Nuclear densitometry is a technique used in civil construction and the petroleum industry, as well as for mining and archaeology purposes, to measure the density and inner structure of a test material. The processes uses a nuclear density gauge, which consists of a radiation source that emits particles and a sensor that counts the received particles that are either reflected by the test material or pass through it. By calculating the percentage of particles that arrive at the sensor, the gauge can be calibrated to measure the density.

In geotechnical engineering, a nuclear densometer or soil density gauge is a field instrument used to determine the density of a compacted material. The device uses the interaction of gamma radiation with matter to measure density, either through direct transmission or the "backscatter" method. The device determines the density of material by counting the number of photons emitted by a radioactive source (cesium-137) that are read by the detector tubes in the gauge base. A 60-second time interval is typically used for the counting period.

Different variants are used for different purposes. For density analysis of very shallow objects such as roads or walls, a gamma source emitter such as ¹³⁷Cesium is used to produce gamma radiation. These isotopes are effective in analyzing the top 10 in (25 cm) with high accuracy. ²²⁶Radium is used for depths of 330 yd (300 m). Such instruments can help find caves or identify locations with lower density that would make tunnel construction hazardous.

Nuclear density gauges are typically operated in one of two modes:

Direct transmission: The retractable rod is lowered into the material through a pre-drilled hole. The source emits radiation, which then interact with electrons in the material and lose energy and/or are redirected (scattered). Radiation that loses sufficient energy or is scattered away from the detector is not counted. The denser the material, the higher the probability of interaction and the lower the detector count. Therefore, the detector count is inversely proportional to material density. A calibration factor is used to relate the count to the actual density.

Backscatter: The retractable rod is lowered so that it is even with the detector but still within the instrument. The source emits radiation, which then interact with electrons in the material and lose energy and/or are redirected (scattered). Radiation that is scattered towards the detector is counted. The denser the material, the higher the probability that radiation will be redirected towards the detector. Therefore, the detector count is proportional to the density. A calibration factor is used to correlate the count to the actual density.

Many devices are built to measure both the density and moisture content of material. This is important to the civil construction industry specifically as both are essential to verifying suitable soil conditions to support structures, streets, highways, and airport runways.

A nuclear densometer is used on a compacted base to establish its percentage of compaction. Before field tests are performed, the technician performs a calibration on the gauge which records the 'standard count' of the machine. Standard counts are the amount of radiation released by the two nuclear sources inside the machine, with no loss or leakage. This allows the machine to compare the amount of radiation released to the amount of radiation received. With the use of a 3/4" diameter rod a hole is created in the compacted base by hammering the rod into the base to produce a hole that the densometer's probe can be inserted into. The densometer is placed on top of the hole, and then the probe is inserted into the hole by unlocking the handle at the top of the probe. One source produces radiation that interacts with the atoms in the soil, and is then compared to the standard count, to calculate the density. The other source interacts with hydrogen atoms to calculate the percentage of water in the soil.