[Historical Photographs] First Recognizable Cosmic Ray Particles in Cloud Chamber Photographs

Among photographs taken in a small horizontal cloud chambers, particle tracks with little deflection in magnetic field of 1500 Gauss was recorded. These tracks are not associated in the direction with the radioactive source under investigation, and they appears to enter the chamber from elsewhere. The author identified these tracks as cosmic rays and estimated the horizontal flux to be 1.2 per square centimetre per minute. 

In the image above, false colour (green) shows the tracks of a relatively slow cosmic ray electron which traversed a considerable distance in the illuminated part of the chamber. The measured momentum of this particle is 7.3 MeV/c

In the image above, false colour (green) shows the tracks of a single cosmic ray particle which was not significantly deflected in the magnetic field while beta particles from a radioactive source bends so much the curvature is effectively a circle. 

In the image above, false colour (green) shows the tracks of a pair of undeflected and almost parallel tracks dipping sharply relative to the plane of the chamber. These were no doubt members of a cascade.  

Reference

D. Skobelzyn, Leningrad, Z. Phys. 54, 686. (1929)