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Signal yields of keV electronic recoils and their discrimination from nuclear recoils in liquid xenon

E. Aprile et al. (XENON Collaboration)
Phys. Rev. D 97, 092007 – Published 25 May 2018
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  • INTRODUCTION
  • DATA AND ANALYSES
  • RECOMBINATION FACTOR AND FLUCTUATION
  • ELECTRONIC AND NUCLEAR RECOILS…
  • CONCLUSION
  • ACKNOWLEDGMENTS
    • References

    Abstract

    We report on the response of liquid xenon to low energy electronic recoils below 15 keV from beta decays of tritium at drift fields of 92  V/cm, 154  V/cm and 366  V/cm using the XENON100 detector. A data-to-simulation fitting method based on Markov Chain Monte Carlo is used to extract the photon yields and recombination fluctuations from the experimental data. The photon yields measured at the two lower fields are in agreement with those from literature; additional measurements at a higher field of 366  V/cm are presented. The electronic and nuclear recoil discrimination as well as its dependence on the drift field and photon detection efficiency are investigated at these low energies. The results provide new measurements in the energy region of interest for dark matter searches using liquid xenon.

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    • Received 2 October 2017

    DOI:https://doi.org/10.1103/PhysRevD.97.092007

    © 2018 American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Particle dark matter
    1. Techniques
    Particle detectors
    Particles & Fields

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    Vol. 97, Iss. 9 — 1 May 2018

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    Images

    • Figure 1
      Figure 1

      Tritiated methane rate starting from the first injection. The left and right vertical black dashed lines represent the first and second tritium injection separately. The periods of circulating the xenon from the bulk liquid and from the gas phase are marked by the red and blue regions, respectively. The horizontal black solid line shows the background rate before tritium injection with the uncertainty in the rate, within 1σ, given by the black shaded region.

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    • Figure 2
      Figure 2

      The uniform spatial distribution of tritium beta-decay events (black dots), under the drift field of 366  V/cm, overlaid with the S1 light yield (photoelectrons per keV energy deposition) relative to that of the center (FV#4) from the Kr83m (41.5 keV) calibration. The radial positions used in this analysis are not corrected for the field distortion at the bottom corner of the TPC [27], thus the event locations shown in this plot are those detected at the liquid surface. To avoid the systematic effects due to non-uniform drift field, we chose 50% of events from the FV in the central part of the TPC for the following analysis. The central volume, after removing the top and bottom parts, is further divided into seven small FVs, each with a different S1 photon detection efficiency which increases from the top to the bottom.

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    • Figure 3
      Figure 3

      Anticorrelation between S1 and S2 signals for events at different energies. A linear anticorrelation fit is applied to obtain the g1 and g2 values. This plot is for the central fiducial volume (FV#4) at 366  V/cm. The energy spectrum of the NR component and the 39.6 and 80.2 keV gamma lines from inelastic scattering are obtained from the Geant4 simulation and converted to photons and electrons separately, according to the NR and ER models in NEST. We then subtracted the average charge/light yields from the NR component to obtain the correct yields for the two pure gamma lines. The 41.5 keV events from Kr83m are from the combination of two transitions (32.1 and 9.4 keV) [30].

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    • Figure 4
      Figure 4

      The g1 and g2 values for seven FVs (see FV numbering in Fig. 2) at three different field configurations. The g1 values depend only on the detector geometry and PMT quantum/collection efficiency, and they are consistent at the three different fields. The g1 values increase toward the lower part of the detector. The g2 values depend on the liquid purity (electron lifetime) and also on the electron extraction efficiency, thus show lower values at a lower extraction field. The Eex values are obtained based on a 2D finite element simulation in COMSOL, with an uncertainty of 0.1  kV/cm due to the uncertainty of the liquid level. Using a parallel plate approximation will result in extraction fields 0.3  kV/cm higher than those from simulation. The electron extraction efficiency across the liquid-gas interface, calculated based on the ratio between g2 and the single electron gas gain, is 96±2% for a Va of 4.4 kV (Eex=9.7  kV/cm), and is about 84±6% at 3.6 kV (Eex=7.9  kV/cm).

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    • Figure 5
      Figure 5

      The signal detection efficiencies in this study for subvolume 1, 4, 7 as a function of recoil energy are shown in blue, purple and red, respectively. The inset shows the signal detection efficiency from the S1 coincidence requirement (black), which is the dominant contribution to the overall detection efficiency and is a function of detected photon number. The shaded regions represent the 15.4%–84.6% credible region.

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    • Figure 6
      Figure 6

      The comparison between the MC fit result (left-middle) and the data (left-bottom), in the form of the 2-D distribution in Log10(S2/S1) vs S1 parameter space for FV#4 at 366  V/cm field. S1 spectra (top-left panel) and S2 spectra at different S1 slices (right panels) are shown together with the 15.4%–84.6% credible region from the fit in red. A goodness-of-fit test using the method in [39] is performed upon the S2 spectra matching, which gives a p-value of 0.01, 0.16, 0.10, 0.37, 0.38 for the S2 spectra in S1 slices of 0–10 PE, 10–20 PE, 20–30 PE, 30–40 PE, 40–50 PE, respectively. The low p-value for the S2 spectrum matching in S1 slice of 0–10 PE is caused by the uncertainty of acceptance modeling.

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    • Figure 7
      Figure 7

      The best estimates for photon yields ⟨nph⟩/E, charge yields ⟨ne⟩/E, and mean recombination fraction ⟨r⟩ as a function of deposited energy obtained from the fit are shown in the (a–c), (d–f), and (g–i), respectively, for three drift fields. The solid lines represent the mean values and the shaded regions indicate the 15.4% to 84.6% credible regions of ⟨nph⟩/E, ⟨ne⟩/E and ⟨r⟩. The dot-dashed lines indicate the fitting uncertainties. Predictions from NEST v0.98 [25] (dashed blue lines) and measurements from LUX [23] (red solid lines and shaded regions) are shown for comparison where available.

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    • Figure 8
      Figure 8

      The best estimate for the recombination fluctuation Δr as a function of deposited energy. The panels (a), (b) and (c) show the recombination fluctuations for the three drift fields scanned in the study. The solid lines represent the mean values and the shaded regions indicate the 15.4% to 84.6% credible regions of Δr. The dot-dashed lines indicate the fitting uncertainties. Measured Δr values at 180  V/cm from LUX are shown in panel (b) in red solid (mean) and shaded region (uncertainty).

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    • Figure 9
      Figure 9

      (Top) ER (red) and NR (gray) bands from the CH3T and AmBe241 data at 366  V/cm in FV#7, which has the largest g1 value among the seven FVs shown in Fig 2. The S2 signal is corrected for the electron lifetime. The mean and ±2σ values of the ER band and the median of the NR band are fit by a power law plus a first order polynomial. (Bottom) The ER leakage fractions obtained by counting the number of events below the NR median, divided by the total number of ER events in each bin, for three different FVs from the top to the bottom of the detector at 366  V/cm drift field. The equivalent nuclear recoil energy is calculated based on the S1 signal following the method in [26].

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    • Figure 10
      Figure 10

      The total ER leakage below the nuclear recoil median for different photon detection efficiencies at three measured drift fields. We chose events with S1 corresponding to between 100 and 400 photons generated in liquid xenon. Such a range gives nuclear recoil equivalent energies between approximately 11 and 34 keVnr.A smaller ER leakage fraction is observed at a higher photon detection efficiency. The impact of the drift fields on the ER rejection is insignificant for the three fields reported here.

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