X-Ray Fluorescence Analytical Techniques - CNSTN : Centre ...

II.1 Excitation Mode
II.1.1 Direct Tube Excitation
Because of the simplicity of the instrument and the availability of a high photon output
flux by using direct tube excitation, the X-ray fluorescence spectrometer equipped with an Xray
tube as direct excitation source is gaining more and more attention from manufactures as
well as from analytical chemists. The spectrometer is more compact and cheaper compared to
secondary target systems. Of course, the drawback is still the less flexible selection of
excitation energy. However, by using an appropriate filter between tube and sample, one can
obtain an optimal excitation. The understanding of the process of continuum excitation and
the possibility to obtain a good estimate of the continuum excitation spectrum originating
from the tube has minimized the problems associated with quantization, so that very
satisfactory quantitative analysis can be carried out. The most popular X-ray tube used in
direct excitation ED spectrometer is the side window tube for reasons of simplicity and safety.
With direct tube excitation, low powered X-ray tubes (< 100 W) can be used. These air cooled
tubes are very compact, less expensive, and only require compact, light, inexpensive, highly
regulated solid state power supplies. In a WD spectrometer, on the other hand, high-power
tubes (3-4 kW) are essential to compensate for the losses in the crystal and collimator. With
the low-power tubes used in ED spectrometer, better excitation of light elements (i.e. low-Z
element), analysis of smaller samples, small spot analysis, and compact systems can be
obtained. The use of X-ray tubes with a multi-element anode having a thin layer of low-Z
element (e.g. Cr) sputtered onto a heavy element target (e.g. Mo) has been reported.
Optimized excitation can be obtained by operating the multi-element anode tube at different
voltages to switch between the excitation by the light element and the heavy element targets.
II.1.2 Secondary Target Excitation
The principle of secondary target excitation was developed to avoid the intense
Bremsstrahlung continuum from the X-ray tube by using a target between tube and sample
(Figure II.2).
Figure II.2: Schematic illustration of secondary target excitation.
The ratio of the intensity of the characteristic lines to that the continuum in secondary
target excitation is much higher than that in direct tube excitation because the continuum part
of the excitation spectrum of the secondary target is generated only by scattering. One can
excite various elements efficiently by selecting a secondary target that has characteristic lines