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REVIEW 1 major objections 7 minor 14 references

Influence of Self-Absorption on Pulse Shape Discrimination in Organic Glass Scintillators

T0 review · 1 major / 7 minor · reviewed 2026-08-15 · deepseek-v4-flash

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Pith's one-line read Organic glass scintillators lose neutron-gamma discrimination as they grow because self-absorption cuts the number of detected photoelectrons; their normalized discrimination quality remains constant.

desk verdict Solid incremental OGS characterization: normalized FOM is a useful size-scaling metric, but the 'intrinsic' claim needs direct pulse-shape measurements beyond the 25 mm sample. read the letter →

arxiv 2505.08513 v1 pith:UXGZU2JU submitted 2025-05-13 physics.ins-det

Lukasz Adamowski , Martyna Grodzicka-Kobylka , Tomasz Szczesniak , Agnieszka Syntfeld-Każuch , Lukasz Swiderski , Adam Kisiel This is my paper · ORCID
classification physics.ins-det PACS 29.40.Mc
keywords organicglassscintillatorself-absorptionpulseshapediscriminationneutron-gammafigureofmeritphotoelectronyieldchargecomparisonmethodsizeeffects
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

Organic glass scintillators are promising solid alternatives to flammable liquid scintillators for detecting neutrons and gamma rays, but their performance depends on detector size. This paper claims that the loss of neutron-gamma discrimination in taller scintillators is caused by self-absorption of the scintillation light, which reduces the number of photoelectrons reaching the photomultiplier, and not by any change in the pulse shape itself. The evidence comes from five cylinders of the same diameter with heights from 25 mm to 125 mm: as height grows, both photoelectron yield and figure of merit fall, while the figure of merit normalized to the square root of the photoelectron number stays constant. If the claim is right, detector performance at a given energy can be predicted from a single measurement, and self-absorption sets a concrete upper limit on useful detector size.

What carries the argument

The load-bearing quantity is the normalized figure of merit, $\mathrm{FOM}/\sqrt{N_{\mathrm{phe}}}$, which stays constant across all five heights and is proposed as an intrinsic scintillator property. It is built from charge-comparison pulse shape discrimination, where each pulse gets a parameter $\mathrm{PSD} = (Q_{\mathrm{long}} - Q_{\mathrm{short}})/Q_{\mathrm{long}}$, and the figure of merit is the separation of the neutron and gamma centroids divided by the sum of their full widths at half maximum. The decisive experimental design is a set of five cylinders with identical 25.4 mm diameter and heights from 25 mm to 125 mm, so the self-absorption path length grows with height without any stacked-sample interface losses. The pulse shape itself is examined with a delayed single-photon coincidence setup and fitted with a three-exponential decay using a genetic algorithm, showing no significant change in decay times across energy and supporting the interpretation that FOM loss is statistical rather than a shape change.

What would settle it

Measure the three-exponential decay times and intensities of neutron- and gamma-induced pulses on a 125 mm cylinder with the same delayed single-photon setup; if the fast, medium, and slow component intensities differ from the 25 mm sample beyond uncertainties, then FOM loss is not purely a photoelectron-statistics effect and normalized FOM is not fully intrinsic.

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Extended reading notes

Core claim

The central claim is that light self-absorption inside an organic glass scintillator degrades neutron-gamma pulse shape discrimination purely by cutting photoelectron statistics, not by altering the scintillation pulse shape. At 300 keVee the figure of merit drops from $2.37 \pm 0.07$ for the 25 mm sample to $1.70 \pm 0.05$ for the 125 mm sample, while the photoelectron yield falls from $4310 \pm 420$ to $1760 \pm 330$ per MeV. The ratio $\mathrm{FOM}/\sqrt{N_{\mathrm{phe}}}$ stays within $0.066$ to $0.074$, that is, constant within uncertainties. The paper therefore concludes that normalized FOM at a given energy is an intrinsic property of the scintillator, that optimal charge-comparison gates are independent of detector size, and that self-absorption imposes a limit on the maximum useful detector size. The samples retain higher discrimination quality than the EJ-276 plastic comparison and remain broadly comparable to liquid EJ-309.

Load-bearing premise

The argument assumes the scintillation pulse shape does not change as the detector grows, so all loss of discrimination is blamed on fewer detected photons; that assumption is inferred from the constant normalized figure of merit, not measured on the taller samples.

Editorial extensions

If this is right

  • Practical detector design must treat self-absorption as a size ceiling: FOM at 300 keVee falls steadily from $2.37$ at 25 mm to $1.70$ at 125 mm, and the trend continues.
  • A single measurement of normalized FOM at a given energy, combined with the photoelectron-yield scaling curve, predicts the FOM of untested detector heights.
  • Optimal charge-comparison gates are size-independent but energy-dependent; for wide energy ranges, choosing gates tuned to the lowest energy avoids a catastrophic loss of separation at low energy with only a small penalty at high energy.
  • With FOM above 1 even for the tallest tested sample, organic glass scintillators remain usable at moderate sizes and, thanks to short pulses, can sustain higher count rates than trans-stilbene crystals.

Reading between the lines

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

  • A direct measurement that would sharpen the claim is a single-photon pulse-shape run on a 125 mm cylinder; the paper's height-independence of pulse shape is inferred, not measured.
  • If normalized FOM is truly size-invariant, the same one-sample-plus-scaling characterization could be applied to other self-absorbing scintillators, reducing the number of prototypes needed to predict field performance.
  • The linear rather than exponential falloff of yield with height hints that reflective boundaries and source geometry, not simple Beer-Lambert attenuation, set the effective path length; a light-transport model could separate those contributions.
  • Because optimal gates vary with energy in every sample tested, a fixed-gate instrument may silently lose low-energy separation; an energy-dependent or two-gate readout scheme is a testable engineering response.
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Editorial analysis

A structured set of objections, weighed in public.

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

Referee Report

1 major / 7 minor

Summary. The manuscript reports a systematic study of five cylindrical organic glass scintillator samples of identical 25.4 mm diameter and heights from 25 mm to 125 mm, plus a trans-stilbene reference. Using the Bertolaccini method it measures photoelectron yield, and using a charge-comparison pulse-shape-discrimination (PSD) analysis with scanned gates it measures the neutron-gamma Figure of Merit (FOM) over energies from 100 to 1000 keVee. It finds that both photoelectron yield and FOM decrease with increasing scintillator height, while the FOM normalized by the square root of the number of photoelectrons remains approximately constant. The decrease is attributed to light self-absorption. The paper also reports Bollinger-Thomas pulse-shape measurements for the 25 mm OGS and for trans-stilbene, fitted with a genetic algorithm to extract three exponential decay components, and compares OGS and stilbene decay times and intensities.

Significance. If the central claim holds, the constant normalized FOM would be a practically useful result: it implies that, at a given deposited energy, the neutron-gamma discrimination performance of an OGS detector can be predicted from a single photoelectron-yield measurement, and that the degradation of PSD with size is driven purely by photoelectron statistics rather than by a change in the intrinsic pulse shape. The dataset is valuable: five physical heights, repeated measurements with re-coupling, propagated uncertainties, and a consistent analysis pipeline. The comparison with EJ-276 plastic and trans-stilbene adds context. The paper's direct measurements of yield and FOM are sound; the main caveat is that the 'intrinsic property' conclusion rests on an assumption about pulse-shape invariance that is supported only indirectly.

major comments (1)
  1. [Section 2.2 and Table 1] The FOM values in Table 1 are obtained with fixed gates (short = 66 ns, long = 350 ns), which are not exactly the optimal gates for all samples according to Table 2 (for example, at 300 keVee the optimal long gate for the 55, 78, and 102 mm samples is 330 or 350 ns). The fixed-gate choice affects the absolute FOM values but not the overall trend. This is a minor methodological point, but it means that the normalized FOM values in Table 1 are not all evaluated at the maximum FOM for each sample. The authors should either state that the fixed gates are representative and that the maximum-FOM comparison in Table 2 gives the same qualitative result, or justify the use of fixed gates for the intrinsic-property claim.
minor comments (7)
  1. [Section 3.1] In the paragraph discussing the linear relation between yield and size, the text mentions 'the 136Cs source', which appears to be a typo for '137Cs'.
  2. [Section 3.1] The sentence 'In case of the scintillators used in our research peak on the right side represents pulses induced by fast neutrons...' is missing a comma after 'research' and should be rephrased for clarity.
  3. [Figure 13 caption] The caption states that 'R2 and reduced χ2 values were calculated to confirm that the results are reliable' but the actual values are not reported anywhere in the text or tables. Please include them or remove the sentence.
  4. [Table 2] The footnote says that the uncertainty of the gates can be estimated as the step size (4 ns for short, 50 ns for long), but the text in Section 3.1 says the averaged gates were rounded to 2 ns and 10 ns, respectively. Clarify which uncertainty is meant to be quoted.
  5. [Section 3.2] The phrase 'the uncertainties of decay times were estimated as 2σ of Gaussian fit' in the note to Table 3 is slightly ambiguous: it should specify that the Gaussian is fit to the distribution of parameter values obtained from the repeated genetic-algorithm runs, and that the reported value is the mean of that Gaussian.
  6. [Throughout] The paper uses 'we' extensively in the results sections; this is acceptable in this venue, but the manuscript would benefit from a brief list of the analysis software versions and a statement on data availability, since the custom Python software is described but not deposited.
  7. [References] Reference [6] is an arXiv preprint; if a published version exists by the time of submission, it should be cited. Also, reference [4] is a web page; please add a retrieval date.

Circularity Check

0 steps flagged · score 0.0 of 10

No circular derivation: normalized-FOM constancy is an empirical ratio of measured quantities; the single-height pulse-shape measurement is a limitation, not a circular step.

full rationale

The paper's central claims are based on direct measurements, not on a fitted parameter disguised as a prediction. The normalized FOM is computed as the measured FOM divided by the square root of the measured photoelectron number (Table 1); its approximate constancy across heights is an empirical observation, not an identity enforced by construction. The conclusion that self-absorption reduces neutron-gamma discrimination is supported by the measured decrease of both FOM and photoelectron yield with scintillator height, and the statement that normalized FOM is an intrinsic property is an interpretation of those measurements rather than a definitional restatement. The paper does invoke its own prior work, notably [5] for earlier OGS characterization and [6] for the offline analysis method, but these citations support methodology and context, not the central result; the present data and analysis stand independently. A legitimate weakness is that Bollinger-Thomas pulse shapes were measured only for the 25 mm OGS (Section 3.2), and the size-independence of pulse shape is inferred indirectly from constant normalized FOM and size-independent optimal gates, with the conclusion stating only that 'we found no evidence that the pulse shape itself undergoes any change' (Section 4). This is an evidentiary gap and a correctness-risk concern, not circularity: no equation in the paper reduces the predicted quantity to an assumed input, and no fitted parameter is renamed as a prediction. Therefore the derivation chain is self-contained with respect to circularity.

Assumptions & free parameters 5 free parameters · 4 assumptions · 0 invented entities

The main scaling conclusions rest on standard measurement techniques and the assumption that pulse shape is size-independent. The fitted decay parameters are characterization outputs, not inputs to the scaling claim.

free parameters (5)
  • Fast decay time τ1 = OGS gamma 1.95 ns, OGS neutron 2.32 ns, stilbene gamma 4.98 ns, stilbene neutron 5.88 ns
    Fitted to Bollinger-Thomas pulse shape data via genetic algorithm (Section 3.2, Table 3).
  • Medium decay time τ2 = OGS gamma 10.0 ns, OGS neutron 24.2 ns, stilbene gamma 23 ns, stilbene neutron 40.5 ns
    Fitted to Bollinger-Thomas pulse shape data via genetic algorithm (Section 3.2, Table 3).
  • Slow decay time τ3 = OGS gamma 73 ns, OGS neutron 132 ns, stilbene gamma 107 ns, stilbene neutron 291 ns
    Fitted to Bollinger-Thomas pulse shape data via genetic algorithm (Section 3.2, Table 3).
  • Component intensities I1, I2, I3 = OGS gamma 85%, 9%, 6%; OGS neutron 70%, 17%, 13%; stilbene gamma 89%, 6%, 5%; stilbene neutron 60%, 19%, 21%
    Derived from fitted amplitudes and decay times via Eq. 6.
  • Baseline offset C in Eq. 5 = Not reported numerically
    Nuisance parameter in the exponential fit, included in the genetic algorithm fitting.
assumptions (4)
  • domain assumption The scintillation pulse shape is described by a sum of three exponential decays (Eq. 5).
    Assumed functional form for fitting; no derivation is given.
  • domain assumption The Charge Comparison Method with optimized short/long gates measures the true pulse shape discrimination capability.
    Standard method; the paper relies on this to define FOM.
  • domain assumption The Bollinger-Thomas delayed coincidence setup provides accurate single-photon timing of scintillation pulses.
    Standard experimental method cited from [11].
  • domain assumption Photoelectron yield measured via the Bertolaccini method is an accurate measure of light output.
    Standard method cited from [9].

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Cite this review

Pith. "Pith review of Influence of Self-Absorption on Pulse Shape Discrimination in Organic Glass Scintillators." pith.science (2026). https://pith.science/paper/UXGZU2JU

@misc{pith2026250508513,
  author       = {Pith},
  title        = {Pith review of: Influence of Self-Absorption on Pulse Shape Discrimination in Organic Glass Scintillators},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/UXGZU2JU}},
  note         = {Machine review of arXiv:2505.08513}
}
read the original abstract

Organic glass scintillators are an interesting alternative to liquid scintillators, offering many advantageous characteristics with few drawbacks. In this paper we investigate the influence of light self-absorption in the organic glass scintillator on its pulse shape discrimination capability. With five scintillators of different heights but same diameter, we measure photoelectron yield and Figure of Merit in neutron-gamma discrimination. The decrease of both values with increasing size is attributed to light self-absorption, while normalized Figure of Merit remains constant. The choice of gates for charge comparison method is discussed. We also use genetic algorithm to estimate decay times and intensities of fast, medium, and slow components of light pulse shapes measured with Bollinger-Thomas setup. We compare the results to trans-stilbene reference sample.

Figures

Figures reproduced from arXiv: 2505.08513 by the authors.

Figure 1
Figure 1. Photo of trans-stilbene (left) and five organic glass scintillators (right) used in this research. [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Energy spectra measured for 25-mm OGS with three calibration sources: [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 3
Figure 3. Experimental setup for photoelectron yield measurement. [PITH_FULL_IMAGE:figures/full_fig_p003_3.png] view at source ↗
Figures from the paper (10 more)
Figure 4
Figure 4. Figure 4: Experimental setup for neutron-gamma discrimination performance measurement. [PITH_FULL_IMAGE:figures/full_fig_p004_4.png]
Figure 5
Figure 5. Figure 5: Experimental setup for light pulse shape measurement. [PITH_FULL_IMAGE:figures/full_fig_p004_5.png]
Figure 6
Figure 6. Figure 6: The decrease of photoelectron yield of OGS (green circles) compared to EJ-276 (blue crosses) [PITH_FULL_IMAGE:figures/full_fig_p005_6.png]
Figure 7
Figure 7. Figure 7: Example of gamma and neutron induced pulses recorded by digital analyzer with gates visual [PITH_FULL_IMAGE:figures/full_fig_p006_7.png]
Figure 8
Figure 8. Figure 8: Example of PSD parameter histogram. CTR is a centroid of a peak and FWHM is a full [PITH_FULL_IMAGE:figures/full_fig_p006_8.png]
Figure 9
Figure 9. Figure 9: Example of FOM dependence on short and long gates for variety of energies in 25 mm organic [PITH_FULL_IMAGE:figures/full_fig_p011_9.png]
Figure 10
Figure 10. Figure 10: OGS FOM in wide energy range 100-1000 keVee decreases with scintillator height. Several [PITH_FULL_IMAGE:figures/full_fig_p012_10.png]
Figure 11
Figure 11. Figure 11: Average best FOM (left) and FOM at fixed gates short = 58 ns, long = 300 ns (right) of OGS [PITH_FULL_IMAGE:figures/full_fig_p012_11.png]
Figure 12
Figure 12. Figure 12: The pulse shapes of gamma and neutron induced pulses in OGS (left) and trans-stilbene (right) [PITH_FULL_IMAGE:figures/full_fig_p013_12.png]
Figure 13
Figure 13. Figure 13: Example of the pulse shape (top) and the distributions of parameters (bottom), in case of [PITH_FULL_IMAGE:figures/full_fig_p013_13.png]

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Reference graph

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