Cloud-Chamber Studies of the Production of Light "Positive Particles" near Negative Beta-Ray Emitters

A series of experiments was performed with a cloud chamber to investigate the nature of the particles producing the positive tracks near negative beta-ray emitters reported in various investigations. Special emphasis was laid on the questions of whether the tracks are (1) due to electrons reentering the source and (2) due to electrons emerging from the source multiply scattered to show an apparent positive curvature. While the second possibility can account for only a negligible fraction of the tracks, it is found that, depending upon the source arrangement and the manner of investigating cloud-chamber photographs, a considerable fraction of observed "positive" tracks may be due to electrons which reenter or appear to reenter the source. With stereoscopic analysis and an appropriate source arrangement, the fraction of spurious positive tracks may be reduced to a few percent. With these precautions we find for ${\mathrm{P}}^{32}$ a ratio of positive particles to electrons of 3×${10}^{-4\mathrm{}}$. This ratio is increased by covering the source with carbon or lead. It is shown that the estimate of two electron masses previously obtained for the mass of these particles by comparing their multiple scattering with that of electrons was not influenced by the difference in the momentum distributions of the two species of particles used in the determination.