A neutron moisture meter is a
moisture meter utilizing
neutron scattering. The meters are most frequently used to measure the water content in soil or rock. The technique is non-destructive, and is sensitive to moisture in the bulk of the target material, not just at the surface.
Water, due to its hydrogen content, is an effective
neutron moderator, slowing high-energy neutrons. With a source of high-energy neutrons and a detector sensitive to low-energy neutrons (
thermal neutrons), the detection rate will be governed by the water content of the soil between the source and the detector. The
neutron source typically contains a small amount of a
radionuclide. Sources may emit neutrons during
spontaneous fission, as with
californium; alternatively, an
alpha emitter may be mixed with a light element for a nuclear reaction yielding excess neutrons, as with
americium in a
beryllium matrix.
Pershin, A. A.; Glushkova, L. T. (1980). "Nomenclature and design principles of neutron moisture meters and means of calibrating them". Measurement Techniques. 23 (4): 348–351.
doi:
10.1007/BF00831296.
S2CID121099209.