Cosmic-ray ionization in the lower atmosphere

Abstract

The response of a pressurized argon-filled ionization chamber to charged particles in the cosmic radiation near 50°N geomagnetic latitude has been determined as a function of atmospheric depth from mountain altitudes to sea level. The γ contributions to the various ionization chamber readings were subtracted out by utilizing concurrent determinations of the γ ionization from γ-ray energy spectra obtained with a scintillation spectrometer. Conversion of relative response to absolute free air ionization is accomplished by normalizing the mean chamber reading at sea level to a calculated value of 2.1±0.1 ion pairs cm−3/sec−1 atm−1 (STP), derived from existing experimental data on muon, electron, and proton fluxes and energy distributions. The curve of absolute ionization versus atmospheric depth has the same shape as that obtained by Millikan in 1932, but with significantly lower absolute values. It is inferred that many present determinations of free-air ionization, primarily conducted at higher altitudes, may be as much as 50% too high.