  A long term operation of Multi-Strip Multi-Gap Resistive Plate Chambers
(MSMGRPC) with gas mixtures based on C2H2F4 and SF6 leads to aging effects,
observed as depositions on the surface of the resistive electrodes. Moreover,
enhanced depositions and higher noise rates were evidenced around the nylon
spacers used for defining the gas gaps between the resistive electrodes. The
aging effects are reflected in an increase of the dark current and dark
counting rate, with negative impact on the long term performance of the chamber
and data volume in a free running readout mode operation. MSMGRPC prototypes
designed with a direct gas flow through the gas gaps and minimization of the
number of spacers in the active area were developed as mitigation solution.
Prototypes with this new design and different granularities were assembled
using fishing line as spacers and investigated for aging effects. Although a
significant reduction in the dark current and dark counting rate was evidenced,
dark counting rate localized around the fishing line spacers remains. In this
paper, a new generation of direct flow chambers based on discrete spacers is
presented. The results of their aging investigations show that, even at lower
gas flows, the aging effects become negligible.
