  As part of the R2D2 (Rare Decays with Radial Detector) R&D, the use of a gas
detector with a spherical or cylindrical cathode, equipped with a single anode
and operating at high pressure, was studied for the search of rare phenomena
such as neutrinoless double-beta decay. The presented measurements were
obtained with a cylindrical detector, covering gas pressures ranging from 1 to
10 bar in argon and 1 to 6 bar in xenon, using both a point-like source of
$^{210}$Po (5.3 MeV $\alpha$ ) and a diffuse source of $^{222}$Rn (5.5 MeV
$\alpha$). Analysis and interpretation of the data were developed using the
anodic current waveform. Similar detection performances were achieved with both
gases, and comparable energy resolutions were measured with both sources. As
long as the purity of the gas was sufficient, no significant degradation of the
measured energy was observed by increasing the pressure. At the highest
operating pressure, an energy resolution better than 1.5% full-width at
half-maximum (FWHM) was obtained for both gaseous media, although optimal noise
conditions were not reached.
