Cosmicray bursts and shower spread under large thicknesses of lead.

The frequency of occurrence of cosmic-ray bursts under large thicknesses of lead and iron has been measured as a function of the thickness, using a small ionization chamber with and without shielding material round its sides. Experiments have also been carried out with two small chambers side by side, the frequency of occurrence of bursts simultaneously in both chambers being observed under different thicknesses of lead. While the burst-rate—thickness curves are, on the whole, fairly flat beyond the first maximum, there are indications of a second maximum at large thicknesses in the case of lead. From the experimental results information has also been obtained about the mean and the maximum angular spread of showers, and the increase in shower spread with increasing size of shower.