Compton Crystal, diffraction

On a more distant horizon, experimental gamma-ray astronomy has to find ways to further push the limits of resolution and sensitivity At energies above one MeV, Compton telescopes can provide angular resolutions of fractions of degrees and sensitivities of a several 10"" ph cm s . At energies below the MeV, tunable crystal diffraction telescopes can achieve sensitivities of a few times 10"" ph cm s and angular resolutions of the order of 15". 

Note also that balanced filters will not exclude Compton scattering, which differs in wavelength by only 0.05 A or less from the wavelength of the diffracted beam (Eq. 4-3) and which will therefore generally fall inside the pass band. And if the radiation forming the diffracted beam is, for example, Cu Aix, then neither a monochromating crystal nor balanced filters will exclude diffusely scattered Cu Kci caused, for example, by thermal vibration of the atoms. 

Absorption means diminution of coherent x-ray intensity in the crystal through inelastic processes such as atomic absorption and fluorescence, photoelectron emission, and Compton effect extinction means intensity diminution due to loss through diffraction by fortuitously oriented mosaic blocks. The simple extinction expression due to Darwin, given in Eq. (18), is only a rough approximation more accurate treatments will be mentioned in what follows. In Eq. (17) the absorption factor is expressed in terms of the linear absorption coefficient /inn (calculated from tabulated values of the elemental atomic or mass absorption coefficients, updated values of which will appear in Vol. IV of International Tables,2 the path length f, of the incident ray from the crystal surface to the point of diffraction r, and the path length t2 of the diffracted ray from that point to the crystal surface. 

The scattering of radiation, i,e, its diffusion in different directions, which has two components, one coherent with the incident radiation Thompson scattering) and the other incoherent with a longer wavelength Compton scattering). The term coherent indicates a precise relation between the phases of the incident and scattered wave. The coherent scattering by a periodic structure, hence by a crystal, is called diffraction. This may be elastic or inelastic (Section 3,1.1),... 

In the case of a crystalline phase in an amorphous matrix, a rough measure of their ratio can be obtained if the compositions are known. This is accomplished by summing the powder diffraction line intensities, subtracting the amorphous (background) intensity, and correcting for Compton-modified scattering. The fact that there is a continuous spectrum of order from well-crystallized to noncrystalline phases is an inherent limitation to the method. 

At the same time, a diffraction grating with a period of about 1 A has the nature that the interatomic distances in crystals are about this size. As the interatomic distances are approximately 10 m and the size of even the smallest crystal is 10 m (repetition is 10 in the majority of cases), the crystal can be considered infinite. If a beam of X-rays falls on a crystal, under the action of an electromagnetic wave the atoms electrons begin to oscillate and scatter secondary radiation of the same wavelength in all directions (compare with Compton-effect, Section 6.6). As the atoms in a crystal are ordered, these secondary waves are coherent and interfere this defines the diffraction effect. 

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