Mark 1 / Little Boy
The minimum amout of fissionable material required to sustain a fission chain reaction is called a critical mass. The fissionable material in the atomic bomb dropped on Hiroshima was uranium 235. This U-235 was kept in two pieces, each smaller than a critical mass. To trigger the bomb, a conventional explosive forced the two pieces together, instantly creating more than a critical mass. This was called a gun-barrel bomb.
In addition to calculations on uranium and plutonium fission, chain reactions, and critical and effective masses, work needed to be done on the ordnance aspects of the bomb, or the "Gadget" as it came to be known. Two subcritical masses of fissionable material would have to come together to form a supercritical mass for an explosion to occur. Furthermore, they had to come together in a precise manner and at high speed.
Measures also had to be taken to ensure that the highly unstable subcritical masses did not predetonate because of spontaneously emitted neutrons or neutrons produced by alpha particles reacting with lightweight impurities. The chances of predetonation could be reduced by purification of the fissionable material and by using a high-speed firing system capable of achieving velocities of 3,000 feet per second. A conventional artillery method of firing one subcritical mass into the other (above) was under consideration for uranium-235, but this method would work for plutonium only if absolute purification of plutonium could be achieved.
Early experiments on both uranium and plutonium provided welcome results. Uranium emitted neutrons in less than a billionth of a second -- just enough time, in the world of nuclear physics, for an efficient explosion. Emilio Segre later provided an additional cushion with his discovery in December 1943 that, if cosmic rays were eliminated, the subcritical uranium masses would not have to be brought together as quickly as previously thought; nor would the uranium have to be as pure. Muzzle velocity for the scaled-down artillery piece could be lower, and the gun could be shorter and lighter.
Segre's tests on the first samples of plutonium demonstrated that plutonium emitted even more neutrons than uranium due to the spontaneous fission of plutoniurn-240. Both theory and experimental data now agreed that a bomb using either element would detonate if it could be designed and fabricated into the correct size and shape. But many details remained to be worked out, including calculations to determine how much uranium-235 or plutonium would be needed for an explosive device.
An experiment to determine the cross section of uranium-235 for fast neutrons. The target is the small pile of cubes of uranium hydride. The uranium target is surrounded by larger blocks of beryllium tamper. Bacher's experimental physics division patiently generated the essential cross section measurements needed to calculate critical and efficient mass. The same group utilized particle accelerators to produce the large numbers of neutrons needed for its cross section experiments. Bacher's group also compiled data that helped identify tamper materials that would most effectively push neutrons back to the core and enhance the efficiency of the explosion. Despite Los Alamos's postwar reputation as a mysterious retreat where brilliant scientists performed miracles of nuclear physics, much of the work that led to the atomic bombs was extremely tedious.
The chemists' job was to purify the uranium-235 and plutonium, reduce them to metals, and process the tamper material. Only highly purified uranium and plutonium would be safe from predetonation. Fortunately, purification standards for uranium were relatively modest, and the chemical division was able to focus its effort on the lesser known plutonium and make substantial progress on a multi-step precipitation process by summer 1944. The metallurgy division had to turn the purified uranium-235 and plutonium into metal. Here, too, significant progress was made by summer as the metallurgists adapted a stationary-bomb technique initially developed at Iowa State College.
In March 1944, two uranium guns were ordered. Field tests performed with uranium-235 prototypes in late 1944 eased doubts about the gun-type method to be employed in the uranium bomb. It was clear that the uranium-235 from Oak Ridge could be used in a gun-type nuclear device to meet the 01 August 1945 deadline Groves had given General Marshall and the Joint Chiefs of Staff. The plutonium produced at such expense and effort at Hanford (right), however, would not fit into wartime planning unless a breakthrough in implosion technology could be found. Weapon design for the uranium gun-type bomb was frozen in February 1945. Confidence in the weapon was high enough that a test prior to combat use was seen as unnecessary.
As a result of progress at Oak Ridge and metallurgical and chemical refinements on plutonium that improved implosion's chances, the nine months between July 1944 and April 1945 saw the American bomb project progress from doubtful to probable. The August 1 delivery date for the "Little Boy" uranium bomb certainly appeared more likely than it had when Groves briefed George Marshall. There would be no implosion weapons in the first half of 1945 as Groves had hoped, but developments in April boded well for the scheduled summer test of the "Fat Man" plutonium bomb. And new calculations provided by Hans Bethe's theoretical group gave hope that the yield for the first weapon would be in the vicinity of 5,000 tons of TNT rather than the 1,000-ton estimate provided in fall 1944.
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