The paper describes the construction and the performance of a GM counter designed specially for the measurement of low-energy (40 eV- 120 eV) photon fluxes. The successful operation is determined by the strength and transmissivity of the counter window as well as the cross section of the counting gas. Various tests with different radiations and counting gases indicate that the counter behaves in a predictable way and that the primary cause of a count is the production of an ion pair arising from the absorption of a soft x-ray photon. Because the discharge of the counter corresponds to the stopping of a single photon the counter may serve as a detector of absolute photon fluxes of soft radiation.
D. L. Ederer and D. H. Tomboulian, "The Performance of a Geiger-Mueller Counter in the 100-300 Å Region," Appl. Opt. 3, 1073-1078 (1964)