Nuclear moisture density gauges have been in use for over thirty years. They offer the user a quick and accurate means of determining density and moisture content of soils, asphalt, rooftops and concrete and they all essentially use the same design and basic approach to determining these measurements.
All moisture density gauges use two different radioactive sources to produce two different types of radiation. One of the radioactive sources, Cesium 137, emits gamma ray photon radiation to determine density, while the other radioactive source, Americium 241 (combined with non-radioactive Beryllium), emits neutron radiation to determine moisture content.
As seen in the depiction these sources are double encapsulated before being installed inside the gauge. This double encapsulation undergoes extensive integrity testing and is virtually impenetrable, forming a solid core of metal around the sources. The encapsulated source is then fused at the base of the metal source rod, giving the operator three levels of distinct metal shielding from the source.
The sources themselves have been solidified in a way that prevents them from powdering or leaking. Note the physical size of the density source bead – it is approximately half the size of a piece of rice. The moisture pellet is smaller than a baby aspirin. The double encapsulated metal housing around these sources is about the size of a pencil eraser.
Both sources have been laser fused inside the gauge, leaving no way to gain access. There has never been a case where the metal encapsulation has failed or been penetrated.
As seen in the depiction, the gauges all have a handle at the top of metal rods. These long rods are called source rods and depth rods. Some models house these rods in an enclosed tower.
The handle at the top of the rods has a release mechanism that is depressed or triggered to allow the source rod to be lowered out of the gauge into positions starting at the base of the gauge and continuing at predetermined notched positions below the surface. These notched positions can be readily seen along the spine of the depth rod (gauges with a tower assembly will have a ruler-like strip along the side showing rod position). The notches are usually spaced 1-2 inches apart along an 8-12 inch rod, allowing the operator to choose the notch that matches the depth of the desired measurement.