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arxiv: 2605.02468 · v2 · submitted 2026-05-04 · ✦ hep-ex

Recognition: unknown

Background of the BULLKID detector array operated with moderate shield on surface

A. Acevedo-Renter\'ia, A. Cruciani, A. D'Addabbo, A. Mazzolari, A. Monfardini, A. Tartari, C. Bonomo, C. M. A. Roda, C. Puglia, D. A. Crovo, D. Delicato, D. L. Helis, D. Nicol\`o, D. Pasciuto, D. Quaranta, E. V\'azquez-J\'auregui, F. Carillo, F. Cescato, F. Ferraro, F. Paolucci, F. Simon, G. Del Castello, G. Signorelli, K. Zhao, L. Bandiera, L. E. Ardila-Perez, L. Malagutti, L. Pesce, M. Calvo, M. Cappelli, M. del Gallo Roccagiovine, M. De Lucia, M. Folcarelli, M. Giammei, M. Grassi, M. Romagnoni, M. Vignati, R. Caravita, R. Gartmann, S. Fu, S. Roddaro, T. Lari, T. Muscheid, U. Chowdhury, V. Guidi

Pith reviewed 2026-05-08 17:12 UTC · model grok-4.3

classification ✦ hep-ex
keywords BULLKIDkinetic inductance detectorscryogenic detectorsdark matter searchcoherent neutrino scatteringsurface backgroundradiation shieldinglow energy spectrum
0
0 comments X

The pith

BULLKID silicon detector array operated on surface with moderate shielding shows low-energy background compatible with simulations down to 600 eV.

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

The paper reports results from running the BULLKID array of silicon absorbers in a surface lab using both external and internal shields. Over 290 hours, the measured background rate between 2 keV and 600 eV matches Monte Carlo expectations at (6.8 ± 0.4 stat. ± 0.1 syst.) × 10^4 counts per keV per kg per day when central detectors are read out with surrounding units serving as veto. The high-energy spectrum also follows the predicted shape, including X-ray lines from lead. A rise appears below 600 eV that exceeds the simulation but does not display the usual signatures of low-energy excesses seen elsewhere. These findings matter for developing detectors that can perform light dark matter or neutrino scattering measurements without deep underground sites.

Core claim

With external and internal radiation shields in place, the BULLKID array of 60 cubic silicon absorbers of 0.34 g each, sensed by cryogenic kinetic inductance detectors, records a low-energy background spectrum from 15 elements that is compatible with simulations at the level of (6.8 ± 0.4 stat. ± 0.1 syst.) × 10^4 counts / keV kg days from 2 keV down to 600 eV. The region between 225 eV and 600 eV shows a background rise in disagreement with the simulations, while not sharing some of the key traits of the low energy excess observed in other cryogenic experiments. The high energy spectrum shape is in overall agreement with the simulations and displays the typical particle-induced X-ray lines.

What carries the argument

An array of silicon particle absorbers with central units used for background measurement and surrounding units acting as an active veto, operated together with moderate external and internal radiation shields.

If this is right

  • Background levels in the keV range become predictable by simulation when moderate shielding and veto are applied, allowing reliable estimates for surface-based runs.
  • Surrounding detector elements successfully reject a substantial fraction of ambient background events.
  • High-energy particle interactions produce X-ray emissions from the lead shield that match the expected pattern.
  • Surface operation of low-threshold cryogenic arrays becomes practical for light dark matter or coherent neutrino scattering searches.

Where Pith is reading between the lines

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

  • If the unexplained rise below 600 eV can be reduced by additional material or geometry changes, the detector threshold could be lowered further for lighter dark matter particles.
  • Surface testing with this shielded veto setup could shorten development time for similar cryogenic detectors before moving to underground sites.
  • The specific form of the low-energy rise may point to surface or material effects that differ from those in other experiments and could be isolated with targeted runs.

Load-bearing premise

The Monte Carlo simulations accurately include every relevant surface background component and the surrounding detectors function as a perfect veto without leaving unmodeled contributions.

What would settle it

A measured rate or spectral shape between 600 eV and 2 keV that differs from the simulated value by more than the combined statistical and systematic uncertainties quoted in the paper.

Figures

Figures reproduced from arXiv: 2605.02468 by A. Acevedo-Renter\'ia, A. Cruciani, A. D'Addabbo, A. Mazzolari, A. Monfardini, A. Tartari, C. Bonomo, C. M. A. Roda, C. Puglia, D. A. Crovo, D. Delicato, D. L. Helis, D. Nicol\`o, D. Pasciuto, D. Quaranta, E. V\'azquez-J\'auregui, F. Carillo, F. Cescato, F. Ferraro, F. Paolucci, F. Simon, G. Del Castello, G. Signorelli, K. Zhao, L. Bandiera, L. E. Ardila-Perez, L. Malagutti, L. Pesce, M. Calvo, M. Cappelli, M. del Gallo Roccagiovine, M. De Lucia, M. Folcarelli, M. Giammei, M. Grassi, M. Romagnoni, M. Vignati, R. Caravita, R. Gartmann, S. Fu, S. Roddaro, T. Lari, T. Muscheid, U. Chowdhury, V. Guidi.

Figure 1
Figure 1. Figure 1: Top left) The BULLKID detector operated in this measurement. It consists of a silicon wafer, 3 inches in diameter and 5 mm thick, carved into 60 dice and installed in a copper holder. The two remaining copper holders do not contain any detector and are used solely for mechanical reasons. The array of KIDs is on the side opposite to the dice.Bottom left) The detector is placed inside a lead pot for shieldin… view at source ↗
Figure 2
Figure 2. Figure 2: Top) Distribution of noise samples after applying the matched filter over the whole duration of the data taking for KID-47. The RMS is 0.38 mrad and corresponds to 33 eV after calibration. Bottom) Distribution of KID-47 and 49 pulse response to the phonon leakage produced by energy de￾positions on the adjacent die of KID-48. Three distinct time intervals in which the signal was not detected have been ex￾cl… view at source ↗
Figure 3
Figure 3. Figure 3: Left) View of the external shield surrounding the cryostat in which the detector is installed. Right) Geant4 representation of the experiment, where the lead bricks and the support table are shown in yellow. The BULLKID detec￾tor is represented in blue. over, we find that neutrons are a sub-leading contributor to the background by approximately two orders of mag￾nitude, as seen in view at source ↗
Figure 5
Figure 5. Figure 5: Combined energy spectrum of the two main dice for a total exposure of 290 h × 0.68 g, from 2 keV to 85 keV (blue data points). The lead-induced X and γ-rays listed in Tab. I are indicated with vertical dashed lines. The simulated high energy spectrum is superimposed on the data and the bottom panel shows the difference between the measured and the simulated spectra. An energy dependent discrepancy is obser… view at source ↗
Figure 6
Figure 6. Figure 6: Top: Energy spectra after 290 h of exposure for the dice sensed by KID-47 (orange) and KID-49 (green). Af￾ter applying the event selection based on the pulse shape dis￾crimination and on the anti-coincidence veto the two spectra fully overlap, once corrected for selection efficiency. Bottom: Combined energy spectrum of the two main dice for a total exposure of 290 h × 0.68 g (red data points). The energy s… view at source ↗
Figure 8
Figure 8. Figure 8: Time evolution of the counting rate measured be view at source ↗
Figure 7
Figure 7. Figure 7: Top) Acquired KID map showing the working con￾ditions of the different KIDs in terms of their SNR (i.e. the optimization of their readout power). The maps are shown for respectively the main (first) acquisition of this work (left), the spectrum is presented in view at source ↗
Figure 9
Figure 9. Figure 9: Comparison of the BULLKID’s spectrum presented in this work (solid red line) with other experiments of the field (see view at source ↗
Figure 10
Figure 10. Figure 10: Combined efficiencies (both trigger and analysis) view at source ↗
Figure 11
Figure 11. Figure 11: Top: 2-dimensional parameter space defined by the variables (ψ1)R and (ψ2)R for KID-47. The red-blue col￾orscale represents LED events known to illuminate the cor￾rect die, while the gray points are unwanted background to be rejected. The red rectangle represents the event selec￾tion achieved by independently requiring |(ψ1)R| < 2.5 and |(ψ2)R| < 2.5, which yields high efficiency at the cost of mod￾erate … view at source ↗
read the original abstract

We present the operation with moderate radiation shield in a surface laboratory of BULLKID (BULky and Low-threshold Kinetic Inductance Detector), a cryogenic detector for searches of light Dark Matter or Coherent Elastic Neutrino-Nucleus Scattering. The detector consists of an array of 60 cubic silicon particle absorbers of 0.34 g each, sensed by cryogenic kinetic inductance detectors. The analysis presented focuses on data from 15 elements of the array, with two central units used to evaluate the background and with their surrounding elements used as veto. The low energy spectrum resulting from an exposure of 290 hours to ambient backgrounds, acquired with the use of external and internal radiation shields, is compatible with the simulations at the level of $(6.8\pm0.4\,{\rm stat.}\pm0.1\,{\rm syst.})\times10^4$ counts / keV kg days from 2 keV down to an energy of 600 eV. The region between 225 eV and 600 eV shows a rise in background in disagreement with the simulations, while not sharing some of the key traits of the low energy excess observed in other cryogenic experiments. The high energy spectrum shape is in overall agreement with the simulations and displays the typical particle-induced X-ray emission of the surrounding lead.

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. This manuscript presents results from operating the BULLKID array of 60 silicon absorbers read out by kinetic inductance detectors in a surface laboratory with moderate radiation shielding. The analysis focuses on 15 elements, using two central absorbers to measure the background spectrum while employing the surrounding 13 elements as a veto. After an exposure of 290 hours, the authors report a background level of (6.8 ± 0.4 statistical ± 0.1 systematic) × 10^4 counts per keV per kg per day between 2 keV and 600 eV, which is compatible with their Monte Carlo simulations. They observe a rise in the background rate in the 225–600 eV range that deviates from the simulations but does not share key features with low-energy excesses reported by other cryogenic experiments. The high-energy spectrum is in agreement with simulations and exhibits typical particle-induced X-ray emission from the lead shielding.

Significance. Should the central claims be substantiated, this work is significant as it provides one of the first detailed background characterizations for a KID-based detector array targeting low-energy rare events. The quantitative agreement with independent simulations, including statistical and systematic uncertainties, and the use of an internal veto strategy represent strengths. It offers practical insights for surface-based testing of such detectors and identifies a potential new background component at sub-keV energies that warrants further study. This contributes to the field by benchmarking background levels for future underground deployments or improved shielding designs.

major comments (2)
  1. [Abstract and data analysis section] The headline result of spectral compatibility with simulations from 2 keV to 600 eV is predicated on the surrounding detectors functioning as an effective veto for the central absorbers. The manuscript does not report the coincidence window used, the energy-dependent veto efficiency, or the accidental coincidence rate. A direct comparison of the spectrum with and without the veto applied would help validate this assumption and rule out leakage of surface-induced events into the central detectors.
  2. [Simulation comparison section] While the Monte Carlo is stated to include ambient and surface backgrounds, no sensitivity study or variation of key parameters (e.g., surface contamination levels, lead shield leakage, or low-energy neutron/X-ray contributions) is presented. Given the noted disagreement below 600 eV, such a study is necessary to assess whether the simulations fully capture the relevant physics or if an unmodeled component is present.
minor comments (2)
  1. [Results figures] The energy spectrum plots would be clearer if the simulation uncertainty bands were overlaid on the data points for direct visual assessment of the agreement.
  2. The units in the background rate expression could be written more conventionally as counts keV^{-1} kg^{-1} day^{-1} to avoid ambiguity in the slash notation.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the thorough review and valuable suggestions. We address the major comments below and have made revisions to the manuscript to incorporate the requested information and clarifications.

read point-by-point responses

  1. Referee: [Abstract and data analysis section] The headline result of spectral compatibility with simulations from 2 keV to 600 eV is predicated on the surrounding detectors functioning as an effective veto for the central absorbers. The manuscript does not report the coincidence window used, the energy-dependent veto efficiency, or the accidental coincidence rate. A direct comparison of the spectrum with and without the veto applied would help validate this assumption and rule out leakage of surface-induced events into the central detectors.

    Authors: We agree that these details are important for validating the veto strategy. The original manuscript omitted explicit reporting of these parameters. In the revised version, we will add a description of the coincidence analysis, specifying the time window used for vetoing, the calculated veto efficiency as a function of energy, and the estimated accidental coincidence rate. We will also include a figure showing the background spectrum before and after applying the veto to demonstrate its effectiveness in reducing the event rate. revision: yes

  2. Referee: [Simulation comparison section] While the Monte Carlo is stated to include ambient and surface backgrounds, no sensitivity study or variation of key parameters (e.g., surface contamination levels, lead shield leakage, or low-energy neutron/X-ray contributions) is presented. Given the noted disagreement below 600 eV, such a study is necessary to assess whether the simulations fully capture the relevant physics or if an unmodeled component is present.

    Authors: We acknowledge the value of a sensitivity study to better understand the disagreement below 600 eV. While a comprehensive variation of all parameters would require significant additional computational resources, we will include in the revised manuscript a limited sensitivity analysis focusing on the impact of varying surface contamination levels and low-energy X-ray contributions. This will help assess whether the observed excess can be attributed to unmodeled components or variations in the input parameters. revision: partial

Circularity Check

0 steps flagged

No significant circularity: measured background rates compared to independent Monte Carlo

full rationale

The paper reports an experimental measurement of background count rates from 290 hours of data taking with the BULLKID array, using two central detectors for the spectrum and surrounding elements as veto. The quoted rate of (6.8±0.4 stat.±0.1 syst.)×10^4 counts / keV kg days is extracted directly from the observed data spectrum between 2 keV and 600 eV and compared to separate Monte Carlo simulations of ambient and surface backgrounds. No equations, ansatzes, or self-citations reduce this measured rate to a fitted parameter defined by the same dataset. The high-energy spectrum shape is likewise compared to simulation without self-referential derivation. Veto usage and simulation fidelity are assumptions whose validation is external to the reported numbers; they do not create a definitional loop. This is a standard data-vs-simulation comparison with no load-bearing self-definition or fitted-input-called-prediction.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The central claim rests on standard assumptions of radiation transport modeling and veto efficiency rather than new free parameters or invented entities.

axioms (2)
  • domain assumption Monte Carlo simulations accurately model ambient radiation backgrounds and detector response in a surface laboratory with moderate shielding.
    Compatibility statement in abstract assumes this modeling fidelity.
  • domain assumption Surrounding detector elements provide an effective veto that isolates true background events in the central units.
    Analysis uses two central units for background evaluation with surrounding elements as veto.

pith-pipeline@v0.9.0 · 5789 in / 1325 out tokens · 61992 ms · 2026-05-08T17:12:15.454309+00:00 · methodology

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