The Production of Photons Relative to Ionization by Collision in a Townsend Gap

Measurements were made in a Townsend gap to determine the average number of photons liberated by an electron for each secondary electron it liberates. In ${\mathrm{H}}_2$, this number was found to decrease rapidly with increasing ratio of field strength to gas pressure in the range from 50 to 150 volts/cm/mm, and at pressures of the order of 1 mm. Determination of the absolute number of photons per electron depends on the unknown efficiency of the photoelectric cell used to count the photons. However, this number is estimated as of order unity. The effective radiation has an absorption coefficient of 0.55 ${\mathrm{cm}}^{-1\mathrm{}}$ at a pressure of 1 mm of Hg. This large coefficient indicates that only those photons of rather high energy were counted. There is corroborative evidence that the brass surface of the photoelectric cell had a high work function. Thus only high energy photons would be expected to register.