FROM CURRENT LITERATURE

On Rutherford’s Works on the Passage of $\alpha$ Rays Through Matter

The books received from abroad (L. Graetz, Die Atomtheorie (1920), and K. Fajans, Die Radioaktivität¹ (1919)) contain accounts of Rutherford’s remarkable investigations, which appeared in 1919 (Philosoph. Mag., June 1919). Making use of the data contained in the books just mentioned, we shall give an account of these works.²

As is known from the work of Bohr and his followers, we must imagine atoms as consisting of a positive nucleus, complexly built up of positive and negative charges and having a radius $R$ of the order of $10^{-13}$ cm (for gold $R = 3 \cdot 10^{-13}$), while around the nucleus electrons move in circular closed orbits at various distances from the nucleus. For hydrogen, the closest possible position of the sole electron to the nucleus is, according to Bohr, $1.1 \cdot 10^{-8}$ cm.

An $\alpha$ particle is a positive helium nucleus, endowed with two charges and having dimensions of the order of $10^{-13}$ cm.

If an $\alpha$ particle flies through a gas, then only in the case of direct

between the $\alpha$ particle and the nitrogen nucleus interactions are observed, and we can imagine a case in which an $\alpha$ particle strikes the nucleus of a nitrogen atom centrally; in this case it is possible for the nucleus of the atom to be liberated in the form of a free atomic nucleus.

If we assume that an $\alpha$ particle strikes the nucleus of a hydrogen atom centrally, then the latter, after the collision, must acquire, as Darwin showed ¹), a velocity 1.6 times greater than the velocity of the $\alpha$ particle. In its flight an $\alpha$ particle is capable of producing a glow on a fluorescent screen over a distance equal to the range (Reichweite). It may be assumed that, at the same velocity, both the $\alpha$ particle and the hydrogen nucleus, or, as Rutherford calls them, $H$-particles, produce glows over the same distance; and since the range $R$ is proportional to the cube of the velocity $V$ ($R = KV^3$, $K$ a constant), it is clear that the $H$-particles obtained from hydrogen atoms in the central collision of an $\alpha$ particle with a hydrogen nucleus must have a range $(1.6)^3$, or approximately 4 times greater than that of the $\alpha$ particle.

These phenomena were quite clearly discovered by Marsden ²), who obtained $\alpha$ rays from the emanation of $Ra$ enclosed in a thin-walled tube permeable to $\alpha$ particles. The tube was placed in hydrogen at atmospheric pressure, where the range of the $\alpha$ particle was 24 cm. However, upon careful observation at a distance of 80 cm from the tube, isolated flashes of the zinc sulphide screen from the impact of a particle—scintillations—could be observed. That these scintillations were caused by fast $H$ particles was discovered by Rutherford through a direct measurement of their charge-to-mass ratio.

For the hydrogen nucleus the ratio of the charge in coulombs to the mass of the nucleus in grams should have been 96,540, while in the experiment it turned out to be 100,000. Thus the agreement obtained was complete.

Marsden and Lantsberry (1915) observed the liberation of $H$-particles when $\alpha$ rays passed through a substance containing hydrogen, for example sealing wax. For the liberation of an $H$-particle to occur, a central impact of the $\alpha$ particle on the nucleus is necessary, and this explains why the percentage number of $H$-particles in relation to $\alpha$ particles is small.

Marsden calculated that for $10^{-5}$ $\alpha$ particles, each of which collides with $10^{-4}$ atoms of $H$, only one $H$-particle with a large range is formed.

It is very interesting that such $H$-particles are sometimes formed in cases where there is no hydrogen in the surrounding medium. Thus, for example, $H$-particles are emitted by a $Ni$ plate coated with radium $C$, which, in order to remove $H_2$ and $H_2O$, had been heated to $150^\circ$. Thus it turns out that radioactive bodies themselves can emit $H$-particles.

Besides radium $C$, as Rutherford showed, $H$-particles are emitted by radium emanation free from $RaC$, and the increase in the number of $H$-particles emitted from the emanation is considerably faster than the increase of $RaC$.

To determine whether the gas content affects the emission of $H$-particles, Rutherford filled the space in which the radioactive substance was located with dry air, $CO_2$, $O_2$, and finally $O_2$ saturated with vapors

¹) C. G. Darwin. Philos. Mag. 27. p. 499—1914.
²) E. Marsden. Philos. Mag. 27 p. 824—1914. Marsden and Lantsberry. Philos. Mag. 30 p. 240—1915.

$H_2O$, and it turned out that the greatest number of $H$-particles is obtained in air, and the more of them, the higher the vacuum.

Even when the space was filled with pure $N_2$, Rutherford showed an increase of $H$-particles by 25% in comparison with air, i.e., by approximately that amount by which the content of nitrogen molecules per unit volume increases in these two cases.

It was then rigorously demonstrated that the $H$ particles arose in the gaseous medium during the passage of $\alpha$ particles through it; moreover, the $H$-particles arising in $N_2$, just like the $H$-particles arising in $H_2$, had a range four times greater than that of the $\alpha$ particles.

From all this it was natural to conclude that, in the latest experiments, Rutherford had succeeded in causing the nucleus of $N_2$ to disintegrate and in obtaining from it a nucleus corresponding to the nucleus of $H_2$.

Thus, for the first time, an experimental basis was obtained for the assertion that the nuclei of the elements are complex formations.

P. Lazarev.

1. Graetz’s book is being translated and will be published by the State Publishing House; Fajans’s book has been translated and will be published by the publishing department of the People’s Commissariat of Health. ↩

2. At present Rutherford’s original papers have been received. They will be reviewed in the next issue of Uspekhi Fizicheskikh Nauk. ↩