  The time-dependent gain variation of detectors incorporating Thick Gas
Electron Multipliers (THGEM) electrodes was studied in the context of
charging-up processes of the electrode's insulating surfaces. An experimental
study was performed to examine model-simulation results of the aforementioned
phenomena, under various experimental conditions. The results indicate that in
a stable detector's environment, the gain stabilization process is mainly
affected by the charging-up of the detector's insulating surfaces caused by the
avalanche charges. The charging-up is a transient effect, occurring during the
detector's initial operation period; it does not affect its long-term
operation. The experimental results are consistent with the outcome of
model-simulations.
