From Current Literature

Artificial Decomposition of Light Elements

Sir E. Rutherford and J. Chadwick. Artificial Disintegration of the light Elements. Phil. Mag. 39, p. 809 (1921).

In the preceding issue of Uspekhi we already reported the preliminary results of this work in the form in which they were set forth in a brief note published by the authors in Nature1. In the article under review the authors communicate a number of interesting and important details.

When Rutherford first succeeded in detecting the appearance of H-particles from nitrogen2, the scintillations which these particles produced on a ZnS screen were so weak that it was difficult to determine with certainty the maximum range of the particles. As a preliminary figure Rutherford gave 28 cm, i.e. a value coinciding with the range of H-particles from hydrogen.

Recently Rutherford has used a microscope with optics so greatly improved that the actual counting of particles has been considerably facilitated, and the results obtained have become more definite and constant.

It turned out, first of all, that the H-particles obtained from a mixture of \(H_2 + CO_2\) and from paraffin have a maximum range of 29 cm. Beyond this range no scintillations were observed. Meanwhile, the maximum range of the particles from nitrogen was 40 cm. Thus it may be stated definitely that the particles arising in nitrogen are by no means due to the presence of impurities of hydrogen or of any hydrogen-containing compounds. At the same time the task of investigating the artificial decomposition of other elements was made easier. The arrangement of the experiment was such that the total absorption between the source and the screen was equivalent to 32 cm of air. Owing to this, the possibility of H-particles from any hydrogen impurities reaching the screen was completely excluded, and it was possible not to concern oneself with removing all traces of hydrogen (even plates of mica and paraffin were used as absorbing screens). The results of the investigation are given in the table placed below.

In addition, a series of heavier elements was investigated: chlorine—from MgCl₂, potassium—from KCl; calcium from CaO, titanium from Ti₂O₃, iron, copper, silver, gold. These elements do not give particles with a range greater than 32 cm in air; the question of whether they give particles with a smaller range has not yet been investigated.

Nitrogen and aluminum were subjected to more detailed study. First of all, for nitrogen the dependence of the number of scintillations on the absorption of α-particles was investigated (the particles had a range of 7.0 cm in air). It turned out that whereas at an absorption of 12 cm of air the number of particles from N₂ was significantly smaller than from a mixture of CO₂ + H₂, beyond 29 cm scintillations were no longer observed at all in the latter, while in nitrogen they could still be traced up to an absorption of 40 cm. Next, the dependence of the number and range of the particles on the velocity of the incident α-particles was studied. As we shall see below, the investigation of this dependence should shed some light on the very mechanism of the disintegration. It turned out that, in a first approximation, the range of the particles arising in nitrogen is proportional to the range of the incident α-particles: α-particles with ranges of 8, 6, 7, and 6 cm of air cause particles with ranges respectively of 50, 40, and 34 cm. The number of particles also increases with the range of the α-particles.

Experiments with aluminum first of all showed that the range of the particles in this case is approximately twice as great as in nitrogen. The dependence between the magnitude of the range of the particles arising in aluminum and the α-particles, as for nitrogen, proved to be approximately a simple proportionality. However, owing to the small number of scintillations, it is very difficult to determine the maximum range accurately and to establish this dependence with any precision.

In his first work Rutherford showed that the H-particles obtained in hydrogen move almost entirely in the direction of flight of the α-particles producing them. By analogy one might have expected the same in other elements as well. However, in aluminum a remarkable fact was observed for the first time: it turned out that the particles are ejected not only forward, but also backward, and moreover in equal numbers. This effect was discovered by the following two experiments:

1. The active deposit was applied to an aluminum (instead of copper) disk. The aluminum plate in front of the source was removed. Under these conditions particles with large range could be produced only by α-particles penetrating into the aluminum, i.e., flying away from the ZnS screen.

2. The active deposit was applied to a silver disk, turned with its active side away from the ZnS screen. In front of the disk (on the active side) aluminum was placed.

In both cases scintillations corresponding to large ranges were observed on the screen. Their number was of the same order as when particles were ejected in the direction of flight of the α-particles, though somewhat smaller. The maximum range proved, however, to be 67 cm instead of 90 cm.

Similar experiments were carried out with nitrogen. The results showed that in this case the number of particles in the backward direction, if they exist at all, must be exceedingly small (less than \( \frac{1}{50} \) of the number of particles in the forward direction).

As for the nature of these particles with long ranges, in the case of nitrogen it had already been shown earlier that they are H-atoms1. The nature of the particles arising in other elements has not yet been investigated, but one can hardly doubt that everywhere we are dealing with H-particles of different velocities. In any case, these particles can only be products of the disintegration of the atomic nuclei of the elements.

In interpreting the results of his experiments, Rutherford first of all notes that H-particles are given only by those elements whose atomic weights are expressed by the general formulae \(4n + 2\) and \(4n + 3\) (for example: boron—\(11 = 2 \cdot 4 + 3\), nitrogen—\(14 = 3 \cdot 4 + 2\), etc.). Conversely, elements with atomic weights of the form \(4n\) (carbon, oxygen) definitely do not give H-particles. This once again confirms the already established conception that the principal components of the nuclei of the elements are the nuclei of hydrogen and helium.

Further, it is necessary to explain the ejection of H-atoms with velocities considerably greater than those which could be expected from the simple theory of impact. Here the first thought that suggests itself is that the very process of destruction of the nucleus has the character of an explosion, so that the \(\alpha\)-particles play only the role of detonators. The fact that aluminum ejects particles in all directions is a convincing support for such a hypothesis. However, it is contradicted by another fact observed in aluminum and in nitrogen, namely, that the velocity of the H-particles is approximately proportional to the velocity of the incident \(\alpha\)-particles. It should be noted that, owing to the difficulty of the experiments, it is impossible to determine exactly the range (and consequently the velocity) of the H-atoms. Thus this fact cannot be considered fully established—which is also noted by Rutherford himself. Nevertheless, he represents the mechanism of splitting as the pushing of H-atoms out of the nucleus. In order to explain the excess velocity and energy (in nitrogen a range of 40 cm, in aluminum—90 instead of 28), as well as the ejection from aluminum of particles in the backward direction, Rutherford constructs a special model of the nucleus. Namely, he imagines that H-nuclei are satellites revolving around the main mass of the nucleus. Such a system will, of course, be unstable, and therefore Rutherford makes the further assumption that at very small distances in the nucleus the repulsive forces are replaced by attractive ones, so that an \(\alpha\)-particle in its motion is at first repelled by the nucleus and then attracted. Rutherford supports this hypothesis only by referring to the fact that otherwise it is difficult to understand the stability of a nucleus composed of positive charges alone. If this is so, then in the case when the \(\alpha\)-particle imparts to the H-satellite an impulse in the direction toward the nucleus, under the influence of the perturbing attraction of the latter the satellite describes an open orbit and flies out of the nucleus in a direction opposite to the flight of the \(\alpha\)-particle. In all other cases it is pushed out in the direction of motion of the \(\alpha\)-particle. The excess energy is drawn from the internal energy of the nucleus. Rutherford calculates this energy liberated in the splitting. Indeed, knowing the range of the H-particles, one can compute their velocity by Geiger’s formula, and consequently also their energy. For H-particles from aluminum emitted forward, the kinetic energy obtained is \(2.81V\), where \(V\) is the velocity of the \(\alpha\)-particle, i.e. 1.4 times greater than the kinetic energy of the \(\alpha\)-particle. It is curious to note that the ejection of H-particles from aluminum does not occur if the range of the \(\alpha\)-particles is less than 5 cm. If one calculates the energy of such an \(\alpha\)-particle, converts this energy to an electron, and expresses it in volts, the result is about 6,000,000 volts, whereas the energy required to remove an electron from the \(K\)-shell is only 2,200 V.

E. Shpol’skii.

1. Rutherford. Proc. Roy. Soc. A, 97, p. 374 (1920); for the Russian translation, see Uspekhi Fizicheskikh Nauk, vol. II, issue 2, p. 194. ↩↩

2. Rutherford. Phil. Mag., 37, p. 581 (1919). ↩