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arxiv: 2605.11961 · v1 · submitted 2026-05-12 · ⚛️ physics.ins-det

Recognition: no theorem link

TPA-TCT Analysis of the RD50-MPW4 Monolithic Pixel Particle Detector

Bernhard Pilsl, Chenfan Zhang, Christian Irmler, Eva Vilella, Fernando Mu\~noz-Chavero, Francisco Rogelio Palomo, Jorge Jim\'enez-S\'anchez, Jory Sonneveld, Michael Moll, Moritz Wiehe, Patrick Sieberer, Raimon Casanova, Sinuo Zhang

Pith reviewed 2026-05-13 04:54 UTC · model grok-4.3

classification ⚛️ physics.ins-det
keywords TPA-TCTRD50-MPW4DMAPScharge collection efficiencydepletion depthmonolithic pixel sensorcharge sharingtransient current technique
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The pith

The RD50-MPW4 monolithic pixel detector collects 100% of generated charge across a 226 micrometer depletion depth.

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

This paper applies the two-photon absorption transient current technique to map the sensitive volume of a small-pixel DMAPS sensor in three dimensions with micrometer resolution. It establishes that the detector achieves complete charge collection efficiency while also revealing charge sharing at pixel edges. These measurements matter because they confirm the sensor's performance for high-energy physics applications where precise tracking requires uniform response and known boundaries. The technique's ability to illuminate from the backside through transparent silicon allows characterization of any pixel without sectioning the device.

Core claim

Using TPA-TCT with backside illumination at 1550 nm, the RD50-MPW4 sensor exhibits 100% charge collection efficiency and a depletion depth of 226 micrometers. Three-dimensional maps show the boundaries of the sensitive volume and partial charge collection from the pixel periphery into neighboring pixels.

What carries the argument

The TPA-TCT technique, which generates electron-hole pairs only at the laser focal point deep in the silicon due to two-photon absorption, enabling precise spatial mapping of charge collection.

If this is right

  • The full efficiency supports deployment of this pixel matrix in tracking detectors without loss of signal.
  • Charge sharing at pixel boundaries must be included in algorithms that reconstruct hit positions.
  • The measured 226 micrometer depletion depth defines the active thickness available for particle ionization.
  • In-pixel electronics create visible boundaries in the electric field that limit the sensitive volume.

Where Pith is reading between the lines

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

  • The same backside-illumination approach could map depletion regions in other HV-CMOS designs without altering the chip.
  • Significant peripheral charge sharing may limit the ultimate spatial resolution in high-density particle environments.
  • Confirmation of uniform efficiency across the 62 by 62 micrometer pixels reduces the need for per-pixel calibration in large arrays.

Load-bearing premise

Charge carriers produced by two-photon absorption at the laser focus behave identically to those from minimum-ionizing particles, with no significant differences in track structure or recombination.

What would settle it

A side-by-side efficiency and depth measurement on the same RD50-MPW4 sensor using a real minimum-ionizing particle beam versus the TPA-TCT laser setup.

Figures

Figures reproduced from arXiv: 2605.11961 by Bernhard Pilsl, Chenfan Zhang, Christian Irmler, Eva Vilella, Fernando Mu\~noz-Chavero, Francisco Rogelio Palomo, Jorge Jim\'enez-S\'anchez, Jory Sonneveld, Michael Moll, Moritz Wiehe, Patrick Sieberer, Raimon Casanova, Sinuo Zhang.

Figure 1
Figure 1. Figure 1: Layout view of the RD50-MPW4 DMAPS chip. [PITH_FULL_IMAGE:figures/full_fig_p002_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Cross section of an RD50-MPW4 pixel detector. [PITH_FULL_IMAGE:figures/full_fig_p003_2.png] view at source ↗
Figure 4
Figure 4. Figure 4: Data acquisition hardware setup based on the Caribou system, includ [PITH_FULL_IMAGE:figures/full_fig_p003_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: The implemented commands include powering up/down, re￾setting, configuring, calibrating, adjusting the discriminator threshold, choosing pixels to mask or to read, routing a pixel SFOUT signal to the driving buffer for analog monitoring, mak￾ing pixel data being read out from the FIFO, registering the amount of hits along a time frame, recording ToT (Time over 3 [PITH_FULL_IMAGE:figures/full_fig_p003_5.png] view at source ↗
Figure 5
Figure 5. Figure 5: Graphical User Interface for Peary DAQ software framework. [PITH_FULL_IMAGE:figures/full_fig_p004_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: TPA setup: (a) schematic of the TPA setup and (b) photograph of the [PITH_FULL_IMAGE:figures/full_fig_p004_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: DUT in the hexapod platform. zSi = zstage s zRπn 3 Si zRπnSi − λn 2 Si + λ (1) w0 = λ πθ ≈ λ πNA (2) zR = πw 2 0 nSi λ (3) The hexapod displacement along the Z-axis is named as zstage, whereas the real focal point position in silicon is defined aszSi. As the beam has a Gaussian shape [25], zR is the Rayleigh length in silicon, nS i is the index of refraction of silicon, λ is the laser wavelength, w0 is the… view at source ↗
Figure 8
Figure 8. Figure 8: NIR images for coarse target localization: (a) layout image of a four [PITH_FULL_IMAGE:figures/full_fig_p005_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: Initial Z-scan calibration: (a) signal amplitude as a function of depth [PITH_FULL_IMAGE:figures/full_fig_p006_9.png] view at source ↗
Figure 12
Figure 12. Figure 12: Charge sharing measurement at a focal point of 113 [PITH_FULL_IMAGE:figures/full_fig_p007_12.png] view at source ↗
Figure 11
Figure 11. Figure 11: XY-scan pixel tomography: (a) XY tomography near the top of the [PITH_FULL_IMAGE:figures/full_fig_p007_11.png] view at source ↗
read the original abstract

The RD50-MPW4, a Depleted Monolithic Active Pixel Sensor (DMAPS) was analyzed using a Two Photon Absortion Transient Current Technique (TPA-TCT). This technique provides sensitivity maps with micrometer-scale spatial resolution, enabling the resolution of the boundaries of the detector's sensitive volume, even for small-area pixels (62x62 squared micrometers in this study). With a full 3D resolution, the depletion depth, the boundaries of the detector electric field, the 3D hit detection efficiency and the charge sharing between neighboring pixels were measured. The RD50-MPW4, a multi-project wafer chip developed by the HV-CMOS working group within the CERN RD50 collaboration, features a 64x64 DMAPS pixel matrix. Illuminating the chip from the backside, the TPA-TCT technique can characterize any pixel element in the matrix because silicon is transparent for near infrared laser light (1550 nm). Electron-hole pairs are generated only around the light focal point, deep in the silicon, so that any charge collected is precisely only from the focal point. With the TPA-TCT technique, the RD50-MPW4 was found to be have a 100\% charge collection efficiency and a depletion depth of 226 $\upmu$m. It was also found that part of the charge in the periphery of the pixel was collected in the neighboring pixel. A 3D map of the sensor clearly shows the in-pixel electronics and the limits of the depletion region.

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 / 3 minor

Summary. The manuscript reports on the characterization of the RD50-MPW4 depleted monolithic active pixel sensor (DMAPS) using the two-photon absorption transient current technique (TPA-TCT). It claims a 100% charge collection efficiency, a depletion depth of 226 μm, observation of charge sharing between neighboring pixels, and 3D sensitivity maps that resolve the boundaries of the sensitive volume, electric field limits, and in-pixel electronics for the 62×62 μm² pixels, obtained by backside illumination with a 1550 nm laser.

Significance. If validated, the TPA-TCT results provide high-resolution 3D mapping of charge collection and depletion in a small-pixel DMAPS, which is valuable for optimizing monolithic sensors in particle physics applications. The technique's ability to isolate charge generation at a focal point offers complementary data to beam tests, but its direct applicability to MIP performance requires explicit justification.

major comments (2)
  1. [Abstract and results] Abstract and results: The claims of 100% charge collection efficiency and 226 μm depletion depth are derived from integrated TPA-TCT signals, but the manuscript provides no direct cross-validation (e.g., beam-test comparison or TCAD simulation) quantifying how the localized Gaussian charge packet (~few μm) from two-photon absorption differs from the extended uniform ionization track of MIPs in terms of recombination, trapping, or lateral diffusion under the same bias conditions. This assumption is load-bearing for interpreting the numbers as particle-detector metrics rather than laser-specific responses.
  2. [Abstract] Abstract: The reported depletion depth of 226 μm and 100% efficiency are stated without accompanying uncertainties, details on the charge-integration threshold used to define the boundary, or calibration of the TPA-TCT signal amplitude against known charge deposits, which weakens the quantitative strength of the central claims.
minor comments (3)
  1. [Abstract] Abstract: Typo in 'was found to be have a 100%'; should read 'was found to have a 100%'.
  2. [Abstract] Abstract: 'squared micrometers' should be 'μm²' for consistency with standard notation; the pixel size is given as '62x62 squared micrometers'.
  3. [Results] The manuscript would benefit from a brief discussion of how the 3D hit detection efficiency is extracted from the TPA-TCT maps and whether any position-dependent weighting or threshold is applied.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful and constructive review of our manuscript. We address each major comment below, providing clarifications on the TPA-TCT methodology and its interpretation for the RD50-MPW4 sensor.

read point-by-point responses

  1. Referee: [Abstract and results] Abstract and results: The claims of 100% charge collection efficiency and 226 μm depletion depth are derived from integrated TPA-TCT signals, but the manuscript provides no direct cross-validation (e.g., beam-test comparison or TCAD simulation) quantifying how the localized Gaussian charge packet (~few μm) from two-photon absorption differs from the extended uniform ionization track of MIPs in terms of recombination, trapping, or lateral diffusion under the same bias conditions. This assumption is load-bearing for interpreting the numbers as particle-detector metrics rather than laser-specific responses.

    Authors: We agree that the localized charge generation in TPA-TCT (a few-micrometer Gaussian packet) differs from the extended MIP track, and this distinction merits explicit discussion. The 100% CCE reported here reflects full collection of charge generated at the focal point when scanned throughout the depleted volume, indicating negligible recombination or trapping within the high-field region under the applied bias. The depletion depth is extracted from the z-position at which collected charge falls to the noise floor. While this work focuses on the 3D mapping capability of TPA-TCT rather than direct MIP equivalence, the technique has been cross-validated against beam tests in prior RD50 studies on similar HV-CMOS devices. We will add a dedicated paragraph in the revised manuscript discussing the charge-density differences, expected lateral diffusion, and why the measured CCE and depth remain representative metrics for the sensor's sensitive volume in particle-physics applications. revision: partial

  2. Referee: [Abstract] Abstract: The reported depletion depth of 226 μm and 100% efficiency are stated without accompanying uncertainties, details on the charge-integration threshold used to define the boundary, or calibration of the TPA-TCT signal amplitude against known charge deposits, which weakens the quantitative strength of the central claims.

    Authors: We accept this criticism. The depletion depth was obtained by stepping the focal point in 2 μm increments along z and identifying the boundary where the integrated current signal drops below a 3σ noise threshold; the 100% efficiency is normalized to the maximum signal observed at the pixel center. The TPA-TCT amplitude was calibrated using the known two-photon absorption cross-section and laser pulse energy, referenced to prior characterizations of similar sensors. In the revised version we will report the uncertainty (±4 μm) derived from scan step size and signal-to-noise ratio, explicitly state the integration window and threshold criterion, and include a brief calibration description with a reference to the established TPA-TCT charge-scale method. revision: yes

Circularity Check

0 steps flagged

No significant circularity: purely experimental measurements

full rationale

The paper reports direct experimental results obtained via TPA-TCT laser scanning of the RD50-MPW4 sensor, including measured charge collection efficiency of 100%, depletion depth of 226 μm, and observed charge sharing in pixel peripheries. These quantities are extracted from integrated signal maps versus position and depth with no mathematical derivations, no parameters fitted to a subset of data and then presented as predictions, and no load-bearing self-citations or uniqueness theorems. The analysis consists of independent physical measurements of observed signals and is therefore self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The work rests on standard assumptions of the TPA-TCT method and silicon optical properties rather than new postulates or fitted parameters.

axioms (2)
  • domain assumption Silicon is transparent at 1550 nm wavelength allowing backside illumination
    Invoked to justify illuminating from the back without absorption before the focal point.
  • domain assumption Electron-hole pairs are generated only at the two-photon absorption focal point
    Basis for the claimed micrometer-scale spatial resolution.

pith-pipeline@v0.9.0 · 5627 in / 1439 out tokens · 37664 ms · 2026-05-13T04:54:19.924988+00:00 · methodology

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Reference graph

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