  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 increased. By a lucky coincidence of cancelling
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 Some images from historical documents from our Los Alamos archives are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 Through this work, these data have now been
archived in the international experimental nuclear reaction data library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.
