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REVIEW 4 major objections 5 minor 18 references

Time Resolution Characterization of 4H-SiC LGADs with a ${}^{90}$Sr Source

T0 review · 4 major / 5 minor · reviewed 2026-08-05 · deepseek-v4-flash

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Pith's one-line read 4H-SiC LGADs time beta particles from a 90Sr source to 61 ps, matching silicon LGADs and pointing to charge collection as the next optimization target.

desk verdict First beta-source timing of a 4H-SiC LGAD, but the 61 ps headline rests on an unverified 41 ps reference and the internal arithmetic doesn't quite close; worth refereeing nonetheless. read the letter →

arxiv 2509.04638 v1 pith:GBWRZQYH submitted 2025-09-04 physics.ins-det hep-ex

Tao Yang , Yashas Satapathy , Ben J. Sekely , Abraham Tishelman-Charny , Greg Allion , Gil Atar , Philip Barletta , Carl Haber
show 4 more authors
Steve Holland John F. Muth Spyridon Pavlidis Stefania Stucci
This is my paper · ORCID
classification physics.ins-dethep-ex
keywords 4H-SiCLGADtimeresolution90Srbetasourceminimumionizingparticles4Dtrackingchargecollection
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

This paper reports the first timing measurement of 4H-SiC Low Gain Avalanche Detectors (LGADs) with beta particles from a 90Sr source, which stand in for minimum-ionizing particles. It claims a time resolution of 61 ps, comparable to standard silicon LGADs and better than earlier silicon-carbide PIN detectors. The authors argue that the remaining gap to silicon is set by limited charge generation—lower ionization yield, modest internal gain, and a partially depleted drift layer—rather than by carrier drift, which is faster in SiC. If correct, the result opens a path to timing detectors for future 4D tracking that tolerate higher voltages and harsher radiation than silicon.

What carries the argument

The measurement uses a stacked coincidence setup: a 90Sr beta source sends particles through a thin silicon LGAD (the timing reference) and then through the 4H-SiC LGAD, both read out by fast transimpedance amplifiers and a high-speed oscilloscope. Arrival times are extracted with constant-fraction discrimination at 40% of pulse amplitude for both devices, and the spread of the time difference is fit with a Gaussian. Because the two detectors are triggered by the same beta, the silicon reference's known 41 ps jitter can be removed in quadrature to isolate the SiC device's resolution. The SiC LGAD itself is a P++/N+/N- diode with a 0.5 µm gain layer, a 75 µm drift layer, and a field plate, op

What would settle it

Build a coincidence setup with two 4H-SiC LGADs of identical design and measure their time difference directly; if the per-device resolution derived from that measurement disagrees with the 61 ps obtained by subtracting the silicon reference, the reference resolution or the correlation assumption is wrong. Alternatively, measure the silicon LGAD's resolution in the stacked geometry with a second silicon LGAD to check whether the 41 ps value holds there.

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Extended reading notes

Core claim

The paper's central claim is that a 300 µm diameter 4H-SiC LGAD operated at 500 V times beta particles from 90Sr with 61 ps resolution, extracted by measuring the spread of arrival-time differences against a 50 µm silicon LGAD with known 41 ps resolution and subtracting in quadrature. This places SiC LGADs in the same timing class as silicon LGADs (~50 ps) for MIP-like particles. The paper further claims that the current resolution is limited by the amount of charge the device collects: beta particles deposit less energy in SiC than in Si, the internal gain is only 7–8, and the 75 µm drift layer is not fully depleted at the available bias. It concludes that improving charge collection—throug

Load-bearing premise

The result rests on the reference silicon LGAD timing exactly as well inside the stacked beta setup as it did standalone, and on the two detectors' timing errors adding in quadrature with no correlation between them.

Editorial extensions

If this is right

  • 4H-SiC LGADs can serve as timing detectors for minimum-ionizing particles in future 4D trackers, with tens-of-picosecond precision.
  • Increasing internal gain and fully depleting the drift layer should improve the 61 ps resolution, because charge collection, not drift speed, is the stated bottleneck.
  • At equal collected charge, SiC LGADs should match or outperform Si LGADs, since they run at higher bias and have faster carrier drift.
  • The higher voltage tolerance and low leakage of SiC make this technology a candidate for operation in extreme or high-radiation environments.

Reading between the lines

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

  • The 61 ps number depends on the assumption that the silicon reference's 41 ps resolution is unchanged inside the stacked geometry and that the two detectors' timing jitters are uncorrelated. Since both see the same beta track, correlated energy deposition could bias the quadrature subtraction; a coincidence measurement between two identical 4H-SiC LGADs would settle this without relying on an exte
  • If charge collection is indeed the limiting factor, the paper's charge-versus-resolution curve predicts a concrete scaling: devices with larger gain or thicker depleted drift layers should move down that curve, providing a direct target for the next fabrication round.
  • The through-hole stack reduces scattering, but multiple scattering in the first detector still changes the beta's direction and energy at the second; a Monte Carlo of energy deposition in the stack could quantify how much this affects the extracted resolution.
  • SiC's wide bandgap implies lower leakage current and likely better radiation tolerance at high fluence; testing the timing resolution after irradiation is the natural next step, but is not part of this paper.
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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

4 major / 5 minor

Summary. The paper reports timing measurements of a 300 µm diameter 4H-SiC LGAD using beta particles from a 90Sr source. The authors stack a 50 µm BNL Si LGAD as a timing reference above the 4H-SiC LGAD, record coincident pulses on a 2.5 GHz oscilloscope, extract times with 40% constant-fraction discrimination, and fit the distribution of ΔT = t_SiC − t_Si with a Gaussian. They quote σ_ΔT = 76 ± 2 ps and, using a previously measured Si reference resolution of about 41 ps, deconvolve the Si contribution in quadrature to obtain a 4H-SiC LGAD time resolution of 61 ps. They also compare collected charge and timing as a function of bias voltage, concluding that the present timing is limited by the relatively small collected charge in the SiC device rather than by carrier drift. The central claims are that this is the first demonstration of MIP-like timing in a 4H-SiC LGAD at the tens-of-picosecond level and that charge generation, not drift velocity, is the current bottleneck.

Significance. If the reported 61 ps value is robust, the result is significant for 4D tracking and timing detector R&D: it would demonstrate that 4H-SiC LGADs can time minimum-ionizing-like particles at a level comparable to standard Si LGADs, with the added potential of higher voltage tolerance and faster carrier drift. The paper's strength is that it is a direct two-detector differential measurement with a realistic beta source, and the charge-versus-timing trend in Fig. 10 independently supports the qualitative interpretation that signal charge is the limiting factor. The study also builds on the authors' earlier UV-TCT work, which already indicated excellent intrinsic timing. However, the headline number rests entirely on a quadrature subtraction whose inputs are not internally consistent and whose reference value is not verified in the stacked configuration. The qualitative conclusion may survive those concerns, but the exact numerical claim needs further support.

major comments (4)
  1. [Section IV] The arithmetic in the central extraction is inconsistent. With σ_ΔT = 76 ± 2 ps and σ_Si = 41 ps, quadrature subtraction gives sqrt(76² − 41²) = 64 ps, not 61 ps as quoted in the abstract and Section IV. To obtain 61 ps one would need σ_Si ≈ 45 ps in the stacked measurement. The paper must correct this arithmetic or clearly state which inputs produce 61 ps; as written, the headline value does not follow from the stated numbers.
  2. [Sections III–IV] The reference resolution σ_Si ≈ 41 ps is taken from a previous standalone determination and applied to the Si LGAD inside the stacked beta setup. No in-stack calibration or cross-check is presented. The through-hole PCBs, scattering from the stack, and coincidence selection for particles that traverse both detectors can all change the reference jitter. Please provide an in-situ reference measurement (e.g., a third detector, a split-signal self-calibration, or quantitative simulation) or otherwise bound the systematic shift. Without this, the deconvolved 61 ps number is not secured.
  3. [Section IV] The quadrature relation σ_ΔT² = σ_Si² + σ_SiC² assumes that the timing errors of the two detectors are uncorrelated. For a continuous 90Sr spectrum, the same beta particle deposits correlated energies in the two layers; residual amplitude-dependent time walk or amplitude-modulated jitter can produce covariance between t_Si and t_SiC. Positive covariance would make the true σ_SiC larger than the quadrature estimate. The authors should estimate or bound this correlation, for instance by repeating the analysis in narrow energy/amplitude bins, or justify why the correlation is negligible.
  4. [Section IV, Figs. 9–10] No total uncertainty is quoted for the final 61 ps value. The statistical uncertainty on σ_ΔT is only 2 ps, but the uncertainty on σ_Si is not stated, and the correlation issue above is unquantified. An error budget is needed before the number can be compared with Si LGADs. In addition, Fig. 10 shows points without error bars, so the proposed charge-scaling trend is not quantitatively assessable.
minor comments (5)
  1. [Fig. 8 caption and Section IV] The caption says panel (a) uses a '10%–90% fraction,' while the text says the minimum is reached for fractions between 30% and 50%. Please clarify what fraction is plotted and how the CFD fraction is defined.
  2. [Fig. 10] The axes and legends appear truncated in the figure, and the units are not visible. Please add a complete axis label with units and error bars, and state the charge integration method used.
  3. [Section IV] There is a typo: 'due to due to the lower ionization energy deposition' should be 'due to the lower ionization energy deposition.'
  4. [Section II and IV] The gain of 7–8 is taken from the authors' own UV-TCT work in Ref. [16]. This is not circular because the present time resolution is directly measured, but stating that the gain value comes from a separate measurement and is not re-derived here would improve transparency.
  5. [Abstract/Conclusion] The phrase 'time resolutions of 61 ps were achieved' is used in both the abstract and conclusion, but the paper reports one operating point. Please specify the bias voltage and CFD fraction when quoting the value.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the 61 ps value is a direct deconvolution of a measured ΔT distribution using an independently pre-determined reference resolution; self-citations are supporting but not load-bearing.

full rationale

The extracted 4H-SiC LGAD time resolution is obtained as σ_SiC = sqrt(σ_ΔT² - σ_Si²), where σ_ΔT = 76 ± 2 ps is directly measured from the Gaussian fit of the stacked-detector ΔT distribution (Section IV, Fig. 8), and σ_Si ≈ 41 ps is a pre-existing measurement of the BNL Si LGAD reported in Section III. Neither quantity is fitted to produce the target result: σ_Si is not adjusted to make σ_SiC come out to 61 ps, and the raw ΔT spread is reported as measured. The self-citations (device gain of 7–8 from UV-TCT [16], the analysis method from [6], prior fabrication [15]) characterize the device or reference a standard technique, and they are not used to manufacture the headline number. Two genuine concerns are present but do not constitute circularity: (1) the quadrature subtraction assumes the Si LGAD retains its standalone 41 ps resolution in the stacked coincidence setup and that the two detectors' timing errors are uncorrelated, which is an external-validity assumption rather than a definitional reduction; and (2) the quoted numbers are numerically inconsistent, since sqrt(76² - 41²) = 64 ps, not 61 ps, and reproducing 61 ps would require σ_Si ≈ 45 ps in situ. These are correctness, reproducibility, and support issues, not circular reasoning: the inputs are not defined in terms of the target result, and no fitted parameter is renamed as a prediction. The paper's central derivation is therefore self-contained with respect to circularity, even though its precision is open to scrutiny.

Assumptions & free parameters 1 free parameters · 3 assumptions · 0 invented entities

The measurement chain rests on three external or self-cited inputs: the Si LGAD reference resolution (41 ps from prior work), the assumption of uncorrelated jitter for quadrature subtraction, and the UV-TCT gain value (7-8) used to interpret charge limitation. None is re-derived in this paper. The only hand-chosen analysis parameter is the CFD fraction (40%), selected from the data as the apparent optimum.

free parameters (1)
  • CFD fraction = 40% (0.4)
    The fraction of pulse amplitude used for constant-fraction discrimination was set to 40% because the measured σ_ΔT was smallest for fractions between 30% and 50%; this data-driven choice affects the reported 61 ps value.
assumptions (3)
  • domain assumption Quadrature addition of independent timing jitters: σ_ΔT² = σ_Si² + σ_SiC²
    Used in Section IV to deconvolve the SiC resolution; assumes zero correlation between the reference and device jitters, which is not verified.
  • domain assumption Reference Si LGAD time resolution is about 41 ps and remains valid in the stacked beta setup
    Section III states the value was previously determined with a 90Sr source; no in-situ recalibration or uncertainty is given.
  • domain assumption Gain of the 4H-SiC LGAD is 7-8 as measured by UV-TCT
    Section II cites ref [16], a same-group paper, for the gain value used to explain the charge limitation; not independently verified here.

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Cite this review

Pith. "Pith review of Time Resolution Characterization of 4H-SiC LGADs with a ${}^{90}$Sr Source." pith.science (2026). https://pith.science/paper/GBWRZQYH

@misc{pith2026250904638,
  author       = {Pith},
  title        = {Pith review of: Time Resolution Characterization of 4H-SiC LGADs with a $^90$Sr Source},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/GBWRZQYH}},
  note         = {Machine review of arXiv:2509.04638}
}
abstract

This work presents timing measurements of 4H-SiC Low Gain Avalanche Detectors (4H-SiC LGADs) using beta particles from a ${}^{90}$Sr source. The 4H-SiC LGADs exhibit fast signal responses, and a time resolution of 61~ps was achieved, comparable to that of standard Si LGADs. The present limitation in the time resolution of 4H-SiC LGADs appears to stem from limited charge generation. Nevertheless, their higher voltage tolerance and faster carrier drift suggest that, with increased charge collection, their timing performance could approach or even surpass that of Si LGADs. These results demonstrate the strong potential of 4H-SiC LGADs as a robust platform for precision timing in future 4D tracking detectors, while also highlighting that signal charge is the dominant factor currently limiting their performance, indicating that further optimization of gain and drift structures will be essential for future development.

Figures

Figures reproduced from arXiv: 2509.04638 by the authors.

Figure 1
Figure 1. Photograph of the 300 µm thick 4H-SiC LGAD (left) and diagram of the cross-sectional view of the device (right). (a) (b) VBD VGL [PITH_FULL_IMAGE:figures/full_fig_p001_1.png] view at source ↗
Figure 2
Figure 2. (a) I–V and (b) C–V characteristics of the 300 [PITH_FULL_IMAGE:figures/full_fig_p001_2.png] view at source ↗
Figure 3
Figure 3. Photograph of the 90Sr source test setup. The Si LGAD was mounted on the upper circuit board, and the 4H-SiC LGAD was placed on the lower circuit board. 4 H-SiC LGAD Si LGAD Oscilloscope 2.5 GHz 20 Gsa/s HV SMU HV SMU - + - + 90Sr [PITH_FULL_IMAGE:figures/full_fig_p002_3.png] view at source ↗
Figures from the paper (5 more)
Figure 5
Figure 5. Figure 5: Sampled waveforms with the 90Sr source: (a) pulse signal detected by the Si LGAD; (b) pulse signal detected by the 4H-SiC LGAD [PITH_FULL_IMAGE:figures/full_fig_p002_5.png]
Figure 6
Figure 6. Figure 6: Charge collection distribution of the 90Sr source and Landau-Gaussian convolution fit results: (a) Si LGAD at 250 V; (b) 4H-SiC LGAD at 500 V. III. TEST SETUP To measure the time resolution of the 4H-SiC LGAD, a 50 µm thick Si LGAD with a gain of 10-20 from Brookhaven …
Figure 7
Figure 7. Figure 7: (a) Collected charge of the Si LGAD as a function of bias voltage, [PITH_FULL_IMAGE:figures/full_fig_p003_7.png]
Figure 9
Figure 9. Figure 9: Time resolution of the Si LGAD and 4H-SiC LGAD as a function of [PITH_FULL_IMAGE:figures/full_fig_p003_9.png]
Figure 10
Figure 10. Figure 10: Time resolution of the Si LGAD and 4H-SiC LGAD as a function [PITH_FULL_IMAGE:figures/full_fig_p004_10.png]

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