We investigated the performance of synthetic high purity monocrystalline diamond radiation detectors fabricated with TiW, Cr/Au and a novel metallization technique utilising diamond-like carbon tunnelling junction and Pt/Au as electrical contacts. The investigation was carried out under irradiation with 60Co gamma-rays, 90Sr electrons and 241Am alpha-particles. The experimental results with respect to I-V dark current levels, irradiation photocurrent, signal-to-noise ratio and time response have been compared and discussed. Results show an ohmic behaviour of the DLC/Pt/Au contact, a charge collection efficiency of 100% at an applied electric field of 0.377 V/ mum for all electrical contacts (under 90Sr electrons irradiation) at a correspondent dark current value of less than a picoAmpere. The single crystal CVD DLC/Pt/Au diamond radiation detector reported here shows spectroscopic energy resolution of 1.1% at an applied voltage of +450 volts (0.9 V/mum) under 5.5 MeV alpha particle irradiation. No dasiamemorypsila or dasiapumpingpsila effect was observed for the DLC/Pt/Au contact at positive bias; the signal was also stable (fluctuations below 0.5%) and reproducible under 60Co irradiation, with a signal-to-noise ratio > 10000:1 and a linearity in the dose rate response.