Through mid-1945 the Los Alamos Laboratory continued to pursue a strategy of overlapping approaches in its programs to test and refine critical mass and perform other calculations affecting bomb design and deployment. These activities included work in R-, G-, and F-Divisions with critical and subcritical assemblies, nuclear constant and other measurements by R-Division, and theoretical fine-tuning by T-Division, along with a backburner effort on the Super in F-Division. In addition, new projects were begun after mid-1944, among them a series of tests on critical assembly by G-Division, work on a high-power Water Boiler (nicknamed Hypo) to be used by F-Division for critical mass measurements, and the development of a spectacular assembly suggested by Frisch, which, unlike other experimental assemblies, went critical with prompt neutrons alone.

Besides diverting resources from implosion development and preparations for the Trinity test, these projects were sometimes quite dangerous and often risked the loss of precious fissionable materials. Nonetheless, they were mounted with considerable enthusiasm, and characteristically subjected to empirical testing. As had been the case in early nuclear constant measurements by P-Division and in the metallurgy program, researchers attempted to compensate for the lack of 239Pu by working with 239U, in hopes that the results could be extrapolated to reveal the properties of the heavier element.

Continuation of the Critical Mass Studies

Short of the real explosion, there was no way to determine precisely the extent of supercriticality achieved in the gun- or implosion-assembled bomb, or to measure other chain reaction properties, such as the neutron population growth rate.