arxiv: 2605.13572 · v1 · submitted 2026-05-13 · ⚛️ physics.ins-det

Recognition: 2 theorem links

· Lean Theorem

Upgrade of the Belle II Vertex Detector with Depleted Monolithic CMOS Active Pixel Sensors

Belle II VTX Collaboration

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Pith reviewed 2026-05-14 18:02 UTC · model grok-4.3

classification ⚛️ physics.ins-det

keywords Belle IIvertex detectorDMAPSOBELIXCMOS active pixel sensorsradiation toleranceSuperKEKBpixel pitch

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The pith

The OBELIX depleted monolithic sensor supports Belle II's vertex detector upgrade for six times higher luminosity.

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

The Belle II collaboration is designing an upgraded vertex detector using five or six layers of depleted monolithic active pixel sensors to handle a luminosity increase to 6 times 10^35 per square centimeter per second. The OBELIX chip, based on the TJ-Monopix2 design in 180 nm CMOS, targets a pixel pitch of 33 micrometers, time stamping in 50 to 100 nanoseconds, and radiation tolerance to 5 times 10^14 neutron equivalent per square centimeter and 100 Mrad. Beam tests and irradiation studies on the predecessor show that keeping the sensor below 40 degrees Celsius allows it to manage hit rates up to 120 megahertz per square centimeter. This approach keeps the total material budget around 3 percent of a radiation length while supporting trigger rates up to 30 kilohertz.

Core claim

The paper presents the OBELIX sensor as the core component for the Belle II VTX upgrade, derived from TJ-Monopix2, and demonstrates through beam test and irradiation studies of the predecessor that it can operate at the required performance levels if the temperature is maintained below 40 degrees Celsius after irradiation to 5 times 10^14 neq per square centimeter, enabling hit rates up to 120 MHz per square centimeter at the target luminosity.

What carries the argument

The OBELIX chip, a depleted monolithic active pixel sensor with 33 micrometer pitch and dedicated digital periphery for the Belle II trigger system.

Load-bearing premise

Single-chip beam-test results from the TJ-Monopix2 will translate directly to the full OBELIX chip and the complete VTX system under high-rate and radiation conditions at SuperKEKB.

What would settle it

A measurement showing that the complete OBELIX chip or assembled VTX exceeds 40 degrees Celsius or fails to maintain efficiency at 120 MHz per square centimeter after irradiation to 5 times 10^14 neq per square centimeter would disprove the performance claims.

Figures

Figures reproduced from arXiv: 2605.13572 by Belle II VTX Collaboration.

**Figure 2.** Figure 2: Simulation of the temperature gradient reached along the structure of the iVTX ladder. 0.3% 𝑋∕𝑋0 per layer. From the OBELIX design advances, the power dissipation in the iVTX is expected to reach up to 200 mW∕cm2 . Several cooling options are being investigated to manage the high power density and the degradation of chip performance with increasing temperature after irradiation. One such option is a pass… view at source ↗

**Figure 4.** Figure 4: The layout of the 2 × 2 TJ-Monopix2 CMOS pixel sensor. (𝑇sensor), due to an opening in the copper plate at the chip position [PITH_FULL_IMAGE:figures/full_fig_p004_4.png] view at source ↗

**Figure 5.** Figure 5: shows the hit detection efficiency of TJ-Monopix2 measured at maximum bias voltage after irradiation, as a function of the discriminator threshold (top) and the sensor temperature (bottom). Measurements are shown for both DC- and AC-coupled pixel variants irradiated with either 24 MeV protons or 90 MeV electrons, reaching NIEL fluences of up to 5 × 1014 𝑛eq∕cm2 . The efficiency as a function of threshold e… view at source ↗

**Figure 6.** Figure 6: Floorplan illustration of the OBLIX chip [PITH_FULL_IMAGE:figures/full_fig_p005_6.png] view at source ↗

**Figure 7.** Figure 7: Conceptual layout of the ITT and its integration with the Belle II detector. 6. Inner Timing Tracker Concept The proposed upgrade of the Belle II VTX and new central drift chamber (CDC), which will exclude the inner layers, introduces a gap in the region between the outermost vertex layers and the innermost CDC superlayer. It may leave tracks traversing this transition region partially unmeasured. This ef… view at source ↗

read the original abstract

The Belle II experiment, operating at the asymmetric SuperKEKB $e^+e^-$ collider, is preparing an upgrade of its vertex detector to cope with an increased luminosity of $6 \times 10^{35}$ cm$^{-2}$s$^{-1}$. The upgraded vertex detector (VTX) will consist of five or six layers of depleted monolithic active pixel sensors (DMAPS), with a total material budget of approximately $3\%$ $X/X_0$. The OBELIX chip, derived from the TJ-Monopix2 sensor and fabricated using Tower Semiconductor 180 nm CMOS technology, is being developed for this upgrade. It features a 33 $\mu$m pixel pitch with a time-stamping binning of $50-100$ ns, along with a dedicated digital periphery compatible with the Belle II trigger system, supporting rates up to 30 kHz. The sensor is designed to operate under the high background conditions expected at the target luminosity, with radiation tolerance up to $5 \times 10^{14}$ $n_{eq}$/cm$^2$ and 100 Mrad, while targeting a power density of about 200 mW/cm$^2$. This corresponds to hit rates up to 120 MHz/cm$^2$. Beam test and irradiation studies of TJ-Monopix2 demonstrate that the operating sensor temperature should stay below $40^\circ$C after irradiation up to $5 \times 10^{14}$ $n_{eq}$/cm$^2$. This report reviews the proposed VTX concept, sensor performance, and ongoing R$\&$D activities.

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

This is a competent status report on the Belle II VTX upgrade but adds no new measurements and leans on untested extrapolation from TJ-Monopix2 single-chip data.

read the letter

The paper is mainly a technical overview of the planned five- or six-layer DMAPS vertex detector for Belle II at 6e35 luminosity. It does a clear job stating the target specs for the OBELIX chip—33 µm pitch, 50-100 ns time binning, 30 kHz trigger compatibility, 200 mW/cm² power, and tolerance to 5e14 neq/cm² plus 100 Mrad—while pointing to the TJ-Monopix2 beam-test and irradiation results that set the <40°C operating limit after irradiation to support 120 MHz/cm² hit rates. That reference to prior work is useful context and keeps the claims grounded in existing data rather than invention. The design choice to stay in the Tower 180 nm process and retain the same pixel pitch also looks pragmatic for schedule and risk reduction. The soft spot is the one flagged in the stress-test note. OBELIX adds a dedicated digital periphery for the Belle II trigger, yet the paper gives no quantitative comparison of power density, local heating, or post-irradiation charge collection between the two chips. There are also no full-system simulations or multi-layer heat-flow results shown for the complete VTX geometry under combined high-rate and radiation conditions. If the periphery raises local power or changes dynamics, the temperature bound from single-chip TJ-Monopix2 tests may not carry over directly. This is a status report rather than a new result, so it is mainly useful to people already following Belle II instrumentation or DMAPS development for high-background flavor experiments. It deserves peer review in an instrumentation journal because the upgrade matters for the experiment’s future physics reach and referees could usefully comment on the integration risks. I would not cite it in my own work and would only bring it to a reading group if the group specifically covers detector R&D plans.

Referee Report

2 major / 2 minor

Summary. The paper presents the proposed upgrade of the Belle II vertex detector (VTX) to five or six layers of depleted monolithic active pixel sensors (DMAPS) to handle SuperKEKB luminosities up to 6×10^35 cm^{-2}s^{-1}. The OBELIX chip, derived from TJ-Monopix2 and fabricated in Tower 180 nm CMOS, features a 33 μm pixel pitch, 50-100 ns time-stamping, and a dedicated digital periphery compatible with the Belle II 30 kHz trigger. It targets radiation tolerance to 5×10^14 n_eq/cm² and 100 Mrad, a power density of ~200 mW/cm² supporting hit rates up to 120 MHz/cm², and a total material budget of ~3% X/X0. Beam-test and irradiation results on the TJ-Monopix2 predecessor are cited to establish an operating temperature limit below 40°C post-irradiation.

Significance. If the extrapolation from predecessor data holds, the work is significant for enabling the Belle II VTX upgrade with a low-material, high-rate DMAPS solution. It builds directly on validated TJ-Monopix2 performance in the same process node, providing a concrete path to the required hit-rate and radiation tolerance while maintaining trigger compatibility. The grounding in existing beam-test data is a strength, though full-system validation remains pending.

major comments (2)

[Abstract / OBELIX sensor description] Abstract: The claim that TJ-Monopix2 beam-test and irradiation results establish a <40°C operating limit after 5×10^14 n_eq/cm² (supporting 120 MHz/cm² hit rates) is load-bearing, yet the manuscript provides no quantitative comparison of power density, digital activity, or heat distribution between TJ-Monopix2 and the OBELIX design with its added digital periphery. If the periphery increases local power or alters post-irradiation dynamics, the temperature bound may not translate.
[VTX upgrade concept] VTX concept section: No full-system thermal or rate simulations are presented for the five- or six-layer geometry under combined high-background and radiation conditions at SuperKEKB. Multi-layer heat flow, mechanical integration effects on temperature, and end-to-end efficiency after irradiation are not quantified, leaving the feasibility of the 200 mW/cm² target and <40°C limit unverified for the complete detector.

minor comments (2)

[Abstract] Abstract: Quantitative error bars, efficiency numbers, or post-irradiation hit-efficiency values from the TJ-Monopix2 studies are not provided, making it difficult to assess the margin on the <40°C recommendation.
[Sensor design] The manuscript would benefit from a table comparing key parameters (power density, digital periphery power, timestamp resolution) between TJ-Monopix2 and OBELIX to clarify the extrapolation.

Circularity Check

0 steps flagged

No circularity; performance bounds rest on external experimental data from predecessor sensor

full rationale

The paper is a design proposal and review for the OBELIX DMAPS chip in the Belle II VTX upgrade. Its central claims (temperature limit below 40°C after 5×10^14 neq/cm² irradiation and compatibility with 120 MHz/cm² hit rates) are explicitly attributed to prior beam-test and irradiation results on the TJ-Monopix2 predecessor, which are external experimental measurements rather than quantities derived from equations or fits inside this manuscript. No self-definitional steps, fitted inputs renamed as predictions, or load-bearing self-citations of unverified theorems appear; the document contains no internal derivation chain that reduces to its own inputs. The noted differences (added digital periphery, full multi-layer geometry) are described qualitatively but do not create circularity because they are not used to redefine or force the predecessor bounds.

Axiom & Free-Parameter Ledger

1 free parameters · 1 axioms · 1 invented entities

The report relies on the assumption that predecessor sensor test results scale to the new chip and full detector; no new free parameters are fitted in the abstract, and no invented entities beyond the named OBELIX chip are introduced.

free parameters (1)

target power density
Specified as approximately 200 mW/cm² for the OBELIX sensor under expected hit rates.

axioms (1)

domain assumption Beam-test results from TJ-Monopix2 apply to the OBELIX chip after irradiation
The abstract directly invokes these studies to set the 40°C temperature limit for the upgraded sensor.

invented entities (1)

OBELIX chip no independent evidence
purpose: Dedicated DMAPS sensor for the Belle II VTX upgrade
New chip derived from TJ-Monopix2 with added digital periphery for Belle II trigger compatibility.

pith-pipeline@v0.9.0 · 5589 in / 1459 out tokens · 112923 ms · 2026-05-14T18:02:31.274789+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

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