X-Ray Monochromators

Two main types of focusing configurations are: 

The Johann Geometry (semi-focusing)

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

• Cleavage
• Machining

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:

• Single machining Johansson
• Double machining Johansson

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. 

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 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.