<P> The bomb was dropped at approximately 08: 15 (JST) 6 August 1945 . After falling for 44.4 seconds, the time and barometric triggers started the firing mechanism . The detonation happened at an altitude of 1,968 ± 50 feet (600 ± 15 m). It was less powerful than the Fat Man, which was dropped on Nagasaki, but the damage and the number of victims at Hiroshima were much higher, as Hiroshima was on flat terrain, while the hypocenter of Nagasaki lay in a small valley . According to figures published in 1945, 66,000 people were killed as a direct result of the Hiroshima blast, and 69,000 were injured to varying degrees . Of those deaths, 20,000 were members of the Imperial Japanese Army . </P> <P> The exact measurement of the yield was problematic since the weapon had never been tested . President Harry S. Truman officially announced that the yield was 20 kilotons of TNT (84 TJ). This was based on Parsons's visual assessment that the blast was greater than what he had seen at the Trinity nuclear test . Since that had been estimated at 18 kilotons of TNT (75 TJ), speech writers rounded up to 20 kilotons . Further discussion was then suppressed, for fear of lessening the impact of the bomb on the Japanese . Data had been collected by Luis Alvarez, Harold Agnew, and Lawrence H. Johnston on the instrument plane, The Great Artiste, but this was not used to calculate the yield at the time . </P> <P> After hostilities ended, a survey team from the Manhattan Project that included William Penney, Robert Serber, and George T. Reynolds was sent to Hiroshima to evaluate the effects of the blast . From evaluating the effects on objects and structures, Penney concluded that the yield was 12 ± 1 kilotons . Later calculations based on charring pointed to a yield of 13 to 14 kilotons . In 1953, Frederick Reines calculated the yield as 13 kilotons . This figure became the official yield . </P> <P> In 1962, scientists at Los Alamos created a mockup of Little Boy known as "Project Ichiban" in order to answer some of the unanswered questions, but it failed to clear up all the issues . In 1982, Los Alamos created a replica Little Boy from the original drawings and specifications . This was then tested with enriched uranium but in a safe configuration that would not cause a nuclear explosion . A hydraulic lift was used to move the projectile, and experiments were run to assess neutron emission . Based on this and the data from The Great Artiste, the yield was estimated at 16.6 ± 0.3 kilotons . After considering many estimation methods, a 1985 report concluded that the yield was 15 kilotons ± 20% . </P>

The 2 radioactive elements that were chosen for the atomic bomb were