Investigation of background electron emission in the LUX detector

Abstract

Dual-phase xenon detectors, as currently used in direct detection dark matter experiments, have observed elevated rates of background electron events in the low energy region. While this background negatively impacts detector performance in various ways, its origins have only been partially studied. In this paper we report a systematic investigation of the electron pathologies observed in the LUX dark matter experiment. We characterize different electron populations based on their emission intensities and their correlations with preceding energy depositions in the detector. By studying the background under different experimental conditions, we identified the leading emission mechanisms, including photoionization and the photoelectric effect induced by the xenon luminescence, delayed emission of electrons trapped under the liquid surface, capture and release of drifting electrons by impurities, and grid electron emission. We discuss how these backgrounds can be mitigated in LUX and future xenon-based dark matter experiments.

A study of events with photoelectric emission in the DarkSide-50 liquid argon Time Projection Chamber

“…The hypothesis of photoelectric extraction in the liquid was also suggested by other investigators in relation to xenon detectors [10,25,26]. For instance, in Ref.…”

Constraints on effective field theory couplings using 311.2 days of LUX data

“…Data from the 14 C calibration was used to validate each criterion. Events with an overabundance of pulses preceding or following either the S1 or S2-such as single photons or single electrons emitted from the detector's grids or delayed releases from impurities [40]-were removed, as these events are more likely to have misidentified S1 or S2 signals. Data selection criteria are applied based on the S1 PMT hit-patterns as well as the shape of the S1 pulse; these remove events where S1s may originate from light leaking in from outside the TPC walls and misidentified S1s, respectively.…”

“…Radiogenic impurities in the bottom PMT array and RFR TPC walls-namely 238 U, 232 Th, 60 Co, 40 K and their daughters-may produce γ − X, in addition to back-scattering events originating from the cathode grid wires. Because these events appear superficially as normal single scatters, we are unable to obtain a set of known γ − X events.…”

Enhancing the sensitivity of the LUX-ZEPLIN (LZ) dark matter experiment to low energy signals

“…We consider also the impact of a higher rate of 10 mHz in our analysis. These additional events may be due to radioactivity from the grids and other detector materials near the top and bottom of the active volume, and from spurious emission of multi-electron clusters from gate and cathode [3,20,38]. In our treatment we allocate equal emission rates and spectra to the two grids for simplicity: although the gate has higher fields at the wire surfaces and more field-lines connecting to the extraction region, the cathode was not passivated to mitigate electron emission (although the cleanliness procedures were strict also in this case).…”

“…If the S1 pulse cannot be unambiguously tagged, then the event is classed as 'S2-only'. Sources of few-electron background signals contribute to the observed rate of S2-only events, especially those originating at the electrode grids-such as radioactive backgrounds where the scintillation is obscured and spurious electron emission from cathodic electrodes-or electron-train pileup [3]. In these events it is not possible to measure the ionization drift time and, hence, z-fiducialization is in principle lost.…”