Absorption of penetrating cosmic ray particles in gold

Abstract

A considerable amount of experimental data on the energy loss of cosmic-ray particles in metal plates is now available. Much of this, however, represents work carried out before the separate nature of the hard and soft components was fully understood, so that in many cases unsuitable conditions make the interpretation of the results difficult. The soft component is known to consist of electrons, and these predominate in the cosmic-ray energy spectrum for energies less than 2 x 10 8 e-volts. It has been verified for these electrons that the energy loss by ionization (Corson and Brode 1938) and by radiative collisions (Blackett 1938) is in close agreement with the theoretical predictions. At energies greater than 2 x 10 8 e-volts, very few electrons are found at sea level, and for all higher energies the majority of the particles are now considered to be mesotrons. These, together with an uncertain, but small, number of protons form the hard component of the cosmic rays. Absorption measurements for the hard component are more difficult than for the electrons, since the particles are, in general, of higher energy and the loss of energy in an absorbing plate is very much smaller. The early observations (Blackett and Wilson 1937; Crussard and Leprince Ringuet 1937; Wilson 1938 a ) lead to the conclusion that at an electron energy ( E e = 300 Hρ )* of about 5 x 10 8 e-volts, the total energy loss of penetrating particles was very small—of the same order as that due to ionization alone —but that at higher energies, E e ~ 1.5 x 10 9 e-volts, a considerable additional energy loss took place, which did not appear to be attributable to the inclusion of electrons in the measurements (Wilson 1938 a ). With the comparatively thin absorbing plates used, however, it was not certain that all electrons had been excluded from these measurements.