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Measurement of ambient radon progeny decay rates and energy spectra in liquid argon using the MicroBooNE detector

P. Abratenko et al. (The MicroBooNE Collaboration)
Phys. Rev. D 109, 052007 – Published 20 March 2024
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  • INTRODUCTION
  • MICROBOONE DETECTOR AND DATASETS
  • LOW-ENERGY RECONSTRUCTION
  • ANALYSIS PROCEDURE
  • MONTE CARLO SIMULATION
  • RESULTS AND DISCUSSION
  • CONCLUSION
  • ACKNOWLEDGMENTS
    • References

    Abstract

    We report measurements of radon progeny in liquid argon within the MicroBooNE time projection chamber (LArTPC). The presence of specific radon daughters in MicroBooNE’s 85 metric tons of active liquid argon bulk is probed with newly developed charge-based low-energy reconstruction tools and analysis techniques to detect correlated Bi214−Po214 radioactive decays. Special datasets taken during periods of active radon doping enable new demonstrations of the calorimetric capabilities of single-phase neutrino LArTPCs for β and α particles with electron-equivalent energies ranging from 0.1 to 3.0 MeV. By applying Bi214−Po214 detection algorithms to data recorded over a 46-day period, no statistically significant presence of radioactive Bi214 is detected, and a limit on the activity is placed at <0.35  mBq/kg at the 95% confidence level. This bulk Bi214 radiopurity limit—the first ever reported for a liquid argon detector incorporating liquid-phase purification—is then further discussed in relation to the targeted upper limit of 1  mBq/kg on bulk Rn222 activity for the DUNE neutrino detector.

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    • Received 6 July 2023
    • Accepted 23 February 2024

    DOI:https://doi.org/10.1103/PhysRevD.109.052007

    Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.

    Published by the American Physical Society

    Physics Subject Headings (PhySH)

    1. Techniques
    Cosmic ray & astroparticle detectorsElectromagnetic calorimetersMulti-purpose particle detectorsParticle data analysisParticle detectors
    Particles & Fields

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    Vol. 109, Iss. 5 — 1 March 2024

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    Images

    • Figure 1
      Figure 1

      The MicroBooNE cryogenic circulation system, including the modifications made to include a Ra226 source inline with the flow of new liquid argon for special R&D periods described in this analysis. Gaseous and liquid argon flow is represented by green and blue lines, respectively, with arrows indicating the direction of flow. A filter bypass switch enabled a special flow configuration in which recondensed argon flowed directly into the cryostat without first passing through the filters. A second switch following the LAr pump determined whether circulating LAr flowed through the full-sized filter skid or the smaller 30%-sized filter skid [44].

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    • Figure 2
      Figure 2

      A section of a MicroBooNE LArTPC event display from data. Each vertical column of pixels represents a single wire. The rainbow color spectrum denotes charge collected per 500 ns ADC time-tick, with redder colors indicating higher charge densities. A grouping of hits across 10 wires from a randomly-selected area of interest spanning about 3 cm is represented in the zoomed inset as a set of pink boxes, where the extent of each box along the vertical (time) axis conveys the hit’s RMS width. A cosmic muon track can be seen to the right.

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    • Figure 3
      Figure 3

      Distribution of integrated hit cluster charge on the collection plane and the middle induction plane for potential match candidates, for cases requiring matches in only 2–3 planes (top) and all 3 planes (bottom). Matches falling within the red regions in the top plot are rejected.

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    • Figure 4
      Figure 4

      Reconstruction efficiency as a function of energy deposited by electrons in Monte Carlo simulations. Both standard MicroBooNE settings (blue) and low-threshold settings (red) are shown. The solid markers represent hit-finding performance for wire signals on the collection plane, while the unfilled markers represent 3D blips that were plane matched on at least two planes. The presence of nonfunctional wires on each plane limits the maximum achievable efficiencies.

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    • Figure 5
      Figure 5

      Ionization charge as a function of corresponding energy deposition for electrons distributed uniformly in the MicroBooNE active volume. Error bars show the average variations due to nonuniformities in the electric field from accumulated space charge.

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    • Figure 6
      Figure 6

      Energy resolution from simulated electrons in the MicroBooNE TPC using low-threshold reconstruction settings. For the fit, defined in Eq. (3), deposited energy E is in units of MeV.

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    • Figure 7
      Figure 7

      A Bi214−Po214 decay signal candidate in an event display, including backgrounds from potential de-excitation γ rays emitted following the decay.

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    • Figure 8
      Figure 8

      Schematic illustrating the selection regions on the collection plane for a BiPo decay candidate. Dashed lines represent readout channels, and hits are represented as circles. Drawing is not to scale.

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    • Figure 9
      Figure 9

      Distributions of ΔT for Bi214−Po214 candidates in the signal and background selection regions for a period during which Rn222 was actively being added to the LAr.

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    • Figure 10
      Figure 10

      Background-subtracted and fitted ΔT distributions for the Rn-doping data for a period when the filter was bypassed (blue) and the preceding period where the full filtration system was employed (black). The lifetime τ in both fits is fixed to the known Po214 mean lifetime of 237  μs.

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    • Figure 11
      Figure 11

      Measured rate of BiPo decays per readout within the fiducial volume, plotted against the time of the event relative to the start of each Rn-doping data-taking period. Statistical errors are represented by solid lines, while systematic errors from uncertainties in the fit methodology as described in the text are represented by shaded regions.

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    • Figure 12
      Figure 12

      The reconstructed βBi energy spectrum. The shaded region represents MC statistical uncertainty, while the blue dotted line is the MC spectrum with a −5% energy scale shift. The value of Qβ=3.27  MeV for Bi214 is indicated.

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    • Figure 13
      Figure 13

      Reconstructed αPo energy spectrum, in electron-equivalent units, following the background-subtraction procedure. Due to the uncertainty in the α QY in LAr, additional samples are generated using NEST’s empirical model [74] with a ±20% scaling applied to the QY. The LArG4 MC, which by default uses particle dE/dx from Geant4 [69] as input to the Modified Box model to calculate recombination, is shown for comparison.

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    • Figure 14
      Figure 14

      The fitted ΔT distribution from unbiased nonbeam data taken during a 46-day period of standard operating conditions in MicroBooNE.

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    • Figure 15
      Figure 15

      The efficiency-corrected BiPo rate measured in 48-hour periods throughout the beam-external dataset. Error bars on each data point are dominated by statistical uncertainties.

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