15-24 April 1943
OUTLINE OF PRESENT KNOWLEDGE
[J. Robert] Oppenheimer
Materials and Schedules: . . . The isotope 25 [235U] will support a chain reaction because neutrons of all energies can cause fission in it and because there are no known competing processes. . . It has been shown that there is no appreciable fraction of neutrons delayed by more than 10–5 sec. It (25) is being produced in two ways.
Lawrence's group [Berkeley] is separating the isotope 25 by mass spectrographic means. It is planned to have 500 tanks of two each installed by January 1, 1944. [t is expected that each arc will give 100 milliamps of 28 [238U] and 3 milliamps of enriched beam.
Urey's group is separating 25 by a diffusion process [Columbia University]. . .
The element 49 [239Pu] is produced from 28 by the absorption of neutrons. The material is to be produced on a large scale by the Chicago pile. 300 gms per day is hoped for by Jan. 1945.
Isotope 23 [233U] can be produced by putting thorium around a pile. The yield is small, 5% of 49, for a carbon pile. The yield would be 20% for a deuterium pile.
Energy Release: The destructive effect of the gadget is due to radiative effects and the shock wave generated by the explosion. . The shock wave effect seems to extend over the biggest area and would be, therefore, most important. The area devastated by the shock wave is proportional to the 2/3 power of the energy release and may be simply calculated by comparing the energy release with that of TNT. If the reaction would go to completion, then 50 kg of 25 would be equivalent to 10 tons of TNT. Actually it is very difficult to obtain a large percentage of the potential energy release.
Detonation: The second major difficulty facing us is connected with the question of detonation. . It is important that no neutron should start a premature chain reaction. . . Possible sources of neutrons are 1) Cosmic ray neutrons . . . and 2) Spontaneous fission neutrons. . .
EXPERIMENT RESULTS AND DESCRIPTION OF AVAILABLE EQUIPMENT
Experimental Nuclear Research Facilities: . . . We shall have a cyclotron, obtained from Harvard, which should give us about 50 µa of 10 MeV deuterons. . . .
Two pressure Van de Graaffs have been obtained from Wisconsin. . . .
Illinois has loaned us a Cockcroft-Walton outfit which when used as a D-D [deuteron-deuteron] source, delivers 300 µa of 0.3 MeV deuterons producing some 108 n/see.
Neutrons may also be produced from chain reactions. Fermi's pile operates conveniently at 100 watts, at which power it gives 1013 n/see or about 5 x 105 n per sec per cm2. These include both fast and thermal neutrons. . . .
The natural source situation is not completely clear. but we are obtaining from Chicago the following sources: 200 mc pressed Ra-Be mixed source, yielding 2 x 106 n/see; 500 mc RdTh for a photo source which should yield about 5 x 106 n/see of .9 MeV with Be; 2000 mc pressed Ra-B mixed source, yielding about 5 x 106 n/see. . .
THE CHAIN-REACTING PILE
The first chain reacting pile was built in the fall of 1942. It contained 6 tons of metal, 40 tons of oxide, and 400 tons of graphite. The shape was a sphere of 26' diameter with the best materials in the center. . . . This first chain reaction was obtained on December 2, 1942. . . .
The present chain reacting pile is designed for convenient performance of experiments. Its dimensions are 20' x 22' x 18' and it has a removable 33" section in the center. It is shielded to a factor 104 - 105 by a 5' concrete wall. On top, a 6' graphite column for a source of thermal neutrons projects through the shield.
The pile has two types of uses. First it is a relatively intense and very stable source of neutrons. The intensity can be controlled within 0.1%. . . .
The other main use of the pile is to measure changes in the critical position of the control rod due to insertion of various materials in the pile. This is especially useful for rapid determination of absorption cross sections.
EXPECTED DAMAGE OF THE GADGET
Comparison with TNT: The most striking difference between the gadget and a TNT charge is in the temperatures generated. The latter yields temperatures of a few thousand degrees whereas the former pushes the temperature as high as [tens of millions of degrees]. . . .
The actual damage depends much on the objective. Houses begin to be smashed under shocks of 1/10 to 1/5 of an atmosphere. For objects such as steel supported buildings and machinery, greater pressures are required and the duration of the shock is very important. If the duration of the pressure pulse is smaller than the natural vibration period of the structure, the integral of the pressure over the duration T of the impulse is significant for the damage. If the pulse lasts for several vibration periods. the peak pressure is the important quantity. . . .
Other Damage: The neutrons emitted from the gadget will diffuse through the air over a distance of 1 to 2 km, nearly independent of the energy release. Over this region, their intensity will be sufficient to kill a person,
The effect of the radioactive fission products depends entirely on the distance to which they are carried by the wind. If 1 kg of fission products is distributed uniformly over an area of about 100 square miles, the radioactivity during the first day will represent a lethal dose (=500 R units): after a few days, only about 10 R units per day are emitted, If the material is more widely distributed by the wind, the effects of the radioactivity will be relatively minor.