Study of neutron response using time of flight technique in ISMRAN detector

D.K. Mishra; L.M. Pant; P.K. Netrakanti; R. Dey; R. Sehgal; S.P. Behera; V. Jha

REVIEW 4 major objections 5 minor 10 references

Study of neutron response using time of flight technique in ISMRAN detector

T0 review · 4 major / 5 minor · reviewed 2026-08-07 · deepseek-v4-flash

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Pith's one-line read This paper establishes a time-of-flight method for measuring fast-neutron energy response and capture time in the ISMRAN detector array, reporting a capture time of 68.29 ± 9.48 μs that matches the delayed signature of inverse beta decay.

desk verdict A credible TOF-based neutron response measurement for ISMRAN, with a capture-time result that is real but currently overinterpreted as the Gd capture time. read the letter →

arxiv 2506.13826 v1 pith:U6IGANSK submitted 2025-06-15 physics.ins-det nucl-ex

R. Dey , P.K. Netrakanti , D.K. Mishra , S.P. Behera , R. Sehgal , V. Jha , L.M. Pant This is my paper · ORCID

classification physics.ins-detnucl-ex PACS 29.40.Mc29.30.Hs

keywords time-of-flightneutronspectroscopyplasticscintillatorbarsgadoliniumcapturereactorantineutrinodetectioninversebetadecaybackgroundAm-Besourcetime

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 a measurement of how fast neutrons deposit energy in the ISMRAN detector, a 9×10 array of plastic scintillator bars wrapped in gadolinium-coated foil, using a time-of-flight technique with an americium-beryllium source. By tagging the 4.438 MeV gamma ray that accompanies each source neutron, the authors convert neutron flight times into kinetic energies and map them to proton-recoil light yields. They then use the tagged neutrons as prompt events and search the rest of the array for delayed neutron-capture candidates, obtaining a characteristic capture time of 68.29 ± 9.48 μs. That time matches the Monte Carlo prediction and is close to the delayed-neutron capture time expected in inverse beta decay events, which matters because it means the capture-time signature alone cannot separate fast-neutron background from genuine reactor antineutrino events; additional spatial and energy-shape cuts are needed.

What carries the argument

The central mechanism is neutron tagging by time of flight. A $^{241}$Am-$^{9}$Be source emits a 4.438 MeV gamma ray in about 60% of disintegrations, and detecting that gamma in a CeBr$_3$ scintillator provides the start time; the stop time comes from the first column of the ISMRAN array, so the flight time to each bar separates gammas from neutrons. From the flight distance $L$ and time $t$, the neutron kinetic energy is obtained from $E_n = \frac{1}{2}m(L/t)^2$, written as $E_n = \alpha^2 L^2/t^2$ with $\alpha = 72.3\ \sqrt{\text{eV}}\,\mu\text{s/m}$. The proton-recoil light yield in the bars is then fitted to an empirical formula $E_{\rm dep} = A E_n - B(1 - e^{-C E_n^D})$. For the capture-time measurement, the tagged neutron in the first column acts as the prompt event and delayed candidates in the remaining bars are required to satisfy energy and multiplicity cuts; the $\Delta T_{\rm cap}$ distribution is fitted by a single exponential (neutron thermalization and capture) plus a constant (accidental background), yielding the characteristic capture time.

What would settle it

Measure the $\Delta T_{\rm cap}$ distribution separately for low-energy and high-energy tagged neutrons; if the 68 μs component is genuine capture on gadolinium it should be independent of the neutron's initial energy, whereas if thermalization contributes significantly the fitted time constant will shift between energy bins. A second check is to replace the gadolinium-coated wrappers with plain reflective foil and observe whether the measured $\tau$ disappears or changes; if it does not, the fitted exponential is not dominated by gadolinium capture.

Watch

Extended reading notes

Core claim

The central claim is that the ISMRAN array's response to fast neutrons can be measured end-to-end with a data-driven time-of-flight method, and that the measured neutron capture time distribution is characterized by $\tau = 68.29 \pm 9.48\ \mu$s. The experiment uses a $^{241}$Am-$^{9}$Be source, with a cerium bromide detector registering the 4.438 MeV de-excitation gamma ray as the start signal and the first column of plastic scintillator bars as the stop signal. With the gamma and neutron bands cleanly separated in time of flight, the neutron kinetic energy is computed from flight distance and time, and the resulting proton-recoil energy deposition is parametrized by an empirical formula. The same tagged neutrons serve as a prompt population; delayed events recorded in the rest of the array within 1000 μs, selected by energy-sum, per-bar energy, and number-of-bars cuts, produce a capture-time distribution whose exponential-plus-constant fit gives the reported $\tau$. On these measurements the paper builds the claim that fast-neutron capture is indistinguishable in time from inverse $\beta$ decay delayed captures, and therefore background rejection must rely on spatial and energy-distribution variables rather than timing alone.

Load-bearing premise

The load-bearing assumption is that the delayed-event sample selected by the energy-sum, per-bar energy, and bar-multiplicity cuts is dominated by neutron capture on gadolinium and that its capture-time distribution is a single exponential, so the fitted 68.29 μs is the true gadolinium capture time rather than an effective average over several capture processes.

Editorial extensions

If this is right

The measured $\tau$ of 68.29 ± 9.48 μs can be used to set the delayed-coincidence window for inverse beta decay candidate selection in ISMRAN, since true delayed events should follow the same capture-time distribution.
The proton-recoil light-yield parametrization from Eq. (3) gives input for simulating fast-neutron backgrounds, improving the modeling of correlated background in reactor ON and OFF running.
Because fast-neutron captures and inverse beta decay delayed captures have indistinguishable time constants, the array must rely on its segmented geometry, energy-ratio variables, and machine-learning classification to separate the two populations.
The time-of-flight tagging method itself is a reusable, data-driven way to measure neutron capture times in a segmented scintillator array without a dedicated neutron beam.

Reading between the lines

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

Editorial inference: the same prompt-delayed pairing could be applied with the tagged neutron's kinetic energy as an additional variable, allowing a two-dimensional (energy, capture-time) model that separates thermalization from pure capture and could tighten background rejection.
Editorial inference: if applied to a detector with variable gadolinium loading or without gadolinium, the method could turn the measured $\tau$ into an in-situ diagnostic of capture-material fraction.
Editorial inference: the reported single-exponential fit could be tested against a two-component hydrogen-plus-gadolinium model; the fitted amplitudes would reveal whether the current cuts are actually gadolinium-dominated before the array is used for reactor antineutrino monitoring.
Editorial inference: the approach could be transferred to other segmented plastic-scintillator antineutrino detectors, since it uses only a compact Am-Be source and a gamma trigger rather than a pulsed neutron beam.

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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

Plain take: this is a useful detector-characterization measurement, not a breakthrough. The new bits are the TOF-based neutron response on the full 90-bar ISMRAN array and the capture-time distribution measured by tagging neutrons in the first column and looking for delayed events in the rest of the array. That combination is new relative to the group's earlier prototype and D-D/D-T work, and the technique is sensible. The TOF gamma-neutron separation in Fig. 2/3 looks clean, and the response parametrization in Fig. 5 is plausible, with reasonable fit errors.

The soft spot is the capture-time interpretation. Section 2.2 fits ΔT_cap with a single exponential that explicitly includes both thermalization and capture, then calls the resulting tau = 68.29 ± 9.48 us "the characteristic neutron capture time" and claims it is "very similar" to IBD delayed neutron capture. But a single exponential that lumps thermalization with capture is an effective time constant for Am-Be neutrons under your cuts, not necessarily the Gd capture time. The problem is compounded by the abstract: "good agreement with GEANT4" is never shown. No simulated curve, no simulated tau value, no goodness-of-fit. That makes the agreement claim unverifiable. The summary's statement that fast-neutron capture distributions are indistinguishable from νe events goes beyond what the data support, because the selected delayed sample may include captures on H or other materials, and the source energy/spatial distribution differs from IBD neutrons. These are fixable in revision: show the GEANT4 prediction, separate thermalization from capture in the fit if possible, or at minimum label tau as an effective time constant and discuss what it means for background rejection.

Systematic uncertainties are absent, which is a real gap for a measurement paper. The response parametrization would benefit from at least an estimate of the TOF binning smearing and source-position uncertainties. None of this is fatal; the central measurement is credible and the TOF tagging is a legitimate way to study correlated backgrounds.

Who is this for? ISMRAN collaborators and anyone working on segmented plastic scintillator detectors with Gd-coated bars. It deserves a serious referee, but I would want the authors to address the capture-time interpretation and provide the missing GEANT4 comparison before it is accepted. The paper is honest in its methods and clearly builds on the group's own prior work, so I have no concern about the citation pattern.

Referee Report

4 major / 5 minor

Summary. The paper reports TOF-based measurements of the fast-neutron energy response of the ISMRAN plastic-scintillator array using an Am-Be source with a CeBr3 detector triggering on the 4.438 MeV gamma ray. The measured energy deposition versus neutron kinetic energy is fitted with the empirical formula of Eq. (3), and the Delta-T_cap distribution between neutron-tagged prompt events in the first column and delayed candidates in the rest of the array is fitted with an exponential plus a constant term, yielding tau = 68.29 +/- 9.48 microseconds. The authors claim good agreement with GEANT4 simulation and argue that the results will be useful for discriminating correlated fast-neutron background from true IBD events.

Significance. If the response parametrization and the capture-time measurement are robust, they provide useful inputs for ISMRAN background rejection and validate a data-driven method for tagging neutron captures. The paper's strengths include a clear gamma-neutron separation in the TOF spectra (Figs. 2 and 3), a direct measurement of the proton-recoil response curve, and a plausible data-driven approach to the capture-time distribution. However, the absence of a quantitative GEANT4 comparison and the effective rather than pure Gd nature of the fitted tau currently limit the strength of the claims, especially the comparison with IBD delayed-neutron capture times.

major comments (4)

[Section 2.2, Fig. 6(b)] The fit function is described only as "an exponential term for the neutron thermalization and capture time in PSBs and a constant term." Consequently, tau = 68.29 +/- 9.48 microseconds is an effective time constant for the combined thermalization-plus-capture process, not the Gd capture time alone. The abstract's characterization as a "characteristic neutron capture time" and the summary's statement that the distribution is "indistinguishable from those of nu_e events" are therefore not justified by the fit. Please separate the thermalization component (e.g., with a two-exponential fit or a GEANT4 template with known capture time) or explicitly relabel tau as an effective time constant and adjust the physics claims accordingly.
[Abstract and Section 2.2] The statement that tau is "in good agreement with GEANT4 based MC simulation" is not supported by any quantitative comparison in the paper: no simulated Delta-T_cap spectrum, no simulated tau value, and no goodness-of-fit metric are shown. Please include the GEANT4 prediction overlaid on Fig. 6(b) and report the fitted MC tau with its uncertainty and the pull relative to the data.
[Section 2.2, delayed-event selection] The delayed-event cuts (Esum 2.6-10 MeV, Nbars 4-14, per-PSB 0.25-10 MeV) are stated to be benchmarked with GEANT4, but the composition of the selected sample is not demonstrated. If a substantial fraction of captures occurs on hydrogen or other nuclides rather than on Gd, the fitted tau is a weighted average over capture channels and the comparison with the IBD Gd capture time is misleading. Please report the simulated fractions of capture channels in the accepted sample and, if needed, adjust the interpretation.
[Throughout the manuscript] No systematic uncertainties are reported; the quoted errors on the Eq. (3) parameters and on tau appear to be statistical only. The results depend on the source-detector distance L, the T0 calibration from the gamma peak, the CeBr3 energy calibration, and the PSB energy scale. The propagation of these systematic effects into the Edep-versus-En parametrization and into tau should be quantified before the results are used for background discrimination.

minor comments (5)

[Section 2.2] Please write out the explicit fit function used for Fig. 6(b), including the constant term and the fit range; the current description is insufficient to reproduce the fit.
[Fig. 5] The fitted parameter values are typeset in a garbled way ("A : 0.41 0.03 +/- ..." and similar); please correct the formatting so that each parameter and its uncertainty are unambiguous.
[Fig. 4(a)] The panel annotations "pos PS" and "Z0.5" appear incomplete; please clarify that these denote the PSB hit position and the z-window used in the event selection.
[Section 2.2, Fig. 6(a)] The phrase "scaled with the neutron events above Delta-T > 300 microseconds" is unclear; please rephrase to describe how the gamma-tagged distribution was normalized for comparison with the neutron-tagged distribution.
[References] The reference list contains only two self-references; adding standard references for the Am-Be neutron spectrum, the GEANT4 simulation toolkit, and neutron capture on Gd would help readers place the work in context.

Circularity Check

0 steps flagged · score 0.0 of 10

No circular reduction: the TOF energy response and the capture-time distribution are fitted directly from the measured data, with only a non-load-bearing self-citation of an empirical fitting form.

full rationale

The paper's two central measurements are self-contained in the sense relevant to circularity. The fast-neutron kinetic energy is derived from the measured time of flight via Eq. (1) using the known source-to-detector distance, and the Edep-versus-En response is an empirical fit to the present tagged-neutron data shown in Fig. 5. The fitting form, Eq. (3), is attributed to the authors' prior JINST paper [2], but the parameters A, B, C, D are fitted to the new TOF data; the cited prior work is an external characterization of the same detector bars and does not inject the present result. Thus the self-citation is not load-bearing. The capture-time measurement is likewise a direct fit: the Delta-Tcap distribution is formed from tagged neutron prompt events and delayed candidates in different PSBs, and the quoted tau = 68.29 +/- 9.48 microseconds comes from fitting an exponential plus constant to that measured distribution. No simulation value is inserted into the fit, so the result is not forced by construction. The abstract's claim of good agreement with GEANT4 and the statement that the delayed-event selection cuts are GEANT4-benchmarked (Sec. 2.2) are not accompanied by a displayed simulated tau or goodness-of-fit, which is a missing-evidence concern rather than a circular reduction. The caveat that the fitted exponential combines thermalization and capture and may therefore yield an effective time constant affects the physical interpretation of tau for IBD discrimination, but it does not make the measurement circular. No step in the derivation reduces by definition to its own input.

Assumptions & free parameters 6 free parameters · 5 assumptions · 0 invented entities

The paper introduces no new physical entities. Its central claims rest on four fitted response-curve parameters, a set of benchmarked selection cuts, and the assumed validity of the GEANT4 simulation and of a single-exponential capture-time model.

free parameters (6)

A (response curve amplitude) = 0.41 ± 0.03
Fitted to measured Edep vs En data in Fig. 5 using Eq. 3.
B (response curve offset parameter) = 0.67 ± 0.13
Fitted to measured Edep vs En data in Fig. 5 using Eq. 3.
C (response curve exponent scale) = 0.13 ± 0.03
Fitted to measured Edep vs En data in Fig. 5 using Eq. 3.
D (response curve exponent) = 2.54 ± 0.40
Fitted to measured Edep vs En data in Fig. 5 using Eq. 3.
Exponential amplitude in ΔTcap fit
Nuisance parameter in the fit to the capture-time distribution; value not quoted.
Constant background rate in ΔTcap fit
Nuisance parameter representing accidental coincidences in the ΔTcap distribution; value not quoted.

assumptions (5)

standard math Classical non-relativistic TOF formula En = (1/2)m(L/t)^2 is valid for the neutron energies considered.
Eq. 1 uses this formula; the highest tagged neutron energy is roughly 8 MeV, so the non-relativistic approximation introduces a small but unquantified error.
domain assumption The 4.438 MeV gamma from the 9Be(alpha,n)12C reaction provides the emission time reference for the associated neutron.
The TOF start is defined by CeBr3 detection of this gamma; this standard nuclear physics fact is used in Sec. 2.1.
domain assumption GEANT4 simulation correctly models neutron transport, capture, and the detector response in ISMRAN.
Sec. 2.2 says selection criteria are benchmarked with GEANT4 and the measured tau agrees with GEANT4, but the simulation is not independently validated in this paper.
domain assumption The scintillation light yield in PSB is described by the empirical parametrization of Eq. 3 from prior work.
Eq. 3 is taken from ref [2] and assumed to hold for the ISMRAN bars.
domain assumption The ΔTcap distribution can be described by a single exponential decay plus constant background.
The fit in Fig. 6 uses this form, but the text notes thermalization and capture are combined, so a single exponential is a simplification.

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Cite this review

Pith. "Pith review of Study of neutron response using time of flight technique in ISMRAN detector." pith.science (2026). https://pith.science/paper/U6IGANSK

@misc{pith2026250613826,
  author       = {Pith},
  title        = {Pith review of: Study of neutron response using time of flight technique in ISMRAN detector},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/U6IGANSK}},
  note         = {Machine review of arXiv:2506.13826}
}

abstract

We report the measurements of the fast neutron energy response in Indian Scintillator Matrix for Reactor Anti-Neutrinos (ISMRAN) detector consisting of an array of 9$\times$10 Plastic Scintillator Bars (PSBs) at BARC, Mumbai. ISMRAN is an above ground detector setup at $\sim$13 m from the Dhruva reactor core for the detection of reactor anti-neutrinos (${\overline{\ensuremath{\nu}}}_{e}$) via the inverse beta decay (IBD) process. The dominant sources of reactor-related background in the vicinity of the detector are high energy $\gamma$-rays and fast neutrons. Therefore, a good understanding of fast neutron response in PSB is an essential pre-requisite for suppression and discrimination of the fast neutron background from IBD events. Kinetic energies of the fast neutron were determined using the Time-of-Flight (TOF) technique, which is used to get the scintillation light yield due to recoiling proton in PSB. We also measured the fast neutron capture time distribution in ISMRAN array using a novel technique involving TOF of the measured fast neutrons. The observed characteristic neutron capture time ( $\tau$ ) of 68.29 $\pm$ 9.48 $\mu$s is in good agreement with GEANT4 based MC simulation. These experimentally measured results will be useful for discriminating correlated and uncorrelated (accidental) background events from the true IBD events in reactor ON and OFF conditions inside the reactor hall.

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Reference graph

Works this paper leans on

10 extracted references · 3 canonical work pages

[1]

Scherzinger

J. Scherzinger. et al.: The light-yield response of a NE-213 liquid-scintillator detector measured using 2-6 MeV tagged neutrons. Nuclear Instruments and Methods in Physics Research Section A. 840, 121-127 (2016), doi: org/10.1016/j.nima.2016.10.011

work page doi:10.1016/j.nima.2016.10.011 2016
[2]

P. K. Netrakanti. et al.: Measurements using a prototype array of plastic scintillator bars for reactor based electron anti-neutrino detection. Nuclear Instruments and Methods in Physics Research Section A. 1024, 166126 (2022), doi: org/10.1016/j.nima.2021.166126

work page arXiv 2022
[3]

Scherzinger

J. Scherzinger. et al.: Tagging fast neutrons from an ^ 241 Am/ ^ 9 Be source. Applied Radiation and Isotopes. 98, 74-79 (2015), doi: org/10.1016/j.apradiso.2015.01.003

doi:10.1016/j.apradiso.2015.01.003 2015
[4]

R. Dey. et al.: Characterization of plastic scintillator bars using fast neutrons from D-D and D-T reactions. Journal of Instrumentation. 16, P08029 (2021), doi: https://iopscience.iop.org/article/10.1088/1748-0221/16/08/P08029

work page doi:10.1088/1748-0221/16/08/p08029 2021
[5]

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Show all 10 references

[9]

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Pith tools

Reviewed August 7, 2026 · model on record in the stance chip above.

[1] [1]

Scherzinger

J. Scherzinger. et al.: The light-yield response of a NE-213 liquid-scintillator detector measured using 2-6 MeV tagged neutrons. Nuclear Instruments and Methods in Physics Research Section A. 840, 121-127 (2016), doi: org/10.1016/j.nima.2016.10.011

work page doi:10.1016/j.nima.2016.10.011 2016

[2] [2]

P. K. Netrakanti. et al.: Measurements using a prototype array of plastic scintillator bars for reactor based electron anti-neutrino detection. Nuclear Instruments and Methods in Physics Research Section A. 1024, 166126 (2022), doi: org/10.1016/j.nima.2021.166126

work page arXiv 2022

[3] [3]

Scherzinger

J. Scherzinger. et al.: Tagging fast neutrons from an ^ 241 Am/ ^ 9 Be source. Applied Radiation and Isotopes. 98, 74-79 (2015), doi: org/10.1016/j.apradiso.2015.01.003

doi:10.1016/j.apradiso.2015.01.003 2015

[4] [4]

R. Dey. et al.: Characterization of plastic scintillator bars using fast neutrons from D-D and D-T reactions. Journal of Instrumentation. 16, P08029 (2021), doi: https://iopscience.iop.org/article/10.1088/1748-0221/16/08/P08029

work page doi:10.1088/1748-0221/16/08/p08029 2021

[5] [5]

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[7] [7]

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