The average energy, $\epsilon$, which is required to produce an ion pair in xenon by use of x-rays of wave-length between 0.12A and 0.26A as a source of energy, was determined. Two measurements of the power were made; one with a lead disk thermopile which absorbed 97 percent of the incident radiation and the other with a thin silver disk thermopile which absorbed 8.9 percent. The ionization measurements were made with a glass ionization chamber which was filled with xenon at a pressure of 2.22 atmospheres. This chamber absorbed 15.9 percent of the incident radiation. The value of $\epsilon$ was found to be 21.3±0.8 electron-volts.

• Received 4 October 1941

DOI:https://doi.org/10.1103/PhysRev.60.852