A detecting element which consists of a dense colloid suspension of nanoparticles is sensitive to ionising radiation both particulate and electromagnetic; at room temperature. The detector output is measured directly as a voltage and does not require intermediate photon detection. A large detecting surface area increases the probability of interaction between radiation and detector. The nanoparticle material does not need to be a semi-conductor but when exposed to ionisation radiation behaves basically the same as solid-state semiconductor detectors; Silicon, Germanium, Cadmium telluride or Cadmium zinc telluride (CZT). Ionising radiation causes a release of electrons. The electrons promoted into the conduction band are localized in the bulk or near the surface of the nanoparticle; no dopant or electrode bias is required. The spatial resolution of the detector depends only on the interval between electrodes of simple measuring array. It is possible to construct this array with micron or sub-micron spacing using conventional electronic manufacturing techniques. The existing prototype has an electrode interval (≈ 1mm) which is approximately the same as inter-pixel pitch in a typical CZT detectors. Many colloid materials are available which makes it possible to develop and modify the device easily, for example Gadolinium coating for high neutron capture. The device offers a cheaper alternative to existing detectors.