Nuclear density gauges are instruments used to measure the density and moisture content of materials, particularly in construction and civil engineering. They work by emitting low levels of radiation, typically from isotopes like cesium-137 or americium-241, which penetrate the material being tested. The amount of radiation that is reflected back to the gauge helps determine the density and moisture levels. These gauges are commonly used in the field to ensure proper compaction of soil and asphalt, which is crucial for the stability of structures like roads and buildings. They provide quick and accurate measurements, making them valuable tools for engineers and construction professionals.