The spectral analysis of X-rays emitted by a sample after irradiation is both a powerful qualitative and quantitative analytical technique. It is based on the following phenomenon -- an atom relaxes after excitation by emitting X-ray radiation at specific wavelengths, which reveal the identity of the emitting species.
For this spectral analysis, Saint-Gobain Crystals supplies two key components:
- The monochromating crystals
- The scintillation detectors
An X-ray spectrometer basically consists of:
- An excitation source which may be
- Either a primary X radiation, in which case one refers to X-ray fluorescence spectrometry.
- Or an electron beam, inducing a so-called direct emission, used in
microprobes and scanning electron microscopes.
A monochromating crystal which is used to disperse the various spectral components of the incident beam.
A detector in order to measure the intensity of the various spectral lines as singled out by the monochromator.
The detector offered by Saint-Gobain Crystals & Detectors is an X Scintibloc® which combines an Nal(TI) scintillator directly coupled to a photomultiplier with a low absorbing MIB or beryllium entrance window (see brochure "Scintillation detectors" or refer to our scintillation website at www.detectors.saint-gobain.com).
MONOCHROMATING CRYSTALS
A monochromating crystal behaves in X-ray spectrometry as does a diffraction grating in optics. When rotated with respect to the incident polychromatic beam, it will diffract the spectral component ë along the è direction to satisfy Bragg 's law, namely:
2d sinU = nl
where integer n refers to the diffraction order.
Hence, the most important characteristic of a monochromating crystal is the double atomic spacing 2d,which gives the largest wavelength to be diffracted.
The range of monochromators supplied by Saint-Gobain Crystals & Detectors can be found in the table below, along with the usual surface finish, corresponding within our control means, to the best intensity-resolution compromise. The optimum depends on each specific case and strongly reflects the nature of the set-up.
PRODUCTS AVAILABLE
Monochromating crystals can be supplied in the two following shapes:
- Flat
- Curved onto a holder
Flat plates can be supplied unmounted or mounted into holders suitable for industrial X-ray fluorescence spectrometers. Other types of holders may also be supplied on request. The standard orientation accuracy provided is 10 minutes. On special request a one-minute accuracy can be ensured.
The curved plates used in such instruments as microprobes and scanning electron microscopes are always supplied on tailor-made holders.
MAIN TYPES OF FOCUSING CONFIGURATIONS
Two main types of focusing configurations may be considered:
- The Johann Geometry
- The Johansson Geometry
The Johann Geometry
A thin plate, produced by one of the two following methods:
- Cleavage for LiF (200), PET, TlAP, RbAP, KAP
- Machining for other materials, is further cylindrically curved and glued upon a holder of curvature radius 2R. It is possible to show that a beam emitted by a source at S is approximately focused at F. The source and focus are both located on the so-called Rowland circle whose radius is R.
The Johansson Geometry
Two different types of Johansson configurations, theoretically leading to perfect focusing, are considered:
- Single machining Johansson
- Double machining Johansson
Saint-Gobain Crystals & Detectors will choose the most appropriate technique according to the type of crystal, its dimensions and the radius of Rowland circle to be achieved.
The orientation accuracy is better than 10 minutes, except for the Johann cleaved configuration, where it is 1 minute.
Other types of curvature may be investigated on request. For example, curvatures on holders shaped as a logarithmic spiral, elliptical, parabolic, even spheric designs, interesting in plasma or synchrotron radiation study and astrophysics.
Manufacturing capabilities strongly depend upon the crystal nature and dimensions as well as the curvature radii.