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Low-Energy Radon Backgrounds from Electrode Grids in Dual-Phase Xenon TPCs

D. S. Akerib, A. K. Al Musalhi, F. Alder, B. J. Almquist, S. Alsum, C. S. Amarasinghe, A. Ames, T. J. Anderson, N. Angelides, H. M. Araújo, J. E. Armstrong, M. Arthurs, X. Bai, A. Baker, J. Balajthy, S. Balashov, J. Bang, J. W. Bargemann, E. E. Barillier, A. Baxter, K. Beattie, T. Benson, E. P. Bernard, A. Bernstein, A. Bhatti, T. P. Biesiadzinski, H. J. Birch, E. Bishop, G. M. Blockinger, E. M. Boulton, B. Boxer, C. A. J. Brew, P. Brás, S. Burdin, D. Byram, M. C. Carmona-Benitez, M. Carter, C. Chan, A. Chawla, H. Chen, Y. T. Chin, N. I. Chott, S. Contreras, M. V. Converse, R. Coronel, A. Cottle, G. Cox, D. Curran, J. E. Cutter, C. E. Dahl, I. Darlington, S. Dave, A. David, J. Delgaudio, S. Dey, L. de Viveiros, L. Di Felice, C. Ding, J. E. Y. Dobson, E. Druszkiewicz, S. Dubey, C. L. Dunbar, S. R. Eriksen, A. Fan, N. M. Fearon, N. Fieldhouse, S. Fiorucci, H. Flaecher, E. D. Fraser, T. M. A. Fruth, P. W. Gaemers, R. J. Gaitskell, A. Geffre, J. Genovesi, C. Ghag, J. Ghamsari, A. Ghosh, S. Ghosh, R. Gibbons, M. G. D. Gilchriese, S. Gokhale, J. Green, M. G. D. van der Grinten, C. Gwilliam, J. J. Haiston, C. R. Hall, T. Hall, R. H Hampp, E. Hartigan-O'Connor, S. J. Haselschwardt, M. A. Hernandez, S. A. Hertel, D. P. Hogan, G. J. Homenides, M. Horn, D. Q. Huang, D. Hunt, C. M. Ignarra, R. G. Jacobsen, E. Jacquet, O. Jahangir, R. S. James, K. Jenkins, W. Ji, A. C. Kaboth, A. C. Kamaha, K. Kamdin, M. K. Kannichankandy, K. Kazkaz, D. Khaitan, A. Khazov, J. Kim, Y. D. Kim, J. Kingston, D. Kodroff, E. V. Korolkova, H. Kraus, S. Kravitz, L. Kreczko, V. A. Kudryavtsev, C. Lawes, E. Leason, D. S. Leonard, K. T. Lesko, C. Levy, J. Liao, J. Lin, A. Lindote, R. Linehan, W. H. Lippincott, J. Long, M. I. Lopes, W. Lorenzon, C. Lu, S. Luitz, W. Ma, V. Mahajan, P. A. Majewski, A. Manalaysay, R. L. Mannino, N. Marangou, R. J. Matheson, C. Maupin, M. E. McCarthy, G. McDowell, D. N. McKinsey, J. McLaughlin, J. B. McLaughlin, R. McMonigle, D. -M. Mei, B. Mitra, E. Mizrachi, M. E. Monzani, K. Morå, J. A. Morad, E. Morrison, B. J. Mount, M. Murdy, A. St. J. Murphy, A. Naylor, C. Nehrkorn, H. N. Nelson, F. Neves, A. Nguyen, A. Nilima, C. L. O'Brien, F. H. O'Shea, I. Olcina, K. C. Oliver-Mallory, J. Orpwood, K. Y Oyulmaz, K. J. Palladino, N. J. Pannifer, N. Parveen, S. J. Patton, B. Penning, G. Pereira, E. Perry, T. Pershing, A. Piepke, S. S. Poudel, Y. Qie, J. Reichenbacher, C. A. Rhyne, Q. Riffard, G. R. C. Rischbieter, E. Ritchey, H. S. Riyat, R. Rosero, P. Rossiter, N. J. Rowe, T. Rushton, D. Rynders, S. Saltão, D. Santone, A. B. M. R. Sazzad, R. W. Schnee, G. Sehr, B. Shafer, S. Shaw, W. Sherman, K. Shi, T. Shutt, C. Silva, G. Sinev, J. Siniscalco, A. M. Slivar, R. Smith, A. M. Softley-Brown, M. Solmaz, V. N. Solovov, P. Sorensen, J. Soria, A. Stevens, T. J. Sumner, A. Swain, N. Swanson, M. Szydagis, D. J. Taylor, R. Taylor, W. C. Taylor, B. P. Tennyson, P. A. Terman, D. R. Tiedt, M. Timalsina, W. H. To, Z. Tong, D. R. Tovey, J. Tranter, M. Trask, K. Trengove, M. Tripathi, L. Tvrznikova, U. Utku, A. Usón, A. Vacheret, A. C. Vaitkus, O. Valentino, V. Velan, A. Wang, J. J. Wang, Y. Wang, R. C. Webb, L. Weeldreyer, J. T. White, T. J. Whitis, K. Wild, M. Williams, J. Winnicki, M. S. Witherell, L. Wolf, F. L. H. Wolfs, S. Woodford, D. Woodward, C. J. Wright, Q. Xia, X. Xiang, J. Xu, Y. Xu, M. Yeh, D. Yeum, J. Young, W. Zha, C. Zhang, H. Zhang, T. Zhang, Y. Zhou

Abstract

The dual-phase xenon time projection chamber (TPC) is a powerful technology to detect rare interactions such as scatters of dark matter particles on nuclei. In particular, the built-in gain of ionization signals in a dual-phase TPC makes it sensitive to events in the few-electron regime, as expected from low-mass dark matter interactions. The pursuit of this low-energy sensitivity through ionization-only signal detection has so far been hindered by excessive electron backgrounds observed across experiments. Much of this background is attributed to the plate-out of $^{222}$Rn decay chain isotopes on the high voltage electrode grid surfaces that span the full cross section of the TPC. This work presents a first-principle model constructed for this background, the predictions of which are consistent with data from the LZ and LUX experiments. We then discuss mitigation strategies of this background in future dual-phase TPCs and the possibility of applying this grid background model to ionization-only dark matter searches.

Low-Energy Radon Backgrounds from Electrode Grids in Dual-Phase Xenon TPCs

Abstract

The dual-phase xenon time projection chamber (TPC) is a powerful technology to detect rare interactions such as scatters of dark matter particles on nuclei. In particular, the built-in gain of ionization signals in a dual-phase TPC makes it sensitive to events in the few-electron regime, as expected from low-mass dark matter interactions. The pursuit of this low-energy sensitivity through ionization-only signal detection has so far been hindered by excessive electron backgrounds observed across experiments. Much of this background is attributed to the plate-out of Rn decay chain isotopes on the high voltage electrode grid surfaces that span the full cross section of the TPC. This work presents a first-principle model constructed for this background, the predictions of which are consistent with data from the LZ and LUX experiments. We then discuss mitigation strategies of this background in future dual-phase TPCs and the possibility of applying this grid background model to ionization-only dark matter searches.
Paper Structure (19 sections, 10 figures, 5 tables)

This paper contains 19 sections, 10 figures, 5 tables.

Table of Contents

  1. Introduction
  2. Radon-Induced Grid Backgrounds
  3. Origins of $^{222}$Rn-chain Plate-out Backgrounds
  4. $^{222}$Rn Chain Decays Relevant for Low-Energy Searches
  5. Grid Model Construction
  6. Spatial Distribution of $^{222}$Rn Daughters and Decay Radiation
  7. Light and Charge Yields
  8. Light Collection
  9. Electron Transport and S2 Response
  10. Model Comparison with LZ and LUX Data
  11. Grid Radon Backgrounds in LZ
  12. Grid Radon Backgrounds in LUX
  13. Discussions
  14. Mitigation of Grid Backgrounds
  15. S2-Only Grid Background
  16. ...and 4 more sections

Figures (10)

  • Figure 1: The decay chain for $^{222}$Rn, with half-lives and decay modes. We have highlighted in blue the set of decays that are expected to be observed during TPC operation due to $^{210}$Pb plateout on the wires during grid production, and have left in black the set of decays that may be observed due to in-situ $^{222}$Rn emanation during TPC operation (see discussion in Sec. \ref{['subsec:SourcesOfRadonChainPlateouBackgrounds']}).
  • Figure 2: A close-up of the BACCARAT dedicated geometry used for simulating events. Teeth are added to model wire surface roughness, and both "embedded" and "surface" layers of the geometry are included. An inner layer used to generate embedded decays in a no-teeth, smooth-wire geometry is also shown, but is not used to generate any of the models explicitly presented in this work.
  • Figure 3: Top: a side-view diagram illustrating how the field line structure around the gate (left) and cathode (right) impacts tracks of electrons starting at the grid wires. Green tracks indicate electrons that drift toward the extraction region, and red tracks are electrons lost to the RFR. Bottom: simulation-based estimates of the charge survival fractions for the LZ cathode, as a function of azimuthal location $\phi$ around the wire circumference. Here, 0$\degree$ implies the top of the wire, and 180$\degree$ implies the bottom of the wire. Purple (green) points show the survival fraction for a location laterally very close to (far from) a wire crossing on the LZ cathode, with lateral positions indicated in the accompanying top-down diagram. These are computed using Garfield++ Garfieldpp with high-field diffusion parameters estimated from Ref Boyle.
  • Figure 4: Top: Data-model comparisons for example gate model, with component breakdowns. This model assumes an extraction efficiency of 0.78 and a near-gate $g_{1}$ of 0.077, and embedding fraction $f_{e}=1.0$. A normalization $^{210}$Po rate of 11.8 mBq (corresponding to the +1$\sigma$ scenario in Table \ref{['table:AlphaModelParametersTable']}) is used. The y-axis is normalized to the livetime and the total grid wire surface area, accounting for data quality cut acceptances. See Table \ref{['table:SaturationRegionValues']} for wire geometry information and Fig. 7.23 from Ref. LinehanThesis for additional embedding scenarios. Bottom: comparison of the gate data with a family of models created by taking variations of model parameters in Table \ref{['table:BaseModelParametersTable']}.
  • Figure 5: Top: Data-model comparisons for an example cathode model, with component breakdowns. This model assumes an extraction efficiency of 0.78 and a near-cathode $g_{1}$ of 0.1, and an embedding fraction $f_{e}=1.0$ for the $^{210}$Pb-chain daughters. For the cathode, the normalization $^{218}$Po/$^{214}$Pb/$^{210}$Po rates are 2.6 mBq/17.5 mBq/9.6 mBq, and in-situ-deposited daughters are modeled as landing on the top 10$\%$ of the wire with an $f_{e}=0.0$. The y-axis is normalized to the livetime and the total grid wire surface area, accounting for data quality cut acceptances. Bottom: comparison of the cathode data with a family of models created by taking variations of model parameters in Table \ref{['table:BaseModelParametersTable']}.
  • ...and 5 more figures