The Absorption of the Penetrating Component of the Cosmic Radiation

The nature of the penetrating component of the cosmic radiation has been studied by making G. M. counter measurements in a cavern at a depth below rock of approximately 60 m water equivalent. Observations on the vertical counting rate were made inside and outside the cavern with various thicknesses of lead between the counters. In agreement with the results of previous investigators the counting rate with no absorber between the counters is approximately one-twentieth of the counting rate outside. The small absorption of the penetrating ionizing radiation at this depth in 550 g/${\mathrm{cm}}^2$ of lead shows that the penetrating component is to be associated with an ionizing particle. It is therefore not necessary to assume the transmission of cosmic-ray energy to these large depths by nonionizing particles, e.g., neutrinos. Comparison of the absorption of the penetrating component inside and outside the cavern shows that the penetrating ionizing radiation undergoes a hardening on transmission through large thicknesses of material. The percentage of the radiation of the less penetrating type is very nearly the same inside and outside the cavern (25 percent to 30 percent, respectively). This suggests that such soft radiation is in approximate equilibrium with the penetrating component even at the earth's surface. The character of the absorption curve for the soft component under a depth of 60 m water equivalent shows that it must be possible for the penetrating component to generate secondaries of considerable energy.