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

Since the first development of hectometre-scale soil moisture estimation using the epithermal(-dominated) neutron intensity from moderated Cosmic-Ray Neutron Sensing (CRNS) detectors, researchers have hypothesised that concurrent lower-energy, thermal(-dominated) neutron measurements with bare (unmoderated) detectors could also be useful for environmental sensing. Early studies in this field have highlighted the potential of neutrons observed with bare detectors 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 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 emphasise the complex and differing response of thermal-dominated, bare and epithermal-dominated, moderated neutron detectors over a range of environmental conditions with respect to their signal response and their sensitive measurement footprint. Following these results, we conclude that a paradigm shift is necessary on how we interpret these two types of neutron signals. Moderated and bare detector signals should not be treated as being independent from specific hydrogen pools due to a complex and sometimes counter-intuitive response to changes in soil moisture, biomass or snow water equivalents. 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 detector. At the same time, the simulation results shed further light on empirical findings made in previous studies, set a baseline for an improved interpretation of locally observed neutron signals in future studies, and support the future exploration of potential environmental monitoring applications of bare and moderated detectors in the context of CRNS.