Radioisotope instrument has been designed for non-dispersive X-ray fluorescence logging of elements with close atomic numbers and X-ray energies of 5-12keV. It contains plutonium-238 sources, two SRM-19 proportional counters and balanced filters. Its diameter is 52mm. Gamma-quantum impluses with equal energies have different exit amplitudes of proportional counters due to differences (about 200V) in supply voltage. It gives the possibility to obtain different count velocities with two filters, simultaneously using a dual spectrometer, one singlechannel communication line between it and the counters and a subtractive scheme. -English summary