A new monitoring system based on the monitoring of natural radiation emitted by the material in the separation zone of a jig compartment has been developed and tested in parallel with a radiometric density meter and a conventional float. The authors investigated the correlation between the separation density monitored by the meter and the intensity of the natural radiation. The measuring head of the radiometric density meter consists of a ¹³⁷Cs radiation source and a detector in the form of a scintillation counter. The signal from the detector is measured over a period of 0.15 s at the end of each cycle of pulsations (1.2 s) when the material is compressed. The control systems were installed in the second compartment of the O M20-type jig. The aim of control was to stabilise the separation density at desired values. The separation process was monitored by a radiometric density meter (RDM) to indicate changes in the separation density over a given period of time. The RDM was installed close to the upper edge of the product overflow wall to measure the density of the material separation layer reporting in half to the product and in half to the refuse. A conventional float, indicating the position of the heavy fraction in the bed, was used as a basic sensor in the control system. After first experiments the RDM replaced the float as a main sensor in the closed loop control. In the third experiment a new monitor, based on the measurement of the natural radiation emitted by the material (NRM) accumulating below the product overflow wall was used. A good correlation between the NRM indications and the RDM measurements indicates that the radiometric density meter RDM can be replaced effectively by the NRM, especially in control systems where separation density is stabilised at desired values.