[en] Apparatus for determining the concentration of a radioactive constituent in a sample of material is claimed. It comprises: a radiation detector; a weightometer; and an electronic means to calculate the ratio of the intensity of the radiation monitored by the detector to the mass of the sample. Apparatus for sorting samples of a material on a conveyor in accordance with the concentration of a radioactive constituent is also claimed. It comprises: a low energy souce of gamma radiation positioned above the conveyor; a gamma detector positioned below the conveyor which can receive gamma radiation from the source and the constituent; an electronic calculator adapted to receive the output signals from the detector and to produce a signal representative of the ratio of the intensity of received characteristic radiation from a sample to the mass of a sample; and sorting means responsive to the output signal for controlling the destination of a sample on leaving the conveyor