WE have recently taken two thousand pairs of photographs of the cloud-chamber tracks of particles emitted from radium E. The rays entered through a small aperture from a source situated outside the chamber, across the centre of which was stretched a foil of aluminium of mass 0.046 gm./cm.2. The applied uniform magnetic field was about 220 gauss, and almost all the tracks showed a curvature indicating a negative charge. In a total of some 12,000 tracks, not more than twenty were found which proceeded from the source and which had a curvature in the opposite direction from that of the negative (β-particles. Applying further very rigorous criteria of perfection, we can select three tracks A, B and C, of which A and B have Hρ about 2,400, while the value for C is the highest recorded at 3,600.
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CHAMPION, F., AHMED, A. Existence and Nature of the ‘Positive Particles’ near a Source of Radium E. Nature 168, 780 (1951). https://doi.org/10.1038/168780a0