A simple method is described for the determination of uranium and thorium in gological materials. The samples are irradiated
in a reactor with resonance and fast neutrons behind a cadmium filter. Compared with an irradiation with the whole reactor
neutron spectrum, the matrix activities are reduced to about 1%, those of uranium (239Np) and thorium (233Pa) to about only 50 and 25%, respectively. This relative diminution of matrix activities allows the γ-measurement of239Np and233Pa as early as after two days' cooling time; in samples with high uranium contents the determination of233Pa requires one month's cooling time. This non-destructive procedure yields a detection limit of 0.1 ppm for uranium and thorium
in samples of 200 mg, with an error of ±5%.