  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.

Instrumentation and Detectors;Instrumentation and Methods for Astrophysics;High Energy Physics - Experiment