New measurement of the relative scintillation efficiency of xenon nuclear recoils below 10 keV

Liquid xenon is an important detection medium in direct dark matter experiments, which search for low-energy nuclear recoils produced by the elastic scattering of WIMPs with quarks. The two existing measurements of the relative scintillation efficiency of nuclear recoils below 20 keV lead to inconsistent extrapolations at lower energies. This results in a different energy scale and thus sensitivity reach of liquid xenon dark matter detectors. We report a new measurement of the relative scintillation efficiency below 10 keV performed with a liquid xenon scintillation detector, optimized for maximum light collection. Greater than 95% of the interior surface of this detector was instrumented with photomultiplier tubes, giving a scintillation yield of 19.6 photoelectrons/keV electron equivalent for 122-keV $\gamma$ rays. We find that the relative scintillation efficiency for nuclear recoils of 5 keV is 0.14, staying constant around this value up to 10 keV. For higher energy recoils we measure a value of 0.21, consistent with previously reported data. In light of this new measurement, the XENON10 experiment's upper limits on spin-independent WIMP-nucleon cross section, which were calculated assuming a constant 0.19 relative scintillation efficiency, change from $8.8\times {10}^{-44}\hspace{0.5em}{\mathrm{cm}}^2$ to $9.9\times {10}^{-44}\hspace{0.5em}{\mathrm{cm}}^2$ for WIMPs of mass 100 GeV/$c^2$, and from $4.5\times {10}^{-44}\hspace{0.5em}{\mathrm{cm}}^2$ to $5.6\times {10}^{-44}\hspace{0.5em}{\mathrm{cm}}^2$ for WIMPs of mass 30 GeV/$c^2$.

Figure 1

Schematic diagram of the experimental setup. Incoming 1-MeV neutrons scatter in the LXe detector and are tagged by the EJ301 neutron scintillator for different scattering angles of ${48}^{°}$, ${62}^{°}$, $70.5^{°}$, and $109.5^{°}$. The paraffin and lead are used to shield the EJ301 neutron scintillator from direct neutrons and $\gamma$ rays.

Figure 2

Schematic diagram of the LXe detector used for the ${\mathcal{L}}_{\mathrm{eff}}$ measurement. Visible are four of the six PMTs used to view the 1-in.${}^3$ active LXe volume.

Figure 3

The scintillation light spectrum of 122-keV $\gamma$ rays from ${}^{57}\mathrm{Co}$, used to calibrate the electronic recoil energy scale. This calibration gives a scintillation yield of 19.64 p.e./keV.

Figure 4

The distribution of triggered events in PSD vs. TOF space from the data set at $70.5^{°}$. An “upper” band and “lower” band are readily identifiable in the data and correspond to nuclear recoils and electronic recoils, respectively. The peak at the lower left near $\mathrm{TOF}=0$, due to $\gamma$ rays that Compton scatter in the LXe before striking the EJ301, is easily vetoed by the PSD cut. A population of accidental triggers (see text) having a flat TOF spectrum is visible in both bands and contributes background events within the neutron peak. The LXe spectra of events within the left box are used as the expectations of this background. The width of the right box (10 ns) is chosen to accept neutrons that interact in any region of the finitely sized detectors.

Figure 5

Selected results of the Monte Carlo simulations, which do not include the accidentals background. (a) The spectrum of events tagged at $70.5^{°}$, scaled with the measured value of ${\mathcal{L}}_{\mathrm{eff}}$ giving the electron-equivalent energy (keVee), convoluted assuming Poisson statistics for the number of p.e. and multiplied by the simulated trigger efficiency curve. The green histogram is the total spectrum, and the black circles indicate the true materials background. The red dashed line is an exponential fit to the high-energy region of the green histogram; its agreement with the true materials background confirms the validity of this technique's use in the real data. The shaded blue area shows the spectrum of true elastic single scattered neutrons. (b) The spectrum of events tagged at $109.5^{°}$. The data are shown in the original, recoil equivalent energy scale (keVr) without Poisson convolution. The materials background in this region departs from the exponential behavior seen at lower energies and distorts the position of the peak from true single scatters, at 20 keV. The red dashed lines are the result of an exponential+Gaussian fit. The Gaussian component, centered at $22.94\pm 4.34$ keV, is used as the “true” energy of the Gaussian component in the real spectrum.

Figure 6

Energy spectrum measured at the four angles used in this study: (a) $-{48}^{°}$, (b) $-{62}^{°}$, (c) $-70.5^{°}$, and (d) $-109.5^{°}$. The data are shown after subtracting both accidentals and materials backgrounds. Error bars are the combined errors of the original spectra, accidentals, and materials background, included in the Gaussian fits indicated by the solid curves.

Figure 7

Measured ${\mathcal{L}}_{\mathrm{eff}}$ values as a function of Xe nuclear recoil energy. Symbols correspond to $(\circ )$ this work; $(\square )$ Chepel et al. [27]; $(▵)$ Aprile et al. [21]; $(\diamond )$ Akimov et al. [28]; (×) Bernabei et al. [29]; $(\nabla )$ Arneodo et al. [30]. The solid gray curve is the result from a recent best-fit analysis of XENON10 AmBe source data and MC [31]. Also shown is the theoretical prediction of Hitachi (dashed line) [16].

Figure 8

The upper limit on the WIMP-nucleon spin-independent cross section based on the 58.6 live days of XENON10's WIMP search, shown with a flat ${\mathcal{L}}_{\mathrm{eff}}=0.19$ (solid). An ${\mathcal{L}}_{\mathrm{eff}}$ function consistent with the results of this study, applied to the same XENON10 data is shown as well (dashed).