REVIEW 3 major objections 5 minor 24 references
AstroPix_v3 measures a 35.4 keV most‑probable energy deposit for 120 GeV protons, implying a 133 µm effective depletion depth at −150 V bias.
Reviewed by Pith at T0; open to challenge. T0 means a machine referee read the full paper against a public rubric. the ladder, T0–T4 →
T0 review · deepseek-v4-flash
2026-08-03 04:20 UTC pith:SADOWL3P
load-bearing objection Solid, useful beam-test measurement of AstroPix v3, but the energy scale rests on a three-point quadratic calibration that needs a stronger cross-check before I'd trust the 133 µm depth to 5 µm. the 3 major comments →
Beam Test Performance of AstroPix sensor with 120 GeV protons
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
Using 120 GeV protons, the authors measured the Time‑over‑Threshold (ToT) spectra of individual AstroPix_v3 pixels, fitted each with a Landau function convolved with a Gaussian, and extracted the most‑probable value (MPV) per pixel. After applying the per‑pixel gamma‑ray calibration curves, the average MPV across all pixels at the beam center was 35.4 keV, with a statistical spread of ±1.3 keV and a total uncertainty of ±1.8 keV after adding systematic effects. This energy corresponds to an effective depletion depth of 133 µm, quoted as 133±5 µm (statistical) and 133±6 µm (total). The pixel‑to‑pixel variation in the measured energy was 3.9%, which the authors attribute to local differences i
What carries the argument
The analysis rests on a per‑pixel ToT‑to‑energy calibration constructed from photopeaks of 241Am (59.5 keV) and 133Ba (31 and 81 keV), fitted with a second‑order polynomial after an injection‑voltage scan showed that this functional form agrees with the empirical nonlinear response over the relevant ToT range. For each beam hit, the ToT is read out through the mock‑strip row/column architecture and matched into individual pixels. The proton data are fitted pixel‑by‑pixel with a Landau function convolved with a Gaussian to isolate the MPV, which is converted to energy via the calibration curve and then to depletion depth using the theoretical most‑probable energy loss of 120 GeV protons in si
Load-bearing premise
The per‑pixel energy calibration is a second‑order polynomial forced through exactly three gamma photopeaks (31, 59.5, and 81 keV), and its functional form was validated only with a voltage‑injection scan whose charge response is not yet fully characterized; a bias in this calibration scales both the reported energy and the inferred depletion depth.
What would settle it
Measure the deposited energy of the same 120 GeV proton beam with an independently calibrated AstroPix_v3 chip, or use a calibrated 90Sr beta source to compare the extracted MIP MPV; alternatively, measure the depletion depth directly via capacitance‑voltage profiling of the deep n‑well/p‑substrate junction. If the resulting MIP energy or depletion depth disagrees with 35.4 keV or 133 µm by more than the quoted uncertainties, the calibration chain is biased.
If this is right
- AstroPix_v3 can detect MIP signals with a most‑probable energy deposit around 35 keV, well inside its 25–200 keV dynamic range, satisfying a key requirement for calorimeter imaging layers.
- A uniform 3.9% pixel‑to‑pixel spread suggests that the effective depletion depth varies only slightly across the sensor, which is favorable for consistent energy measurement in a large array.
- The measured depletion depth of about 133 µm can be used in detector simulations to model energy deposition in AstroPix layers for both space‑based gamma‑ray telescopes and electron‑ion collider calorimetry.
- The sensor's capability to produce a beam profile with 500 µm pixels provides a practical way to characterize beam position and width when a dedicated tracking telescope is unavailable.
- The demonstrated Landau‑plus‑Gaussian fitting procedure offers a straightforward method for extracting MIP energies from ToT data in future beam tests of AstroPix_v4 and later versions.
Where Pith is reading between the lines
- Because the inferred depletion depth is directly proportional to the calibrated energy, any systematic bias in the ToT‑to‑energy conversion – for instance from the not‑yet‑fully‑characterized injection response – scales the reported 133 µm linearly; an independent cross‑check with a calibrated electron source or capacitance‑voltage profiling would tighten the result.
- Only 61% of the 1,120 pixels had clear photopeaks at all three calibration energies and thus entered the calibration; if the uncalibrated pixels have a systematically different response, the reported average MIP energy could be slightly biased toward well‑behaved pixels.
- The exclusion of the first high‑intensity run hints at rate‑dependent dead‑time and saturation effects; for the high‑rate environment of a collider calorimeter, the pixel dead time at high occupancy will need dedicated characterization.
- A testable extension would be to measure the same MIP energy at several bias voltages (e.g., −50 V to −350 V) and check whether the extracted depletion depth scales as the square root of the bias, as expected for a one‑sided p‑n junction.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper reports a beam test of the AstroPix_v3 high-voltage CMOS pixel sensor using a 120 GeV proton beam at the Fermilab Test Beam Facility. Per-pixel Time-over-Threshold (ToT) to energy calibration is performed with three gamma-ray photopeaks from 241Am and 133Ba sources (31, 59.5, and 81 keV). The authors measure the most probable value (MPV) of the energy deposit for MIPs on a pixel-by-pixel basis, obtaining an average MPV of 35.4 ± 1.3 keV (statistical), which they interpret via a theoretical energy-loss calculation as an effective depletion depth of 133 ± 5 μm (systematics increase the total uncertainty to ±6 μm). The paper also presents beam-profile measurements from the sensor, and excludes one high-rate run from the energy analysis. The central claim is that AstroPix_v3 can detect MIP signals with ~35 keV most-probable energy deposit, corresponding to an approximately 133 μm depleted active region at -150 V bias.
Significance. If the result is correct, it provides a useful validation of AstroPix for both the ePIC Barrel Imaging Calorimeter and future gamma-ray space missions, and it gives a quantitative depletion-depth estimate that can feed detector simulations. The measurement chain is mostly transparent: the energy scale is anchored to independent gamma-ray sources, the Landau+Gaussian fits are described, and the conversion to depletion depth uses an external model for energy loss in silicon. The main weakness is the three-point calibration curve, which is the least secure link in the chain because a coherent curvature bias would directly scale all measured MIP energies and hence the inferred depletion depth. The manuscript is concise and addresses a relevant detector R&D question, but the calibration validation and the treatment of the excluded run and double-counted pixels need strengthening before the quantitative headline can be fully trusted.
major comments (3)
- [Section 3.2, systematics paragraph] The per-pixel ToT-to-energy calibration uses a second-order polynomial through exactly three gamma photopeaks (31, 59.5, 81 keV). The MIP MPV (~35 keV) lies between the 31 and 59.5 keV points, so the interpolation is central. The injection study in Figure 5 compares a polynomial with the empirical function of Ref. [9] in the injection-voltage domain, but the text states that the injection-to-charge response is 'not yet fully characterized.' Thus the injection scan does not validate the absolute energy scale or the polynomial curvature on the energy axis. The 1% calibration systematic assigned in Section 3.2 is asserted rather than derived from a fourth energy point or an alternative functional form. Because a curvature bias shifts all pixel energies coherently, a few-percent error changes the depletion depth outside the quoted uncertainty (e.g., a 4% energy-scale error moves 133 μm to ro
- [Section 3.2, paragraph 2] The first overnight run is excluded from the energy analysis because of a period of very high beam intensity (20–30 million particles per spill) that increased dead time and caused data loss. The text says that the ToT spectrum for each pixel 'can still be well described by a Landau function convoluted with a Gaussian function,' yet the run is removed without a quantitative criterion. If the high-rate condition only caused inefficiency and did not distort the per-hit ToT response, the run could in principle be included or at least used as a consistency check. Please either (a) demonstrate that the high-rate condition did not bias the extracted MPV (e.g., by comparing stable periods within the run, or by showing that the excluded run's fitted MPVs are consistent with the stable runs), or (b) define a quantitative selection criterion based on measured dead-time or hit-loss fraction. As wri
- [Section 3.2, Figure 11] The text states that there are 153 and 253 calibrated pixels from the two runs with an overlap of 97 pixels, giving a 'total of 406 pixels.' However, Figure 11 shows 411 entries. More importantly, because the two runs share 97 pixels, the distribution in Figure 11 includes those pixels twice, so the entries are not independent. The quoted pixel-to-pixel variation (3.9%) and the average MPV are then affected by double-counting. Please clarify whether Figure 11 is a per-pixel distribution or a per-run-per-pixel distribution, and if the latter, quantify the effect of the overlap (e.g., by averaging the two runs for the shared pixels or by reporting both per-pixel and per-run values). Otherwise the statistical uncertainty on the 35.4 keV average is slightly overstated in precision.
minor comments (5)
- [Section 3.1] The Gaussian fits to the beam projections show very large chi2/ndf values (1818/29 and 4108/32) and the fit parameters (e.g., 'Mean 0.015 ± 6.409', 'Sigma 0.015 ± 5.686') appear numerically implausible. The profile is likely not well described by a single Gaussian, or the parameter labels are mis-rendered. Please add a panel with residuals, or describe why the fits are acceptable despite the chi2 values, and correct the displayed parameter values if they are artifacts.
- [General] The abstract and conclusion state '35.4 keV' and '133±5 µm', while the final result including systematics is 35.4±1.8 keV and 133±6 µm. Please be consistent about which uncertainty is being quoted in the abstract and at the end of Section 3.2.
- [Section 3.2] The text says 406 pixels while Figure 11 reports 411 entries; reconcile the number or explain the difference (e.g., entries vs. unique pixels).
- [Section 2.2] The text says 'deposited MIP energy is expected to lie between 31 keV and 59.5 keV based on the 90Sr energy measurements.' Reference [9] is cited for the empirical function, but the 90Sr measurement is not otherwise described in this paper. A brief statement of the 90Sr result (or a citation to it) would make the expectation self-contained.
- [Title page] The first affiliation contains a typo: '9700 S. ass Avenue' should likely be '9700 S. Cass Avenue.'
Circularity Check
No significant circularity: the MIP energy and depletion depth are derived from independent gamma-source calibration and an external energy-loss model.
full rationale
The paper's central claim — an average MIP MPV of 35.4 keV and an effective depletion depth of 133±5 µm — is not circular. The ToT-to-energy calibration is obtained from external gamma-ray photopeaks (31, 59.5, 81 keV from 241Am and 133Ba), fit per-pixel with a second-order polynomial. This calibration is independent of the 120 GeV proton data. The proton ToT distributions are then measured and converted to energy using that calibration; the measured MPV lies in the interpolated range, but it is a genuine measurement, not a fitted parameter or a quantity that defines the calibration. The effective depletion depth is derived from the measured MPV using the standard most-probable energy loss for 120 GeV protons in silicon, cited to PDG and Bichsel — an external theoretical/empirical model with assumptions that do not include the measured result. The injection study is used only to compare functional forms for the calibration, and the paper explicitly states the injection-to-charge response is 'not yet fully characterized'; this is a stated limitation on the absolute charge scale, not a circular derivation. Self-citations to prior AstroPix work ([9], [15]) provide context and previous performance figures but are not load-bearing for the beam-test measurement, and no cited uniqueness theorem or fitted input is used to force the result. The quoted '1% calibration systematic' is asserted rather than rigorously derived, but that is an uncertainty-estimation weakness, not circularity under the criteria here. Overall, the derivation chain is self-contained and externally anchored.
Axiom & Free-Parameter Ledger
free parameters (2)
- Per-pixel second-order calibration polynomial coefficients =
Not tabulated; fitted per pixel to 3 gamma lines
- Landau+Gaussian MPV per pixel =
35.4 keV (average after calibration)
axioms (4)
- domain assumption 120 GeV protons behave as minimum-ionizing particles in silicon.
- domain assumption The most probable energy loss of a 120 GeV proton in silicon as a function of thickness is given by the Bichsel/PDG calculation.
- domain assumption The ToT spectrum of a MIP in a thin silicon layer follows a Landau distribution convolved with a Gaussian resolution function.
- standard math The gamma-ray energies of 241Am (59.5 keV) and 133Ba (31 and 81 keV) are known and produce full-energy photopeaks in the sensor.
read the original abstract
AstroPix is a high-voltage CMOS (HV-CMOS) monolithic active pixel sensor (MAPS) developed for precision gamma-ray imaging and spectroscopy in the medium energy regime, as well as for precise shower imaging and tracking in the Barrel Imaging Calorimeter (BIC) of the Electron Proton/Ion Collider (ePIC) detector at the future Electron-Ion Collider (EIC). We present beam test results of the AstroPix v3 sensor using a 120 GeV proton beam at the Fermilab Test Beam Facility (FTBF), performed as part of the broader experimental campaign for the BIC prototype calorimeter. The sensor's 500 um pixel pitch enabled precise measurement of the beam profile, providing important information for calorimeter performance studies. Using the measured 120 GeV proton data, we measure the energy deposit of minimum ionizing particles and use them to extract the corresponding effective depletion depth.
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