This International Standard applies to equipment consisting of – borehole logging probe equipped with a collimated radioisotope (gamma) source (during the actual measurements only) and a detector unit to measure scattered gamma radiation; – coaxial cable with winch and depth control; – other instruments and devices (power supply, pulse converter/amplifier, ratemeter, recorder, signal processing and readout units). This standard defines the terminology, specifies the types of apparatus, design and general technical requirements, specific radiation performance, electrical, mechanical and environmental performance requirements. It also defines test and calibration procedures and covers electrical safety and radiation protection issues. Further, it gives recommendations about items that should be included in the manufacturer's operation and maintenance documentation (or certificate). The purpose of this standard is to specify design requirements and performance characteristics of nuclear instrumentation used in boreholes to determine bulk rock density in situ. With suitable response charts the measurements can be equated to rock lithology and porosity.