 As our nation entered the 1930s, the Empire State Building opened in New York City, Franklin Roosevelt became President, and Prohibition was repealed. In science, Ernest Lawrence invented the cyclotron, a charged particle accelerator, and received the Nobel Prize for it in 1939. Frederick Joliet Curie and Irene Joliet Curie were the first to report that they had artificially produced radioactive material after discovering radioactive phosphorus in an aluminum foil that was irradiated with alpha particles. Two years later, John Lawrence was the first to use an artificially produced radionuclide for a medical application in leukemia patients. He used Phosphorus 32 that was created in his brother's cyclotron for these treatments. In Health Physics, the American physicist Robley Evans was the first to measure exhaled radon, radium excretion, and radium body content of former radium dial painters. The National Bureau of Standards used this information to establish the first official radiation exposure standard, called a tolerance level for radium, and was set at one-tenth of a microgram. Over the next decades, more than 2,000 former radium dial painters would be studied making it one of the largest living cohorts of exposed individuals. National and international organizations continued to develop scientifically-based radiation protection guidelines. By 1934, the first radiation dose limit was suggested, a tolerance dose limit of two-tenths of a rentgen per day, or 25 rentgens per year. At this level, a person could be continuously exposed to radiation and develop no observable health effects or harm. As the decade ended, the splitting of an atom was first observed. One year later, Lisa Meitner and Otto Frisch theoretically explained the process and named it Fission by analogy with the biological fissioning of living cells. At the same time, a research and development project to produce the world's first nuclear weapons, dubbed the Manhattan Project, began.