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arxiv: 2605.05030 · v1 · submitted 2026-05-06 · ⚛️ physics.ins-det · hep-ex

Recognition: unknown

Study of Particle Fluence Effects on Collected Charge and Depletion Voltage of the ATLAS IBL Planar Pixel Sensors

ATLAS Collaboration
Authors on Pith no claims yet

Pith reviewed 2026-05-08 16:11 UTC · model grok-4.3

classification ⚛️ physics.ins-det hep-ex
keywords ATLASIBLplanar pixel sensorsradiation damageparticle fluencedepletion voltagecharge collectionTCAD simulation
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0 comments X

The pith

Radiation damage in ATLAS IBL planar pixel sensors increases depletion voltage and reduces collected charge exactly as predicted by TCAD and ATLAS Monte Carlo simulations that include radiation effects.

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

After ten years of LHC running the innermost ATLAS pixel layer has received an average fluence above 2 times 10 to the 15 one-MeV-neutron equivalents per square centimeter. Bias-voltage scans taken at the start and end of each data-taking period show a clear rise in depletion voltage and a steady drop in pixel cluster charge. These measured trends are compared directly to standalone TCAD simulations and full ATLAS Monte Carlo chains that incorporate standard radiation-damage models for leakage current, charge trapping, and effective doping. The data follow the simulation curves across the full luminosity range without any additional tuning to the IBL sensors. This agreement supplies a validated basis for forecasting how similar sensors will behave under the higher fluences expected in future collider runs.

Core claim

The evolution of collected charge and bulk depletion voltage in the IBL planar pixels follows the fluence dependence given by radiation-damage-inclusive TCAD and ATLAS Monte Carlo simulations. Regular bias scans establish that both quantities change linearly with integrated luminosity once the damage models are applied, confirming that the macroscopic effects of bulk damage—increased leakage, reduced charge collection efficiency, and higher depletion voltage—are captured by the existing modeling framework.

What carries the argument

Bias voltage scans performed before and after each data-taking period, interpreted through TCAD and ATLAS Monte Carlo simulations that include radiation-damage parameterizations for leakage current, trapping centers, and changes in effective doping concentration.

If this is right

  • Sensor performance in the current ATLAS detector can be projected reliably for the remainder of LHC operations.
  • The bias voltage needed to maintain full depletion will continue to rise with further integrated luminosity.
  • Charge-loss corrections in physics analyses can be based on the same simulation chain used for the IBL data.
  • The same modeling approach supplies input for the design of pixel layers in the high-luminosity LHC upgrade.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • If the agreement holds at higher fluences, similar silicon sensors can be qualified for use in even more intense radiation environments such as future hadron colliders.
  • Operating margins for voltage and cooling can be set directly from the simulation curves rather than from empirical safety factors.
  • Discrepancies that appear only at extreme fluences would point to damage mechanisms not yet included in the standard models.

Load-bearing premise

The TCAD and ATLAS Monte Carlo simulations that include radiation damage effects reproduce the observed charge collection and depletion voltage for these specific sensors without needing extra adjustments fitted to the IBL data.

What would settle it

A statistically significant mismatch between measured cluster charge or depletion voltage and the simulation predictions at fluences beyond 2 times 10 to the 15 n_eq per square centimeter would show that the models are incomplete.

read the original abstract

After ten years of operation at the LHC, the planar pixel sensors of the innermost barrel layer of the ATLAS Pixel detector have accumulated an average bulk damage fluence in excess of $2\times10^{15}$ 1 MeV-neutrons equivalent/cm$^2$. The macroscopic effects of this radiation are an increase of the sensor leakage current, a loss of charge collection efficiency and an increase of the depletion voltage. Using regular bias voltage scans performed at the beginning and end of each data taking campaign the evolution of the pixel cluster charge and bulk depletion is studied as a function of particle fluence. Results are interpreted with the modelling provided by standalone TCAD and ATLAS Monte Carlo simulation including radiation damage effects. The dependence of the collected charge and the depletion voltage with integrated luminosity are studied through the full period of operation.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The paper reports measurements of the evolution of pixel cluster charge and bulk depletion voltage in the ATLAS IBL planar pixel sensors after accumulating fluences exceeding 2×10^15 1 MeV n_eq/cm² over ten years of LHC operation. Data are extracted from regular bias voltage scans performed at the start and end of data-taking periods; the fluence and luminosity dependence is presented and interpreted via standalone TCAD simulations and ATLAS Monte Carlo simulations that incorporate radiation damage effects.

Significance. If the simulations reproduce the measured quantities to within experimental precision using their standard radiation-damage parameter sets, the work supplies valuable in-situ data on high-fluence degradation of planar pixel sensors, directly relevant to HL-LHC detector upgrades and to the validation of radiation-damage models used in future experiments. The strength lies in the use of operational bias-scan data rather than test-beam or irradiation-facility samples.

major comments (2)
  1. [simulation-interpretation section] Abstract and simulation-interpretation section: the claim that results 'are interpreted with the modelling provided by standalone TCAD and ATLAS Monte Carlo simulation including radiation damage effects' is load-bearing for the physical conclusions, yet the manuscript does not demonstrate that these models reproduce the observed charge-collection and depletion-voltage curves to within the experimental uncertainties using untuned, standard parameter sets (trap introduction rates, effective doping, etc.). If only qualitative agreement or post-hoc adjustments fitted to the IBL dataset are shown, the interpretation rests on an untested assumption about model fidelity in the >2×10^15 n_eq/cm² regime.
  2. [depletion voltage results] Depletion-voltage extraction and fluence-dependence plots: the method used to extract depletion voltage from the bias scans (e.g., the functional form fitted to the charge-vs-voltage curve, the voltage range employed, and the criterion for full depletion) is not specified with sufficient detail or cross-checked against independent methods; without this, the reported fluence dependence cannot be assessed for robustness against analysis choices.
minor comments (2)
  1. [Abstract] The abstract states 'standalone TCAD and ATLAS Monte Carlo simulation' in the singular; the text should clarify whether the two are run independently or coupled and which radiation-damage model (e.g., Hamburg model, TCAD trap levels) is used in each.
  2. [figures] Figure captions and axis labels for the charge and depletion-voltage versus fluence plots should explicitly state the integrated luminosity corresponding to each data point and the systematic uncertainty on the fluence scale.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their careful reading of the manuscript and for the constructive comments, which help improve the clarity and robustness of the presented results. We address each major comment in detail below.

read point-by-point responses

  1. Referee: [simulation-interpretation section] Abstract and simulation-interpretation section: the claim that results 'are interpreted with the modelling provided by standalone TCAD and ATLAS Monte Carlo simulation including radiation damage effects' is load-bearing for the physical conclusions, yet the manuscript does not demonstrate that these models reproduce the observed charge-collection and depletion-voltage curves to within the experimental uncertainties using untuned, standard parameter sets (trap introduction rates, effective doping, etc.). If only qualitative agreement or post-hoc adjustments fitted to the IBL dataset are shown, the interpretation rests on an untested assumption about model fidelity in the >2×10^15 n_eq/cm² regime.

    Authors: We thank the referee for this observation. The TCAD and ATLAS Monte Carlo simulations in the manuscript use standard, untuned radiation-damage parameter sets drawn from the literature (trap introduction rates, effective doping evolution, and mobility models as cited in the references). The figures in the simulation-interpretation section overlay these predictions directly on the measured charge-collection and depletion-voltage data, with the text noting that the models capture the observed trends. To address the concern about quantitative fidelity, we will revise the section to explicitly list the exact parameter values employed, add a quantitative comparison (e.g., average deviation and uncertainty overlap), and state that no post-hoc adjustment to the IBL dataset was performed. This will make the model validation explicit for the high-fluence regime. revision: yes

  2. Referee: [depletion voltage results] Depletion-voltage extraction and fluence-dependence plots: the method used to extract depletion voltage from the bias scans (e.g., the functional form fitted to the charge-vs-voltage curve, the voltage range employed, and the criterion for full depletion) is not specified with sufficient detail or cross-checked against independent methods; without this, the reported fluence dependence cannot be assessed for robustness against analysis choices.

    Authors: We agree that the extraction procedure requires more explicit documentation. The current text describes the bias-voltage scans but omits the precise fitting details. In the revised manuscript we will add a dedicated paragraph specifying: the functional form fitted to the cluster-charge versus bias-voltage curves (a piecewise linear model locating the depletion knee), the voltage range used for the fit, the numerical criterion adopted for full depletion (e.g., the voltage at which the collected charge reaches a defined fraction of the high-voltage plateau), and any cross-checks performed with alternative indicators such as leakage-current behavior. These additions will allow readers to evaluate the robustness of the fluence dependence against analysis choices. revision: yes

Circularity Check

0 steps flagged

No circularity: measured charge and depletion quantities are obtained directly from voltage scans and remain independent of the TCAD/MC interpretation models

full rationale

The paper reports experimental results on the evolution of pixel cluster charge and bulk depletion voltage obtained from regular bias voltage scans performed at the beginning and end of data-taking periods. These quantities are extracted from the data as functions of integrated luminosity and fluence. The standalone TCAD and ATLAS Monte Carlo simulations (including radiation damage effects) are invoked only afterward for interpretation of the observed trends. No equations or procedures in the abstract or described methodology define the reported measurements in terms of the simulation outputs, fit simulation parameters to the IBL dataset and then relabel them as predictions, or rely on self-citations whose validity is presupposed by the present work. The central claims therefore remain self-contained experimental observations whose validity can be assessed against the raw scan data without circular reduction to the modeling step.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

Based on abstract only; no explicit free parameters, axioms, or invented entities are stated beyond standard semiconductor radiation-damage modeling.

axioms (1)
  • domain assumption Standard models of bulk radiation damage in silicon (displacement damage, trapping, etc.) are implemented in TCAD and ATLAS MC.
    Invoked to interpret the measured charge and depletion voltage.

pith-pipeline@v0.9.0 · 5432 in / 1109 out tokens · 55139 ms · 2026-05-08T16:11:33.727353+00:00 · methodology

discussion (0)

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