On the “Hardness” of X-Ray Tubes.

(F. Dessauer. Erzeugung harter Röntgenstrahlen. Phys. Zeitschr., 14 (1913), p. 246).

The production of homogeneous X-rays of great hardness is a very important problem both scientifically and in purely practical terms. Quite recently attempts have been made to use, for the solution of this problem, the powerful appliances of modern X-ray technology. Such an attempt has already led to extremely interesting positive results1.

However, anyone who has worked practically in the field of radiology knows well that a technical “X-ray tube” gives a whole complex of rays of different hardness. Therefore the terms “hard” or “soft” tube refer to a certain average, total effect, detected by one or another technical instrument. Thus, even the radiation from the hardest tube always includes soft rays as well.

As is known, the discharge of an inductor has the character of a system of rapidly damped electrical oscillations, whose period is determined by the capacitance and self-induction of the secondary circuit2. Consequently, the glow of the tube is—

is “flickering,” depending not only on the interruptions of the current in the primary circuit, but also on these natural oscillations in the secondary circuit. Dessauer discovered this circumstance by the following simple experiment. X-rays were directed at a lead screen and, after passing through a slit 1 mm wide, fell on a photographic film fastened to a rapidly rotating disk. As a result, the film showed not a continuous blackening, but a whole system of bands. It was found that the total time of luminescence of the tube during a single induction impulse is approximately \(4 \cdot 10^{-5}\) sec., i.e., by the end of precisely this interval of time the oscillations have completely died out. The question arises whether the rays producing each of these bands are homogeneous with respect to hardness, or whether they likewise consist of a mixture of rays of different penetrating power. To answer this question, the following experiment was set up: the slit in the fluorescent screen was covered by a ladder-shaped aluminum body, through which the X-rays passed before reaching the photographic plate. With this arrangement, the soft rays should be absorbed by a comparatively small layer of aluminum, whereas sufficiently hard rays can pass through the entire aluminum body.

It turned out that at first, when the tube lights up, very hard rays arise, which become progressively softer as the emission approaches its end. The last portion of the radiation contains only very soft rays. Consequently, we may consider that the separate components of the complex of rays given by an X-ray tube are produced not simultaneously, but predominantly successively: first hard rays, then medium ones, and finally soft rays.

E. Shpolsky.

1. For example: F. Dessauer, Verh. d. Deutsch. Phys. Ges. Heft 16/17, 1917.
   Naturwissenschaften, Heft 12, 1918.
   Abstract by S. Rzhevkin, Uspekhi Fiz. Nauk, vol. I, issue 3–4, p. 251 (1918). ↩

2. Duddel, Journal of the Röntgen Society, 1918.
   F. Dessauer, Phys. Zeitschr. 13, p. 1101 (1912).
   G. W. Kaye, X-Rays, p. 62. London, 1917. ↩