New Evidence for the Positive Electron

J. Chadwick et al.

Editor’s Note

Here James Chadwick, along with Patrick Blackett and Giuseppe Occhialini, confirms the existence of a positively charged anti-particle of the electron, initially reported by Carl Anderson from cloud-chamber experiments. Chadwick and colleagues placed pieces of polonium and beryllium just outside a cloud chamber, and near a lead target placed just inside the chamber. Alpha particles from the polonium excited neutrons or gamma rays from beryllium that in turn stimulated the emission of particles from lead. These left tracks in the cloud chamber with the properties expected for a positively charged electron. Further experiments strongly suggested that the unknown particles had the same mass as electrons, but a positive charge. They were later called positrons, the first known example of antimatter.　　中文

THE experiments of Anderson1 and of Blackett and Occhialini2 on the effects produced in an expansion chamber by the penetrating radiation strongly suggest the existence of positive electrons—particles of about the same mass as an electron but carrying a positive charge.　　中文

Some observations of the effects produced by the passage of neutrons through matter, and the experiments of Curie and Joliot3 in which they observed retrograde electron tracks in an expansion chamber, led us to consider the possibility that positive electrons might be produced in the interaction of neutrons and matter, and we have recently obtained evidence which can be interpreted in this way.　　中文

A capsule containing a polonium source and a piece of beryllium was placed close to the wall of an expansion chamber. On the inside of the wall was fixed a target of lead about 2.5 cm. square and 2 mm. thick. This lead target was thus exposed to the action of the radiation, consisting of γ-rays and neutrons, emitted from the beryllium. Expansion photographs were taken by means of a stereoscopic pair of cameras. A magnetic field was applied during the expansion, its magnitude being usually about 800 gauss.　　中文

Most of the tracks recorded in the photographs were, from the sense of their curvature, clearly due to negative electrons, but many examples were found of tracks which had one end in or near the lead target and showed a curvature in the opposite sense. Either these were due to particles carrying a positive charge or they were due to negative electrons ejected in remote parts of the chamber and bent by the magnetic field so as to end on the lead target. Statistical examination of the results supports the view that the tracks began in the target and therefore carried a positive charge.　　中文

Strong evidence for this hypothesis was acquired by placing a metal plate across the expansion chamber so as to intercept the paths of the particles. Only a few good photographs have so far been obtained in which a positively curved track passes through the plate and remains in focus throughout its path, but these leave no doubt that the particles had their origin in or near the lead target and were therefore positively charged. In one case the track had a curvature on the target side of the plate, a sheet of copper 0.25 mm. thick, corresponding to a value of H of 12,700; on the other side the curvature gave a value H =10,000. This indicates that the particle travelled from the target through the copper plate, losing a certain amount of energy in the plate. The change in the value of H in passing through the copper is roughly the same as for a negative electron under similar conditions. The ionising power of the particle is also about the same as that for the negative electron. These observations are consistent with the assumption that the mass and magnitude of the charge of the positive particle are the same as for the negative electron.　　中文

The manner in which these positive electrons are produced is not yet clear, nor whether they arise from the action of the neutron emitted by the beryllium or from the action of the accompanying γ-radiation. It is hoped that further experiments now in progress will decide these questions.　　中文

Our thanks are due to Mr. Gilbert for his help in the experiments.　　中文

(131, 473; 1933)

J. Chadwick, P. M. S. Blackett, G. Occhialini: Cavendish Laboratory, Cambridge, March 27.

References:

1. Anderson, Science, 76, 238 (1932).

2. Blackett and Occhialini, Proc. Roy. Soc., A, 139, 699 (1933).

3. Curie and Joliot, L’Existence du Neutron (Hermann et Cie, Paris).