the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 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.- Preprint
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-1495', Anonymous Referee #1, 08 Apr 2026
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RC2: 'Comment on egusphere-2026-1495', Anonymous Referee #2, 01 May 2026
The manuscript presents a simulation-based assessment of bare and moderated CRNS detector responses under varying environmental conditions. The topic is technically relevant to the CRNS community, and the use of detector response functions is potentially useful. However, in its current form, the manuscript reads primarily as a technical sensitivity study, with limited demonstration of broader methodological or practical impact. I also have concerns regarding the justification of some modelling choices, the treatment of uncertainty, and the accessibility of the manuscript to a wider GI readership. I therefore do not recommend this manuscript for publication. However, if the Editor considers the topic suitable for GI, the authors could be invited to submit a substantially revised manuscript.
Specific comments:
L103–105: It is not clear to me why soil moisture and absolute humidity need to be fixed here. Please expand the explanation. Is the detector not sensitive to those variables?
L106–108: My understanding is that, due to lower costs, the BF3 tubes from Hydroinnova, such as the CRS1000/B, tend to be more popular for deployment, especially at larger volume or in developing countries with financial constraints. Why not base the results on the CRS1000/B design? Would the CRS1000 design not further limit the potential reach of this study?
L112–113: How would that choice affect our understanding of actual co-located setups in situ? It would seem more appropriate to model what is effectively “seen” in the field.
L144–145: Why is the detector simulated in URANOS like a detector, while plants are simulated as a “gas layer”? Have the authors tested the impacts of this simplification on the overall results? They earlier mention the potential for biomass estimation using a bare detector, so it seems appropriate to mimic the actual site conditions as accurately as possible in order to evaluate such potential.
Section 2.2.3: How did the authors consider the fact that snow density is different from liquid water density? It appears that the proposed scenarios are based on liquid water layers only.
Section 3.2.2: The authors have not clearly explained why this transfer function is needed, only that it was lacking for bare detectors. What is the intended usage of this equation? Are users expected to switch from moderated to bare detectors?
Results section: In general, I found several statements to lack a follow-up implications sentence to help the general reader understand the contribution of this paper. The manuscript appears to have been written primarily for CRNS detector specialists. Even hydrologists, agricultural scientists, and many non-specialist CRNS users may struggle to follow this manuscript.
Figure 7, right-column panels: It seems that the region where there is large variability in the signal, between approximately 1 and 10 BWE, is also where fewer simulated cases were carried out in the study. Would it improve the results to simulate this range with more points?
L398–413: There is no discussion about how the plant “gas layer” simplification could have driven these differences against empirical estimates. Could the authors discuss this?
L433–435: How could uncertainty in the non-linear equation hinder this improvement? The non-linear equation adds significant complexity compared with the linear approach developed by Baatz. It is not clear to me that this added complexity significantly changes the response to BWE sufficiently to justify abandoning the linear approach. This needs to be further investigated. Are the data points in Figure 8b significantly different from those in Figures 8c or 8d?
L480–482: Knowing that the neutron counts observed by the sensor follow a Poisson distribution, with mean = N and standard deviation = sqrt(N), and generally assuming that a considerable number of neutrons will have interacted with the soil moisture “source”, this would reduce the overall “leftover count/signal” to be attributed to other processes. Lower effective counts mean higher overall uncertainty. Yet here, and in similar papers trying to assess the nature of CRNS measurements of other quantities simultaneously, the manuscript does not adequately account for the uncertainty propagated to these other “observed” processes if the soil moisture signal is removed from the overall signal. The authors should address this.
Conclusions: This section is heavily written for readers with high expertise in CRNS and perhaps particle transport physics. It seems to target a very specialised and niche audience. If I am simply a typical CRNS user, it is unclear what I gain from this study. For example: should I replace my CRS1000/B with a CRS1000? What happens if I only have the moderated tube? Which conditions are more or less recommended for deploying both tubes from a CRS1000? How do I simultaneously utilise the signal from both moderated and bare tubes if their footprints are different?
Citation: https://doi.org/10.5194/egusphere-2026-1495-RC2
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- 1
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.