The Los Alamos Primer by Robert Serber;

Author:Robert Serber;
Language: eng
Format: epub
ISBN: 9780520344174
Publisher: University of California Press

Mc = 5 Kg for U tamper

Mc = 7.5 Kg for Au tamper.

These values of critical masses are still quite uncertain, particularly those for 49. To improve our estimates requires a better knowledge of the properties of bomb materials and tamper: neutron multiplication number, elastic and inelastic cross sections, overall experiments on tamper materials. Finally, however, when materials are available, the critical masses will have to be determined by actual test.

It was not until the early months of 1945 that Los Alamos received amounts of 25 and 49 comparable to their critical masses. The critical masses were determined by a series of “integral experiments.”6 Spheres of active material and U tamper were fabricated. In one series of experiments a neutron source was placed at the center of the core of active material. The number of neutrons emerging from the assembly was greater than if the source were there alone and the multiplication in the number of neutrons could be measured. As more material came in, larger and larger spheres were made. The larger the sphere the larger the multiplication. The critical radius was determined by extrapolating to infinite multiplication. In another series of experiments the cyclotron irradiated the spheres with a burst of fast neutrons and the rate of decay of the number of neutrons in the assembly was measured. The critical radius was found by extrapolating to zero decay rate.

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