Extremely sensitive instruments for measuring faint light, from 900A to 7500A, have been made by combining the essential features of photoelectric cells and Geiger-Müller tube counters, so that the photoelectrons ejected by the radiation are individually counted. With some of the tubes described, photocurrents of 0.05 electrons per second can be measured. Photoelectric surfaces of Sn, Cd, Zn, Cu, brass, Ag, Hg, Mg, ${\mathrm{I}}_2$, and Se, were used for measuring ultraviolet light. Visible-sensitive surfaces were made by coating Cu cathodes with fused NaCN, and with amalgams of Na, K, Cs, Sr, and Na-K, and subsequently directing intense ${\mathrm{H}}^{+}$ bombardment against them, in a hydrogen atmosphere. In this manner, thin films of hydrides of the respective metals were formed by reduction. The KH-on-Hg cathodes were the most sensitive to visible light, while the most sensitive one for ultraviolet light was a thin layer of ${\mathrm{I}}_2$ on Se. Very thin films of about 30 aniline dyes and photographic sensitizers were successively coated on the zinc cathode of a counter. The dye usually shifted the photoelectric threshold toward the ultraviolet, but increased the total sensitivity of the surface. Similar effects were obtained by reversing the direction of field in the counting tube and introducing traces of certain organic liquids into the vicinity of the tungsten wire, which was then the cathode. Thus diethyl-aniline increased the sensitivity by a factor of about 25. Different gases were also tried in the counting tubes; it was found best to use a gas with a high minimum ionization potential, when possible. A counter was used for comparing the total ultraviolet luminosities of several light sources. Thus a particular quartz mercury arc, using about 120 watts, gave 6×${10}^6$ times as much ultraviolet as did a 1000-watt incandescent lamp. Amplifying and recording apparatus is described which is suitable for counting as many as 100 electrons per second, arriving at random times. The paper also discusses criteria for determining the useful electron-counting sensitivity of a counter, the factors limiting its counting speed, an explanation of its mode of operation, and evidence for the existence of other ionic emissions, besides electrons, from counter cathodes. Some possible applications of the counters are mentioned.

• Received 28 September 1932

DOI:https://doi.org/10.1103/PhysRev.42.525