arxiv: 2604.24367 · v1 · submitted 2026-04-27 · ⚛️ physics.ins-det

Recognition: unknown

Investigation of the in-pixel response of the Mupix11 monolithic pixel sensor using a microfocus X-ray beam at Diamond Light Source

A. Brooks, A.E. McDougall, A.J.A. Knight, A.S. Rotelli, D. Bortoletto, D M S Sultan, H. Augustin, L. Vigani, M. Grimes, M.S. K\"oppel, R. Plackett, S. Wood

Pith reviewed 2026-05-07 17:06 UTC · model grok-4.3

classification ⚛️ physics.ins-det

keywords MuPix11HV-MAPSin-pixel responseX-ray beamdetector uniformitypixel sensorMu3emonolithic active pixel sensor

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The pith

The MuPix11 monolithic pixel sensor displays uniform in-pixel response to an 8 keV X-ray beam at nominal voltage and threshold.

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

Researchers used a microfocus X-ray beam to probe the local response inside each pixel of the MuPix11 sensor, which is designed for the Mu3e experiment's tracking system. The 3 micrometer spot size and 8 keV energy mimic the ionization from passing charged particles. Scans at standard operating conditions found that the sensor detects signals equally well no matter where the beam hits within the pixel. This uniformity holds even at the edges. Without reverse bias voltage the efficiency drops at boundaries, showing the importance of proper biasing for reliable performance.

Core claim

High-resolution scans across the pixel matrix of the 70 micrometer thick MuPix11 sensor reveal uniform detector response at nominal operating voltage and threshold. The relative sub-pixel response becomes location-dependent in the absence of reverse bias, with reduced detection rates observed at pixel boundaries.

What carries the argument

The microfocus X-ray beam with 3 μm spot size used to map the sub-pixel photon response by scanning across the pixel matrix.

If this is right

The sensor maintains consistent detection efficiency suitable for precise particle tracking without additional corrections.
Pixel boundaries do not introduce significant efficiency losses under normal bias conditions.
The thinning to 70 μm preserves the uniform response needed for the Mu3e experiment.
Zero-bias operation is unsuitable due to position-dependent response variations.

Where Pith is reading between the lines

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

If the X-ray emulation of MIPs is accurate, similar sensors could be qualified using this non-destructive method before installation.
These findings suggest that boundary effects in monolithic sensors can be mitigated by applying reverse bias, which may generalize to other HV-MAPS designs.
Direct comparison experiments with actual particle beams would further confirm the validity of the X-ray results for real-world use.

Load-bearing premise

The 8 keV X-ray beam with 3 μm spot size accurately emulates the passage of a minimum ionising particle in terms of charge deposition and detection.

What would settle it

A scan at nominal voltage and threshold that shows more than a few percent variation in detection rate between the center and edges of a pixel would contradict the uniformity claim.

Figures

Figures reproduced from arXiv: 2604.24367 by A. Brooks, A.E. McDougall, A.J.A. Knight, A.S. Rotelli, D. Bortoletto, D M S Sultan, H. Augustin, L. Vigani, M. Grimes, M.S. K\"oppel, R. Plackett, S. Wood.

**Figure 1.** Figure 1: Schematic of the HV-MAPS pixel structure implemented in MuPix11 [5]. increasing bias voltage, a depletion region forms around and below the n-well. However, even at 0 V bias, a small built-in potential arising from the p-n junction enables some signal collection. For the MuPix11 this intrinsic voltage is 0.61 V to 0.67 V [6]. Additionally the n-well is biased to a voltage close to positive supply voltage, … view at source ↗

**Figure 2.** Figure 2: SCC and FEB mounted on the motion stage. The SCC provides connections for low- and high-voltage supplies and to the FEB view at source ↗

**Figure 3.** Figure 3: Alignment of the MuPix11 sensor using guidance lasers. in view at source ↗

**Figure 4.** Figure 4: Preliminary two-dimensional scan with 5 µm step size to ensure alignment (bias 0 V, threshold 42.2 mV, 6 seconds of beam per point). line DAQ. A preliminary two-dimensional scan ( view at source ↗

**Figure 5.** Figure 5: Number of hits recorded at 0 V bias. Red boxes indicate per-pixel centre averaging regions (3 µm step size, threshold 42.2 mV, 11.7 seconds of beam per point) view at source ↗

**Figure 7.** Figure 7: Relative MuPix11 response at 0 V sensor bias, showing 1D cuts across the pixels at view at source ↗

**Figure 8.** Figure 8: Diagonal cross section of the two-dimensional plot in Fig. 5 at 0 V, showing the hit rate view at source ↗

**Figure 9.** Figure 9: Number of hits recorded at −30 V bias. Red boxes indicate per-pixel centre averaging regions (3 µm step size, threshold 42.2 mV, 15 seconds of beam per location) view at source ↗

**Figure 10.** Figure 10: A two-dimensional scan of relative hit rate across a pixel and its neighbouring pixels at −30 V sensor bias (3 µm step size, threshold 42.2 mV, 15 seconds of beam per location). To quantify the overall performance difference, the efficiency is combined with the measured hit rates (381±5 Hz at −30 V, 362±6 Hz at 0 V). The effective throughput is therefore 364.5 ± 4.6 Hz and 329.4 ± 5.8 Hz, respectively… view at source ↗

**Figure 11.** Figure 11: Relative MuPix11 response at −30 V sensor bias, with 1D cuts across the pixels at various heights (Poisson error bars) (3 µm step size, threshold 42.2 mV, 15 seconds of beam per point) view at source ↗

**Figure 12.** Figure 12: Number of pixels recording a hit along the vertical slice at view at source ↗

**Figure 13.** Figure 13: Normalised hits recorded by one pixel inside its physical 80 view at source ↗

**Figure 14.** Figure 14: 1D scan across three pixels for bias voltages between 0 V and view at source ↗

**Figure 15.** Figure 15: 1D scan across three pixels at different bias voltages. Intermediate scans between 0 V and view at source ↗

**Figure 16.** Figure 16: 1D scan across three pixels for thresholds between 42 view at source ↗

read the original abstract

MuPix11 is a High-Voltage Monolithic Active Pixel Sensor (HV-MAPS) developed for the tracking system of the Mu3e experiment. The in-pixel photon response of a MuPix11 sensor thinned to 70 {\mu}m was measured using an 8 keV X-ray beam with a 3 {\mu}m spot size at the B16 beamline at Diamond Light Source, emulating the passage of a minimum ionising particle (MIP). At nominal operating voltage and threshold, high-resolution scans across the pixel matrix show the detector response to be uniform. In the absence of reverse bias (0 V), the relative sub-pixel response is location-dependent as a reduced detection rate is observed at pixel boundaries.

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.

Desk Editor's Note private letter to a colleague

X-ray scans show uniform MuPix11 response at nominal bias but the MIP emulation rests on an untested assumption about charge deposition.

read the letter

The paper's central result is that a 70 μm thinned MuPix11 sensor gives a uniform in-pixel response when scanned with a 3 μm 8 keV X-ray spot at the nominal voltage and threshold, while the same scans at zero bias show a clear drop in detection rate at pixel boundaries. This is new data for this specific device and thinning level at Diamond Light Source, and it directly addresses a practical question for the Mu3e tracking layers. The measurement is clean enough to map sub-pixel behavior at high resolution, and the contrast between biased and unbiased operation is a straightforward, useful observation. The work stays focused on what the scans actually show without inflating the claims. The soft spot is the leap from X-ray data to MIP performance. An 8 keV photon creates a compact charge cloud via photoelectric absorption, unlike the continuous ionization trail plus occasional delta rays from a minimum ionizing particle. The abstract already demonstrates that boundary effects are visible at zero bias, so lateral diffusion and collection near edges could differ between the two cases. Without a quantitative comparison—either to actual beam-test MIP data or to a simulation that matches the observed X-ray maps—the uniformity claim at operating voltage remains tied to the X-ray interaction rather than proven for the particles the sensor will see. The paper is aimed at the Mu3e collaboration and groups working on similar HV-MAPS sensors. Readers who need concrete sub-pixel response numbers for this device will get value from the maps and the zero-bias result. It deserves peer review because the measurement is concrete and relevant to an ongoing experiment; a referee can ask for the missing cross-check or error analysis without the core data being unsound.

Referee Report

1 major / 2 minor

Summary. The manuscript reports experimental measurements of the in-pixel response of the MuPix11 HV-MAPS sensor (thinned to 70 μm) using a 3 μm spot-size 8 keV X-ray beam at the B16 beamline of Diamond Light Source. The central result is that the detector response is uniform across the pixel matrix at nominal operating voltage and threshold, while at 0 V bias the sub-pixel response becomes location-dependent with reduced detection rate at pixel boundaries.

Significance. If the X-ray measurements are representative of MIP interactions, the work supplies high-resolution sub-pixel characterization data directly relevant to the Mu3e tracking system. The controlled microfocus scans provide a useful complement to particle-beam tests by mapping in-pixel variations that are otherwise difficult to resolve.

major comments (1)

[Abstract] Abstract: The assertion that the 8 keV X-ray beam 'emulates the passage of a minimum ionising particle (MIP)' is not supported by any quantitative comparison (e.g., sub-pixel hit-efficiency maps, collected-charge distributions, or boundary-effect metrics) between the X-ray data and MIP test-beam data. Because 8 keV photoelectric absorption produces a compact charge cloud whose depth and lateral diffusion differ from the extended ionization trail of a MIP (including delta rays), the observed uniformity at nominal bias could be specific to the X-ray interaction and may not hold for MIP tracking in the thinned sensor.

minor comments (2)

The manuscript lacks explicit quantitative metrics (standard deviation, uniformity maps, or statistical tests) used to establish 'uniform' response and does not report error analysis or uncertainties on the detection-rate measurements.
Methods details on beam energy calibration, sensor bias and threshold settings, and the precise definition of 'nominal operating voltage and threshold' should be expanded for reproducibility.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the positive evaluation of the work's significance and for the detailed comment on the abstract. We address the concern below and will revise the manuscript accordingly.

read point-by-point responses

Referee: [Abstract] Abstract: The assertion that the 8 keV X-ray beam 'emulates the passage of a minimum ionising particle (MIP)' is not supported by any quantitative comparison (e.g., sub-pixel hit-efficiency maps, collected-charge distributions, or boundary-effect metrics) between the X-ray data and MIP test-beam data. Because 8 keV photoelectric absorption produces a compact charge cloud whose depth and lateral diffusion differ from the extended ionization trail of a MIP (including delta rays), the observed uniformity at nominal bias could be specific to the X-ray interaction and may not hold for MIP tracking in the thinned sensor.

Authors: We agree that the manuscript does not include a direct quantitative comparison (such as hit-efficiency maps or charge distributions) between the X-ray results and MIP test-beam data. This paper is dedicated to high-resolution sub-pixel mapping with the microfocus X-ray beam, which provides spatial resolution difficult to achieve with particle beams. We will revise the abstract to remove the claim that the beam 'emulates' MIP passage and instead state that the measurements supply detailed in-pixel response data relevant to the Mu3e tracking system. We will also add a short clarifying paragraph in the introduction noting the complementary character of these X-ray scans to existing MIP tests, without asserting equivalence of the charge-deposition processes. revision: yes

Circularity Check

0 steps flagged

No circularity: pure experimental measurement of sensor response

full rationale

The paper consists of direct experimental scans of a thinned HV-MAPS sensor using a microfocus 8 keV X-ray beam. Central claims (uniformity at nominal bias/threshold, location dependence at 0 V) are reported observations from data, with no equations, fitted models, predictions, or derivations that reduce to inputs. The phrase 'emulating the passage of a MIP' is an interpretive statement about the measurement technique, not a load-bearing derivation or self-referential fit. No self-citations appear in the provided abstract or description that justify uniqueness or ansatz choices. This is a standard experimental report whose results stand or fall on the data collection itself.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The measurement relies on standard assumptions about X-ray charge deposition in silicon and the equivalence to MIPs; no free parameters or new entities are introduced.

axioms (1)

domain assumption An 8 keV X-ray microbeam produces charge deposition profiles sufficiently similar to those of minimum-ionising particles for response mapping purposes.
Invoked to justify using the X-ray beam as a MIP emulator.

pith-pipeline@v0.9.0 · 5481 in / 1050 out tokens · 79085 ms · 2026-05-07T17:06:02.744898+00:00 · methodology

discussion (0)

Reference graph

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