A method is presented to determine the radio-equilibrium of the uranium–actinium series in rock samples by low energy photon spectroscopy. The equilibrium pattern is indicated by the relative abundance of the isotopes 238U, 231Pa, 226Ra, 222Rn and 210Pb. For whole rock analysis no elaborate sample preparation is needed and the measurements can be performed in the field. On chemically separated material the uranium-234, -235 and -238 ratio can also be determined by the present method. A practical table for radioactive rock analysis, listing intense photons below 250 keV energy, is also included.Measurements on uranium-bearing rocks indicate that radioactive disequilibrium is common, especially in near-surface specimens. This should be taken into consideration in calculating equivalent uranium (eU) values from scintillometric measurements, and in interpreting anomalies by radon geochemistry. The deduced isotopic ratios can be used for age determination and to monitor industrial enrichment of uranium-235.

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