Oliphant took the Frisch–Peierls memorandum to Tizard. As a result, the MAUD Committee was established to investigate further. It directed an intensive research effort. Four universities provided the locations where the experiments were taking place. The University of Birmingham undertook theoretical work, such as determining what size of critical mass was needed for an explosion. This group was run by Peierls, with the help of fellow German refugee scientist Klaus Fuchs. The laboratories at the University of Liverpool and the University of Oxford experimented with different types of isotope separation. Chadwick's group at Liverpool dealt with thermal diffusion, a phenomenon observed in mixtures of mobile particles where the different particle types exhibit different responses to the force of a temperature gradient. Francis Simon's group at Oxford investigated the gaseous diffusion, which works on the principle that at differing pressures uranium 235 would diffuse through a barrier faster than uranium 238. This was determined to be the most promising method. Egon Bretscher and Norman Feather's group at Cambridge investigated whether another element, now called plutonium, could be used as a fissile material. Because of the presence of a team of refugee French scientists led by Hans von Halban, Oxford also had the world's main supply of heavy water, which helped them theorise how uranium could be used for power.