Ruled grating

Unlike the case of diffraction of light by a ruled grating, the diffraction of x-rays by a crystalline solid leads to the observation that constructive interference (i.e., reflection) occurs only at the critical Bragg angles. When reflection does occur, it is stated that the plane in question is reflecting in the nth order, or that one observes nth order diffraction for that particular crystal plane. Therefore, one will observe an x-ray scattering response for every plane defined by a unique Miller index of (h k l). 

Fig. 1 Optical diagram of the prototype system model. The drawing is not to scale and is not to be considered an optical ray diagram. The principal dispersing element is a coarse echelle ruled grating 20 x 40 cm wide. Theoretical double-pass resolution at four normal slits is approximately 0.095 cm-1 actual achievable resolution is approximately 0.009 cm-1.

While ruled gratings are separated into different families according to their blaze angle, interference gratings are categorized by using the modulation frequency, a, defined as ... 

Figure 1. Experimental setup of the pulse shaper AOM, Ge acousto-optic modulator AWG, arbitrary waveform generator CM, cylindrical mirror grating, 200-g/mm ruled grating. Pulses are dispersed by the grating, focused at a CM, shaped at the AOM, collimated at a CM, and recombined at a grating.

From the wave-length A of X-rays measured with a ruled grating and the observed angles 6 at whidbi these same X-rays are reflected from a ciystal one can calculate by Bragg s relation A = 2 i sin 0 the value of d the grating space of the aystal. For caldte at 20 °C it is found that... 

Ruled gratings have sawtooth-shaped grooves and diffract 40 to 80% into the first order on one side of the incident beam. The rest of the light is refleeted in the zero order or diffracted into higher orders and into the first order on the other side of the incident beam. This can result in a number of unpleasant effects. 

Ruled gratings are mainly used in AA instruments. Holographic gratings have less irregularities and therefore lower stray radiation, but this is not so important in AAS as it is, for example, in plasma-AES. From the ruled master gratings (often aluminium) copies can be prepared, which are less expensive. 

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