New data on the absolute cosmic ray ionization in the lower atmosphere

An air-filled ionization chamber has been constructed with a volume of 552 liters and a wall consisting of 12.7 mg/cm² of aluminized plastic wrapped over a rigid, lightweight aluminum frame. A calibration in absolute units, independent of previous Caltech ion-chamber calibrations, was applied to a sealed Neher electrometer for use in this chamber. With this chamber and an older, argon-filled balloon chamber in a C-135 aircraft, we measured the ionization in the atmosphere from 1000 to 40,000 feet altitude. Other measurements off the southern California coast, primarily with the balloon chamber, resulted in a value of 2.60±0.03 ion pairs/cm³ sec atm for the cosmic ray ionization at sea level. The calibration of the 552-liter ion chamber agrees with the old Caltech calibration to within 1%, and the airplane data were consistent with previous balloon measurements in the upper part of our altitude range. We present data on the fluctuations of ionization due to radon and its daughters in the atmosphere. Our new ionization values are lower than previous Caltech values but higher than more recently reported values.