Ruprecht Karls Universität Heidelberg

Cosmic Ray Neutron Sensing

measuring soil moisture non-invasively by cosmic-ray induced neutron

Cosmic Neutron Showers
  • generated constantly in the top atmosphere
  • produce all sorts of high energetic particles, including neutrons
  • in total around 100-150 n/(m2s)
  • in the air and in the soil neutrons are also constantly created

Soil Moisture Sensitivity
  • depending on the amount of water (hydrogen)
  • neutrons are either reflected (dry)
  • or absorbed (wet)
  • the above-ground neutron density scales by soil moisture, air humidity and biomass 

From high energy and nuclear physics to environmental sciences


Soil Moisture Map,
measured at the UFZ Site Schäfteral,
with the Cosmic Ray Rover

from dry regions more neutrons enter the detector than from wet ones
  Measurement Range
  • Neutron scatter off other nuclei in the atmosphere and the soil and form a neutron gas above the ground
  • they can easily travel several hundred meter in air
  • the range of influence is several hundred m2 and 50 cm in depth
  • measurement depth and footprint radius depend on the en

  • fast neutrons are slowed down by a polyethylene casing (moderator), then
  • thermal neutrons are converted by 3Helium gas or 10Boron layers and
  • detected by a wire-based proportional counter
  • several modules increase the detection efficiency

Suited for time resolved field-scale soil moisture determination

  Bachelor and Master theses


EDV Abteilung