Direct methods have been devised for calibrating the quantum efficiencies of two x-ray detectors: a Siemens flow-proportional counter in the energy range 2.6 to 11.2 keV, and a General Electric SPG 5 scintillation counter in the range 5.9 to 50.4 keV. Literature values for x-ray attenuation coefficients of detector components are used only to estimate small corrections for x-ray scattering. The largest known errors in the measurements are statistical errors in photon counting and range from 0.03 to 0.23%. In the energy range 5.9 to 11.2 keV, efficiencies of a lithium-drifted silicon detector measured by comparison with the two calibrated detectors as references agree satisfactorily. Linear attenuation coefficients have been measured for argon, methane and P-10 gas (90% Ar; 10% CH4) in the energy range 2 to 12 keV; these can be used to calculate the efficiency of any flow-proportional counter of known geometry when used with counter gas consisting of a known mixture of argon and methane.
T. C. Loomis and H. D. Keith, "Accurate Calibration of Efficiencies of X-ray Detectors: Flow-Proportional and Scintillation Counters," Appl. Spectrosc. 29, 316-322 (1975)