T. Dessauer. Production of X-rays of Very Great Hardness

(Verh. d. deutsch. Phys. Ges. 16/17, 1917).

Up to the present time the hardest X-rays have been obtained by Dessauer and Winawer (Phys. Zeitschr. 15, 1914) and by Rutherford, Barnes, and Richardson (Phil. Mag. (6), 30, 1915). The absorption coefficient of these rays in aluminum was \(\mu_{Al}=0.30\)¹); rays of such hardness were obtained at voltages of about 170,000 Volt. In order to obtain still harder X-rays it was necessary to construct a transformer or an inductor giving a higher voltage. This task was carried out by Dessauer. The chief obstacle to increasing the voltage of a transformer is the danger of breakdown of the insulation between the secondary and primary windings; the danger of breakdown is especially great if one end of the secondary winding is grounded. In order to avoid breakdown of the insulation, it was often customary to connect the primary winding with the middle of the secondary, owing to

¹) The absorption coefficient is determined by the formula \(J=J_0 e^{-\mu d}\), where \(J_0\) is the intensity of the X-rays before passing through an aluminum plate of thickness \(d\), and \(J\) is the intensity after passing through this plate.

as a result of which the voltage difference was reduced by half and the danger of puncture was confined to the ends of the coil. In order to reduce still further the danger of breakdown of the insulation and thereby to attain the highest possible voltages, Dessauer employs the following arrangement. The secondary winding is divided into two parts, and its middle, \(c\), is connected to ground. The transfer of energy from the primary winding to the secondary is accomplished by means of two intermediate transformers, \(H_1\) and \(H_2\), which do not raise the voltage, i.e. have an equal number of turns in the primary and secondary windings. The middles of the windings of the main transformers, \(T_1\) and \(T_2\), connected with the secondary windings of the intermediate transformers \(H_1\) and \(H_2\), are joined to the middles \(d\) and \(e\) of the high-voltage secondary windings. In this way, the dangerous voltage difference between the secondary and primary windings is reduced to only one quarter of the full voltage of the secondary winding, and the construction of sufficiently reliable insulation is greatly facilitated. The diagram in the right half of the drawing explains the scheme and indicates the voltages between the various points of the transformer, assuming a full voltage in the secondary winding of 100,000 Volt.

Such an installation enabled Dessauer to attain voltages of more than 300,000 Volt and, with the aid of an X-ray tube with an incandescent cathode, to obtain rays of considerably greater hardness than previous investigators had been able to obtain. The results of these investigations are compared in the following table, where, in addition to the absorption coefficient, the frequency and the wavelength of the X-rays are also indicated (the relation between \(\mu\) and \(\lambda\) is taken as follows: \(\mu = b\lambda^{5/2}\), where for aluminum \(b/k = 21.5\)).

Let us indicate, for comparison, that for the \(\gamma\)-rays of radium \(B\) the absorption coefficient \(\mu_d = 0.51\), and for the hard rays of radium \(C\), \(\mu_d = 0.115\); thus, with the aid of X-ray tubes it is possible to obtain rays of the same hardness as the \(\gamma\)-rays of radium, although the extreme limit of hardness of \(\gamma\)-rays has not yet been reached with the aid of an X-ray tube. It is interesting that earlier investigators came to the conclusion that increasing the voltage beyond

145,000 volts does not produce an increase in the hardness of the X-rays. Dessauer has evidently shown that this assertion is incorrect. Dessauer’s investigation permits one to hope that the X-ray tube will be able to replace radium in those cases in which it is used in medicine for therapeutic purposes.

S. Rzhevkin.