. XRF experiments are designed according to different geometries (Figs. 6.2-6.4). When thick specimens are excited by the continuous X-ray spectrum from an X-ray tube, the sensitivity of the method is lowered because of the relatively high background from the scattered continuous radiation from the sample, or its substrate, in the spectral region of the fluorescent radiation (Fig. 6.5a). A secondary target irradiation geometry can be used to partly monochromatize the X-ray tube radiation, and thus to decrease this background scattering (Jaklevic and Giauque 1993) (Fig. 6.4).

Proportional and/or semiconductor X-ray detectors are available for the abovementioned EDXRF systems. The choice of detector and excitation source is very important. In cheaper spectrometers, a radioactive source and a proportional sample

detector

Fig. 6.2 X-ray fluorescence (XRF) geometry radioisotope source detector

Fig. 6.2 X-ray fluorescence (XRF) geometry

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