The spectral analysis of X-rays emitted by a sample after irradiation is both a powerful qualitative and qualitative analytical technique. It is based on the following phenomenon: an atom relaxes after excitation by emitting X-ray radiations at specific wavelengths, which reveal the identity of the emitting species.
For this spectral analysis, Saint-Gobain Crystals supplies two key components:
A monochromating crystal is used to disperse the various spectral components of the beam emitted by the sample. A detector is used to measure the intensity of spectral lines singled out by the monochromator.
X-Ray Monochromator crystals can be supplied in the two following shapes:
The Johann Geometry (semi-focusing)
A thin plate is produced by one of the two following methods:
The thin plate is then curved cylindrically and glued to a curved holder with approximate focusing.
The Johansson Geometry (exact focusing geometry)
Two different types of Johansson configurations, theoretically leading to perfect focusing, are considered:
The thin plate is either curved cylindrically, glued to a curved holder and one face is machined (single machining Johansson) or both faces are machined and then glued to a curved holder (double machining Johansson).
We will offer the best technique according to the type of crystal, its dimensions and the radius of Rowland circle to be achieved.
Other types of curvature may be investigated on request. Following designs can be made: Logarithmic spiral, Elliptical, Conical, Parabolic, Sphere, Toroidal
Manufacturing capabilities strongly depend upon the crystal nature, dimensions as well as the curvature radii.
A monochromating crystal behaves in X-ray spectrometry as does a diffraction grating in optics. When rotated with respect to the incident polychromatic beam (see figure), it will diffract the spectral component along with direction to satisfy Bragg’s law, namely: 2d sin θ = n λ 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 can be found in Monochromator Crystal Properties along with the usual surface finish, 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.
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 combines a Nal(TI) or Lanthanum Bromide scintillator directly coupled to a photomultiplier with a low absorbing MIB or beryllium entrance window.
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|Crystal||Lithium fluoride||Quartz||Indium Antimonide||Silicon||Germanium||Pentaerythritol PET||Ammonium Dihydrogen Phosphate ADP||Beryl||Acid Phthalates||Crystal|
|Thallium TIAP||Rubidium RbAP||Potassium KAP||Cesium CsAP|
|Crystal system||Cubic||Hexagonal||Cubic||Cubic||Cubic||Quadratic||Quadratic||Hexagonal||Orthorhombic||Orthorhombic||Orthorhombic||Orthorhombic||Crystal System|
|Reflecting planes orientations||(200)||(220)||(420)||(1011)||(1010)||(111)||(111)||(220)||(111)||(220)||(002)||(101)||(1010)||(001)||(001)||(001)||(001)||Reflecting planes orientations|
|2d in Å||4.027||2.848||1.801||6.684||8.514||7.480||6.271||3.840||6.532||4.000||8.740||10.648||15.950||25.900||26.120||26.640||26.650||2d in Å|
|Usual surface finish||Cleaved or Treated||Treated||Treated||Polished||Polished||Polished||Polished||Polished||Polished||Polished||Cleaved or Treated||Polished or Treated||Polished||Cleaved||Cleaved||Cleaved||Cleaved||Usual surface finish|
Mo, Fe, Ti
|Mo, Fe||Mo||Cu||Cu||Si||Cu||Cu||Cu||Cu||Al, Si||Mg||Mg||Na, Mg||Na||Na||Na||Calibration elements|
|Common Applications||From K to heavy elements||Heavy elements
|As PET||Quantitative analysis of silicon||Extinction of even order spectral lines||Mg||Na and following elements||F to Al||Na to Al, up to F in emission probes||Na to Al, up to F in emission probes||Na to Al, up to F in emission probes||Common applications|