Nuclear Science for the Manhattan Project and Comparison to Today’s ENDF Data

Abstract

Nuclear physics advances in the US and Britain, from 1939-1945, are described. The Manhattan Project's work led to an explosion in our knowledge of nuclear science. A conference in April 1943 at Los Alamos provided a simple formula used to compute critical masses, and laid out the research program needed to determine the key nuclear constants. In short order, four university accelerators were disassembled and reassembled at Los Alamos, and methods were established to make measurements on extremely small samples owing to the initial lack of availability of enriched 235 U and plutonium. I trace the program that measured fission cross sections, fission emitted neutron multiplicities and their energy spectra, and transport cross sections, comparing the measurements with our best understanding today as embodied in the Evaluated Nuclear Data File ENDF/B-VIII.0. The large nuclear data uncertainties at the beginning of the project, which often exceeded 25-50%, were reduced by 1945 often to less than 5-10%. 235 U and 239 Pu fission cross section assessments in the fast MeV range were reduced with more accurate measurements, and the neutron multiplicity ν increased. By a lucky coincidence of canceling errors, the initial critical mass estimates were close to the final estimated masses. Some images from historical documents from our Los Alamos archives are shown. Many of the original measurements from these early years have not previously been widely available. Through this work, these data have now been archived in the international experimental nuclear reaction data library (EXFOR) in a collaboration with the IAEA and Brookhaven National Laboratory. ContentsI.

Criticality Experiments with Fast 235U and 239Pu Metal and Hydride Systems During the Manhattan Project

“…Additional information on nuclear data from the Manhattan Project is given in a different work in this same issue. 57…”

“…These critical mass estimates should also be compared against calculations of 25 critical mass, given in this issue. 57 The same approach can also be applied to the Lady Godiva experiment performed at Los Alamos in the 1950s. 8 Unlike the deeply subcritical spherical experiments discussed in this section, Lady Godiva was a bare sphere of HEU that was critical.…”

Neutronics Calculation Advances at Los Alamos: Manhattan Project to Monte Carlo

“…Early critical experiments were done using reflected geometries. 5,6 The critical radius was determined by plotting the inverse of the observed neutron multiplication as increasingly more material was added and extrapolating for the point of infinite multiplication. This paper discusses the various computational methods for solving the neutron transport equation during the Manhattan Project, the evolution of a new computational capability, and the invention of stochastic methods, focusing on Monte Carlo, and nuclear data.…”

The Taming of Plutonium: Plutonium Metallurgy and the Manhattan Project

“…The most basic properties of plutonium needed to inform a design of a functioning weapon were unknown beyond its half-life and a crude estimate of its nuclear cross section. 1,2 A functioning design would require knowledge of its density and compressibility. 3,4 Construction of a given design would require limits on impurities and the ability to construct a geometricallystable shape that would withstand the rigors of assembly, transport, and deployment.…”