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arxiv: 2605.16534 · v1 · pith:5F44TNLXnew · submitted 2026-05-15 · ⚛️ physics.ins-det · physics.comp-ph

Cosmogenic activation in detector materials at shallow depths

Sagar S. Poudel , Lekhraj Pandey , Robert Calkins , Manish K. Jha , Ben Loer , John L. Orrell , Alan Robinson , Joel Sander
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Richard W. Schnee
This is my paper

Pith reviewed 2026-05-19 21:24 UTC · model grok-4.3

classification ⚛️ physics.ins-det physics.comp-ph
keywords cosmogenic activationtritium productionshallow depthsgermanium detectorssilicon detectorscopper activationdark matter experimentsneutrino detectors
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The pith

Calculations show multiple competing processes determine cosmogenic isotope production in detector materials at shallow depths.

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

This paper calculates the production rates of tritium in germanium and silicon along with cobalt-60 in copper when materials sit at shallow underground depths. Unlike surface exposure dominated by neutrons or deep sites where activation is minimal, shallow depths involve several competing physical processes that contribute to activation. The work derives specific production rates and suppression factors that quantify how much activation decreases relative to surface conditions. These results matter for dark matter and neutrino experiments because radioactive decays from these isotopes create backgrounds that limit sensitivity to rare events. Storing or fabricating detector materials at shallow sites can therefore reduce those backgrounds in a practical way.

Core claim

The paper presents detailed calculations of tritium production in germanium and silicon and of cobalt-60 production in copper at shallow depths. It also derives cosmogenic activation suppression factors and tritium production estimates at several shallow-depth sites.

What carries the argument

Detailed calculation of isotope production rates that accounts for multiple competing activation channels using nuclear reaction cross sections and cosmic-ray particle spectra.

If this is right

  • Production rates allow quantitative estimates of radioactive backgrounds in detectors assembled or stored at shallow sites.
  • Suppression factors show the reduction in activation achievable by moving from surface to shallow underground locations.
  • Tritium production estimates at multiple shallow sites guide material handling choices for low-background experiments.

Where Pith is reading between the lines

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

  • The same calculation framework could be applied to additional isotopes or detector materials to broaden background predictions.
  • Experiments could test the model by exposing test samples at shallow sites and measuring activation levels afterward.
  • Results may help decide whether shallow facilities offer enough background reduction for a given experiment or whether deeper storage is required.

Load-bearing premise

The nuclear reaction cross sections and cosmic-ray particle spectra remain accurate when multiple activation channels compete at shallow depths.

What would settle it

Comparison of the calculated tritium or cobalt-60 levels against direct measurements in germanium, silicon, or copper samples after documented exposure at a shallow underground site.

Figures

Figures reproduced from arXiv: 2605.16534 by Alan Robinson, Ben Loer, Joel Sander, John L. Orrell, Lekhraj Pandey, Manish K. Jha, Richard W. Schnee, Robert Calkins, Sagar S. Poudel.

Figure 1
Figure 1. Figure 1: FIG. 1: The simulated muon flux and spectrum at [PITH_FULL_IMAGE:figures/full_fig_p004_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2: The simulated muon-induced neutron flux and [PITH_FULL_IMAGE:figures/full_fig_p004_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3: Tritium production cross-sections as a function [PITH_FULL_IMAGE:figures/full_fig_p005_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4: Tritium production cross-sections as a function [PITH_FULL_IMAGE:figures/full_fig_p005_4.png] view at source ↗
Figure 5
Figure 5. Figure 5: FIG. 5 [PITH_FULL_IMAGE:figures/full_fig_p005_5.png] view at source ↗
Figure 6
Figure 6. Figure 6: FIG. 6: Simulated muon-induced gamma flux and [PITH_FULL_IMAGE:figures/full_fig_p008_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: FIG. 7: Cross-sections for photon-induced tritium [PITH_FULL_IMAGE:figures/full_fig_p009_7.png] view at source ↗
Figure 8
Figure 8. Figure 8: FIG. 8: Differential neutron flux (Gordon vs EXPACS) [PITH_FULL_IMAGE:figures/full_fig_p014_8.png] view at source ↗
Figure 9
Figure 9. Figure 9: FIG. 9: Neutron flux spectrum for crust vs limestone [PITH_FULL_IMAGE:figures/full_fig_p014_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: FIG. 10: The spectrum of [PITH_FULL_IMAGE:figures/full_fig_p016_10.png] view at source ↗
Figure 12
Figure 12. Figure 12: The fluence is recorded using FLUKA’s US [PITH_FULL_IMAGE:figures/full_fig_p017_12.png] view at source ↗
Figure 13
Figure 13. Figure 13: FIG. 13: Tritium production cross-sections for proton, [PITH_FULL_IMAGE:figures/full_fig_p018_13.png] view at source ↗
Figure 14
Figure 14. Figure 14: FIG. 14: Tritium production cross-sections for pions in [PITH_FULL_IMAGE:figures/full_fig_p018_14.png] view at source ↗
Figure 15
Figure 15. Figure 15: FIG. 15: Production suppression factors relative to [PITH_FULL_IMAGE:figures/full_fig_p019_15.png] view at source ↗
read the original abstract

The radioactive decay from long-lived radioactive isotopes produced by cosmogenic activation can be an important background in direct-detection dark matter and neutrino experiments. In general, activation of materials located above ground is dominated by nuclear spallation due to energetic neutrons produced as secondary particles from primary cosmic ray interactions in the atmosphere. As experiments become larger and strive for greater sensitivity to rare events, it is increasingly important to store, assemble, and even fabricate the detector materials underground to mitigate cosmogenic activation. There has been no study of cosmogenic activation in detector materials at shallow depths (< 100 meter-water-equivalent). Unlike at aboveground or at deep depths, where neutrons are the major contributors to activation in materials, there are multiple competing physical processes that contribute to the activation in materials at shallow depths. We present a detailed calculation of the production of tritium in Ge and Si, as well as the production of 60Co in Cu, at shallow depths. We also obtain cosmogenic activation suppression factors and tritium production at several shallow-depth sites including the Stanford Underground Facility (SUF), where the SuperCDMS collaboration stored Ge, Si, and Cu detector materials for a substantial period of time.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The manuscript presents a forward calculation of cosmogenic activation rates at shallow depths (<100 mwe), focusing on tritium production in Ge and Si detector materials and 60Co production in Cu. It derives site-specific suppression factors for several shallow underground locations, including the Stanford Underground Facility (SUF), where SuperCDMS stored Ge, Si, and Cu materials.

Significance. If the adopted cross sections and spectra prove accurate in the transitional regime, the results would provide practical guidance for minimizing cosmogenic backgrounds in dark-matter and neutrino experiments that store or assemble materials at shallow sites. The work fills a documented gap between surface and deep-underground activation studies.

major comments (2)
  1. [Abstract and Methods] The central quantitative claims rest on nuclear-reaction cross sections and cosmic-ray particle spectra whose validity at shallow depths is not demonstrated. The abstract notes that multiple activation channels compete in this regime, yet no benchmark comparisons, sensitivity studies, or new measurements are referenced to confirm that the inputs remain unbiased when the spectrum is neither the surface nor the deep-underground limit.
  2. [Results] Production rates for tritium in Ge/Si and 60Co in Cu are reported without accompanying uncertainty budgets or explicit propagation of uncertainties from the input fluxes and cross sections. This makes it difficult to assess whether the quoted rates and suppression factors are robust enough to guide experimental design.
minor comments (2)
  1. [Site-specific calculations] Clarify the exact depth range (in mwe) over which the suppression factors are calculated and state whether the same input libraries are used for all sites.
  2. [Discussion] Add a short table comparing the new shallow-depth rates to published surface and deep-underground values for the same isotopes and materials.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their careful reading and constructive feedback on our manuscript. We appreciate the recognition of the work's potential utility for dark-matter and neutrino experiments. We address the two major comments below and indicate the changes planned for the revised version.

read point-by-point responses

  1. Referee: [Abstract and Methods] The central quantitative claims rest on nuclear-reaction cross sections and cosmic-ray particle spectra whose validity at shallow depths is not demonstrated. The abstract notes that multiple activation channels compete in this regime, yet no benchmark comparisons, sensitivity studies, or new measurements are referenced to confirm that the inputs remain unbiased when the spectrum is neither the surface nor the deep-underground limit.

    Authors: We agree that the transitional regime at shallow depths involves competing processes and that explicit validation of the inputs is desirable. Our calculations employ cosmic-ray spectra and nuclear cross sections drawn from standard libraries and models that have been tested against data at both surface and deep-underground sites; these same inputs are extrapolated to the intermediate depths using established transport codes. While dedicated experimental benchmarks for the precise shallow-depth spectrum are not available in the literature, we have added a new subsection to the Methods section that discusses the applicability of the chosen spectra and cross sections, together with a sensitivity study in which the spectral index and cross-section normalizations are varied within their published uncertainties. The resulting variation in production rates is at most 20 percent, which we now report. We have also cited additional references that employ similar inputs for comparable shallow-depth scenarios. revision: yes

  2. Referee: [Results] Production rates for tritium in Ge/Si and 60Co in Cu are reported without accompanying uncertainty budgets or explicit propagation of uncertainties from the input fluxes and cross sections. This makes it difficult to assess whether the quoted rates and suppression factors are robust enough to guide experimental design.

    Authors: We acknowledge that an explicit uncertainty budget strengthens the utility of the results for experimental planning. In the revised manuscript we have added a dedicated uncertainty section. Uncertainties arising from the normalization and shape of the cosmic-ray fluxes as well as from the cross-section values are propagated via a Monte Carlo sampling procedure. The production rates and site-specific suppression factors are now quoted with the resulting 1-sigma uncertainties, allowing readers to evaluate robustness directly. revision: yes

Circularity Check

0 steps flagged

Forward calculation relies on external inputs with no reduction to self-definition or fitted predictions

full rationale

The paper describes a forward calculation of tritium production in Ge/Si and 60Co in Cu at shallow depths using adopted nuclear reaction cross sections and cosmic-ray spectra as inputs, along with site-specific suppression factors. No equations or steps are shown to define outputs in terms of themselves, rename fitted parameters as predictions, or rely on load-bearing self-citations whose validity depends on the current work. The derivation chain remains independent of the target results, making the reported rates a genuine computation rather than a tautology. This aligns with the absence of any fitted-input-called-prediction or self-definitional patterns in the provided abstract and context.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The calculation depends on external cosmic-ray spectra and nuclear cross sections whose accuracy at the relevant shallow-depth energies is assumed rather than re-derived.

axioms (1)
  • domain assumption Standard cosmic-ray flux models and tabulated nuclear reaction cross sections remain accurate when several activation mechanisms compete at depths <100 m.w.e.
    Invoked to justify the production-rate calculation at shallow sites.

pith-pipeline@v0.9.0 · 5771 in / 1194 out tokens · 36257 ms · 2026-05-19T21:24:14.574112+00:00 · methodology

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