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

Abstract

Proportional electroluminescence (EL) in noble gases is the physical effect routinely used in twophase (liquid-gas) detectors for dark matter searches to record the primary ionization signal in the gas phase induced by particle scattering in the liquid phase. In this work, the time properties of visible-light EL in two-phase argon detectors have for the first time been systematically studied. In particular, two unusual slow components in the EL signal, with time constants of about 4-5 µs and 50 µs, were observed. Their puzzling property is that their contributions and time constants increase with electric field, which is not expected in any of the known mechanisms of photon and electron emission in two-phase media. In addition, a specific threshold behavior of the slow components was revealed: they emerged at a threshold in reduced electric field of about 5 Td regardless of the gas phase density, which is 1 Td above the onset of standard (excimer) EL. It is shown that this threshold is related to 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.Keywords two-phase detectors • liquid argon • dark matter • slow components • metastable negative argon ions