[Paper Review] Electron train backgrounds in liquid xenon dark matter search detectors are indeed due to thermalization and trapping

Peter Sørensen|arXiv (Cornell University)|Feb 15, 2017

Dark Matter and Cosmic Phenomena8 citations

TL;DR

This paper demonstrates that electron train backgrounds in liquid xenon dark matter detectors originate from thermalized and trapped electrons at the liquid-gas interface, with a calculated barrier height of $\phi_b = 0.34 \pm 0.01$ eV. The model achieves quantitative agreement with experimental data, predicts a trap lifetime of $\mathcal{O}(10)$ ms, and suggests mitigation strategies such as high-voltage switching or LED stimulation to suppress the background for low-mass and dark sector dark matter searches.

ABSTRACT

Electron emission from liquid into gaseous xenon is a cornerstone of dark matter search detectors such as ZEPLIN, XENON, LUX and LZ. The probability of emission is a function of the applied electric field E, and electrons which fail to pass from the liquid into the gas have been previously hypothesized to become thermalized and trapped. This article shows, for the first time, quantitative agreement between an electron emission model and existing data. The model predicts that electrons in the liquid must surmount a typical potential barrier phi_b=0.34+-0.01 eV in order to escape into the gas. This value is a factor of about x2 smaller than has previously been calculated or inferred. Knowledge of phi_b allows calculation of the lifetime of thermalized, trapped electrons. The value is O(10) ms, which appears to be compatible with XENON10 observations of electron train backgrounds. As these backgrounds limit the sensitivity of dark sector dark matter searches, possible mitigations are discussed.

Motivation & Objective

• To resolve the long-standing uncertainty about the origin of electron train backgrounds in liquid xenon dark matter detectors.
• To test the hypothesis that unemitted electrons become thermally trapped at the liquid-gas interface and later emit after a delay.
• To quantify the electron emission efficiency and barrier height $\phi_b$ using absolute experimental data.
• To calculate the lifetime of trapped electrons and assess its compatibility with observed XENON10 data.
• To propose practical mitigation strategies to suppress electron train backgrounds in future S2-only dark matter searches.

Proposed method

• A Schottky barrier model is applied to fit absolute electron emission efficiency data from Gushchin et al. (1982), using $\phi_b$ as a free parameter.
• The model calculates the probability of electron emission as a function of applied electric field $E$, with $\kappa$ representing emission efficiency.
• The lifetime of trapped electrons is derived from the energy barrier $\phi_b$ using thermal activation theory, assuming a typical energy distribution and escape rate.
• The model is validated by comparing predicted electron train rates with observed XENON10 data, showing good agreement.
• Mitigation strategies are evaluated using theoretical modeling: high-voltage switching to recapture trapped electrons and LED stimulation with 940 nm photons to assist electron emission.
• An analysis framework is proposed for off-line background suppression using time-structure measurements of single-electron emission ($\lambda$) across the detector.

Experimental results

Research questions

• RQ1 What is the true value of the potential barrier $\phi_b$ that electrons must overcome to escape from liquid xenon into the gas phase?
• RQ2 Is the hypothesis that electron train backgrounds arise from thermally activated emission of trapped electrons consistent with existing experimental data?
• RQ3 What is the predicted lifetime of thermally trapped electrons in liquid xenon, and does it match observed timescales in XENON10?
• RQ4 Can the electron train background be quantitatively modeled and suppressed using hardware or analysis-based mitigation techniques?
• RQ5 To what extent can future S2-only dark matter searches remain viable despite this background?

Key findings

• The electron emission model achieves quantitative agreement with absolute experimental data, yielding a barrier height of $\phi_b = 0.34 \pm 0.01$ eV, which is approximately half of previously estimated values.
• The calculated lifetime of thermally trapped electrons is $\mathcal{O}(10)$ ms, consistent with the observed duration of electron train backgrounds in XENON10.
• The emission efficiency $\kappa$ reaches $\sim 99.9\%$ at $E \approx 7$ kV/cm, suggesting a potential $\times 50$ suppression of electron train backgrounds if such fields are achieved.
• High-voltage switching on a $\mu$s timescale could recapture and re-emit trapped electrons, offering a hardware-based mitigation path.
• LED stimulation at 940 nm (1.3 eV per photon) could assist trapped electrons in overcoming $\phi_b$, with photomultipliers likely insensitive to these photons, enabling selective excitation.
• Off-line analysis using time-structure measurements of single-electron emission ($\lambda$) across the detector could enable background suppression even if hardware solutions fail.