Crystals for X-Ray Spectrometry

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.

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. 

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.