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

IV. X-Ray Production Sources
IV.1 X-Ray Tubes
A variety of radiation sources of sufficient energy, emitting ether particles, γ-rays, or Xrays,
are potential candidates as sources for exciting the elements of interest in a sample to
emit characteristic radiation. The use of sample excitation by electrons is used in electron
probe micro-analysis (EPMA), and excitation by charged particles, like protons, is achieved in
articles-induced X-ray emission (PIXE). Most XRF analyzers have an X-ray tube for sample
excitation.
All modern X-ray tubes owe their existence to Coolidge’s hot-cathode X-ray tube
(Coolidge 1913). It consists essentially of a vacuum sealed glass tube containing a tungsten
filament for the production for electrons, an anode and a beryllium window. From variety of
modifications, two geometries have emerged as the most suitable for all practical purposes:
the end-window tube and the side-window tube, both having their own merits and limitations.
The general requirements are as follows:
1. Sufficient photon flux over a wide spectral range, with increasing emphasis on the intensity
of the low-energy continuum. The actual intense interest in low-Z element analysis
certainly activated research in this direction.
2. Good stability of the photon flux (< 0.1 % at least). Short-term stability is an absolute
requirement for obtaining acceptable precision.
3. Tunable tube potential allowing the creation of the most effective excitation conditions for
each element, because the intensity of the analyte lines varies considerably with excitation
conditions.
4. Freedom from two many interfering lines from the characteristic spectrum of the anode
(scatter peaks).
All X-ray tubes work on the same principle: accelerating electrons in an electrical field
and decelerated them in a suitable anode material. The region of the electron beam in which
this takes place must be evacuated in order to prevent collisions with gas molecules. Hence
there is a vacuum within housing. The X-rays escape from the housing at a special point that
is particularly transparent with a thin beryllium window.
An X-ray tube emits the characteristic radiation of the anode material, in addition to the
Bremsstrahlung radiation; a typical spectrum obtained with an X-ray tube of Rh anode
material is shown in Figure I.14.
The main differences between tube types are in the polarity of the anode and cathode
and the arrangement of the exit window.