On Scattered Rays in the Diffraction of X-Rays

J. Plimpton, On the Scattering of Rays in X-ray Diffraction. Phil. Mg. 42. p. 302. 1921.

The theory of the diffraction of X-rays assumes that the secondary scattered rays which produce the diffraction effect are caused by oscillations of the electrons of the molecules of the crystal, wholly dependent on the primary rays incident upon them, and therefore must be of the same “hardness” as the latter. However, all experiments carried out thus far in the direction of comparing the absorption of scattered rays with the absorption of primary rays indicate a greater softness of the secondary rays. This was attributed, as it were, to the selective ability of the molecules of crystals to scatter rays of greater wavelength more intensely, i.e. in the secondary beam the “softness” is more sharply expressed.

Analogous experiments using filters in the primary beam to obtain monochromatic rays, as well as with γ-rays incident on a crystal of light elements, nevertheless led to results showing greater absorbability of the secondary radiation.

Confirmation of the fact of equal absorbability of the scattered and the primary rays that produce them is of theoretical interest in the sense of explaining the mechanism of secondary radiation, and this prompted Plimpton to verify with the greatest possible care the results of previous experiments and to compare the absorption of secondary and primary X-rays.

The source of X-rays was a Coolidge tube with a rhodium or molybdenum anticathode. The beam of rays fell on a bent mica plate.

The monochromatic beam reflected at a certain angle passed through the scattering body (paraffin or water) and entered an ionization chamber. Special attention was paid to protecting the scattering body and the chamber from extraneous secondary radiation, for which purpose several lead screens were placed in the path of both the primary beam coming from the tube and the beam reflected from the mica in such a way that each subsequent screen stopped the unwanted radiation falling from the preceding ones. The last screen shields the chamber from extraneous rays and is itself placed so that radiation from it itself does not enter the chamber.

The ionization chamber, filled with ethyl bromide, is connected to a Wilson electrometer. Control measurements made in the absence of the scattering body showed that the extraneous radiation amounted to only a few percent of the investigated effect of the scattered rays, although the sensitivity of the apparatus was such that a wire placed in front of the diaphragms of the screens produced secondary radiation so intense that measurements could not be made in its presence. An absorbing aluminum screen was placed either in the path of the primary beam reflected from the mica or in the path of the rays scattered from paraffin (water) in front of the chamber.

The chamber was placed on a circumference at whose center was the scattering body, and could occupy any position on the periphery of this circle; together with the scattering body it could, moreover, rotate along a circumference at whose center was the middle of the reflecting mica plate.

Thus measurements could be made at various angles both of the rays reflected from the mica and of those scattered from the paraffin.

Usually the comparison of absorption was carried out at second-order angles of reflection from the mica, as giving the most intense monochromatic rays.

The results obtained from measuring the ionization, with the aluminum screen placed in the path of the primary rays and of the secondary scattered rays, gave, to an accuracy of 1%, one and the same value; i.e., the wavelength of the scattered rays is apparently the same as that of the primary rays that produce them.

A. Trapeznikov.