Shower production by penetrating cosmic rays

Abstract

The transition curve, giving the cosmic-ray shower intensity under increasing thicknesses of lead, rises to a maximum at about 1.6 cm., falls fairly rapidly to 5 cm., and then falls off more slowly, maintaining a finite intensity at very great thicknesses. Further, the occurrence of showers deep underground has long been established (Follett and Crawshaw 1936; Ehmert 1937). The showers forming the initial part of the transition curve (up to about 5 cm. of lead) are adequately accounted for by cascade multiplication from incident electrons or photons (Bhabha and Heitler 1937). Some other mechanism, however, is required to explain the occurrence of showers under much greater thicknesses. It is generally recognized that the showers forming the tail of the transition curve are associated with the penetrating component, and the theory developed by Bhabha (1938) suggests a probable mechanism, whereby a mesotron knocks-on an electron in a direct collision; the electron subsequently producing a shower through the normal cascade process. It has already been shown that the production of secondaries (small showers) by the penetrating component can be accurately explained by this knock-on mechanism (Wilson, J. G. 1938; Trumpy 1938; Hopkins, Nielsen and Nordheim 1939).