Signal response of bare and moderated cosmic-ray neutron sensors to varying soil and biomass conditions

Abstract. Since the first development of hectometre-scale soil moisture estimation using epithermal neutron intensity from moderated Cosmic-Ray Neutron Sensors (CRNS), researchers have hypothesize that concurrent lower‑energy, thermal neutron measurements with bare (unmoderated) detectors could also be useful for environmental sensing. Early studies in this field have highlighted the potential of thermal neutrons for monitoring biomass, plant traits, and snow water equivalent, while others underlined a soil moisture dependence that can adversely affect their usability. Similarly, varying estimates of the radius and depth of the measurement footprint of thermal neutron observations compared to that of the standard epithermal CRNS observations have been proposed. However, a generalised simulation-based assessment of the signal response and of the footprint of bare detectors for thermal neutrons is currently lacking. Against this background, this study aims to generate an improved understanding of neutron signals recorded by bare and moderated detectors through the simulation of generalised environmental scenarios using a Monte-Carlo neutron transport model. The results emphasize the differing response of thermal (bare) and epithermal (moderated) neutron detectors over a range of environmental conditions and also show differences in their sensitive measurement footprint. For example, we confirm a partially opposing response of bare and moderated detector signals to biomass changes and a generally smaller horizontal measurement footprint of the bare neutron detector. At the same time, the simulation results shed further light on empirical findings made in previous studies, they set a baseline for an improved interpretation of locally observed neutron signals in future studies, and they support the future exploration of potential environmental monitoring applications of bare and moderated detectors in the context of CRNS.