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REVIEW 3 major objections 7 minor 16 references

Radiation effects on surface and bulk properties of ATLAS18 silicon sensors under low- and high-dose gamma irradiation and annealing

T0 review · 3 major / 7 minor · reviewed 2026-08-02 · deepseek-v4-flash

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Pith's one-line read Low-dose gamma irradiation of ATLAS18 strip sensors changes the surface but leaves the silicon bulk undamaged, and high-temperature annealing restores leakage currents.

desk verdict Solid low-dose gamma damage study for ATLAS18 strip sensors, but the headline claim overreaches: V_FD unchanged is only shown up to 15 krad, not the full 0.5–100 krad range. read the letter →

arxiv 2607.13932 v1 pith:6XMM2N2I submitted 2026-07-15 physics.ins-det hep-ex

Marcela Mikestikova , Vitaliy Fadeyev , Pavla Federicova , Petr Gallus , Jana Kozakova , Jiri Kroll , Magdalena Kutova , Jiri Kvasnicka
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Igor Mandic Pavel Tuma Miguel Ullan Yoshinobu Unno Iveta Zatocilova
This is my paper · ORCID
classification physics.ins-dethep-ex
keywords siliconstripsensorsgammairradiationsurfacedamagebulkannealingATLASITkfulldepletionvoltageleakagecurrent
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

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

The reading

This paper works out what gamma radiation does to ATLAS18 silicon strip sensors in the dose range expected during the early years of the ATLAS ITk tracker, and whether the damage can be removed by annealing. Its central result is that for total ionizing doses up to about 100 krad, the rise in leakage current is mostly a surface effect: the full depletion voltage stays unchanged after irradiation and after annealing, which means the bulk silicon is not measurably damaged. Surface current shows signs of saturating around 2 Mrad, while bulk current stays nearly flat over the low-dose range. Annealing near 300 C brings leakage currents close to their pre-irradiation values. If this picture is right, early tracker operation should be stable, with gamma-induced degradation limited and reversible rather than accumulating in the sensor bulk.

What carries the argument

The load-bearing mechanism is the decomposition I_tot = I_bulk + I_surf, combined with the full depletion voltage extracted from capacitance-voltage curves. In unsegmented diodes, a contactable guard ring measures the two currents separately; in miniature strip sensors, the bulk current is inferred as 104 times the per-strip current obtained from the voltage drop across a bias resistor, with the surface current as the remainder. An Arrhenius-type annealing law (activation energy around 0.06-0.11 eV for the low-dose annealing process) is used to compare the thermal stability of surface and bulk components.

What would settle it

Measure the individual leakage current of every strip (or a large random sample) on a mini irradiated to 100 krad and compare the sum with 104 times a single strip's current; a ratio significantly different from 104 would break the bulk/surface split. Alternatively, if full depletion voltage shifts after low-dose irradiation in a more sensitive test, the 'no bulk damage' claim would be contradicted.

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Extended reading notes

Core claim

At low gamma doses relevant to the early ITk phase, the sensor's depletion voltage is unchanged by irradiation and annealing, and the observed leakage-current increase is dominated by the surface current component. The paper reaches this by splitting total current into bulk and surface parts — directly in diodes with a contactable guard ring, and in strip sensors by measuring one strip's current through the bias resistor and multiplying by the 104 strips. At ultra-high gamma doses (hundreds of Mrad), the same devices show genuine bulk damage: depletion voltage drops from about -275 V to about -20 V, and the electric field redistributes into a double-junction structure, consistent with trappe

Load-bearing premise

The mini-sensor bulk current is computed as 104 times the current in one strip, which holds only if all strips carry the same current and inter-strip coupling is negligible; the paper checks bias-resistance uniformity on just six random strips and three edge strips.

Editorial extensions

If this is right

  • During the early ITk period, gamma-induced leakage current in ATLAS18 sensors will come almost entirely from surface states, so it will saturate rather than keep growing with dose.
  • Full depletion voltage will remain stable at low gamma doses, so the sensors' bias settings and charge-collection behaviour should not need compensation for bulk damage.
  • High-temperature annealing above about 250-300 C can restore leakage currents to near pre-irradiation values after low doses, and largely restore the depletion voltage after ultra-high doses.
  • At hundreds of Mrad, pure gamma irradiation can produce bulk damage with space-charge redistribution; this damage is not permanent and responds to high-temperature annealing.
  • The temperature dependence of the leakage current follows the standard activation energy of about 1.21 eV, so existing scaling formulas can be used to extrapolate measurements to different operating temperatures.

Reading between the lines

Editorial extensions of the paper, not claims the author makes directly.

  • Inference: if the ~2 Mrad surface-current saturation holds on full-size sensors, then integrated ionizing dose in the early tracker could rise well above the qualification limit before leakage becomes a problem, since bulk damage from gamma alone is negligible.
  • Inference: the unchanged depletion voltage at low doses implies charge-collection efficiency and spatial resolution should stay unchanged; a direct CCE measurement on irradiated minis, not reported here, would test this cleanly.
  • Inference: the per-strip bias-resistor method could be extended to map strip-to-strip current uniformity on full-size sensors; if deviations appear, the 104x multiplication used to estimate bulk current would need revising.
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Editorial analysis

A structured set of objections, weighed in public.

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

Referee Report

3 major / 7 minor

Summary. The paper reports a combined gamma-irradiation study of ATLAS18 miniature strip sensors (minis) and MD8 diodes, covering low TIDs of 0.5–100 krad relevant to early ITk operation, together with supplementary measurements at higher doses (up to 3 Mrad for minis, 24 Mrad for diodes, and 630 Mrad in prior high-dose studies). The authors measure total, bulk, and surface leakage currents, extract full depletion voltage from CV measurements, perform isochronal and isothermal annealing, and use TCT on high-dose samples. The central claim, stated in Section 4, is that for low TID levels relevant to ITk operation the full depletion voltage V_FD remains unchanged after irradiation and annealing, indicating that the observed effects are predominantly surface-related and do not produce measurable bulk damage. The paper also reports surface-current saturation around 1–2 Mrad, activation energies for annealing and for leakage-current temperature dependence, and high-temperature annealing recovery for both low- and high-dose samples.

Significance. If the central claim holds, it is reassuring for early HL-LHC ITk strip operation: leakage increases would originate mostly from surface states and be recoverable by high-temperature annealing, while bulk damage would be negligible. The paper's strengths include the combination of segmented and unsegmented devices, a broad TID range from 0.5 krad to several hundred Mrad, and the TCT confirmation of bulk recovery after 300°C annealing of high-dose diodes. However, the load-bearing null result on V_FD is not established over the full claimed low-dose range, and the mini bulk-current decomposition is not fully validated. With additional measurements or a suitably restricted claim, the paper would be a valuable experimental contribution to the ITk radiation-damage literature.

major comments (3)
  1. [Section 4, Fig. 9] The central claim that V_FD remains unchanged for low TID levels relevant to ITk operation is only supported by data up to 15 krad. Section 2 defines the relevant low-dose range as 0.5–100 krad, and IV data are shown up to 100 krad in Fig. 5, but the CV measurements of minis in Fig. 9 cover only TIDs up to 15 krad. No V_FD measurements are presented at 25, 50, or 100 krad. The conclusion in Section 4 therefore extrapolates the null result beyond the measured range. This is load-bearing: the claim that early ITk operation is free of bulk effects depends on this null result. The authors should either extend the CV measurements to the full 0.5–100 krad range or explicitly restrict the claim to the measured dose range.
  2. [Section 3.2, Fig. 9] The null result for V_FD has no stated uncertainty or sensitivity. The paper does not report the number of samples per dose, the scatter of the CV curves, or the precision of the V_FD extraction. Without a quantitative bound on how much V_FD could change between 15 and 100 krad, the statement that irradiation 'does not induce measurable bulk damage' is not falsifiable. Please provide at least the sample count and an estimate of the V_FD measurement uncertainty, and ideally an upper limit on any possible V_FD change derived from the CV method's resolution.
  3. [Section 3.1, Fig. 5] The mini bulk-current extraction assumes I_bulk = 104 × I_strip, with I_strip inferred from the voltage drop across the bias resistor. The supporting evidence is the uniformity of the average bias resistance measured on six randomly selected strips and three edge strips (R_bias = (1.584 ± 0.002) MΩ). However, equal bias resistances do not rule out strip-to-strip variations in generation current or interstrip coupling, both of which would directly bias the extraction. Moreover, Fig. 5 shows that the extracted mini bulk current decreases at the upper end of the low-dose range, and the paper itself states that 'the origin of the bulk current decrease at the upper end of the investigated low TID range requires further study.' This unexplained behavior indicates that the bulk/surface decomposition is not yet robust. Please provide a sensitivity analysis of the extraction, or report interstrip
minor comments (7)
  1. [Throughout] The figures generally lack error bars and sample counts. This is particularly important for Fig. 4 and Fig. 5, where the claim that bulk current remains 'significantly smaller' is made without any measure of reproducibility. Please add error bars or state the number of samples and the run-to-run variability.
  2. [Section 3.2, Eq. (1)] The Arrhenius-type relation in Eq. (1) uses T_ref as a free parameter, but the fitting procedure and the uncertainties on E and T_ref are not reported. Please clarify how T_ref is determined and provide uncertainties for the values in Table 1.
  3. [Section 3.4, Table 2] The activation energies for total, bulk, and surface currents are reported to three significant digits, but no uncertainties are given. Given that the text compares these values to the Chilingarov result (1.209 ± 0.007) eV, please provide uncertainties from the fits.
  4. [Conclusion] There is a typo: 'behaior' should be 'behavior'.
  5. [References] Reference [12] is cited as 'This Issue' with no further information. Please provide a complete reference or a DOI.
  6. [Abstract / Section 2] The abstract says 'measurements extending up to a few Mrad,' while the paper reports minis up to 3 Mrad and diodes up to 24 Mrad. Please be consistent, e.g., 'a few Mrad for minis and up to 24 Mrad for diodes.'
  7. [Fig. 11 and Fig. 12] The TCT-derived V_FD is quoted as '-250 to -300 V' in the text and as '-260 V' from the CCE saturation, but no systematic uncertainty is given. Please state the estimated uncertainty from the pulse-shape and CCE methods.

Circularity Check

0 steps flagged · score 0.0 of 10

No circularity found: central claim rests on direct CV/V_FD and IV measurements, not on fitted inputs or self-citations.

full rationale

The paper is an experimental characterization, not a derivation. The headline conclusion that low TID leaves V_FD unchanged is a measurement result from Fig. 9, and the conclusion that effects are surface-related is supported by the measured dominance of the surface-current component and by the unchanged depletion voltage; neither is obtained by feeding a fitted parameter back into the claim. The Arrhenius forms in Eqs. (1)-(2) are descriptive fits used to report activation energies; the extracted E values are not then used to predict V_FD or the dose dependence, so no fitted input is called a prediction. The mini-sensor bulk current is estimated as I_bulk = 104 × I_strip using the measured bias-resistor voltage drop, which is an experimental procedure with stated uniformity checks; the paper itself notes that this 'bulk' current also contains inter-strip surface components, so the limitation is disclosed rather than hidden by construction. References [5]-[7] are comparisons to the authors' prior work and are not the load-bearing support for the new V_FD result, which is shown in this paper's own CV data. The gap between the 15 krad CV measurements and the 0.5-100 krad range stated as 'relevant' is an extrapolation/sensitivity concern, not a circularity. Therefore no circular step can be quoted, and the appropriate score is 0.

Assumptions & free parameters 4 free parameters · 5 assumptions · 0 invented entities

The central claim does not rest on fitted free parameters; the listed fit parameters are descriptive characterizations of annealing and temperature behavior. The load-bearing assumptions are geometric/electrical: guard-ring current separation and the uniform-strip-current model for minis. No new physical entities are introduced.

free parameters (4)
  • Activation energy E_A for leakage current temperature dependence = 1.23, 1.21, 1.20, 1.19, 1.18, 1.16 eV depending on bias (Table 2)
    Fit parameters in Eq. (2) for total, bulk, and surface currents; used for comparison with Chilingarov, not to construct the central claim.
  • Pre-exponential factor A in Eq. (2) = not reported
    Free fit parameter in temperature-dependence fits; descriptive only.
  • Annealing activation energy E in Eq. (1) = 0.064, 0.041, 0.065, 0.113 eV (Table 1)
    Fit parameters describing the annealing-rate temperature dependence; descriptive, not load-bearing.
  • Reference temperature T_ref in Eq. (1) = 94.56–104.58 °C
    Fitted reference temperature corresponding to maximum observed current; descriptive parameter.
assumptions (5)
  • domain assumption 60Co gamma irradiation produces displacement damage only through Compton electrons up to ~1 MeV, creating point defects rather than cluster defects.
    Used in the Introduction to frame bulk damage; standard radiation-physics assumption not verified in this paper.
  • domain assumption The current measured at the guard ring of MD8 diodes represents surface/edge current, while the diode pad current represents true bulk current.
    Section 3.1; relies on guard-ring geometry and TCAD edge-current modeling from Ref. [12].
  • domain assumption For mini sensors, all 104 strips have identical bias resistance and identical voltage drop, so I_bulk = n × I_strip.
    Section 3.1; verified only on six random strips plus three edge strips. This is the load-bearing measurement assumption.
  • domain assumption The standard Shockley–Read–Hall generation model and Chilingarov's temperature dependence I(T) = A T^2 exp(-E_A/2kT) describe the bulk and surface leakage currents.
    Section 3.4, Eq. (2); used to extract activation energies.
  • domain assumption The charge particle equilibrium box ensures uniform energy deposition, so dose values are accurate within the quoted 5% uncertainty.
    Section 2; assumes CPE conditions and dosimetry following ESA/SCC recommendations.

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Pith. "Pith review of Radiation effects on surface and bulk properties of ATLAS18 silicon sensors under low- and high-dose gamma irradiation and annealing." pith.science (2026). https://pith.science/paper/6XMM2N2I

@misc{pith2026260713932,
  author       = {Pith},
  title        = {Pith review of: Radiation effects on surface and bulk properties of ATLAS18 silicon sensors under low- and high-dose gamma irradiation and annealing},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/6XMM2N2I}},
  note         = {Machine review of arXiv:2607.13932}
}
abstract

Silicon strip detectors for the ATLAS Inner Tracker (ITk) at the HL-LHC must withstand harsh radiation conditions, including fluences of up to 1.6E15 1 MeV n$_{eq}$/cm$^{2}$ and total ionizing doses (TID) of up to 66 Mrad. These requirements are met using radiation-hard n+-in-p technology implemented in the ATLAS18 silicon strip sensors currently under production. This work presents a combined study of gamma-irradiation effects in ATLAS18 silicon sensors, including both segmented miniature strip sensors (minis) and unsegmented MD8 diodes fabricated on ATLAS18 production wafers. The samples were irradiated with a $^{60}$Co gamma source to multiple low TIDs between 0.5 and 100 krad, corresponding to the dose range relevant for the early operational phase of the ITk tracker. Additional measurements extending up to a few Mrad were performed to investigate the saturation of surface related damage effects. Post-irradiation characterization included measurements of total, bulk, and surface leakage currents, as well as capacitance-voltage measurements used to extract the full depletion voltage. The thermal stability of radiation-induced defects was studied using isochronal annealing between 80{\deg}C and 300{\deg}C and isothermal annealing at 60{\deg}C and 160{\deg}C. In addition, complementary studies of MD8 diodes irradiated to ultra-high doses of several hundred Mrad, well beyond the ATLAS ITk requirements, are included to investigate possible bulk-related effects induced by pure gamma irradiation and their annealing behavior. The combined analysis of low- and ultra-high-dose irradiation provides a comprehensive picture of surface- and bulk-related gamma-induced effects in ATLAS18 silicon sensors and their thermal evolution.

Figures

Figures reproduced from arXiv: 2607.13932 by the authors.

Figure 1
Figure 1. The miniature sensors in CPE box [PITH_FULL_IMAGE:figures/full_fig_p003_1.png] view at source ↗
Figure 2
Figure 2. Schematic of the test configuration of 𝐼tot and 𝐼bulk in diodes with a contactable guard ring. CV measurements, annealing studies, temperature-dependent measurements, and transient current technique (TCT) analyses. 3.1. IV measurements with separation of bulk and surface currents Special care was taken to determine individual leakage current components, namely the bulk current (𝐼bulk) flowing exclusively through the… view at source ↗
Figure 3
Figure 3. Schematic of the test configuration of 𝐼tot and 𝐼bulk in strip sensors with non-contactable guard ring. confirms that compensating interstrip current through the interstrip resistance is negligible and that the voltage drop can be assumed identical for all strips. Under these conditions, the bulk current was calculated as 𝐼bulk = 𝑛 × 𝐼strip, where 𝑛 = 104 is the number of strips in the mini sensor. The surface curre… view at source ↗
Figures from the paper (10 more)
Figure 4
Figure 4. Figure 4: Evolution of total, bulk and surface currents in MD8-p diodes as a function of TID, measured at 𝑉bias= -300 V [PITH_FULL_IMAGE:figures/full_fig_p005_4.png]
Figure 5
Figure 5. Figure 5: Evolution of total, bulk and surface currents in minis as a function of TID. 𝑉bias = -300 V. area inside the guard ring compared with mini sensors, the ratio of the bulk component to the surface component in the inside current remains constant, as does its evolution wi…
Figure 6
Figure 6. Figure 6: Total current versus annealing time for minis irradiated to 25 krad (60 ◦C) and 10 krad (160 ◦C); 𝑉bias = -500 V [PITH_FULL_IMAGE:figures/full_fig_p006_6.png]
Figure 7
Figure 7. Figure 7: Evolution of total current as a function of annealing temperature for mini irradiated to 15 krad. 𝑉bias = -300 V. Data were fitted by Eq. (1) defects at elevated temperature and is consistent with previous studies on MD8 diodes [7]. The CV characteristics remain unchan…
Figure 8
Figure 8. Figure 8: Total, surface, and bulk currents for MD8 diode irradiated to a high TID of 630 Mrad measured before and after annealing for 1 hour at 300 ◦C (“before annealing” denotes before the 300 °C high-temperature annealing; standard annealing at 60 ◦C for 80 minutes was applie…
Figure 9
Figure 9. Figure 9: CV characteristics of minis irradiated to low TIDs up to 15 krad, measured before annealing and after 1 hour of annealing at 300 ◦C. The measurements were made with the system produced by Particulars [16]. Top-TCT measurements were performed using a red laser with a wa…
Figure 10
Figure 10. Figure 10: CV characteristics of an MD8 diode irradiated to a high TID of 630 Mrad, measured before annealing and after 1 hour of annealing at 200 ◦C and 300 ◦C (“before annealing” denotes before the high-temperature annealing; standard annealing at 60 ◦C for 80 minutes was appl…
Figure 11
Figure 11. Figure 11: Top-TCT results for MD8 diode irradiated to 630 Mrad and subsequently annealed at 300 ◦C. Vbias [V] 0 100 200 300 400 500 Charge[arb.] 0 500 1000 1500 2000 2500 3000 3500 4000 [PITH_FULL_IMAGE:figures/full_fig_p010_11.png]
Figure 12
Figure 12. Figure 12: CCE obtained by integrating the transient current over the first 25 ns. 𝑉FD = 260 V. the depleted volume, while the surface current flows in the depleted inversion layer at the Si-SiO2 interface. In both cases, electrons are the dominant carriers, leading to a similar…
Figure 13
Figure 13. Figure 13: Temperature dependence of total, bulk, and surface currents of mini irradiated to 100 krad and biased to -50 V, -160 V, and -500 V. Measured data are fitted with formula in Eq. (2) [PITH_FULL_IMAGE:figures/full_fig_p011_13.png]

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