Observations on the Tracks of Slow Mesons in Photographic Emulsions

Abstract

Introduction. In recent experiments, it has M. been shown that charged mesons, brought to rest A in photographic emulsions, sometimes lead to the production of secondary mesons. We have now extended these observations by examining plates exposed in the Bolivian Andes at a height of 5,500 m., and have found, in all, forty examples of the process leading to the production of secondary mesons. In eleven of these, the secondary particle is brought to rest in the emulsion so that its range can be determined. In Part 1 of this article, the measurements made on these tracks are described, and it is shown that they provide evidence for the existence of mesons of different mass. In Part 2, we present further evidence on the production of mesons, which allows us to show that many of the observed mesons are locally generated in the ‘explosive’ disintegration of nuclei, and to discuss the relationship of the different types of mesons observed in photographic plates to the penetrating component of the cosmic radiation investigated in experiments with Wilson chambers and counters. 1947 was the year of the pion — flick through Nature volumes 159 and 160 and watch the story unfold. In February of that year, Cecil Powell and Giuseppe Occhialini reported the observation of six star-like patterns in emulsions exposed to cosmic rays. Powell's group had finally found the Yukawa particle, predicted in 1935 to be the carrier of strong force inside the atomic nucleus. In fact, Don Perkins pipped them to the post with the publication of a single, similar star-like event just two weeks earlier [Perkins, D. H. Nature 159, 126–127 (1947)]. Later in the year, another paper from Powell's group announced the first observation of pion decay to a muon — the particle picture was beginning to take shape.