High-resolution X-Ray Spectroscopy

 
For the detection of x–rays or gamma–rays energy dispersive or wavelength dispersive instruments can be used. Gas proportional counters, scintillation detectors or semiconductor diodes belong to the first category while crystal spectrometers belong to the second one. Crystal spectrometers are characterized by a high resolving power of the order of 10-4 which  is  much  better  than the one of energy–dispersive detectors (~10-1–10-2), a high precision and a good sensitivity. In contrast to that the efficiency of the wavelength dispersive detectors is much smaller than the one of energy dispersive detectors, mainly because of the small solid angles characterizing crystal spectrometers.
 
Many different types of crystal spectrometers have been developed until now. Flat crystal spectrometers provide the highest resolution and precision but their luminosity is extremely weak. To increase the solid angle, the crystal plate can be bent, cylindrically or spherically. Bending the crystal results in luminosity enhancements as big as 102–103 with, however, some losses in the resolution, due to the geometrical aberrations related to imperfections in the curvature of the crystal plate and the quasi–mosaicity induced in the crystal lamina by the bending torque.
 
For both flat and curved crystal spectrometers, Laue type (also named transmission type) and Bragg type (reflection type) instruments do exist. In Bragg type spectrometers the diffraction planes are parallel to the surface of the crystal exposed to the incoming radiation, whereas for Laue type spectrometers the diffraction planes are normal to the exposed surface. In the Laue case the photons have thus to travel through the crystal thickness to reach the detector. As a result, the intensity of the diffracted radiation is attenuated, being partly absorbed by the crystal.
 
This absorption grows drastically when the photon energy decreases. For instance, for a 1 mm thick quartz plate, the absorption amounts to 4.0% at 100 keV, 46.3% at 20 keV and 99.3% at 10 keV! For this reason, transmission type crystal spectrometers cannot be employed for photon energies below about 10 keV. On the other hand, due to geometrical constraints related to the spectrometer design, the minimum Bragg angles sustainable by reflection type crystal spectrometers cannot be below about 20 deg. For standard crystals such as quartz, silicon and germanium with typical spacing lattice constants 2d = 3–5 Å, Bragg angles of 20 deg. correspond to photon energies E = 7–12 keV so that Bragg type crystal spectrometers are usually employed for measuring photons below about 10 keV. In that sense Bragg type and Laue type crystal spectrometers can be considered as complementary high–resolution instruments.

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