Neutron density gauge, inside a box with warning stickers on its sides. This equipment is used to detect the moisture content of a material, such as soil, by using radiation. It operates by using a source in the gauge that emits radioactive neutrons. These high energy particles collide with hydrogen atoms in the test material, and lose energy in the process. A detector inside the gauge only counts the number of low-energy neutrons that have resulted from the collisions. The wetter the material, the more neutrons collide with hydrogen atoms (as there are more hydrogen atoms in wet material), and more low-energy neutrons are detected.