  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 $\mu$s and 50 $\mu$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.
