At Birmingham, Oliphant's team had reached a different conclusion. Oliphant had delegated the task to Frisch and Rudolf Peierls, two German refugee scientists who could not work on Oliphant's radar project because they were enemy aliens, and therefore lacked the requisite security clearance. Francis Perrin had defined a critical mass of uranium to be the smallest amount that could sustain a chain reaction, and had calculated it to be about . He reckoned that if a neutron reflector were placed around it, this might be reduced to . In a theoretical paper written in 1939, Peierls attempted to simplify the problem by using the fast neutrons produced by fission, thus omitting consideration of a neutron moderator. He too believed the critical mass of a sphere of uranium to be "of the order of tons".