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REVIEW 3 major objections 5 minor 69 references

Convergence on the Proton Drip-Line in Thulium

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Pith's one-line read Precision mass measurements establish 149Tm as the first proton-unbound thulium isotope, placing the Z=69 proton drip-line at N=80.

desk verdict Solid new mass measurements with a load-bearing caveat: the drip-line location hangs on an unexplained 149Er mass shift. read the letter →

arxiv 2412.10259 v5 pith:2IIABACE submitted 2024-12-13 nucl-ex

B. Kootte , M.P. Reiter , C. Andreoiu , S. Beck , J. Bergmann , T. Brunner , T. Dickel , K.A. Dietrich
show 31 more authors
J. Dilling E. Dunling J. Flowerdew L. Graham G. Gwinner Z. Hockenbery C. Izzo A. Jacobs A. Javaji R. Klawitter Y. Lan E. Leistenschneider E.M. Lykiardopoulou I. Miskun I. Mukul T. Murböck S.F. Paul W.R. Pla{ss} J. Ringuette C. Scheidenberger R. Silwal R. Simpson A. Teigelhöfer R.I. Thompson J.L. Tracy Jr. M. Vansteenkiste R. Weil M.E. Wieser C. Will A.A. Kwiatkowski
This is my paper · ORCID
classification nucl-ex PACS 21.10.Dr27.60.+j
keywords protondrip-linethuliumisotopesMR-TOF-MSmassmeasurementone-protonseparationenergy149TmN=82shellclosure149Er
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 settle where the proton drip-line lies in the thulium chain ($Z=69$): the neutron count at which thulium nuclei stop being able to hold a proton and become proton-unbound. Because very small proton-emission energies imply extremely long tunneling lifetimes, the boundary often cannot be observed directly as a decay; it has to be inferred by weighing the parent and daughter nuclei. The authors use a multiple-reflection time-of-flight mass spectrometer to weigh $^{149}$Tm, $^{150}$Tm, and two states of $^{149}$Er, and they find that $^{149}$Tm is proton-unbound while $^{150}$Tm is proton-bound, putting the drip-line at $N=80$. If this is right, it gives nuclear mass models a firm experimental anchor in a region where their predictions disagree, and it also shows that the $N=82$ neutron shell closure persists at this proton-rich edge.

What carries the argument

The carrying instrument is a multiple-reflection time-of-flight mass spectrometer (MR-TOF-MS), which sends bunched ions bouncing between electrostatic mirrors and turns their flight times into mass ratios; mass-selective retrapping suppresses contaminants and lets the weak $^{149,150}$Tm peaks be fit. The physical identity doing the work is the one-proton separation energy, $$S_p = M(Z,N) - M(Z-1,N) - M({}^1H),$$ with $Q_p = -S_p$; a nucleus is proton-unbound when $S_p \le 0$. In the crowded $A=149$ spectrum, a hyper-exponentially modified Gaussian line shape calibrated on a strong $^{149}$Dy peak separates the low-lying ground state of $^{149}$Er from its dominant 741.7 keV isomer, so the daughter mass entering $S_p$ can be extracted directly from a tail.

What would settle it

A third, technique-independent measurement of the $^{149g}$Er and $^{149m}$Er masses would settle it: if the new value agrees with the 2020 Atomic Mass Evaluation rather than with this paper's heavier value, then $^{150}$Tm's separation energy falls back into the $1\sigma$-of-zero band and the claimed boundary at $N=80$ loses its experimental support, while $^{149}$Tm would still be unbound if its own mass and the $^{148}$Er mass hold.

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Extended reading notes

Core claim

The paper's central claim is that $^{149}$Tm is the first proton-unbound isotope in the thulium chain, with a proton-emission $Q_p = +378 \pm 52$ keV, and that $^{150}$Tm is the last proton-bound isotope. The argument rests on measured masses for both thulium isotopes together with revised masses for $^{149g}$Er and $^{149m}$Er, which the authors find to be about 150 keV heavier than the 2020 Atomic Mass Evaluation. That revision is what makes $^{150}$Tm look bound: using the old $^{149}$Er mass instead, the $^{150}$Tm separation energy would be within $1\sigma$ of zero. The new masses also give a $^{150}$Tm proton-emission $Q$-value of $-202 \pm 99$ keV and fix the empirical $N=82$ shell gap at $Z=69$, supporting the continued existence of that shell closure toward the drip-line.

Load-bearing premise

The load-bearing premise is the newly measured $^{149}$Er mass being about 150 keV heavier than the 2020 Atomic Mass Evaluation, because that shift is what makes $^{150}$Tm look proton-bound; the authors themselves cannot explain the discrepancy and call for a third measurement.

Editorial extensions

If this is right

  • The proton drip-line for $Z=69$ is now fixed at $N=80$ by experiment, rather than left to model interpolation.
  • The $Q_p = +378 \pm 52$ keV value for $^{149}$Tm can be used to compute its ground-state proton-emission partial half-life for the expected $11/2^-$ state, since only the $0^+$ ground state of $^{148}$Er is energetically open.
  • The $\approx 150$ keV shift in the $^{149}$Er masses would, if confirmed, propagate into one-proton separation energies and alpha-decay chains that use those masses as anchors.
  • The $N=82$ shell gap measured at $Z=69$ remains sizeable, consistent with the neighbouring $Z=70$ result and with the persistence of magicity at the proton drip-line.

Reading between the lines

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

  • A third, independent mass measurement of $^{149g}$Er would settle the one unexplained 150 keV discrepancy; if it returned the 2020 evaluation value, $^{150}$Tm would no longer be clearly proton-bound, although $^{149}$Tm would remain unbound on the present data.
  • The alternating bound and unbound pattern seen between $^{149}$Tm and $^{150}$Tm suggests the one-proton separation energy can oscillate with neutron number; a similarly precise remeasurement of $^{147,148}$Tm could reveal whether that pattern continues below $N=80$.
  • If the revised $^{149}$Er mass survives scrutiny, the mass surface in this region may need systematic re-evaluation, because a single 150 keV shift at $N=81$ can change the proton-bound character of a neighbouring odd-$Z$ isotope.
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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 / 5 minor

Summary. The paper reports precision mass measurements of neutron-deficient thulium and erbium isotopes performed with the TITAN MR-TOF-MS at TRIUMF, including first measurements of 149Tm and 150Tm. Combined with new measurements of 149gEr and 149mEr that deviate from the AME2020 values by about 150 keV, the authors derive a positive proton-emission Q-value of +378±52 keV for 149Tm and a negative Q-value (i.e., positive S_p) of -202±99 keV for 150Tm. They conclude that 149Tm is proton-unbound and 150Tm is proton-bound, placing the thulium proton drip-line at N=80, and they further use the new masses to evaluate the N=82 neutron shell gap at Z=69.

Significance. If the main claim is correct, the paper resolves a long-standing experimental gap: the location of the proton drip-line in the thulium chain is pinned down by direct mass measurements, and the extracted Q-value for 149Tm provides an input for proton-emission half-life calculations. The 149Tm unboundness leg is robust: the Q-value of +378±52 keV is more than 7σ away from zero and relies only on the well-established 148Er mass and the new 149Tm mass. The paper also provides useful data on neighbouring Er and Tm isotopes, including transparent uncorrected values in Table II that allow future reanalysis. However, the companion claim that 150Tm is proton-bound is only a 2σ result and is conditional on an unexplained ~150 keV discrepancy in the 149gEr mass relative to AME2020, which the authors themselves flag as needing a third measurement.

major comments (3)
  1. [Section III, 'Last Proton-Bound Isotope Determined from 149m,149gEr and 150gTm'] The conclusion that 150Tm is proton-bound (S_p = 202±99 keV) is load-bearing for the paper's central claim, and it is only a 2σ effect that depends entirely on the TITAN value of the 149gEr mass, ME = -53584(47) keV, rather than the AME2020 value of -53742(28) keV. Using the AME2020 149gEr mass, the same 150Tm mass yields S_p = 44±99 keV, which is within 1σ of zero and does not exclude 150Tm being proton-unbound. The authors state that the cause of this single-mass discrepancy is unknown. I request a quantitative sensitivity analysis of the drip-line conclusion to the 149Er mass, or an independent mass measurement, and a clearly caveated statement that the 'first proton-unbound' conclusion is provisional pending confirmation of the 149gEr mass.
  2. [Abstract and Section III, first paragraph] The abstract states that the measurements 'provide the first experimental confirmation that 149Tm is the first proton-unbound nuclide in the Tm chain,' and Section III repeats that 149Tm is 'the first proton-unbound isotope in the thulium chain.' This is inconsistent with the introduction, which notes that 147Tm is a known ground-state proton emitter. The wording must be clarified, for example by specifying 'first proton-unbound isotope beyond the N=82 shell closure' or 'the drip-line transition between 149Tm and 150Tm.' As written, the headline claim is internally inconsistent with the known status of 147Tm.
  3. [Section III, '149Tm: The First p-Unbound Isotope' and Introduction] The statement 'the present measurements demonstrate that 149Tm is the first proton-unbound nucleus, rather than 148Tm or 150Tm' and the conclusion that '150gTm is the last proton-bound Tm isotope' need to be reconciled with the prior constraint that the drip-line lies between A=148 and A=151. If 148Tm is proton-bound (as implied by that constraint and by 147Tm being unbound), then 148Tm would be more neutron-deficient than 150Tm, so 150Tm cannot be the last proton-bound isotope in an absolute sense. The authors should state explicitly which isotopes around the drip-line are known to be bound or unbound and should avoid absolute 'first' and 'last' language unless it is precisely defined.
minor comments (5)
  1. [Section IV, Conclusion] Typo: 'Additonally' should be 'Additionally.'
  2. [Section II.B] The text lists the excitation energy of 152Tm as -100±250 keV; a negative excitation energy is unphysical and is likely a typo for 100±250 keV.
  3. [Section II.B] The paper says 149Tm and 150Tm represent 'first-time measurements,' but 150Tm was already measured in experiment 1 (Ref. [15]) and is included in Table I (Expt. 1). Please rephrase to reflect that these are first high-precision measurements or first-time measurements in this analysis.
  4. [Figure 3 caption] The caption refers to 'purple ×' and 'blue triangle' markers, but the figure is likely to be rendered in grayscale in print; consider also using different marker shapes or adding text labels to ensure the distinction is clear.
  5. [Table I] For 149gTm and 150gTm, the AME2020 mass excesses are extrapolated values (marked with #), but the TITAN-AME difference column is left blank; adding these differences (or a note explaining the blank) would improve readability.

Circularity Check

0 steps flagged · score 0.0 of 10

No significant circularity: the drip-line location is direct arithmetic from independently calibrated mass measurements, and the flagged 149Er mass discrepancy is an admitted robustness limitation rather than a circular derivation.

full rationale

The central claim that 149Tm is the first proton-unbound Tm isotope is obtained by direct arithmetic on measured masses: Q = M(Z,N) - M(Z-1,N) - M(1H). The 149Tm mass is a first-time MR-TOF-MS measurement calibrated to the AME2020 mass of 149Dy, with 148Er and 1H taken from AME2020, so no fitted parameter is renamed as a prediction. The companion claim that 150Tm is proton-bound uses the newly measured 150Tm mass together with the newly measured 149Er ground-state mass. That 149Er value is obtained two ways: a direct hyperEMG fit of the ground-state peak (-53584±47 keV) and an indirect route through the isomer (-53599±25 keV); both agree with each other, and the direct fit is independent of the paper's own prior Yb work. The 2.9-sigma deviation from the AME2020 149Er mass is explicitly acknowledged by the authors ('The cause of our discrepancy in our measurement of this single mass is unknown, but may warrant a third measurement'), which is a genuine robustness concern for the 150Tm-bound leg, but it is not a circular step. The only self-citation, to the authors' Yb mass measurement [15], is used to bolster confidence in the known 741.69 keV isomer excitation energy, but the ground-state mass used in the main table does not depend on that argument because it comes from the direct fit. External benchmarks (AME2020 masses, NUBASE isomer energies, and the Bayesian analysis of Ref. [30]) provide independent anchors; no uniqueness theorem or ansatz is imported from the authors' earlier work. The N=82 shell-gap claim is, by definition, a weighted combination of measured masses, so it is not a renaming of a known empirical pattern into a prediction. Overall, the derivation is self-contained and no claim reduces by construction to its inputs.

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

The result is an experimental measurement; no free parameters are fitted. The analysis relies on standard assumptions about the mass spectrometer calibration, peak shape model, and the correctness of previously published excitation energies and AME2020 reference masses.

assumptions (5)
  • domain assumption The MR-TOF-MS mass ratio measurements are accurate after calibration against AME2020 reference masses.
    Section II.A: masses are calibrated against well-known species; this is standard practice but unverified internally.
  • domain assumption The hyperEMG lineshape correctly models peak shapes including overlapping isomers.
    Section II.A: all peaks are fit with this functional form, which is validated on high-statistics calibrant peaks.
  • domain assumption The isomeric excitation energies from ENSDF/NUBASE, including extrapolated values for 149,150Tm, are correct within stated uncertainties.
    Section II.B: unresolved isomers are corrected using reported excitation energies, some of which are extrapolated from trends.
  • domain assumption The AME2020 masses of 148Er and 1H are accurate.
    Section III: these are used to compute the proton separation energy of 149Tm.
  • domain assumption Prior SHIPTRAP measurements correctly establish 147,148Tm as proton-bound.
    Section I.A: the drip-line is known to lie between A=148 and A=151 from prior measurements.

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Cite this review

Pith. "Pith review of Convergence on the Proton Drip-Line in Thulium." pith.science (2026). https://pith.science/paper/2IIABACE

@misc{pith2026241210259,
  author       = {Pith},
  title        = {Pith review of: Convergence on the Proton Drip-Line in Thulium},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/2IIABACE}},
  note         = {Machine review of arXiv:2412.10259}
}
abstract

Direct observation of proton emission for very small Q-values is often unfeasible due to the long partial half-lives of the proton emission channel associated with tunneling through the Coulomb barrier. Therefore, proton emitters with very small decay energies may require the masses of both parent and daughter nuclei in order to establish them as proton unbound. Nuclear mass models have been used to predict the proton drip-line of the thulium (Tm) isotopic chain ($Z=69$), but until now the proton separation energy has not been experimentally tested. Mass measurements were performed using a Multiple Reflection Time-Of-Flight Mass Spectrometer (MR-TOF-MS) at TRIUMF's TITAN facility to conclusively map the limit of proton-bound Tm. The masses of neutron-deficient, $^{149}$Tm and $^{150}$Tm, combined with measurements of $^{149m,g}$Er (which were found to deviate from literature by $\approx$150 keV), provide the first experimental confirmation that $^{149}$Tm is the first proton-unbound nuclide in the Tm chain. Our measurements also enable the strength of the $N=82$ neutron shell gap to be determined at the Tm proton drip-line, providing evidence supporting its continued existence.

Figures

Figures reproduced from arXiv: 2412.10259 by the authors.

Figure 2
Figure 2. FIG. 2. Fit of the overlapping peaks in the [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. FIG. 3. Comparison of the measured Tm isotopes with the [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figure 4
Figure 4. FIG. 4. The one-proton separation energy ( [PITH_FULL_IMAGE:figures/full_fig_p006_4.png] view at source ↗
Figures from the paper (1 more)
Figure 5
Figure 5. Figure 5: FIG. 5. Comparison of models and experimental data for the [PITH_FULL_IMAGE:figures/full_fig_p007_5.png]

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Pith tools

Reviewed August 11, 2026 · model on record in the stance chip above.