Mean Free Path of Penetrating Shower Producers

The collision mean free path has been measured in carbon and in lead for both neutral and charged particles capable of producing penetrating showers. The results are practically equal for the neutral and charged particles, and average 160±10 g/${\mathrm{cm}}^2$ in lead, and 81±5 g/${\mathrm{cm}}^2$ in carbon. The indicated cross section in carbon is significantly less than the geometric cross section. The ratio of neutral to charged primaries was found, after correction, to be 0.84±0.13.