Booth, Dunning and von Grosse investigated the gaseous diffusion process. In 1941, they were joined by Francis G. Slack from Vanderbilt University and Willard F. Libby from the University of California. In July 1941, an Office of Scientific Research and Development (OSRD) contract was awarded to Columbia University to study gaseous diffusion. With the help of the mathematician Karl P. Cohen, they built a twelve-stage pilot gaseous diffusion plant at the Pupin Laboratories. Initial tests showed that the stages were not as efficient as the theory would suggest; they would need about 4,600 stages to enrich to 90 percent uranium-235.