Positrons from gamma-rays

Abstract

A report of the direct observation of positrons ejected from lead by hard gamma-rays has been published.(1) A source of radiothorium with its disintegration products was used. To obtain a well-defined beam the gamma-rays were collimated by allowing them to pass through a half-inch hole in a series of lead blocks totaling 18 inches in length. That the beam actually was well defined is shown by the fact that most of the observed tracks appeared to originate within a rather narrow region, about 3 cm wide. In this series of photographs the beam passed vertically downward through the side wall of a Wilson chamber across which were placed a 2 mm lead sheet, and below this a 0.5 mm sheet of aluminum. A magnetic field of 430 gauss normal to the face of the chamber and uniform to within 10 percent was used.