The production of cosmic-ray bursts by mesotrons

Abstract

The frequency of occurrence of cosmic-ray bursts under large thicknesses of iron an d lead has been investigated as a function of the thickness. A definite m aximu m is found to occur in the curves for the smaller bursts, its prominence diminishing w ith bursts of increasing size.The dependence of the burst frequency on the size of th e ionization cham ber an d th e proxim ity of the shielding m aterial to the chamber has also been investigated. The results suggest the production of showers of mesotrons which spread out, and ultim ately give rise to electron showers originating in widely separated areas in th e m aterial above, and round th e sides of, the ionization chamber.I t is now well known th at the cosmic radiation reaching the earth's surface com prises two main ionizing components, a soft component consisting of fast electrons. and a hard component consisting of mesotrons.The fast electron component is absorbed in a few centimetres of heavy material as the result of a cascade process in which each fast electron gives rise to a shower of soft electrons. The frequency of showers of a given size rises to a maximum and then falls again as the thickness of the material is increased, giving the familiar 'Rossi curve'. The processes involved have been fairly extensively investigated with various arrangements of coincidence counters, and with ionization chambers which record bursts of ionization produced by showers, and the main features of the observations have been satisfactorily accounted for by the quantum theory (Froman & Stearns 1938)..The Rossi curve, after an initial fall beyond the maximum, flattens out and then falls off much more slowly, indicating the presence of a harder component, the mesotron component, capable of penetrating several metres of heavy material. The occurrence of large cosmic-ray bursts under great thicknesses of m atter has been ascribed to the following process (Christy & Kusaka 1941). The mesotrons produce fast electrons, either (a) in head-on collisions with atomic electrons, or (6) through the intermediary of hard quanta produced by retardation of the mesotrons in nuclear collisions ('Bremsstrahlung'), these quanta subsequently giving rise to fast electron pairs. The fast electrons then give rise to showers of slower electrons by cascade.on May 9, 2018 http://rspa.royalsocietypublishing.org/ Downloaded from