Bohr argued that it was the uranium-235 isotope that was responsible for fission. Dunning realised that if this was the case, then an atomic bomb would be possible. His thoughts turned to devising a process for uranium enrichment, and by 1940 he was investigating gaseous diffusion, which he felt offered the best route to enrichment on an industrial scale. The researchers at Columbia became the Manhattan Project's Substitute Alloy Materials (SAM) Laboratories. Dunning headed the laboratory division responsible for all aspects of the gaseous diffusion program, including engineering problems, pilot plants and research activities. Four papers co-written with James Rainwater, William W. Havens, Jr., and Chien-Shiung Wu appeared in 1947 and 1948, but much remained classified. Due to the secrecy of this work, Dunning and three of his colleagues were awarded $300,000 each in lieu of patent royalties. For his part, Dunning was awarded the Medal for Merit by President Harry S. Truman. His citation read: