Measuring soil moisture in the field at a scale comparable to that of satellite pixels or model raster cells is a central hydrological problem for calibration and validation. Using the standard gravimetric method, this involves taking many (100-1000) samples, which can only be achieved through expensive field campaigns. There is one way to measure soil moisture at a scale of about 30 hectares and that is, somewhat surprisingly, by measuring fast and slow neutrons at a point.

Our atmosphere is constantly being bombarded by cosmic particles, which cause showers of sub-atomic particles to hit the Earth’s service. Among these particles are fast moving neutrons (20,000km/s). As these neutrons bounce into atoms, they come into thermal equilibrium moving at a speed of about 200km/s. Water is especially good at slowing down neutrons. The ratio of slow and fast neutrons is a robust measure of the amount of water present within a radius of about 300m.This has been known and used for some time by the Helium-3 (³He) based COSMOS sensor. Unfortunately, ³He and COSMOS are relatively expensive and also subject to export limitations due to their nuclear applications.

TU Delft and Microstep MIS have developed a new neutron detector that is based on natural  boron. Natural boron consists for 20% of ¹⁰B, which captures thermal neutrons, after which it splits into a lithium atom and an alpha-particle. The alpha-particle is then detected with a ZnS‑Ag pigment, which produced thousands of photons. These photons are captured by wavelength-shifting optical fibers that guide them to a silicon photon-multiplier. The upshot is that we can build a slow and a fast neutron detector for a total of Euro 500 worth of materials. As these are not subject to export limitations, they will be installed soon in Africa for final testing.