We have characterised the radiation response of thick epitaxial GaAs layers that have been grown by water vapour-based chemical reaction, a new growth technique that has the potential to fabricate high quality thick epitaxial GaAs at low cost and with fast rates. The uniformity of these thick epitaxial layers has been assessed using room temperature photoluminescence mapping. The material shows very low EL2 concentrations and excellent whole-wafer uniformity, in contrast to conventional semi-insulating LEC or VGF grown bulk GaAs. Test detectors have been fabricated from the material and their response has been measured to alpha particle and low-energy gamma ray irradiation as a function of temperature. Pulse shape analysis of the signals produced by alpha particles show a fast and slow component, corresponding to charge transport in the depleted and under-depleted regions of the device. This indicates a shallow depletion layer due to non-intentional doping of the material from residual impurities. However, even outside the depletion region, good charge transport is observed in the material, which is consistent with long carrier lifetimes and a low EL2 concentration. CV measurements have been carried out in the temperature range +20°C to -55°C, which indicate a doping level of ND∼1×10 14 cm -3. The thickness of the depletion region was measured using CV with an applied bias of 20 V, and was found to be approximately 10 μm at room temperature, increasing to >30 μm at -55°C.