An Associated Particle Imaging System for Soil-Carbon Measurements

Andrew Rosenstrom; Arun Persaud; Bernhard Ludewigt; Caitlin Hicks Pries; Charles Gary; Craig Brown; Cristina Castanha; Eoin Brodie; Mauricio Ayllon Unzueta; William Larsen

REVIEW 4 major objections 5 minor 14 references

An Associated Particle Imaging System for Soil-Carbon Measurements

T0 review · 4 major / 5 minor · reviewed 2026-08-14 · deepseek-v4-flash

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Pith's one-line read The paper demonstrates that an associated-particle imaging system based on a DT neutron generator can reconstruct neutron scattering sites in soil in three dimensions, with lateral resolution under 5 cm and depth resolution under 7 cm…

desk verdict Genuine first demonstration of API for soil carbon, but the headline 3D resolution claim is not yet supported by the data shown; the paper deserves a serious referee with major revision. read the letter →

arxiv 1908.00950 v1 pith:V6O72JNU submitted 2019-08-02 physics.ins-det nucl-exphysics.app-ph

Mauricio Ayllon Unzueta , Eoin Brodie , Craig Brown , Cristina Castanha , Charles Gary , Caitlin Hicks Pries , William Larsen , Bernhard Ludewigt

show 2 more authors

Andrew Rosenstrom Arun Persaud

This is my paper · ORCID

classification physics.ins-detnucl-exphysics.app-ph

keywords AssociatedParticleImagingsoilcarbon14MeVneutronsDTneutrongeneratorinelasticscatteringgamma-rayspectroscopytime-of-flighttomography3Dreconstruction

verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

The paper reports first experimental results for an Associated Particle Imaging (API) system aimed at non-destructive measurement of carbon in soil. The central claim is that by detecting the alpha particle emitted back-to-back with each 14.1 MeV neutron from a DT generator, the system can locate neutron scattering centers in three dimensions, and by timing the resulting gamma rays it can resolve depth to within 7 cm and lateral position to within 5 cm. On a prepared soil of sand mixed with 4% carbon by weight, the reconstructed images separate pure sand, soil, and graphite volumes, and gated gamma spectra show the characteristic carbon (4.44 MeV), silicon, and oxygen lines. If these numbers hold, the method would let soil carbon be mapped and re-mapped in place at centimeter scale, without digging cores.

What carries the argument

The carrying mechanism is alpha-tagged neutron time-of-flight, also called associated particle imaging. Each D+T fusion emits a 3.5 MeV alpha particle and a 14.1 MeV neutron in opposite directions; a thin position-sensitive yttrium aluminum perovskite (YAP) scintillator records the alpha's position and arrival time, which gives the neutron's direction and starts a clock. When that neutron scatters inelastically off a nucleus in the soil, the nucleus emits a characteristic gamma ray (for example 4.44 MeV from 12C), and a fast scintillator (LaBr3 or NaI) records the gamma's arrival time and energy. The time difference converts to distance traveled by the neutron, so each detected gamma is assigned a 3D position. A 500-MHz digital DAQ with constant-fraction discrimination supplies the timing, and energy gating of the gamma spectrum selects which element produced each event.

What would settle it

Measure the FWHM of the alpha-to-LaBr3 time difference on the actual stacked soil-box array with the same CFD settings; if it is larger than the 1.3 ns obtained on the 1-inch dry sand sample, the <7 cm depth-resolution claim would not be supported by the data. A complementary check is to place a graphite slab of known thickness at a known depth and verify that the reconstructed 4.44 MeV depth profile has an edge width no larger than about 7 cm.

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Extended reading notes

Core claim

The discovery is that a compact, sealed DT neutron generator equipped with a position-sensitive alpha detector and fast gamma-ray detectors can do 3D element-specific imaging of soil. The paper demonstrates this on a 2x3x3 stack of aluminum boxes filled with a homogeneous sand-worm-casting mixture (4% carbon by weight), with a graphite brick and a pure-sand box as standards. Reconstructed x-y and y-z projections show the separated samples, and gamma-ray spectra selected from each spatial region show the expected inelastic-scattering lines. The paper claims lateral resolution of <5 cm at 60 cm from the source and depth resolution of <7 cm, and it shows that the 4.497 MeV silicon line sits close enough to the 4.44 MeV carbon line that it must be accounted for in carbon quantification.

Load-bearing premise

The claimed <7 cm depth resolution depends on a 1.3 ns timing FWHM measured on a separate 1-inch dry sand sample and reported as part of a forthcoming publication; the paper does not directly show that the same timing holds for the stacked soil-box geometry and the 9-hour run.

Editorial extensions

If this is right

Soil carbon can be mapped in three dimensions non-destructively, with voxels of roughly a few centimeters, removing the need for destructive coring to see depth changes.
Because gamma spectra can be gated to specific regions, unwanted counts from structural materials and shielding are strongly reduced, so cleaner spectra can be obtained from buried volumes.
The same measurement yields gamma lines from carbon, silicon, oxygen, and aluminum, so one scan can report several soil constituents simultaneously.
Repeated measurements of the same location become possible, allowing carbon stock changes over time to be tracked without disturbing the soil.
At full neutron output with a faster alpha readout, the 9-hour acquisition could drop below 15 minutes, making the technique practical for field-scale surveys.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

A practical carbon-quantification workflow will need to deconvolve the 28Si 4.5 MeV line from the 12C 4.44 MeV line; the paper presents spectra but not a calibrated carbon density map, so the quantitative accuracy of the method remains open.
If the timing resolution can be pushed toward the ~1 ns the authors speculate is possible, depth resolution would improve to roughly 5 cm, which would make finer soil stratification visible; the paper does not demonstrate this improvement.
The same region-of-interest gating logic could be extended to the 6.13 MeV oxygen line, potentially mapping soil moisture or oxide content in the same scan, an application the paper does not explore.
The depth-resolution claim rests on a single timing measurement made on a 1-inch dry sand sample; a direct test on the real soil-box geometry would either confirm the 7 cm figure or reveal its limit.

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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Desk Editor's Note

read the letter

My quick take: this is a real first-results paper for a genuinely new application—associated particle imaging applied to soil carbon mapping. The experimental data are concrete, not simulations, and the ability to pull region-selected gamma spectra from soil, sand, and a graphite brick is demonstrated convincingly. The x-y image clearly separates the boxes, and the spectra show distinct C, Si, and O features, even if the 4.4 and 4.5 MeV peaks are close. Credit where due: the instrument is built, the measurements exist, and the authors are appropriately cautious about describing it as first results.

The soft spots are exactly where the reader and stress-test note point. The abstract and conclusion advertise a demonstrated 3D resolution of several centimeters, but the depth resolution (<7 cm) rests entirely on a 1.3 ns FWHM timing resolution measured with a 1-inch dry sand sample in a forthcoming publication. That timing measurement is not shown in this paper, and it is never validated in the actual stacked soil-box geometry. The y-z projection in Fig. 4 explicitly says the graphite and sand boxes are not clearly visible, so there is no direct evidence that the depth coordinate is actually resolved at the 7 cm level in the reported measurement. The gain drift noted in Fig. 5 (LaBr3 resolution degrading from ~1% to ~3% over the long run) also makes it unsafe to assume the timing behavior is unchanged from the short calibration. And resolution values are quoted without uncertainties, which is a minor but fixable omission.

These are load-bearing flaws because the central claim is the 3D resolution. But they are not fatal for an instrument paper at this stage—they are missing validation steps, not contradictions. The physics is standard tagged-neutron work, the detection scheme is sound, and the authors are honest about future work (quantitative carbon extraction, faster readout). The citation pattern is appropriate; the self-cited alpha detector paper is the right reference for component details.

Who this is for: nuclear instrumentation and applied neutron-sensing readers, especially anyone interested in soil monitoring or non-destructive element mapping. It deserves a serious referee—the experimental effort is real and the application is novel. My recommendation: send to peer review. Require the timing calibration data from the forthcoming paper, a depth calibration in the actual geometry, and error bars on the resolution claims before publication. If the authors can show a known interface resolved in the y-z projection, the paper will be solid.

Referee Report

4 major / 5 minor

Summary. The manuscript describes an Associated Particle Imaging (API) system for non-destructive soil-carbon measurements and reports first experimental results from a premixed sand sample containing 4% carbon by weight. A DT neutron generator with a position-sensitive alpha detector, LaBr3 and NaI gamma detectors, and a Pixie-16 digital DAQ are used to reconstruct the 3D location of neutron inelastic-scattering interactions from the alpha-particle position and the time difference between alpha and gamma detection. The authors present reconstructed x-y and y-z projections of a stacked soil-box phantom with graphite and sand standards, gamma-ray spectra extracted from selected spatial regions, and a table of identified inelastic lines. They claim a lateral resolution below 5 cm at 60 cm from the neutron source and a depth resolution below 7 cm, based on a 1.3 ns FWHM timing measurement, and they state that 3D resolution of several centimeters in all dimensions has been demonstrated. The paper also outlines planned improvements to increase count rate and reduce measurement time.

Significance. If the resolution and quantification claims are substantiated, the instrument would offer a genuinely useful capability: non-destructive, three-dimensional mapping of carbon and other light elements in soil with centimeter-scale resolution, on much larger representative volumes than conventional core sampling. The approach is physically direct, uses no fitted parameters, and the paper shows a clean demonstration of region-of-interest gamma spectroscopy with clear separation of carbon, silicon, and oxygen lines. The authors are also candid about limitations, including the weak visibility of the phantom in the y-z projection and energy-resolution degradation due to gain drift. However, the central resolution claims are not yet fully supported by the data shown: the depth-resolution claim depends on a calibration measurement not presented in the manuscript, and the lateral-resolution claim is inferred from geometry rather than measured in the reconstructed images. The quantitative connection to the known 4% carbon content is also not established. These gaps are fixable and do not invalidate the approach, but they are load-bearing for the paper's main conclusions.

major comments (4)

[Section 2.4, Fig. 4] The central claim of a depth resolution below 7 cm rests entirely on the 1.3 ns FWHM timing value reported in Section 2.4, which is said to come from a forthcoming publication using a 1-inch-thick dry sand sample. The timing measurement itself is not shown, and no depth calibration is performed in the stacked soil-box geometry actually used in the experiment. Figure 4 explicitly states that the graphite and sand boxes are not clearly visible in the y-z projection, so the data do not provide independent evidence that the depth coordinate is resolved at the 7 cm level in the reported measurement. Please include the timing distribution, validate the depth scale against a known interface or step in the measured geometry, or soften the depth-resolution claim accordingly.
[Section 2.2, Fig. 3] The claimed lateral resolution of <5 cm at 60 cm is derived from the stated <1 mm position accuracy of the alpha detector through simple geometry, rather than from a measured edge response or line-spread function in the reconstructed image. Figure 3 shows the x-y projection but does not provide a quantitative measure of how sharply the soil, sand, and graphite boundaries are reconstructed. Please report a lateral-resolution measurement obtained from the data, with uncertainties, or explicitly label the current value as an expected geometric limit rather than a demonstrated resolution.
[Section 3, Fig. 5] The title and abstract promise the ability to 'measure carbon in soil,' yet the paper provides no quantitative comparison of the reconstructed carbon signal to the known 4% carbon content of the soil mixture. The gamma spectra in Fig. 5 are normalized and displayed by eye, and no peak-area or carbon-yield analysis is presented. To support the soil-carbon application, please fit the 4.44 MeV carbon peak in the soil spectrum, account for the nearby 28Si line at 4.497 MeV, compare the carbon signal in the soil and graphite regions with the sand blank, and report the resulting carbon mass or density with an uncertainty. Without this step, the quantitative claim of carbon measurement is not demonstrated.
[Section 3, Fig. 5] The paper attributes the degradation of LaBr3 energy resolution from about 1% to about 3% at 4.4 MeV to gain drift during the 9-hour run. Since the depth-resolution claim relies on a timing FWHM measured under different, presumably shorter, conditions, the stability of the timing response over the long measurement is not established. Please provide evidence that the 1.3 ns timing FWHM is valid over the full 9-hour acquisition, or quantify the possible drift in the timing path and its effect on the claimed <7 cm depth resolution.

minor comments (5)

[Section 2.4] There is a typo in 'Measurements ... where used to obtain' which should read 'were used to obtain'; similarly, 'horizontal extend of the sample' should be 'horizontal extent of the sample.'
[Section 4] The sentence 'First experimental results and characterization of capabilities where obtained' should read 'were obtained.'
[Section 3, Table 1 caption] The phrase 'which it hard to resolve' is a typo; it should be 'which is hard to resolve.'
[Section 2.2] The statement that the alpha-detector position accuracy is '<1 mm on the alpha-detector, corresponding to <5 cm at 60 cm' would benefit from a brief geometrical explanation or a reference to the previous publication, since the conversion depends on the distance from the alpha detector to the neutron target and the flight path to the sample.
[General] The y-z projection in Fig. 4 is difficult to interpret because the axis labeling and color scale are not fully described; please add a scale bar, color-bar units, and a description of the coordinate origin relative to the neutron generator and the soil-box array.

Circularity Check

0 steps flagged · score 1.0 of 10

No significant circularity: the 3D resolution claims are unit conversions of independently measured hardware resolutions, not re-derivations of the paper's own inputs.

full rationale

The paper's central quantitative claims—<5 cm lateral resolution and <7 cm depth resolution—are direct consequences of measured hardware quantities stated in the paper. The lateral resolution follows from the alpha-detector position resolution (<1 mm) projected to 60 cm from the neutron source, and the depth resolution follows from the measured 1.3 ns FWHM timing resolution converted through the known 14.1 MeV neutron velocity. Neither quantity is fitted to the soil-box data being reported, and neither is defined by the carbon/silicon/oxygen spectra that constitute the experimental result. The reconstructed images and region-selected gamma spectra are independent measurements obtained with the instrument, not predictions deduced from the resolution numbers. The only self-citation, reference [10], supports component details of the alpha detector and is not load-bearing for the soil-carbon demonstration. The 'forthcoming publication' timing calibration is a legitimate concern about evidential strength, but it is not circular: it is an external empirical input measured in a separate dry-sand sample. No equation in the paper reduces to its own output, no fitted parameter is renamed as a prediction, and no uniqueness theorem or ansatz is imported from the authors' prior work. Thus the derivation chain is self-contained, with only a minor and non-load-bearing self-citation, giving a circularity score of 1.0.

Assumptions & free parameters 0 free parameters · 3 assumptions · 0 invented entities

The central reconstruction depends on standard tagged-neutron kinematics and on the assumption that characteristic gamma lines identify elements. The most fragile assumption is that the measured carbon peak is distinguishable from the nearby silicon peak. No free parameters were fitted.

assumptions (3)

standard math DT fusion emits an alpha particle and a 14.1 MeV neutron at 180 degrees in the center-of-mass frame, so the alpha position defines the neutron direction.
Used in Section 1 and Figure 1 to reconstruct interaction location in 3D from alpha position and time difference.
domain assumption Inelastic neutron scattering on C, Si, and O produces characteristic gamma lines (4.44, 4.497, 6.13 MeV) with sufficient yield for detection.
Basis for element identification in Section 3 and Table 1.
domain assumption The 12C 4.44 MeV peak can be resolved from the 28Si 4.497 MeV peak in soil spectra at the achieved detector resolution.
Required for quantitative carbon measurement; the paper itself notes this is hard to resolve for the NaI detector in Section 3.

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Cite this review

Pith. "Pith review of An Associated Particle Imaging System for Soil-Carbon Measurements." pith.science (2026). https://pith.science/paper/V6O72JNU

@misc{pith2026190800950,
  author       = {Pith},
  title        = {Pith review of: An Associated Particle Imaging System for Soil-Carbon Measurements},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/V6O72JNU}},
  note         = {Machine review of arXiv:1908.00950}
}

abstract

We present first results from experimental data showing the capabilities of an Associated Particle Imaging system to measure carbon in soil and other elements. Specifically, we present results from a pre-mixed soil sample containing pure sand (SiO$_2$) and 4% carbon by weight. Because the main isotopes of all those three elements emit characteristic high-energy gamma rays following inelastic neutron scattering, it is possible to measure their distribution with our instrument. A 3D resolution of several centimeters in all dimensions has been demonstrated.

Figures

Figures reproduced from arXiv: 1908.00950 by the authors.

**Figure 1.** Schematic illustration of API. A 14.1 MeV neutron and an alpha particle are created at a known location. The neutron undergoes inelastic scattering off an isotope in the target, for example a 12C nucleus, which emits a 4.4 MeV gamma ray. The location of the interaction can be determined in 3D from the time difference between the detection of the alpha particle and the gamma ray and from the xy-measurement of the po… view at source ↗

**Figure 2.** Left: image of the DT108-API neutron generator [PITH_FULL_IMAGE:figures/full_fig_p002_2.png] view at source ↗

**Figure 3.** API measurements of soil, graphite, and sand sam [PITH_FULL_IMAGE:figures/full_fig_p003_3.png] view at source ↗

Figures from the paper (1 more)

**Figure 5.** Figure 5: LaBr3 gamma spectra taken with of soil, sand, and graphite samples. See [PITH_FULL_IMAGE:figures/full_fig_p004_5.png]

Discussion (0). Continue with ORCID to comment.

Reference graph

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