Unraveling the puzzle of slow components in gaseous argon of two-phase detectors for dark matter searches using Thick Gas Electron Multiplier

The effect of proportional electroluminescence (EL) is used to record the primary ionization signal (S2) in the gas phase of two-phase argon detectors for dark matter particle (WIMP) searches and low-energy neutrino experiments. Our previous studies of EL time properties revealed the presence of two unusual slow components in S2 signal of two-phase argon detector, with time constants of about 4-5 $μ$s and 50 $μ$s. The puzzle of slow components is that their time constants and contributions to the overall signal increase with electric field (starting from a certain threshold), which cannot be explained by any of the known mechanisms of photon and electron emission in two-phase media. There are indications that these slow components result from delayed electrons, temporarily trapped during their drift in the EL gap on metastable negative argon ions of yet unknown nature. In this work, this hypothesis is confirmed by studying the time properties of electroluminescence in a Thick Gas Electron Multiplier (THGEM) coupled to the EL gap of two-phase argon detector. In particular, an unusual slow component in EL signal, similar to that observed in the EL gap, was observed in THGEM itself. In addition, with the help of THGEM operated in electron multiplication mode, the slow component was observed directly in the charge signal, confirming the effect of trapped electrons in S2 signal. These results will help to unravel the puzzle of slow components in two-phase argon detectors and thus to understand the background in low-mass WIMP searches.