Dempster, Arthur

At a meeting of the American Physical Society at Columbia University on February 17, 1939, they advanced a theory of uranium fission which postulated that not all the uranium employed as target actually fissioned. They believed that less than one percent of their uranium target disintegrated because only one of the three isotopes of uranium was actually capable of fission. This fissionable isotope first discovered in 1935 by Arthur Dempster of the University of Chicago, has an atomic weight of 235 instead of 238 which is the atomic weight of 99.3% of the uranium mixture found in nature. U-238 is extremely stable its half-life has been estimated to be four billion years. 

Arthur Dempster at the University of Chicago discovers uranium-235 isotope. 

Most elements (but not all) could be divided into isotopes in this manner. In 1935, the Canadian-American physicist Arthur Jeffrey Dempster (1886-1950) found, for instance, that uranium, as it occurred in nature, was a mixture of two isotopes even though its atomic weight (238.07) was close to a whole number. It was just that one isotope existed in overwhelming proportion. Fully 99.3 per cent of the uranium atoms had nuclei made up of 92 protons and 146 neutrons or a total mass number of 238. These were uranium-238 atoms. The remaining 0.7 per cent, however, had three fewer neutrons and were uranium-235 atoms. 

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