In September 1941, Allison joined the S-1 Section, which coordinated the early investigations into the feasibility of an atomic bomb. He began building a reactor in the squash courts under the disused stands of Stagg Field. He became head of the Metallurgical Laboratory's chemistry section in January 1942, and in March, his small experimental reactor using beryllium came closer to criticality than the graphite-moderated design of Enrico Fermi's group at Columbia University. During 1942, Compton brought all the research groups working on plutonium and nuclear reactor design at Columbia University, Princeton University and the University of California together at the Metallurgical Laboratory in Chicago. Allison was placed in charge of the experimental work.