A nuclear density gauge is a tool used in civil construction and the petroleum industry, as well as for mining and archaeology purposes.
It consists of a radiation source that emits a cloud of 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 return to the sensor, the gauge can be calibrated to measure the density and inner structure of the test material.
Nuclear density gauges are typically operated in one of two modes:
Direct transmission: The retractable rod is lowered into the mat 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.