It is well known that synthetic diamond exhibits unique properties which make it an attractive material for a broad range of applications in the particle-detection field. In particular, thanks to its radiation hardness and availability in larger sizes, the polycrystalline (pc) diamond sample, grown by chemical vapour deposition (CVD), is especially suitable for online beam-monitoring applications, provided it demonstrates excellent counting performances. The present work reports the counting efficiency of a large 20.0×20.0×0.3 mm3 pcCVD diamond detector irradiated by 62 MeV/nucl. carbon beams. The parameter under study was estimated at different particle rates by employing a combination of different detector counters in a sandwich configuration. Results show a counting efficiency of ∼ 95% up to 0.7 MHz particle rate. The pcCVD diamond detector under study was selected among other prepared devices by studying their leakage current and photocurrent characteristics under X-ray irradiation.