Cosmic-Ray Ionization Bursts in an Unshielded 8-Inch Pressurized Sphere at Sea Level

Two thin-walled ($\frac{1}{16}$-inch steel=1.22 g ${\mathrm{cm}}^{-2\mathrm{}}$) spherical ion-chambers, 8 inches in diameter, filled with best quality (99.8%) commercial cylinder argon at 50 atmospheres, have been used to measure cosmic-ray ionization bursts near sea level. The bursts in one ion-chamber were measured by means of a vibrating reed electrometer and in the other by an electrometer tube and dc feedback amplifier. The results from the two ion-chambers agreed to within two percent except that the smallest bursts could not be detected with the vibrating reed instrument. The integral size-frequency distribution of the bursts is given for sizes from 2×${10}^4$ to 3×${10}^7$ ion-pairs and the corresponding frequencies range from 3×${10}^4$ to 3×${10}^{-4\mathrm{}}$ bursts per hour. The size-frequency distribution has been separated into five components identified respectively with single $\mu$ mesons, electrons, single protons, star processes, and extensive electron showers. These components have been resolved by comparison of the unshielded distribution with that obtained under a 27-cm lead shield and the interpretation of their origin has been confirmed by coincidence experiments involving Geiger counters, a Čerenkov proton selector, and other ion-chambers. The existence of a sharp kink in the size-frequency distribution curve, similar to that reported by Carmichael and Chou in 1939, is confirmed.