Shower Production Under Thick Layers of Various Materials

Observations by G-M counters are reported on showers from lead and iron up to thicknesses of approximately 600 g/${\mathrm{cm}}^2$. As previously noted by others, the transition curves at large thicknesses have approximately the same slope as absorption curves obtained for the general cosmic radiation. Data are presented for the iron-lead and leadiron transition curves beginning at a material thickness of 274 g/${\mathrm{cm}}^2$ in each case. For the iron-lead transition curve, the number of coincidences increases and attains a maximum in approximately 1 cm of lead, after which the counting rate decreases and finally falls on the air-lead transition curve. For the lead-iron transition curve, the number of coincidences decreases for the first increments of added iron, passes through a minimum and then increases to the air-iron transition curve. It is pointed out that the observations are consistent, in a qualitative way, with the multiplicative theory of the origin of cosmic-ray showers provided one assumes that additional shower producing radiation is generated in the lower layers of material.