Pith. sign in

REVIEW 2 major objections 4 minor 22 references

Dark Matter Detector Radioimpurities $^{129}$I and $^{210}$Pb Measured with Accelerator Mass Spectrometry

T0 review · 2 major / 4 minor · reviewed 2026-08-07 · deepseek-v4-flash

Challenge this review Re-run · record.json Download PDF Read on arXiv ↗ Review JSON

Pith's one-line read Commercial NaI powders show a consistent $^{129}$I content, and a Pb$_3$O$_4$ sample meets the $^{210}$Pb limit for use as a lead carrier in dark matter detectors.

desk verdict Sound AMS measurements; the 129I data are solid, the 210Pb carrier ratio is a useful upper limit pending a blank, and the paper deserves review with minor revisions. read the letter →

arxiv 2506.11689 v1 pith:4A2QVKXQ submitted 2025-06-13 physics.ins-det physics.acc-ph

Zuzana Slavkovská , Ferdos Dastgiri , L.Keith Fifield , Michaela B.Froehlich , Michael A.C.Hotchkis , Dominik Koll , Silke Merchel , Stefan Pavetich
show 2 more authors
Stephen G.Tims Anton Wallner
This is my paper · ORCID
classification physics.ins-detphysics.acc-ph
keywords acceleratormassspectrometryNaI(Tl)detectorsdarkmatteriodine-129lead-210radioimpurityleadcarrierradiopurity
topics Dark Matter
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

The paper aims to establish that two radioimpurities that can mimic dark matter signals in sodium-iodide scintillation detectors can be measured reliably by accelerator mass spectrometry: iodine-129, which is intrinsic to the sodium iodide, and lead-210, which must be tracked in any stable lead carrier used to process detector material. The authors measured a consistent $^{129}$I/$^{127}$I ratio of $(2.0 \pm 0.3) \times 10^{-13}$ in three commercially available NaI powders, including unprocessed powder as an AMS target for the first time, and concluded that powder choice has negligible influence on the $^{129}$I background. For $^{210}$Pb, they identified PbO$_2^-$ as the preferred molecular ion and measured a $^{210}$Pb/Pb ratio of $(3.6^{+1.7}_{-1.4}) \times 10^{-15}$ in one Pb$_3$O$_4$ powder, which they conclude meets the limit required when adding 1 mg of stable lead to 1 kg of NaI(Tl) powder. If correct, a suitable lead carrier is available for low-background NaI(Tl) experiments, and $^{129}$I uniformity simplifies powder selection.

What carries the argument

The central mechanism is accelerator mass spectrometry (AMS), direct atom counting of $^{129}$I/$^{127}$I and $^{210}$Pb/Pb rather than decay counting. For $^{129}$I, the load-bearing target choice is unprocessed NaI powder mixed with silver powder, which delivers stable 5 microampere beams and yields ratios identical to chemically converted AgI; for $^{210}$Pb, the load-bearing choices are the PbO$_2^-$ molecular ion, a terminal voltage of 0.9 MV, and charge state 3+ (29.2% yield), which together give enough efficiency to measure ratios near $10^{-15}$. Reference materials with known ratios normalise the measurements, and the argument rests on the agreement between samples and references at these very low isotopic ratios.

What would settle it

Take additional Pb$_3$O$_4$ from the same and different suppliers and measure $^{210}$Pb/Pb with the same protocol, both as powder and after the carrier-preparation chemistry; if any batch exceeds $1 \times 10^{-14}$, the carrier claim fails. Likewise, measure $^{129}$I/$^{127}$I in a commercial NaI powder from a supplier not tested here; a ratio outside $(2.0 \pm 0.3) \times 10^{-13}$ would overturn the uniformity claim.

Watch

Extended reading notes

Core claim

On the paper's own terms, the central discovery is that the $^{129}$I/$^{127}$I ratio is the same, within uncertainty, across three commercial NaI powders and consistent with the value reported for the crystals used by a long-running dark matter experiment, so no tested powder grade is better than another with respect to $^{129}$I background. It further reports that one commercial Pb$_3$O$_4$ powder carries $^{210}$Pb at a ratio of $(3.6^{+1.7}_{-1.4}) \times 10^{-15}$, below the $1 \times 10^{-14}$ target for a 1 mg-per-kg lead addition, while the other two lead oxides tested do not meet that target within uncertainties. The paper also demonstrates that unprocessed NaI works as an AMS target and that PbO$_2^-$ extraction with charge state 3+ yields efficient enough measurement of $^{210}$Pb to characterise candidate carriers, with a machine background below $10^{-14}$.

Load-bearing premise

The conclusion that Pb$_3$O$_4$ works as a lead carrier assumes that the one tested bottle represents all Pb$_3$O$_4$ and that its radioactive-lead content stays low after chemical processing; the authors note that an ideal carrier would be ten times purer.

Editorial extensions

If this is right

  • For the three powders tested, the $^{129}$I contribution to detector background is effectively fixed near 1 mBq per kg of NaI, so switching commercial powder grade will not reduce this particular background.
  • The tested Pb$_3$O$_4$ batch can be used as a $^{210}$Pb-clean lead carrier at the 1 mg-per-kg addition level, provided the measured ratio holds after chemical processing.
  • Unprocessed NaI powder works as an AMS target, removing a chemical-conversion step and making routine radiopurity screening of iodide materials faster.
  • The $^{210}$Pb AMS protocol reaches backgrounds below $10^{-14}$, so other candidate carrier materials can now be screened quantitatively at the required sensitivity.
  • Because the measured Pb$_3$O$_4$ sits near, not far below, the $1 \times 10^{-14}$ limit, an ideal carrier would be at least ten times purer; the paper reports that aged lead materials are being tested for this purpose.

Reading between the lines

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

  • If the $^{129}$I uniformity extends to other commercial suppliers, then the only way to reduce the intrinsic $^{129}$I background further would be to use synthetic or otherwise $^{129}$I-free iodine, not to buy a purer powder grade.
  • The same PbO$_2^-$ AMS protocol could be applied to screen small environmental or archaeological lead samples for $^{210}$Pb, making the method useful beyond dark matter instrumentation.
  • An implicit test of the carrier claim is the chemical yield and contamination during the actual NaI processing; a batch with a low starting $^{210}$Pb/Pb ratio could still introduce $^{210}$Pb if the carrier preparation itself is not clean.
  • If multiple Pb$_3$O$_4$ batches are characterised, a simple procurement rule — accept only ratios below $1 \times 10^{-14}$ — could be applied, which the paper does not explicitly state.
Share X Bluesky LinkedIn Reddit HN

Editorial analysis

A structured set of objections, weighed in public.

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

Referee Report

2 major / 4 minor

Summary. The paper reports accelerator mass spectrometry (AMS) measurements of two radioimpurities relevant to NaI(Tl) dark matter detectors: 129I/127I in three commercial NaI powders and 210Pb/Pb in three lead compounds (Pb3O4, PbO2, and PbO). For 129I, the authors demonstrate that both unprocessed NaI and chemically converted AgI are viable AMS targets, and they measure a consistent ratio of (2.0 ± 0.3) × 10^-13 across the three powders, in agreement with the DAMA/LIBRA value. For 210Pb, they characterize negative molecular ions PbO^- and PbO2^-, select PbO2^- with charge state 3+ at 0.9 MV, and report a 210Pb/Pb ratio for Pb3O4 of (3.6 +1.7/-1.4) × 10^-15, which meets the stated SABRE criterion of 1 × 10^-14 when 1 mg of lead carrier is added to 1 kg of NaI(Tl). The paper concludes that Pb3O4 is a potential lead carrier for SABRE and that commercial NaI powders are indistinguishable from an 129I perspective.

Significance. If the results hold, the paper provides a useful technical capability: direct atom counting of 210Pb at the low ratios required for dark matter detector radiopurity, and a convenient AMS method for 129I using unprocessed NaI powder. The study is also of practical value to the SABRE collaboration by identifying a candidate 210Pb-clean lead carrier. Strengths include the use of external calibration standards with fixed nominal ratios (Woodward iodine, Kuni standard, ERISS-derived lead references), a systematic characterization of charge-state yields, and the use of Feldman-Cousins confidence intervals for low-count data. The main weakness is the absence of a reported procedural blank or machine-background measurement for 210Pb, which undermines the specific quoted ratio as a material property, although the conservative character of the suitability conclusion mitigates the impact.

major comments (2)
  1. [Section 3.2 and Section 4] The 210Pb/Pb ratio of (3.6 +1.7/-1.4) × 10^-15 for Pb3O4 is derived from very low counting statistics (Feldman-Cousins applied for <20 counts), yet no procedural blank or dedicated machine-background measurement is reported. The statement in Section 4 that 'a machine background at VEGA ... significantly lower than 10^-14' is unsupported by explicit data, so the measured counts cannot be unambiguously assigned to 210Pb in the sample; contributions from ion-source memory, scattering, or chemical contamination are not quantified. The authors should either provide a measured blank/background sample (e.g., a lead-free matrix or a known much-lower-ratio material) and subtract it, or explicitly present the Pb3O4 result as an upper limit. The suitability conclusion is conservative—if background contamination were present, the true Pb3O4 ratio would be even lower—but the specific central value and its uncertainty are not established as a material property and should not be used for quantitative comparisons or activity-budget calculations.
  2. [Abstract and Section 2.2] The abstract claims that 'the choice of NaI powder has a negligible influence on the 129I contribution to low-background dark matter experiments,' and Section 2.2 states that 'there is no distinction in quality between different NaI powders.' This overgeneralizes the evidence, which consists of only three specific powders: astro-grade and growth-grade from Sigma-Aldrich and one ANU in-house powder from May and Baker. The conclusion in Section 4 is properly qualified with 'assessed here,' but the abstract and Table 1 caption are not. Please restrict the conclusion to the investigated powders or add a clear scope limitation in the abstract, since manufacturers and production batches may differ.
minor comments (4)
  1. [Abstract] The phrase 'the required lower activity limit' is misleading; the condition is an upper bound on the 210Pb/Pb ratio (1 × 10^-14). It should read 'the required maximum (upper) activity limit.'
  2. [Table 4] For the PbO2 row, the quoted 210Pb atoms per g Pb, (2.3 ± 6.5) × 10^7 at/g, is inconsistent with the isotopic ratio (7.9 ± 2.2) × 10^-15; error propagation gives (2.3 ± 0.6) × 10^7 at/g. This appears to be a misplaced decimal. Please correct and recheck all derived quantities in the table.
  3. [Section 3.2] There are two typos: 'seculiar equilibrium' should be 'secular equilibrium', and 'In oder' should be 'In order'.
  4. [Section 3.1 and Table 2] The paper states that the measured 210Pb/208Pb ratios of the reference samples were used to normalize the results, but it does not report the measured values or their agreement with the nominal ratios. One or two sentences with the measured reference ratios would strengthen confidence in the normalization.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: all calibrations use external fixed standards and the acceptance threshold is an external design value.

full rationale

No circular reasoning is present in this paper's derivation chain. The 129I/127I measurements are normalized to the Woodward iodine standard (1.3 x 10^-14) and the Kuni standard (2.68 x 10^-12), both of which have nominal ratios fixed before the target measurements were made, and the resulting values agree with the external DAMA/LIBRA ratio of (1.7 ± 0.1) x 10^-13 [19]. The central 129I claim, that three commercial NaI powders give a consistent ratio of (2.0 ± 0.3) x 10^-13, rests on internal consistency across three independently prepared samples and on regression to external standards, not on any fitted parameter being renamed as a prediction. The 210Pb measurements are normalized to reference samples A1-A3 and B, produced from an ERISS 210Pb solution with a known activity of 142.8 ± 3.2 Bq/g (Section 3.1), so the reference ratios of 1 x 10^-10 and 1 x 10^-11 are defined before the target Pb3O4 measurement, not inferred from it. The acceptance criterion of 1 x 10^-14 for the 210Pb/Pb ratio of a lead carrier is taken from the SABRE design value in reference [20], an external source, and is not derived from the measured data. Although the paper cites earlier work by overlapping authors for the AMS methods (e.g., references [9], [13], [15], including the Woodward iodine ratio), these citations are not load-bearing in a circular sense: the present paper remeasures charge-state yields itself (Table 3), and the conclusions agree with externally anchored values, so the self-citations do not force the outcomes. The acknowledged limitation that the Pb3O4 result is based on low count statistics and that no explicit procedural blank is reported is a measurement-validity concern about separating signal from machine background, not a circularity; the paper itself states in Section 4 that the machine background at VEGA is significantly lower than 10^-14, and even under the skeptical reading the carrier-suitability conclusion would be conservative. The scope limitation, that only one batch of Pb3O4 was tested and that an ideal carrier should be at least ten times purer, is explicitly acknowledged in Section 4 and does not involve circular reasoning. The derivation chain is self-contained and externally calibrated, so no circular step can be exhibited.

Assumptions & free parameters 0 free parameters · 3 assumptions · 0 invented entities

The paper introduces no fitted parameters or new entities. It relies on standard AMS background assumptions and on external calibration standards with nominal ratios; no circular fitting to the target materials.

assumptions (3)
  • domain assumption Xenon does not form negative ions, so 129I AMS measurements are essentially free of isobaric interference from 129Xe.
    Stated in Section 2.2: 'Isobaric interference is negligible as Xe does not form negative ions.' This underpins the 129I/127I measurements.
  • domain assumption The unstable isobars 210Bi and 210Po have significantly lower concentrations in secular equilibrium than 210Pb, so they do not affect 210Pb counting.
    Stated in Section 3.2: 'both are expected to have significantly lower concentrations in seculiar equilibrium than 210Pb.' This underpins the 210Pb AMS measurements.
  • domain assumption The calibration standards (Woodward iodine with 129I/127I = 1.3e-14, Kuni standard with 2.68e-12, and ERISS-derived reference PbO samples with 210Pb/Pb = 1e-10 and 1e-11) have accurate known ratios.
    Used for normalization in Sections 2.1 and 3.1; if the standards are wrong, all reported ratios scale accordingly.

how reviews work

0 comments
Cite this review

Pith. "Pith review of Dark Matter Detector Radioimpurities $^{129}$I and $^{210}$Pb Measured with Accelerator Mass Spectrometry." pith.science (2026). https://pith.science/paper/4A2QVKXQ

@misc{pith2026250611689,
  author       = {Pith},
  title        = {Pith review of: Dark Matter Detector Radioimpurities $^129$I and $^210$Pb Measured with Accelerator Mass Spectrometry},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/4A2QVKXQ}},
  note         = {Machine review of arXiv:2506.11689}
}
abstract

Sodium iodide crystals doped with thallium NaI(Tl) can be used as detector material for direct dark matter detection by taking advantage of their particle detection properties of scintillation. In order to achieve this, it is crucial that these crystals are of ultra-high purity. Radioimpurities within the crystals may potentially mimic dark matter signals and thus must be quantified, minimised where possible and distinguished from real events. Abundances of radionuclides $^{129}$I and $^{210}$Pb, which are dominant sources of radioimpurities in NaI(Tl) crystals, were measured using accelerator mass spectrometry at the Australian National University (ANU) and the Australian Nuclear Science and Technology Organisation (ANSTO). NaI powder chemically processed to AgI, and, for the first time, unprocessed NaI powder, were shown to be suitable as AMS targets. A consistent $^{129}$I/$^{127}$I ratio of (2.0 $\pm$ 0.3) x 10$^{-13}$ was measured in three different commercially available NaI powders. Therefore, it was concluded that the choice of NaI powder has a negligible influence on the $^{129}$I contribution to low-background dark matter experiments. For $^{210}$Pb, different Pb molecular ion species were assessed with PbO$_2^-$ being the preferred species and applied to investigate different lead oxide compounds for their suitability as Pb carriers. A $^{210}$Pb/Pb isotopic ratio of (3.6 $\pm$ $^{1.7}_{1.4}$) x 10$^{-15}$ was measured in Pb$_3$O$_4$ powder. This met the required lower activity limit of $^{210}$Pb when adding 1 mg of stable lead into 1 kg of NaI(Tl) powder with a desired maximum $^{210}$Pb/Pb isotopic ratio of 1 x 10$^{-14}$. These results indicate the suitability of the investigated Pb$_3$O$_4$ as a potential carrier for incorporation with Pb extracted from NaI used for dark matter experiments.

Figures

Figures reproduced from arXiv: 2506.11689 by the authors.

Figure 1
Figure 1. Comparison of charge state yields at terminal voltages between 0.5 MV and 0.9 MV for charge [PITH_FULL_IMAGE:figures/full_fig_p007_1.png] view at source ↗
Figure 2
Figure 2. shows the course of the stable 208Pb current during the measurements for Pb3O4, PbO2 and PbO powders. The PbO2 samples mixed with Ag powder delivered the highest 208PbO2 beam current of about 470 nA at the beginning of the measurement, followed by Pb3O4 with about 440 nA and pure PbO2 with 370 nA. The current of PbO mixed with Ag was one order of magnitude lower, starting at 20 nA. Pb3O4 was sputtered relatively qui… view at source ↗

Discussion (0). Continue with ORCID to comment.

Reference graph

Works this paper leans on

22 extracted references · 22 canonical work pages

  1. [1]

    Bertone, D

    G. Bertone, D. Hooper, Rev. Mod. Phys. 90 (2018) 045002

    work page 2018

  2. [2]

    Froborg, A.R

    F. Froborg, A.R. Duffy, J. Phys. G: Nucl. Part. Phys. 47 (2020) 094002

    work page 2020

  3. [3]

    Steigman, M.S

    G. Steigman, M.S. Turner, Nucl. Phys. B 253 (1985) 375

    work page 1985

  4. [4]

    Bernabei, P

    R. Bernabei, P. Belli, F. Cappella, et al., Journal of Physics: Conference Series 2586 (2023) 012096

    work page 2023

  5. [5]

    Adhikari, G

    P. Adhikari, G. Adhikari, E.B. de Souza, et al., Eur. Phys. J. C 78 (2018) 490

    work page 2018

  6. [6]

    Amar´ e, S

    J. Amar´ e, S. Cebri´ an, I. Coarasa, et al., Phys. Rev. Lett. 123 (2019) 031301

    work page 2019

  7. [7]

    Barberio, T

    E. Barberio, T. Baroncelli, L.J. Bignell, et al., Eur. Phys. J. C 83 (2023) 878

    work page 2023

  8. [8]

    Garc ´ ıa-Tora˜ no, T

    E. Garc ´ ıa-Tora˜ no, T. Altzitzoglou, P. Auerbach, et al. Applied Radiation and Isotopes 140 (2018) 157-162

    work page 2018

Show all 22 references
  1. [9]

    Froehlich, Z

    M. Froehlich, Z. Slavkovsk´ a, D. Koll, et al., Nucl. Instrum. Methods Phys. Res. B 529 (2022) 18-23

    2022

  2. [10]

    Vivo-Vilches, B

    C. Vivo-Vilches, B. Weiser, X. Zhao, et al., Nucl. Instrum. Methods Phys. Res. B 511 (2022) 51-56. 10

    2022

  3. [11]

    J. T. Kristensen, X. Hou. J. Environ. Radioactiv. 287 (2025) 107705

    2025

  4. [12]

    Wallner, L.K

    A. Wallner, L.K. Fifield, M.B. Froehlich, et al., Nucl. Instrum. Methods Phys. Res. B 534 (2023) 48-53

    2023

  5. [13]

    Hotchkis, D

    M.A.C. Hotchkis, D. P. Child, M.B. Froehlich, et al., Nucl. Instrum. Methods Phys. Res. B 438 (2019) 70-76

    2019

  6. [14]

    Roberts, M.W

    M.L. Roberts, M.W. Caffee, I.D. Proctor, Nucl. Instrum. Methods Phys. Res. B 123 (1997) 367–370

    1997

  7. [15]

    Fifield, M

    L.K. Fifield, M. B. Froehlich, D. Koll, et al., Nucl. Instrum. Methods Phys. Res. B 530 (2022) 8-12

    2022

  8. [16]

    Dracoulis, Nuclear Physics News 9 (1999) 9-19

    G. Dracoulis, Nuclear Physics News 9 (1999) 9-19

    1999

  9. [17]

    Fifield, S.G Tims, J.O

    L.K. Fifield, S.G Tims, J.O. Stone, et al., Nucl. Instrum. Methods Phys. Res. B 294 (2013) 126-131

    2013

  10. [18]

    Wilcken, L.K Fifield, T.T Barrows, et al., Nucl

    K.M. Wilcken, L.K Fifield, T.T Barrows, et al., Nucl. Instrum. Methods Phys. Res. B 266 (2008) 3614-3624

    2008

  11. [19]

    Bernabei, P

    R. Bernabei, P. Belli, A Bussolotti, et al., Nucl. Instrum. Methods Phys. Res. A 592 (2008) 297-315

    2008

  12. [20]

    Antonello, E

    M. Antonello, E. Barberio, T. Baroncelli, et al., Eur. Phys. J. C 79 (2019) 363

    2019

  13. [21]

    Sookdeo, R.J

    A. Sookdeo, R.J. Cornett, X.L Zhao, et al., Rapid Commun. Mass Sp. 30 (2016) 867-872

    2016

  14. [22]

    G.J Feldman, R. Cousins. Phys. Rev. D 57 (1998): 3873. 11

    1998

Pith tools

Reviewed August 7, 2026 · model on record in the stance chip above.