On Scattered Rays in the Phenomenon of X-ray Diffraction.

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

The theory of the diffraction of Roentgen rays assumes that the secondary scattered rays which produce the diffraction effect are caused by vibrations of the electrons of the crystal molecules, 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 point to a greater softness of the secondary rays. This was attributed, as it were, to the selective ability of the crystal molecules to scatter rays with a longer wavelength more intensely, i.e. in the secondary beam the “softness” is more sharply expressed.

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

Confirmation of the fact of equal absorption of scattered rays and of the primary rays producing them is of theoretical interest for explaining the mechanism of secondary radiation, and this prompted Plimpton to check the results of the preceding experiments with all possible care 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 upon 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 shielding the scattering body and the chamber from extraneous secondary radiation; for this purpose several lead screens were placed in the path both of the primary beam emerging from the tube and of the beam reflected from the mica, in such a way that each subsequent screen intercepted the unwanted radiation falling from the preceding ones. The last screen shields the chamber from extraneous rays and is itself positioned so that radiation from the screen itself does not enter the chamber.

The ionization chamber, filled with ethyl bromide, was 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 in its presence could not be carried out. 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 by paraffin (water) in front of the chamber.

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

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

Usually the comparison of absorption was carried out at the angles of second-order reflection from the mica, as giving the purest 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 up to 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.