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The footprint characteristics of cosmic ray thermal neutrons
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  • Jannis Christoph Jakobi,
  • Johan A. Huisman,
  • Markus Otto Köhli,
  • Daniel Rasche,
  • Harry Vereecken,
  • Heye Bogena
Jannis Christoph Jakobi
Forschungszentrum Jülich GmbH

Corresponding Author:[email protected]

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Johan A. Huisman
Forschungszentrum Jülich
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Markus Otto Köhli
Heidelberg University
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Daniel Rasche
German Research Centre for Geosciences
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Harry Vereecken
Forschungszentrum Jülich GmbH
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Heye Bogena
Forschungszentrum Juelich, GmbH
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The advance of the cosmic ray neutron (CRN) sensing method for estimating field scale soil moisture relied largely on simulations of the footprint properties of epithermal neutrons (~0.5 eV - 100 keV). Commercially available CRN probes are usually additionally equipped with a thermal neutron (< 0.5 eV) detector. The potential of these measurements is rarely explored because relevant features of thermal neutrons, such as the footprint and the sensitivity to soil moisture are unknown. Here, we used neutron transport modeling and a river crossing experiment to assess the thermal neutron footprint. We found that the horizontal thermal neutron footprint ranges between 43 and 48 m distance from the probe and that the vertical footprint extends to soil depths between 10 and 65 cm depending on soil moisture. Furthermore, we derived weighting functions that quantify the footprint characteristics of thermal neutrons. These results will enable new applications of thermal neutrons.