Systematic Investigation of Acceptor Removal in HPK LGADs with Modified Gain Layers

Issei Horikoshi; Koji Nakamura; Koji Sato; Mahiro Kobayashi; Masato Terada; Minoru Hirose; Sayuka Kita; Tatsuya Masubuchi; Tomoka Imamura; Yua Murayama

Carbon implantation is the only gain-layer modification that clearly improves radiation tolerance in LGADs.

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 →

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2026-07-01 09:20 UTC pith:IPKQYSTP

load-bearing objection Carbon implantation is presented as the only clear win among the tested gain-layer mods for radiation tolerance in these HPK LGADs, but the abstract gives no data to check the metrics. the 2 major comments →

arxiv 2604.23956 v2 pith:IPKQYSTP submitted 2026-04-27 physics.ins-det hep-ex

Systematic Investigation of Acceptor Removal in HPK LGADs with Modified Gain Layers

Yua Murayama , Mahiro Kobayashi , Tomoka Imamura , Koji Nakamura , Issei Horikoshi , Koji Sato , Masato Terada , Minoru Hirose

show 2 more authors

Tatsuya Masubuchi Sayuka Kita

This is my paper

classification physics.ins-det hep-ex

keywords LGADradiation toleranceacceptor removalcarbon implantationgain layersilicon sensorstiming performanceproton irradiation

verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved

The pith

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

This paper tests several modified gain layers in low-gain avalanche diodes to see which resist acceptor removal best after irradiation. LGADs need this tolerance to keep their internal multiplication field in the harsh radiation of future colliders. Among oxygen-modified, carbon-implanted, and boron-phosphorus compensated designs, only carbon implantation shows a clear benefit. The work also finds that the acceptor-removal rate varies with the type and energy of the irradiating particles.

Core claim

The study finds that carbon implantation in the gain layer provides a clear improvement in radiation tolerance as measured by the acceptor-removal coefficient from IV curves and the voltage needed to restore timing performance, whereas oxygen modifications including partially activated boron and gain-layer compensation do not, and adding compensation to carbon-implanted structures adds no further advantage. The acceptor-removal coefficient also depends on whether protons or reactor neutrons are used for irradiation.

What carries the argument

Acceptor-removal coefficient extracted from IV measurements after irradiation, which quantifies the reduction in active acceptor concentration in the gain layer.

Load-bearing premise

The acceptor-removal coefficient extracted from IV measurements and the operation voltage required to recover timing performance are reliable and comparable metrics of radiation tolerance across the different gain-layer designs tested.

What would settle it

Irradiating additional carbon-implanted and non-carbon LGADs to the same fluence and observing no difference in the acceptor-removal coefficient or required operation voltage would falsify the claim of improvement.

Watch this falsifier — get emailed when new claim-graph text bears on it.

If this is right

Carbon implantation yields better post-irradiation performance than other modifications tested.
Compensated carbon-implanted structures show no advantage over carbon-only structures.
Acceptor removal depends on irradiation particle type and energy.
Oxygen modifications and compensation alone do not significantly improve tolerance.

Where Pith is reading between the lines

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

Designs for future collider timing layers should incorporate carbon implantation to enhance radiation hardness.
The dependence on particle type suggests that tolerance results may vary under different radiation environments.
Further studies could test if other combinations of modifications yield additive benefits beyond those examined.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit.

Desk Editor's Note

Carbon implantation is presented as the only clear win among the tested gain-layer mods for radiation tolerance in these HPK LGADs, but the abstract gives no data to check the metrics.

read the letter

Carbon implantation is the only modification that shows a clear improvement in radiation tolerance here, while oxygen changes, PAB, and compensation do not, and adding compensation to carbon brings no extra gain.

The work reports side-by-side results on HPK prototypes with those specific gain-layer tweaks after proton and reactor-neutron irradiation. It tracks tolerance with the acceptor-removal coefficient from IV curves and the voltage needed to restore timing performance, and it notes that the coefficient varies with particle type and energy. That comparative angle on real manufacturer devices is the main new piece.

It does a straightforward job of testing practical options and using two observables that matter for timing layers. The direct statement that carbon stands out and the others do not is easy to follow.

The soft spots are the lack of any numbers, plots, sample details, or error treatment in what we have, which makes it impossible to judge how solid the ordering really is. The stress-test concern lands: different gain-layer designs start with different doping and field profiles, so an IV-derived removal coefficient or a timing-recovery voltage could move for reasons unrelated to actual acceptor removal. The abstract gives no sign that the measurements were normalized to initial gain or capacitance or that the timing criterion was held constant. If the full paper does not show those controls, the claimed ranking could be partly an artifact of the chosen metrics.

This is for people building radiation-hard LGAD timing detectors. A reader already working on gain-layer hardening would get value from the comparative results once the data and methods are visible.

I would send it to peer review. The topic is relevant and the experimental direction is the right one, but the authors need to supply the measurements and address whether the two observables are comparable across designs.

Referee Report

2 major / 0 minor

Summary. The manuscript reports a systematic study of radiation tolerance in HPK LGAD prototypes with modified gain layers (oxygen-modified, carbon-implanted, boron-phosphorus compensated, and combinations). Radiation tolerance is assessed via the acceptor-removal coefficient extracted from IV curves and the bias voltage required to recover timing performance after proton and reactor-neutron irradiation. The central claim is that carbon implantation is the only modification yielding a clear improvement, while oxygen-related changes (including PAB), compensation alone, and compensated carbon show no significant benefit; the removal coefficient is also reported to depend on particle type and energy.

Significance. If the ordering of radiation tolerance is confirmed with appropriate controls and quantitative data, the result would help prioritize gain-layer engineering strategies for LGADs in high-radiation collider environments.

major comments (2)

[Abstract] Abstract: the claim that carbon implantation alone provides a clear improvement while other approaches do not depends on the acceptor-removal coefficient (from N_A(Φ) = N_A0 exp(−cΦ)) and the timing-recovery voltage being unbiased, design-independent proxies; the abstract supplies no evidence that these metrics were normalized to initial gain, doping profile, or electric-field shape, which are altered by the modifications under test.
[Abstract] Abstract: no numerical values of the removal coefficient, uncertainties, sample sizes, or statistical tests are provided to support the stated ordering of the designs or the particle-type dependence of c.

Simulated Author's Rebuttal

2 responses · 1 unresolved

We thank the referee for the constructive comments on the abstract. We respond point by point below.

read point-by-point responses

Referee: [Abstract] Abstract: the claim that carbon implantation alone provides a clear improvement while other approaches do not depends on the acceptor-removal coefficient (from N_A(Φ) = N_A0 exp(−cΦ)) and the timing-recovery voltage being unbiased, design-independent proxies; the abstract supplies no evidence that these metrics were normalized to initial gain, doping profile, or electric-field shape, which are altered by the modifications under test.

Authors: The abstract summarizes the main findings at a high level. The systematic comparison uses the standard exponential model applied uniformly to all designs, with each structure's initial acceptor concentration serving as its own baseline. We agree that a brief clarification on this point would strengthen the abstract and will revise it accordingly. revision: yes
Referee: [Abstract] Abstract: no numerical values of the removal coefficient, uncertainties, sample sizes, or statistical tests are provided to support the stated ordering of the designs or the particle-type dependence of c.

Authors: Abstracts are constrained by length and conventionally omit detailed numerical results, uncertainties, and statistical information, which belong in the main text. We therefore see no need to alter the abstract on this point. revision: no

standing simulated objections not resolved

Specific numerical values of the acceptor-removal coefficients, uncertainties, sample sizes, or statistical tests (these are absent from the provided abstract text)

Circularity Check

0 steps flagged

Empirical measurements only; no derivation chain present

full rationale

The paper reports results from proton and neutron irradiation tests on HPK LGAD prototypes with modified gain layers. Radiation tolerance is assessed via two direct experimental observables: the acceptor-removal coefficient extracted from IV curves and the voltage needed to restore timing performance. The abstract contains no equations, no fitted parameters presented as predictions, no self-citations, and no derivation steps. The ordering of radiation hardness (carbon implantation only) follows from the measured values themselves. This is a standard experimental comparison with no load-bearing reduction to inputs by construction.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

No mathematical model, free parameters, or invented entities described; the work is purely experimental comparison of sensor prototypes.

pith-pipeline@v0.9.1-grok · 5781 in / 869 out tokens · 21612 ms · 2026-07-01T09:20:40.235836+00:00 · methodology

0 comments

read the original abstract

Low-Gain Avalanche Diodes (LGADs) are fast silicon sensors with internal charge multiplication and are key candidates for precision timing layers in future high-energy hadron colliders. Their operation in harsh radiation environments, however, is limited by acceptor removal in the gain layer, which reduces the active acceptor concentration and degrades the internal electric field required for avalanche multiplication. Improving the radiation tolerance of the gain layer is therefore essential for future 4D tracking applications. In this work, we investigated several LGAD prototypes produced in collaboration with Hamamatsu Photonics K.K. (HPK), featuring modified gain-layer designs, including oxygen-modified, carbon-implanted, and boron--phosphorus compensated structures. The sensors were studied after proton and reactor-neutron irradiation. Radiation tolerance was characterized using the acceptor-removal coefficient extracted from IV measurements and the operation voltage required to recover the timing performance after irradiation. The results show that carbon implantation is the only approach among those studied here that provides a clear improvement in radiation tolerance. In contrast, neither oxygen-related modification, including the Partially Activated Boron (PAB) approach, nor gain-layer compensation alone yields a significant improvement, and the compensated carbon-implanted structure shows no clear advantage over the carbon-only case. In addition, the acceptor-removal coefficient is found to depend on the irradiation particle type and energy.

Figures

Figures reproduced from arXiv: 2604.23956 by Issei Horikoshi, Koji Nakamura, Koji Sato, Mahiro Kobayashi, Masato Terada, Minoru Hirose, Sayuka Kita, Tatsuya Masubuchi, Tomoka Imamura, Yua Murayama.