Crystals for X-Ray Spectrometry

Monochromators

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

Two Key Components for Spectral Analysis

  • Monochromating Crystal is used to disperse the various spectral components of the beam emitted by the sample.

    Supplying a range of crystals: 
    LiF, Quartz or SiO2, InSb, Si, Ge, PET, ADP, Beryl, TlAP, RbAP, KAP, and CsAP  

  • Radiation Scintillation Detector used to measure the intensity of spectral lines singled out by the monochromator.
    A radiation detector in order to measure the intensity of the various spectral lines as singled out by the monochromator. The detector combining Nal(TI) or LaBr3, coupled to a light-sensing device with a low energy entrance window.

Geometry

X-Ray Monochromator crystals can be supplied in the two following shapes:

  • Flat plates – supplied unmounted or mounted onto holders suitable for industrial X-ray fluorescence spectrometers. The standard orientation accuracy provided is 10 minutes. On special request, a one-minute accuracy can be ensured.
  • Curved plates – supplied on tailor-made holders for use in instruments such as microprobes, scanning electron microscopes, Synchrotrons, XFEL, Plasma Physics. Two main types of focusing configurations may be considered: The Johann geometry and The Johansson geometry.

3D curved crystal optics can be provided according to your special finishing requirements: spherical, toroidal, ellipsoidal, conical, others (please inquire)

The Johann geometry (semi-focusing)

A thin plate is produced by one of the two following methods:

  • Cleavage for LiF (200), PET, TIAP, 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 thin plate is then curved cylindrically and glued to a curved holder with approximate focusing.

curvature reticular planes
 
The Johansson Geometry (exact focusing geometry)

Two different types of Johansson configurations, theoretically leading to perfect focusing, are considered:

  • Single machining Johansson

  • Double machining Johansson

The illustrations represent the fabrication steps of Johansson plates in both techniques.

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. For example, curvatures on holders shaped as spiral, elliptic, parabolic, even spheric designs, interesting in plasma or synchrotron radiation study and astrophysics.

Manufacturing capabilities strongly depend upon the crystal nature, dimensions as well as curvature radii. 

single machining curvatures
double machining curvature
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Application

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 (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 setup.

An X-ray spectrometer basically consists of:

An excitation source 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 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.

X-Ray Spectrometer Illustration
 
Monochromator Crystal Properties

Scroll right for additional crystals →

Crystal Lithium fluoride Quartz Indium Antimonide Silicon Germanium Pentaerythritol PET Ammonium Dihydrogen Phosphate ADP Beryl Acid Phthalates
  Thallium TIAP Rubidium RbAP Potassium KAP Cesium CsAP

Chemical Formula

LiF SiO2 InSb Si Ge C(CH2OH2) NH4H2PO4 3BeO,Al2O36SiO2 CO2HC6H4CO2TI CO2HC6H4CO2Rb CO2HC6H4CO2K CO2HC6H4CO2Cs
Crystal system Cubic Hexagonal Cubic Cubic Cubic Quadratic Quadratic Hexagonal Orthorhombic
Parameters

 

a................Å.... 4.027 4.913 6.48 5.431 5.658 6.16 7.530 9.21 6.63 6.55 6.46 6.580
b................Å....   4.913       6.16 7.530 9.21 10.54 10.02 9.61 10.752
c................Å.... 5.405 8.74 7.542 9.17 12.95 13.06 13.33 12.825
ß......................                
Reflecting planes orientations (200) (220) (420) (1011) (1010) (111) (111) (220) (111) (220) (002) (101) (1010) (001) (001) (001) (001)
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
Usual surface finish Cleaved or Treated Treated Treated Polished Polished Polished Polished Cleaved or Treated Polished or Treated Polished Cleaved
Reflectivity Intense Intense Average Good Good Intense Intense Average Intense Intense Average Average Intense Intense Good Good
Calibration elements

Mo, Fe, Ti

Mo, Fe Mo Cu Cu Si Cu Cu Cu Cu Al, Si Mg Mg Na, Mg Na Na Na
Common Applications From K to heavy elements Heavy elements
Lines splitting
As
Ge (111)
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

 

Brochures
XRay-Crystals-Spectroscopy.pdf

Crystals for X-Ray Spectrometry; LiF, Quartz or SiO2, InSb, Si, Ge, PET, ADP, Beryl, TlAP, RbAP, KAP and CsAP

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XRay-Crystals-Spectroscopy