IN examining photographic plates ‘exposed' to cosmic radiation at a height of about 10,000 ft. on the Jungfraujoch and the Pic du Midi, and at about 18,000 ft. on Kilimanjaro and Chacaltaya, we have now observed twenty nuclear ‘explosions' which are accompanied by the emission of slow mesons. In this communication we give a number of photomicrographs of the tracks of these ‘ejected' mesons. An analysis of the observed secondary effects which they produce enables us to show that at least the majority of them interact with nuclei and are therefore negatively charged ; and that on the average they produce more vigorous nuclear disintegrations than most of the negative mesons which stop in the emulsions after being generated outside it. The main features of our present knowledge of charged mesons, with masses in the interval from 100 me to 400 me, can be accounted for in terms of two types of particles of different mass1'2, which can be either positively or negatively charged, the heavier being generated in processes associated with the explosive disintegration of nuclei, and decaying spontaneously to produce the lighter ones. The possibility is not excluded, however, that the phenomena are more complicated than has been visualized hitherto.
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Die Naturwissenschaften (1948)