A fast electron passing through matter will radiate at a greater rate than would be expected if the atoms traversed presented to the electron simply a static field of force. This follows because of the additional possibility of collisions in which an atom is excited. In a similar way energetic quanta will produce pairs at a greater rate. Consequently the unit of length characteristic of the multiplicative production of showers will be decreased by a factor which in the case of air amounts to 17 percent. The probabilities of radiation and pair formation are not proportional to the square of the nuclear charge but are supplemented by terms linear in atomic number (mass absorption law).

• Received 8 March 1939

DOI:https://doi.org/10.1103/PhysRev.55.858