The Radioactivity of the Earth's Crust and Its Influence on Cosmic-Ray Electroscope Observations Made Near Ground Level

Radium analyses of 23 rock specimens from the scene of cosmic-ray observations made by R. A. Millikan and his co-workers discloses a linear relationship between the average radium content of the terrain and the local gamma-ray ionization in the cosmic-ray electroscope. Ionization due to soft gamma-radiation stimulated in the earth's crust by the impact of cosmic rays, or due to the reflection of cosmic rays by the earth's crust is therefore negligible, and the ratio of thorium to uranium is approximately constant in the materials examined. A local gamma-ray ionization of 5.5 ion pairs per sec. per cm³, in air at one atmosphere pressure, is normally associated with a content of 1.0 ×{} 10^-12 g Ra per g rock in the neighboring terrain. It is shown that the gamma-ray ionization n ion pairs per cm³ per sec., at h cm above a broad radioactive surface is n=(2πρmKμ₁)G_μh, where G_μh≡∫1^∞z^-2e^-μhzdz, μ₁ρ is the mass absorption coefficient and m the Ra concentration of the emitter, μ is the gamma-ray absorption coefficient of air, and K is Eve's number. Application to the experimental data suggests that the Th/U ratio is the order of 5 to 10 for many common geological materials.