High-Luminosity Monochromators for X-Rays by B. Ya. Pines

The question of obtaining intense monochromatic beams of X-rays, both for the purposes of X-ray structural analysis and for various investigations in the physics of X-rays, has long interested researchers.

Originally, ordinary flat crystals were used for this purpose. Later, after X-ray spectrographs with a bent crystal of the Johann and Cauchois type[^1] had been invented, such crystals came into use. In this case, crystals bent cylindrically are usually employed; from them the divergent beam of X-rays is reflected and, after reflection, is transformed into a convergent one. However, in this arrangement a point source of X-rays is imaged, at the point of convergence of the reflected rays, as a line parallel to the axis of curvature of the crystal.

It is possible, however, with the aid of a single cylindrically bent crystal, to obtain such focusing in which the point is imaged as a point; for this the crystal must be bent in a direction perpendicular to that chosen in the preceding case. Then, from the divergent beam of X-rays incident on the crystal, only those rays are reflected which fall at one definite angle (the Bragg angle), so that after reflection the divergent beam is replaced by a parallel one. But because the crystal is bent, the line that would be obtained upon reflection from a flat crystal is replaced by a point. In other words, a crystal bent in this way brings the spectral line to a point. Such methods were developed by Hamos[^2] and by Doležek and Tayerle[^3]. However, the luminosity of such a method is considerably lower than in the preceding one. A somewhat different path was followed by the author of the present note,[^4] who attempted to increase the luminosity of an X-ray spectrograph with the aid of a flat crystal, placing it in the most advantageous way relative to the focus of the tube.

Finally, one may use two crystals, each bent separately along a cylindrical surface so that the axes of these surfaces are mutually perpendicular. This method gives convergence of the beam

rays to a point, but, because of the double reflection from two crystals, it will have low luminosity. A similar method has recently been developed by Kirkpatrick and Baez.[5] Thus, we are faced with a dilemma: a high-luminosity method does not give convergence of the beam at a single point, while methods that do give convergence at a single point are not sufficiently luminous. Meanwhile, for the purposes of X-ray structural analysis and for certain problems of X-ray physics it is very important to obtain convergence of the beam at a single point. To achieve this, B. Ya. Pines[6] proposed bending the crystal along the surface of a double curvature and carried out such bending of crystals. It must be said that along this path B. Ya. Pines had predecessors, but, in all likelihood, none of them succeeded in carrying out the bending as well as B. Ya. Pines did. B. Ya. Pines bends the crystal along the surface of a torus. The radius of curvature of the large circumference of this surface is equal to twice the radius of the focal circle, and the small radius of curvature depends on the wavelength of the radiation used and on the lattice constant of the crystal. It must be the larger, the larger the lattice constant and the larger the Bragg angle. Along such a surface, at high temperature, a rock-salt crystal is bent. Using crystals bent in this way, B. Ya. Pines obtained a very high intensity of monochromatized X-rays. This enabled him to reduce appreciably the exposure time when recording structural X-ray photographs in monochromatic light, in comparison with monochromators of other types. The advantage of the new type of monochromator is beyond doubt, and one should wish for its earliest dissemination in our laboratories.

It should be noted that, after B. Ya. Pines, an analogous proposal was made by Dumond.[7]

D. B. Gogoberidze

CITED LITERATURE

1. Borovskii and Blokhin, X-ray spectral analysis, 1939.
2. L. V. Hamos, Ann. d. Phys. (5), 17, 716 (1933).
3. Dolejsek et Tayerle, J. de Phys. et Rad. 9, 465 (1936).
4. D. B. Gogoberidze, ZhTF 9, 2048 (1939).
5. Kirkpatrik and Baez, A. Journ. Optic Soc. Am. 38, 766 (1948).
6. B. Ya. Pines, “High-luminosity monochromators for X-ray structural analysis.” Report at the Third Conference on the Application of X-rays in Industry, Leningrad, 1950. Collection of abstracts of reports, Publishing House of the Academy of Sciences of the USSR, 1950, p. 102. For a detailed communication see the Collection dedicated to the seventieth birthday of Academician A. F. Ioffe, p. 448.
7. Dumond, Rev. Sci. Inst. 21, 188 (1950).