The continued releases of radioactive material from the earthquake-damaged Fukushima Dai-ichi nuclear power station in Japan, with the risks to water, coastal environments, agricultural land, animals and human health have drawn international concern. The incident, together with the Chernobyl disaster a generation earlier, has highlighted the importance of being able to detect, measure and monitor radiation in our environment. This is no easy challenge - the amounts of radioactivity are often low (relative to controlled medical or industrial settings) or highly dispersed through soils, sediments and water. There is also a considerable background radiation all around us, not only from the legacy of human nuclear technology but from natural minerals, gases (eg. radon, a major problem in some regions), cosmic and solar sources. On the other hand, this radioactivity is used widely by earth and environmental scientists to date rocks, monitor sediment movement and geomorphological changes, or the growth rates and life histories of plants and animals. If we are to measure environmental radioactivity, not just to help clean-up and recovery after an accidental release but also to monitor sites, prevent releases and support the safe operation and decommissioning of nuclear facilities (as well as support that range of scientific research needs), then we need continuous improvement of sensors which can detect and quantify radiation sources to higher resolution, lower detection thresholds and shorter measurement times. The current generation of sensors is based on mechanical collimators, a technology similar to the 'pixellated' image sensors in digital cameras, in which the radiation arriving at any point on the surface is used to build up a 2D image of the radiation source. Nuclear physicists at the University of Liverpool have recently developed a new approach for detection of gamma radiation called Compton-geometry imaging. In this approach, two sensors are placed one in front of the other and the measurement is based on the scattering of radiation between them. The technique is powerful because the position of the radiation source is located by mathematically reconstructing the origin of many scattering events, rather than by the physical position of the incident radiation on the collimator surface. This 'electronic' collimation can resolve the position of the source with much greater accuracy and sensitivity than mechanical collimation, has the advantage of being able to locate the source in 3D, and yields smaller, lighter detector equipment with potential savings in measurement time. Currently, only two other research groups in the world are working with this technology. The objective of this proposal is to understand how this powerful new technology can be optimised for environmental gamma radioactivity measurements. Research so far has focused on the development of prototype Compton cameras for industrial and medical applications, which present very different challenges to the environmental conditions described earlier. By combining a world leading expertise in device development in close collaboration with academic and industry end-users in environmental science and engineering, this Technology Proof-of-Concept proposal aims to develop design criteria, optimised system specifications, and a first prototype for a Compton camera which we intend will set a benchmark for the next generation of environmental radioactivity sensors. Imagine being able to locate a radioactive substance beneath the ground and monitor how it moves with changes in water flow or sediment movement. Or to watch, using a portable device, in real-time how plants and animals take up radioactive materials from contaminated soils and move them into the food chain. Star Trek science? Perhaps for now, but the environmental Compton camera that is the long-term goal of this research project moves us a significant step closer towards that vision.