Puzzling time properties of proportional electroluminescence in two-phase argon detectors for dark matter searches

TL;DR

This study reveals unusual slow components in the electroluminescence signals of two-phase argon detectors, with properties that challenge existing understanding and suggest electron trapping on metastable ions as a possible explanation.

Contribution

First systematic investigation of time properties of electroluminescence in two-phase argon, discovering slow components with unique threshold and temperature dependence.

Findings

Two slow EL components with time constants of 4-5 μs and 50 μs were observed.

Slow components emerge at a specific reduced electric field threshold of 4.8 ± 0.2 Td.

The contribution of slow components decreases with increasing temperature, nearly disappearing at room temperature.

Abstract

Proportional electroluminescence (EL) in noble gases is a physical process routinely used in two-phase (liquid-gas) detectors for low-energy astroparticle-physics experiments. In this work, the time properties of visible-light EL in two-phase argon detectors have been systematically studied for the first time. In particular, two unusual slow components in the EL signal, with their contributions and time constants increasing with electric field, were observed. This puzzling property is not expected in any of the known mechanisms of photon and electron emission in two-phase media. Time constants of these components is about 4-5 $\mu$s and 50 $\mu$s. In addition, a specific threshold behavior of the slow components was revealed: they emerged at a threshold in reduced electric field of 4.8 $\pm$ 0.2 Td regardless of the gas phase density, which is about 1 Td above the onset of standard (excimer) EL. There is a conspicuous similarity between this threshold and reduced field threshold of EL in NIR occurring via higher atomic excited states Ar$^{*}(3p^{5}4p)$. An unexpected temperature dependence of slow components was also observed: their contribution decreased with temperature, practically disappearing at room temperature. We show that the puzzling properties of slow components can be explained in the framework of hypothesis that these are produced in the charge signal itself due to trapping of drifting electrons on metastable negative argon ions.