| 31 Mar 2026
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.
Competing interests: Markus Köhli and Jannis Weimar hold leading positions at Styx Neutronica GmbH (Germany), a manufacturer of neutron detectors.
Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.
The authors present a timely and useful modeling study examining how bare and moderated cosmic-ray neutron detectors respond to changing soil moisture, biomass, soil chemistry, bulk density, and above-surface water conditions. A major strength of the manuscript is the move from idealized thermal/epithermal energy-window interpretations toward detector-response-function-based analysis, which makes the results more relevant to real CRNS instruments in the field. The paper also makes an important conceptual contribution by showing that bare detector signals are not equivalent to “pure thermal neutron” behavior, but instead reflect a mixed response that still contains the legacy of the epithermal stage of neutron transport. Overall, I found the manuscript strong, well motivated, and likely to be of interest to the CRNS community. I believe it is suitable for publication after the following revisions are addressed.
Major comments
Practical workflow / guidance for CRNS users.
As a long-time CRNS user, I would strongly appreciate a short practical workflow figure or summary box that helps readers decide when a simpler transfer-function framework is sufficient and when the more complete formulations are needed. At present, the manuscript does a very good job showing where complexity arises, but it offers less guidance on how practitioners should respond to that complexity in routine use. This would be especially helpful for newer users, for whom Eq. 6 may appear quite complex relative to the traditional appeal of CRNS as a robust and operationally simple soil-moisture method. The manuscript already demonstrates that biomass, water layers, bulk density, and detector configuration can all modify the soil-moisture response, and that the modified UTS (Eq. 6) can improve the representation of biomass effects, although its broader validation remains outside the scope of the study.
I would encourage the authors to add a brief “recommended use cases” workflow along the following lines:
I think even a simple schematic like this would make the paper much more helpful to practitioners. It would also help balance an important tension in the paper: on one hand, the manuscript correctly highlights important limitations and complexities; on the other hand, most users still need practical guidance on when those complexities materially affect soil-moisture estimation and when simpler transfer functions remain adequate.
Suggested CRNS workflow for transfer-function choice
Minor Comments
“researchers have hypothesize” → “researchers have hypothesized”.
“the simulation results shed further light on empirical findings made in previous studies, they set a baseline…”
Consider revising to avoid the comma splice, for example:
“the simulation results shed further light on empirical findings made in previous studies, set a baseline…”
or split into two sentences.
“the simulations results” → “the simulation results”.
“However, In this study” → “However, in this study”.
“A detailed description of the different homogeneous setups in given in the following sections.”
→ “A detailed description of the different homogeneous setups is given in the following sections.”
“Fig. Fig. 1a-c” → “Fig. 1a–c”.
Duplicate “Fig.” should be removed.
“The use of a homogeneous material layers…”
→ “The use of homogeneous material layers…”
“different amount of above-ground biomass” → “different amounts of above-ground biomass”.
“increments .” → remove extra space before the period.
“with an vertically expanding soil layer” → “with a vertically expanding soil layer”.
“In the case of a thermal neutrons…” → “In the case of thermal neutrons…”
Also, “continuos decrease” → “continuous decrease.”
“and thus, exhibits a lower signal-to-noise ratio making it less favourable…”
This sentence reads awkwardly. Consider:
“and thus exhibits a lower signal-to-noise ratio, making it less favourable…”
“previous studies(e.g.,” → add missing space: “previous studies (e.g.,”
The manuscript alternates a bit between “bare detector,” “bare neutron detector,” “thermal neutrons observed with bare detectors,” and “bare detector signal.” I would recommend tightening terminology so that “thermal neutrons” and “bare detector signals” are not used interchangeably, especially since one of the paper’s conceptual points is that they are not strictly the same thing.