Dunning remained in charge at Columbia until 1May 1943, when the Manhattan District took over the contract from OSRD. By this time Slack's group had nearly 50 members. His was the largest group, and it was working on the most challenging problem: the design of a suitable barrier through which the gas could diffuse. Another 30 scientists and technicians were working in five other groups. Henry A. Boorse was responsible for the pumps; Booth for the cascade test units. Libby handled chemistry, Nier analytical work and Hugh C. Paxton, engineering support. The Army reorganized the research effort at Columbia, which became the Special Alloyed Materials (SAM) Laboratories. Urey was put in charge, Dunning becoming head of one of its divisions. It would remain this way until 1March 1945, when the SAM Laboratories were taken over by Union Carbide.