  We describe a novel gaseous detector concept for heavy-ion tracking and
imaging: the Optical Parallel-Plate Avalanche Counter (O-PPAC). The detector
consists of two thin parallel-plate electrodes separated by a small (typically
3 mm) gap filled with low-pressure scintillating gas (i.e. CF4). The
localization of the impinging particles is achieved by recording the secondary
scintillation, created during the avalanche processes within the gas gap, with
dedicated position-sensitive optical readouts. The latter may comprise arrays
of collimated photo-sensors (e.g. SiPMs) that surround the PPAC effective area.
We present a systematic Monte Carlo simulation study used to optimize the
geometry of the OPPAC components, including SiPMs effective area, collimator
dimensions, and operational conditions. It was found that the optimal design
for 10x10 cm^2 OPPAC detector comprises four arrays, each of them counting a
total of 15-20 individual photo-sensors. This configuration provides a
localization capability with a resolution below 1 mm and good response
uniformity. An experimental investigation successfully demonstrated the proof
of principle stage of an O-PPAC prototype equipped with a single array of 10
photo-sensors, separated by 6 mm. The performance of the prototype was
investigated with an LED light, under 10-,12-C beam irradiation, and with a
low-intensity 241-Am alpha-particle source. The experimental data obtained with
the prototype is compared to the results obtained by systematic Monte Carlo
simulations.
