[en] A steady-state neutron source technique for determining the thermal neutron absorption cross section of geological samples is presented. It needs for analysis about 340 cm³ of crushed material. No sample saturation is required. Variations of scattering properties of samples are monitored and taken into account in the interpretation procedure. Calibration of the system is achieved using a few calibration samples prepared from pure reactor graphite and boric acid. A total time of analysis is well below 2 h per sample. Uncertainty of the cross section determination with this technique has been estimated at less than 5%. Measured cross sections for several kinds of rocks have been compared with values obtained with other methods. (author)