Similarity between Cosmic Rays and Gamma Rays

Abstract

As heretofore indicated,¹ it was in the fall of 1926 that Millikan and Cameron began to use high pressure electroscopes in order to increase the sensibility of their cosmic ray measurements. They built at first two such electroscopes and filled them to pressures of 8 atmospheres and 30 atmospheres respectively. Their first published results¹ were obtained with the 8-atmosphere filling, and they then assumed that for these hard rays the observed ionisation would be proportional to pressure. By directly comparing, however, a little later, similar spherical electroscopes of 1600 c.c. capacity filled to 1 atmosphere and to 8 and 30 atmospheres respectively, they were surprised to find that the ionisation shown in the 8-atmosphere electroscope was but about five times, and that in the 30-atmosphere electroscope was but 13.8 times that in the 1-atmosphere electroscope. These facts were published in one of their 1930 publications,² but since the authors were then interested merely in the variation in the ionisation in a given electroscope with depth beneath the top of the atmosphere, they made no attempt to discuss the reasons for these low factors. They did, however, by direct comparisons find that these factors were the same for the gamma rays of radium and thorium as for the cosmic rays, thus bringing to light another significant similarity in behaviour of these two types of radiations.³