Gamma irradiation of ATLAS18 ITk strip sensors affected by static charge

C. Fleta; C. Jessiman; C. Klein; E. Staats; J. Fernandez-Tejero; J. Keller; J.Kroll; J. Kvasnicka; M. Mikestikova; M. Ullan

REVIEW 2 major objections 3 minor 8 references

Gamma irradiation at very small doses erases electrostatic-charge-induced electrical failures in ITk strip sensors, indicating the effects self-cure within one or two days of operation in the high-luminosity collider.

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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T0 review · deepseek-v4-flash

2026-08-01 11:12 UTC pith:RME7EDU3

load-bearing objection Gamma TID really does cure static-charge-induced QC failures in ATLAS18 strip sensors at few-krad doses, but the 'one or two days' extrapolation to HL-LHC conditions rests on an untested rate/temperature equivalence. the 2 major comments →

arxiv 2607.19936 v1 pith:RME7EDU3 submitted 2026-07-22 physics.ins-det hep-ex

Gamma irradiation of ATLAS18 ITk strip sensors affected by static charge

M. Mikestikova , V. Fadeyev , P. Federicova , J. Fernandez-Tejero , C. Fleta , P. Gallus , C. Jessiman , J. Keller

show 8 more authors

C. Klein T. Koffas J.Kroll J. Kvasnicka E. Staats P. Tuma M. Ullan Y. Unno

This is my paper

classification physics.ins-det hep-ex PACS 29.40.Wk

keywords silicon strip sensorsn+-in-pelectrostatic chargetotal ionizing dosegamma irradiationbreakdown voltageinterstrip isolationquality control

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.

Silicon strip sensors for the upgraded collider occasionally fail electrical quality-control tests because static charge trapped on their surfaces creates local high-field regions, causing early breakdown or loss of interstrip isolation. The authors asked whether the ionizing radiation the sensors will receive in real operation would erase these defects. By exposing affected sensors to gamma rays from a 60Co source, they found that a total ionizing dose of just 1.5–11 krad—corresponding to one or two days in the detector—completely cured every tested sensor, while unirradiated reference sensors stayed defective. The result implies that static-charge problems are a temporary, pre-installation phenomenon and will not compromise long-term tracker operation.

Core claim

The paper reports that gamma irradiation removes the negative electrical effects of electrostatic charge on n+-in-p silicon strip sensors of the ATLAS18 design. Early breakdown (breakdown voltage below the 500 V specification) disappeared after a delivered dose of 11 krad in one sensor and after only 1.5 krad in a reference sensor. Low interstrip isolation, visible as low bias-resistance regions in full strip tests, completely vanished after 11 krad in two sensors and after 3 krad in a reference sensor. An unirradiated reference sensor did not recover. The authors conclude that the effects of static charge completely disappear after a dose equivalent to one or two days of operation in the re

What carries the argument

The clearing agent is total ionizing dose (TID) delivered by a 60Co gamma-ray source inside a charged-particle-equilibrium box to ensure uniform energy deposition. The paper uses electrical diagnostics—current–voltage curves and full strip tests of bias resistance—to track recovery, and a radiation simulation to convert measured krad doses into days of operation at the outermost barrel radius, where TID is lowest. The key relation is the one between accumulated TID and the disappearance of the charge-induced defect; it is this mapping that makes the result relevant to real running.

Load-bearing premise

The load-bearing premise is that the effect of a gamma dose delivered at room temperature and 1 krad/min is the same as the effect of the real mixed-field radiation at operating temperatures below -30 °C, so the simulated conversion from krad to days of operation is valid.

What would settle it

Take a set of sensors with measured high surface charge and early breakdown, expose them to 1.5 krad from a 60Co source at the same dose rate as in this study, and verify whether IV and full-strip tests return to specification. If any sensor retains early breakdown or low interstrip isolation after the dose, the claim of a universal self-cure would be refuted; a controlled measurement of surface potential before and after low-dose irradiation would provide a more direct test of the charge-clearing effect.

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

If this is right

Sensors that fail electrical quality control because of electrostatic charge can be expected to recover fully within the first one or two days of real operation, without any intervention.
The reference-sensor results show that transport, storage, and handling alone do not cure the defects; the ionization itself is responsible.
The tiny dose required (1.5–3 krad) is far below the radiation budget anywhere in the inner tracker, so the cure will occur very early and before any performance impact accumulates.
The leakage-current increase observed after irradiation is largely a room-temperature surface effect; at the operating temperature below -30 °C the bulk component remains small, so the cure does not introduce a current penalty.
The recovery is complete for both failure modes studied—early breakdown and low interstrip isolation—in all tested sensors.

Where Pith is reading between the lines

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

The same small-dose gamma treatment could be adopted as a deliberate recovery step for sensors that fail quality control due to static charge, potentially replacing or shortening the current dry-storage and UV-light recovery procedures.
Because the cure dose is so small, it is plausible that even sensors with undetected charge spots will heal before their first physics run; this would eliminate a latent risk not caught by screening.
The mechanism may act on other charge-induced instabilities, such as slow leakage-current drift; the paper did not explicitly test long-term current stability after irradiation, so this remains open.
A direct measurement of surface potential before and after low-dose irradiation would strengthen the extrapolation from the gamma lab study to the mixed hadron field of the real detector.

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

Gamma TID really does cure static-charge-induced QC failures in ATLAS18 strip sensors at few-krad doses, but the 'one or two days' extrapolation to HL-LHC conditions rests on an untested rate/temperature equivalence.

read the letter

Short version: this is a small but direct empirical study showing that a few krad of gamma irradiation cures two static-charge-related QC failure modes in ATLAS18 ITk strip sensors. The effect looks real in the data, and the practical conclusion — that these failures should self-cure early in HL-LHC operation — is plausible, but the paper's 'one or two days' extrapolation is weaker than the headline suggests.

What's new and what works: the study targets a known production problem — sensors failing QC due to electrostatic charge and local trapped charge — and asks whether the TID the sensors will receive in the experiment would fix them before installation matters. That question is sensible and hasn't been answered in the cited literature. The experiment is straightforward: three early-breakdown sensors, three low-isolation sensors, two kept as references, irradiation at 60Co with a CPE box. The before/after IV and full strip test plots are convincing. Both irradiated breakdown sensors recovered by 11 krad; one reference recovered by 1.5 krad. Both irradiated low-isolation sensors recovered by 11 krad; the low-isolation reference only fully recovered after 3 krad. The direction of the effect is consistent and the doses are small. This is a useful empirical result for the ITk QC program and any silicon strip detector with similar passivation.

The soft spots are proportionate. The sample is six sensors, and there is no uncertainty quantification on the extracted breakdown voltages or strip resistances. Raw data would help. The larger issue is the environmental extrapolation. The 60Co irradiations were done at room temperature and ~1 krad/min; the ITk sensors will see a mixed hadron field at roughly 1 rad/min and below -30 C. The paper assumes the cure depends only on total dose. That assumption is plausible but not shown. If the mechanism is radiation-induced conductivity or charge neutralization in the passivation, both temperature and dose rate can matter, and the 1.5–3 krad threshold could shift. The 'one or two days' claim additionally relies on the ATLAS radiation simulation for the minimum-TID region, not on any measurement in this paper. Also, the low-isolation reference R1-W620-R partially self-healed during storage (26 to 18 affected strips), so its full recovery at 3 krad includes a spontaneous component; without a time-matched non-irradiated control, that particular data point is a bit softer than the others. These are limitations, not fatal flaws. The core observation stands.

Bottom line: this is a conference proceeding with a narrow but genuine new result. It deserves a proper referee, mostly to push for uncertainty quantification, raw data, and an honest discussion of the rate/temperature extrapolation. I'd take the 'self-cure within days' conclusion as a working hypothesis rather than a proven statement. Send it to peer review with those requests.

Referee Report

2 major / 3 minor

Summary. The paper reports a gamma-irradiation study of ATLAS18 ITk strip sensors that had failed electrical QC tests due to electrostatic charge. Six sensors were selected: three with early breakdown and three with low interstrip isolation. Two sensors from each category were irradiated with 60Co gamma rays to 11 krad, while the remaining two served as non-irradiated references for the first phase. After 11 krad, the irradiated sensors recovered. The reference sensors were then irradiated to lower doses: 1.5 krad cured the breakdown on R1-W650-R, while the low-isolation reference R1-W620-R required 3 krad. The authors conclude that static-charge effects disappear after a TID corresponding to one or two days of HL-LHC operation, providing confidence that such failures will self-cure early in the experiment.

Significance. If the conclusion holds, this result is practically important for the ATLAS ITk strip detector: it would mean that static-charge-induced early breakdown and low interstrip isolation, which occasionally appear during QC, are not a concern for operation. The experimental observations are direct, include non-irradiated references for the initial phase, and involve no fitted parameters or model-dependent analysis. The main limitation is that the irradiation conditions (60Co gamma rays, ~1 krad/min, room temperature) differ from the HL-LHC environment (mixed hadron field, low dose rate, below -30°C), and the paper does not provide data bridging this gap. The claim that recovery occurs in 'one or two days' therefore rests on an assumed equivalence that is not demonstrated.

major comments (2)

[Sec. 2 and Sec. 4] The conversion of the 1.5–3 krad curing threshold to 'one or two days of operation' in Sec. 4 uses the TID simulation of Ref. [5] but assumes that 60Co gamma irradiation at ~1 krad/min and room temperature reproduces the effect of the HL-LHC mixed hadron field at low dose rate and below -30°C. The paper reports no low-dose-rate or low-temperature irradiations, and the charge-neutralization mechanisms relevant to static-charge removal (radiation-induced conductivity, charge trapping and annealing) are known to depend on dose rate and temperature. This extrapolation is load-bearing for the headline conclusion; it should either be supported by additional measurements or the claim should be restricted to the tested conditions.
[Sec. 3, Table 1, Fig. 4] The reference sensor R1-W620-R, used for the low-interstrip-isolation category, improved spontaneously from 26 to 18 affected strips during dry storage before irradiation (Table 1). Since no unirradiated control was retained for the post-irradiation period, the disappearance of the remaining low-isolation area after a total dose of 3 krad cannot be unambiguously attributed to the TID. Continued spontaneous recovery is a plausible alternative or contributing factor. Please provide a time-matched control or explicitly quantify the spontaneous component to support the claim that irradiation alone cured this sensor.

minor comments (3)

[Sec. 3] The conclusion that the effects 'completely disappear' is based on only two sensors per failure mode, and the reference sensor showed partial spontaneous improvement. Suggest rewording to 'in the tested samples' and adding a sentence about the limited statistics.
[Sec. 2] The time interval between irradiation and the post-irradiation electrical measurements is not stated. Adding a timeline would help assess the possible contribution of spontaneous recovery, especially for R1-W620-R.
[Ref. [5]] Reference [5] is a TWiki page. For a published proceedings, please cite a versioned document or include the simulation parameters (geometry, fluence, dose profile) so that the 1.5 krad/day conversion can be independently checked.

Circularity Check

0 steps flagged

No circularity: measured TID thresholds are converted to HL-LHC days using an external ATLAS radiation simulation, not derived from the paper's own inputs.

full rationale

The paper is an empirical irradiation study. It reports direct measurements of IV curves and full-strip bias resistance before and after 60Co gamma irradiation at specific TID values, and the conclusion is simply that the previously observed electrical failures disappear after 1.5–11 krad. The conversion of those TID values into 'one or two days' of real experiment operation is taken from an external ATLAS radiation simulation [5], not from any fitted parameter or from the authors' own derived quantity. No parameter is fitted to the outcome, no uniqueness theorem or ansatz is imported from the authors' prior work, and the central claim does not reduce to any input by construction. The only overlapping self-citation, Ref. [4], is used for supplementary information about sensor recovery techniques and is not load-bearing for the irradiation result. Possible concerns about extrapolating from room-temperature, 1 krad/min gamma irradiation to the colder, mixed-hadron, lower-dose-rate HL-LHC environment are external-validity risks rather than circularity. Accordingly, the derivation chain is self-contained with respect to the measurements, and no circular step is present.

Axiom & Free-Parameter Ledger

0 free parameters · 4 axioms · 0 invented entities

No free parameters or invented entities; the paper is a direct experimental study. The main unproven inputs are the equivalence of gamma irradiation to the HL-LHC environment, the TID-to-days simulation, and the room-temperature-to-cold extrapolation.

axioms (4)

domain assumption 60Co gamma irradiation in a CPE box at 1 krad/min reproduces the ionization effects of the HL-LHC mixed radiation field relevant to surface/oxide trapped charge.
Load-bearing for extrapolating from the gamma test to the real experiment; stated in Section 2 but not validated.
domain assumption The ATLAS Radiation Simulation [5] conversion from TID to days of operation in the outer-barrel central region is accurate.
Converts measured krad values into 'one or two days' in the conclusions; the cited source is a public TWiki page, not independently auditable.
domain assumption Room-temperature IV and full-strip measurements after irradiation indicate performance at the -30C operating condition.
All measurements were at room temperature; no cold tests were performed.
domain assumption Reference samples transported and handled identically to irradiated samples isolate irradiation as the cause of recovery.
The control assumption is reasonable, but one reference (R1-W620-R) partially recovered before irradiation, weakening the control.

pith-pipeline@v1.3.0-alltime-deepseek · 4808 in / 8905 out tokens · 89951 ms · 2026-08-01T11:12:54.314025+00:00 · methodology

0 comments

read the original abstract

Construction of the new all-silicon ITk, developed by the ATLAS collaboration to be able to track charged particles produced at the HL- LHC, started in 2021 and is expected to continue until 2028. The ITk detector will include ~18,000 highly segmented and radiation hard n+-in-p silicon strip sensors, which are being manufactured by HPK. Upon their delivery, the ATLAS ITk strip sensor collaboration performs detailed measurements of sensors to monitor quality of all fabricated pieces. QC electrical tests include IV and CV tests, full strip tests, and a measurement of the long-term stability of the sensor leakage current. While most sensors demonstrate excellent performance during QC testing, we have nevertheless observed that a number of sensors from several production batches failed the electrical tests. Accumulated data indicates a strong correlation between observed electrical test failures and high electrostatic charge measured on the sensor surface during initial reception tests. This electrostatic charge enhances the risk of "Local trapped charge" events during manufacturing, shipping, and handling procedures, resulting in failed electrical QC tests. In the presented study, we have investigated whether the TID expected in the real experiment can effectively resolve early breakdown or low interstrip isolation caused by the electrostatic charge. Selected charge-affected sensors were irradiated with gamma rays from a 60Co source for a number of TID values. The results of this study indicate that the negative effects of the electrostatic charge on the critical sensors characteristics disappear after a very small amount of an accumulated TID, which actually corresponds to one or two days in the experiment. This finding gives us confidence in mitigating the issue of electrostatic charge during the operation of the ITk strip sensors in the real experiment.

Figures

Figures reproduced from arXiv: 2607.19936 by C. Fleta, C. Jessiman, C. Klein, E. Staats, J. Fernandez-Tejero, J. Keller, J.Kroll, J. Kvasnicka, M. Mikestikova, M. Ullan, P. Federicova, P. Gallus, P. Tuma, T. Koffas, V. Fadeyev, Y. Unno.

**Figure 1.** Figure 1: shows the full strip test results of bias resistance (RBIAS) of the R1-W635 and R1-W620-R sensors tested just before planned irradiation. The low interstrip isolation manifests as areas with low RBIAS values in the full strip test measuring setup. The required value of RBIAS according to the ATLAS specification is 1.5±0.5MΩ [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗

**Figure 2.** Figure 2: IV characteristics measured before and after gamma irradiation for R1-W617 (a) and for the reference sample R1-W650-R (b). As gamma irradiation resolved issues successfully during the initial planned step of irradiation with a TID of 11 krad (approximately one week in the real experiment), no further irradiations to higher TID values were needed. Instead, the reference samples were irradiated to an even lo… view at source ↗

**Figure 3.** Figure 3: Full strip tests of R1-W635 (left) and R3-W1014 (right) measured after gamma irradiation of 11 krad [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗

**Figure 4.** Figure 4: Full strip tests of the R1-W620-R sensor measured after a total delivered ionizing dose of 1.5 krad (left) and 3 krad (right). 4 Conclusions The results of this study indicate that the negative effects of the electrostatic charge accumulated on the sensor surface completely disappear after a very small amount of delivered total ionizing dose, which corresponds to one or two days of operation in the real ex… view at source ↗

discussion (0)

Reference graph

Works this paper leans on

8 extracted references

[1]

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