Cosmic-Ray Bursts under Lead at Sea Level

The integral size-frequency distributions of ionization bursts near sea level were measured in a thin-walled ($\frac{1}{16}$ inch steel), 8-inch diameter, spherical ion-chamber filled with argon at 50 atmos pressure, under 14 different thicknesses of lead ranging from 0.11 cm to 22 cm. The measurements, along with those already published for the ion chamber with no shield and with a 27-cm lead shield, represent some 15 000 hours of recording. The range of size covered extends from about 3×${10}^4$ to ${10}^8$ ion pairs. In a subsidiary experiment the size-frequency distributions of bursts directly associated with extensive air showers were measured and to these were added, at each thickness of the shield, the bursts due to single $\mu$ mesons, protons, and stars derived from the analysis already published. These totals were then subtracted from the gross size-frequency distributions. The bursts remaining are almost wholly due to the electrons and photons of the soft component and to the radiative and knock-on processes of $\mu$ mesons. Transition curves of these bursts (Rossi curves) are given for selected sizes, corresponding (on the average) to showers of 1, 2, 4, 8,..., 512 electrons crossing the ion chamber. There is no sign of a second maximum in these transition curves. There is strong evidence that the cascades which give rise to the "first" maximum originate from single electrons or photons incident on the lead shield and hence these experimental results are suitable for a straightforward comparison with cascade theory in an energy range extending to some 50 Bev. Indications of a transition effect of the bursts attributed to stars are noted.