In 1943, Teller and Robert Oppenheimer persuaded Critchfield to come to the Manhattan Project's Los Alamos National Laboratory, where he joined the Ordnance Division under Captain William Parsons on the gun-type fission weapons, Little Boy and Thin Man.

[3] During Critchfield's graduate studies, Teller's colleague George Gamow[4] introduced him to Hans Bethe, with whom he wrote a paper in 1938, which analyzed the nuclear fusion of protons into deuterons.

[5] The next year, Bethe showed that this process is a key link in the proton-proton chain reaction and the CNO cycle, which are the major ways that nuclear energy is released in the solar core and in massive stars.

[9][10] In 1942, after a brief stay at Harvard University, Critchfield went to the Carnegie Institution of Washington, where he continued his ballistic studies, which resulted in three patents for improved sabot designs.

[11] Because of his experience with ballistics, Teller and Robert Oppenheimer persuaded Critchfield to come to the Manhattan Project's Los Alamos National Laboratory in 1943,[12] where he joined the Ordnance Division under Captain William Parsons.

[17] Here, with Leland S. Bohl, he invented and patented the natural shape balloon,[18] and participated, with Ney and his student Sophie Oleksa, in an early search for primary cosmic ray electrons.

Critchfield accepted this offer, but ran into a storm of political and media criticism over the conflict of interest involved in heading an agency that did $4 million worth of business with Convair each year.

[23][24] In 1961, Critchfield accepted a professorship at the University of Wisconsin, but before he moved to Madison, his friends at Los Alamos, J. Carson Mark and Norris Bradbury offered him a position there that he took instead.