Personal Recollections of Early Theoretical Cosmic Ray Work

Abstract

My work on cosmic ray theory began with the paper on Bremsstrahlung and pair production by high energy electrons and photons, published jointly with Bethe. The cross sections for these processes proved to be unexpectedly high. A few years later Blackett and Occhialini discovered the cosmic ray showers in cloud chamber experiments. Blackett thought at first that these were nuclear explosions caused by very high energy particles. I saw at once that in a not too thick layer of a preferentially heavy material an incident electron must cause repeated processes of Bremsstrahlung which in turn led to pair production and so forth. If the energy of the incident electron was high enough, this inevitably must lead to a great number of electrons emerging from the bottom of the layer of material. Thus the showers would find a very natural explanation without introducing new physical processes. I discussed the matter with Bhabha. If I am not mistaken, the discussion took place in England. We settled down to work and very soon had a theory of “cascade showers”. It is remarkable that at the same time Carlsson and Oppenheimer published the same results obtained by a very different mathematical method.