Cloud Chamber Search for Particles Ionizing Less Than an Electron

Abstract

Since ionization due to a charged particle varies with the square of its charge, a very clean cloud chamber can be used to search for particles having charges, $\mathrm{ze}$, much less than the electronic charge. Similar remarks hold true for magnetic monopoles of relativistic speed. A horizontal cloud chamber constructed by J. A. Bearden was employed, in the gamma flux from a radium source, to search for such particles. The sensitive time was about 1.5 seconds. The background was at least 60 times less than values commonly reported for "clean" chambers. No subionizers with $z$ in the range $\frac{1}{6}$ to \textonehalf{} were seen during a time in which about 900 electron tracks were recorded, one million gamma rays traversed the chamber, and 3 million disintegrations were available to give rise to such particles.A vertical chamber 12 in. high was used to look for subionizers in the cosmic ray flux. The background ranged from 0.05 to 1.0 drop per cc, with a usable sensitive time of 0.3 second. Technique for a more limited search was developed; use was made of the fact that background drops are single and well separated, while there are many clumps of ions along a track. No subionizers appeared in a body of photographs containing 550 electron and $\ensuremath{\mu}$-meson tracks.It was shown by study of "ghost tracks" that neutral nuclei are a major source of the background commonly encountered in tall chambers.