Radon-induced backgrounds in the NEXT-100 experiment

A. Brodoline; A. Castillo; A.F.B. Isabel; A. Larumbe; A.L.M. Silva; A. Marauri; A. Mart\'inez; A. Para; A. Pazos; A.P. Marques

Reviewed by Pith at T0; open to challenge.

T0 means a machine referee read the full paper against a public rubric. The mark states how deep the mechanical check went, never who wrote it. the ladder, T0–T4 →

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T0 review · glm-5.2

Radon Background Cut to Negligible Levels in NEXT-100

2026-07-05 07:21 UTC pith:NVSAF67F

load-bearing objection Solid radon background characterization for NEXT-100; the topological suppression factor is the load-bearing claim and needs full-text verification. the 2 major comments →

arxiv 2604.19616 v3 pith:NVSAF67F submitted 2026-04-21 hep-ex

Radon-induced backgrounds in the NEXT-100 experiment

NEXT Collaboration: C. Cortes-Parra , P. Novella , G. Mart\'inez-Lema , H. Almaz\'an , V. \'Alvarez , L. Arazi , I.J. Arnquist , F. Auria-Luna

show 105 more authors

S. Ayet Y. Ayyad C.D.R. Azevedo F. Ballester J.E. Barcelon M. del Barrio-Torregrosa J.M. Benlloch-Rodr\'iguez F.I.G.M. Borges A. Brodoline N. Byrnes A. Castillo E. Church M. Cid X. Cid C.A.N. Conde F.P. Coss\'io R. Coupe E. Dey P. Dietz C. Echeverria M. Elorza R. Esteve R. Felkai L.M.P. Fernandes P. Ferrario P. Ferrero Manche\~no F.W. Foss Z. Freixa J. Garc\'ia-Barrena J.J. G\'omez-Cadenas J.W.R. Grocott R. Guenette J. Hauptman C.A.O. Henriques J.A. Hernando Morata P. Herrero-G\'omez V. Herrero C. Herv\'es Carrete Y. Ifergan A.F.B. Isabel B.J.P. Jones F. Kellerer L. Larizgoitia A. Larumbe P. Lebrun F. Lopez N. L\'opez-March R. Madigan R.D.P. Mano A. Marauri A.P. Marques J. Mart\'in-Albo A. Mart\'inez M. Mart\'inez-Vara R.L. Miller K. Mistry J. Molina-Canteras F. Monrabal C.M.B. Monteiro F.J. Mora K.E. Navarro D.R. Nygren E. Oblak I. Osborne J. Palacio B. Palmeiro A. Para I. Parmaksiz A. Pazos J. Pelegrin M. P\'erez Maneiro M. Querol J. Renner I. Rivilla C. Rogero L. Rogers B. Romeo C. Romo-Luque E. Ruiz-Ch\'oliz P. Saharia F.P. Santos J.M.F. dos Santos M. Seemann I. Shomroni A.L.M. Silva P.A.O.C. Silva A. Sim\'on S.R. Soleti M. Sorel J. Soto-Oton J.M.R. Teixeira S. Teruel-Pardo J.F. Toledo C. Tonnel\'e S. Torelli J. Torrent A. Trettin P.R.G. Valle M. Vanga P. V\'azquez Cabaleiro J.F.C.A. Veloso J.D. Villamil L.M. Villar Padruno J. Waiton A. Yubero-Navarro

This is my paper

classification hep-ex

keywords neutrinoless double-beta decayNEXT-100radon backgroundtime projection chamberxenon-136topological selectionbackground suppression

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.

The NEXT-100 experiment uses a high-pressure xenon gas time projection chamber to search for neutrinoless double-beta decay. This paper reports on the first low-background run, focused on measuring how much radon gas contaminates the detector and contributes to fake signals. The key finding is that a topological cut—requiring events to show only one double-electron-like track—reduces the radon-induced background index to approximately 4×10⁻⁵ counts/(keV·kg·yr), which is one order of magnitude below the total expected radiogenic background. The detector operates in a virtually radon-free environment thanks to the lab's radon abatement system.

Core claim

The central mechanism is a topological selection criterion applied to radon progeny (particularly Bi-214 decays on the cathode surface). By requiring only one double-electron-like track in fully contained events, the collaboration demonstrates that radon-induced backgrounds can be suppressed to a level where they no longer dominate the experimental sensitivity.

What carries the argument

High-pressure electroluminescent time projection chamber (TPC) using Xe-136; topological event selection based on track morphology (double-electron-like signature); radon abatement system at the Canfranc Underground Laboratory (LSC).

Load-bearing premise

The efficiency of the topological cut and its systematic uncertainty are assumed to be well-modeled; if the rejection of radon-induced topologies is less effective than claimed, the final background index would be higher.

What would settle it

If the topological selection efficiency for signal events is lower than assumed, or if radon progeny produce topologies that mimic the double-electron signature more often than predicted, the background index would rise above the claimed value.

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

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

Solid radon background characterization for NEXT-100; the topological suppression factor is the load-bearing claim and needs full-text verification.

read the letter

Here's my read on the NEXT-100 radon background paper. The bottom line: this is a straightforward, well-executed background characterization that reports the first low-background run of NEXT-100 at 3.95 bar. The collaboration measures Rn-222 activity, Bi-214 rates, and the radon-induced background index in the 0νββ ROI, then shows that a topological cut brings it down to ~4×10⁻⁵ counts/(keV·kg·yr), about an order of magnitude below the total radiogenic budget. That's a useful, concrete result for the field and for the NEXT program specifically. The airborne radon correlation analysis — showing the detector runs in a virtually radon-free environment thanks to the LSC abatement system — is clean and convincing. The pre-cut background index of (7.3±1.5±0.8)×10⁻⁴ is quoted with both statistical and systematic errors, which is standard and appropriate. No circularity concerns; the radon activity is measured independently and the background index comes from detector data. The stress-test concern about the topological cut is the right thing to worry about. The final value is quoted as ~4×10⁻⁵ with no uncertainty, unlike the pre-cut number. The ~18× suppression factor almost certainly depends on Monte Carlo modeling of Bi-214 surface decays on the cathode — and surface-decay topology is genuinely hard to simulate well (backscattering, emission angles, plate-out depth). If the simulation underestimates how often surface Bi-214 decays produce single-track topologies that leak through the cut, the final index is underestimated. That said, this concern is about quantification precision, not about whether the suppression is real. Gas TPC topology discrimination against radon progeny is a known technique, and a factor of ~18 is plausible. The issue is whether the systematic uncertainty on that factor has been properly propagated, and we can't tell from the abstract alone. This is a paper for the rare-event physics community — people working on 0νββ searches, background modeling, and gas TPC design. It deserves a serious referee who can check the full derivation of the post-cut background index, the topological cut efficiency, and especially the systematic uncertainty treatment for the surface-decay simulation. I'd want the referee to verify that the ~4×10⁻⁵ carries a real uncertainty and that the MC validation for Bi-214 surface topologies is adequate.

Referee Report

2 major / 3 minor

Summary. This manuscript reports the measurement of radon-induced backgrounds in the NEXT-100 experiment, a high-pressure xenon-136 electroluminescent TPC searching for neutrinoless double-beta decay. The authors measure the internal Rn-222 activity, the Bi-214 event rate from progeny plate-out on the cathode, and the corresponding background index in the neutrinoless double-beta decay region of interest. They find that a topological selection requiring a single double-electron-like track suppresses the radon background index from (7.3±1.5±0.8)×10⁻⁴ to approximately 4×10⁻⁵ counts/(keV·kg·yr), placing it an order of magnitude below the total radiogenic background expectation. The analysis also correlates airborne radon levels with detector rates to conclude that the detector operates in a radon-free environment. This review is based on the abstract only, as the full text was not available.

Significance. Radon backgrounds are a critical concern for tonne-scale double-beta decay experiments, and a quantitative assessment of their impact in NEXT-100 is of clear importance to the field. The measurement of internal Rn-222 activity with both statistical and systematic uncertainties, the correlation with airborne radon levels, and the demonstration of topological suppression of Bi-214 backgrounds are all valuable contributions. The claim that the radon-induced background is reduced to one order of magnitude below the total radiogenic expectation is a strong, falsifiable result that directly informs the NEXT-100 sensitivity projection. Credit is due for providing a data-driven pre-cut rate and for quantifying the pre-cut background index with both statistical and systematic errors.

major comments (2)

The final post-topological-cut background index is quoted as '~4×10⁻⁵ counts/(keV·kg·yr)' with no statistical or systematic uncertainty, in contrast to the pre-cut value which carries both. This is the central result of the paper. Without an uncertainty, the reader cannot assess whether the claimed one-order-of-magnitude suppression relative to the total radiogenic background is robust. The authors should provide the full error budget for this quantity, or, if the value is simulation-derived, explicitly state the dominant systematic sources (e.g., Bi-214 surface-decay topology modeling) and their impact on the final number. If the uncertainty is genuinely negligible relative to the total radiogenic background, this should be justified quantitatively rather than stated by omission.
The topological suppression factor of approximately 18× (from 7.3×10⁻⁴ to 4×10⁻⁵) is the load-bearing step in the argument. The abstract does not specify whether this factor is derived from data or Monte Carlo simulation. For Bi-214 decays plated on the cathode surface, the topology of the observed electron tracks depends sensitively on the emission angle distribution, backscattering from the cathode material, and the depth profile of the plated progeny. If the suppression factor relies on simulation, the manuscript must describe the validation of the surface-decay model and quantify the systematic uncertainty associated with it. If the factor is data-driven, the method should be described. This distinction is essential for evaluating the correctness of the final background index.

minor comments (3)

The abstract uses '~4×10⁻⁵' for the final background index. If the full text provides a precise value with uncertainties, the abstract should be updated to include them for consistency with the pre-cut value.
The abstract states that the detector operates in a 'virtually radon-free environment.' A quantitative criterion for this claim (e.g., an upper limit on the external radon contribution to the background index) would strengthen this statement.
The phrase 'after selection of the fully contained events' for the pre-cut background index and the subsequent topological selection could be clarified: are these sequential cuts, and what is the efficiency of the containment selection? This is a presentation issue that the full text likely addresses.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for a careful and constructive review. The referee's two major comments both concern the uncertainty budget and methodology underlying the post-topological-cut background index (~4×10⁻⁵ counts/(keV·kg·yr)), which is indeed the central result of the paper. We address each point below.

read point-by-point responses

Referee: The final post-topological-cut background index is quoted as '~4×10⁻⁵ counts/(keV·kg·yr)' with no statistical or systematic uncertainty, in contrast to the pre-cut value which carries both. This is the central result of the paper. Without an uncertainty, the reader cannot assess whether the claimed one-order-of-magnitude suppression relative to the total radiogenic background is robust. The authors should provide the full error budget for this quantity, or, if the value is simulation-derived, explicitly state the dominant systematic sources (e.g., Bi-214 surface-decay topology modeling) and their impact on the final number. If the uncertainty is genuinely negligible relative to the total radiogenic background, this should be justified quantitatively rather than stated by omission.

Authors: The referee is correct that the post-cut background index should carry a full uncertainty breakdown, consistent with the pre-cut value. In the current manuscript, the post-cut value of ~4×10⁻⁵ counts/(keV·kg·yr) is derived from Monte Carlo simulation of Bi-214 surface decays on the cathode, with the dominant systematic arising from the surface-decay topology model. The reason the uncertainty was not quoted in the abstract is that it is subdominant: the total radiogenic background expectation in NEXT-100 is ~4×10⁻⁴ counts/(keV·kg·yr), so even a factor-of-two systematic on the post-cut radon value would not change the conclusion that radon is subdominant. However, we agree that this should be stated explicitly rather than by omission. In the revised manuscript, we will provide the full statistical and systematic uncertainty on the post-cut value, including the dominant systematic sources (Bi-214 surface-decay topology modeling, emission angle distribution, and depth profile of plated progeny). We will also add a quantitative justification for why the uncertainty does not affect the one-order-of-magnitude conclusion. revision: yes
Referee: The topological suppression factor of approximately 18× (from 7.3×10⁻⁴ to 4×10⁻⁵) is the load-bearing step in the argument. The abstract does not specify whether this factor is derived from data or Monte Carlo simulation. For Bi-214 decays plated on the cathode surface, the topology of the observed electron tracks depends sensitively on the emission angle distribution, backscattering from the cathode material, and the depth profile of the plated progeny. If the suppression factor relies on simulation, the manuscript must describe the validation of the surface-decay model and quantify the systematic uncertainty associated with it. If the factor is data-driven, the method should be described. This distinction is essential for evaluating the correctness of the final background index.

Authors: We agree that the origin of the suppression factor must be clearly stated. The suppression factor is derived from Monte Carlo simulation of Bi-214 decays on the cathode surface, not from data. The simulation models the full chain of Bi-214 surface decay, including emission angle distributions, backscattering from the cathode material, and the depth profile of plated progeny. The referee correctly identifies the key systematic sources. In the revised manuscript, we will: (1) explicitly state that the suppression factor is simulation-derived; (2) describe the surface-decay model in detail, including the treatment of emission angles, backscattering, and progeny depth profiles; (3) describe any validation of the model against available data or cross-checks; and (4) quantify the systematic uncertainty on the suppression factor from each of these sources. We note that the pre-cut rate of (7.3±1.5±0.8)×10⁻⁴ is data-driven, providing an anchor for the simulation, but the post-cut value and suppression factor themselves rely on the MC topology modeling. This distinction will be made clear. revision: yes

Circularity Check

0 steps flagged

No circularity detected: the derivation chain is a straightforward measurement-to-background-index pipeline with no self-referential reduction.

full rationale

The abstract describes a standard experimental measurement chain: (1) internal Rn-222 activity is measured from detector data and cross-checked against airborne radon correlation; (2) the Bi-214 rate above 400 keV is measured directly; (3) the background index in the ROI is derived from this rate after event selection; (4) a topological cut is applied to reduce it further. At no point is an output quantity defined in terms of itself, nor is a fitted parameter repackaged as a prediction. The concerns raised by the skeptic — unquantified uncertainty on the final value, dependence on simulation of surface Bi-214 topologies — are correctness and systematic-uncertainty risks, not circularity. The derivation is self-contained against measured data. No circularity is present.

Axiom & Free-Parameter Ledger

2 free parameters · 2 axioms · 0 invented entities

The paper is an experimental measurement; no new entities or ad-hoc axioms are introduced. The free parameters are experimental cuts and thresholds, not theoretical parameters fitted to data.

free parameters (2)

Topological cut threshold/definition = Not specified in abstract
The specific definition of 'one double-electron-like track' and its selection threshold are parameters that determine the background rejection factor.
Energy threshold for visible events = 400 keV
The choice of 400 keV as the visible energy threshold for the Bi-214 rate measurement is a parameter that affects the measured rate.

axioms (2)

domain assumption The detector simulation accurately models the response to radon progeny plate-out on the cathode surface.
The background index in the 0νββ ROI is likely derived using a combination of data and simulation; the simulation's fidelity to surface events is a load-bearing assumption.
domain assumption The radon abatement system of the LSC provides a stable, low-radon environment.
The conclusion that the detector operates in a 'virtually radon-free environment' depends on the continued operation and effectiveness of this system.

pith-pipeline@v1.1.0-glm · 5497 in / 1973 out tokens · 137504 ms · 2026-07-05T07:21:18.967292+00:00 · methodology

0 comments

read the original abstract

The NEXT-100 detector at the LSC aims at the first competitive search for the \bbnonu decay using a high-pressure \Xe{136} electroluminescent time projection chamber. The first low-background run of NEXT-100 at 3.95 bar has been devoted to the measurement of the radon-induced backgrounds impacting this search. The contributions from both the internal and external airborne radon have been evaluated. The internal \Rn{222} activity is found to be (0.95$\pm$0.04(stat)$\pm$0.09(sys)) Bq/m$^3$, while no traces of \Rn{220} have been observed. Most of the \Rn{222} progeny plate-out on the surface of the cathode of the detector, leading to a rate of Rn-induced \Bi{214} of (0.97$\pm$0.05(stat)$\pm$0.10(sys)) Hz for visible energies above 400 keV. The corresponding background index in the \bbnonu region of interest is evaluated as (7.3$\pm$1.5(stat)$\pm$0.8(sys))$\times10^{-4}$ counts/(keV$\cdot$kg$\cdot$yr) after selection of the fully contained events. This background index is reduced to $\sim$4$\times10^{-5}$ counts/(keV$\cdot$kg$\cdot$yr) by applying a topological selection requiring only one double-electron-like track in the events. This value is one order of magnitude below the total radiogenic background expectation in NEXT-100. By analyzing the correlation of the airborne radon activity and the measured rate of events in NEXT-100, it is concluded that the detector operates in a virtually radon-free environment thanks to the radon abatement system of the LSC.

Review history (2 revisions) →

Radon-induced backgrounds in the NEXT-100 experiment

Core claim

What carries the argument

Load-bearing premise

What would settle it

discussion (0)