At Los Alamos, whose Theoretical Division was Feynman assigned to?

At Los Alamos, Richard Feynman was assigned to Hans Bethe’s Theoretical Division.

The division, usually called the T Division, was central to the Manhattan Project’s effort to understand and design nuclear weapons. Bethe, already an accomplished nuclear theorist at Cornell, directed the division after Robert Oppenheimer established the Los Alamos laboratory in New Mexico. Feynman joined the project in 1943 and was assigned there as a relatively young physicist.

Feynman impressed Bethe and became a group leader within the division. His work included supervising human computers, organizing calculations with IBM punched-card equipment, and studying neutron behavior in weapons and reactor systems. He also worked with Bethe on a formula for estimating the explosive yield of a fission bomb, later known as the Bethe–Feynman formula.

The alternatives reflect nearby figures from the same wartime story. Robert Serber contributed important theoretical work, Edward Teller worked at Los Alamos on thermonuclear ideas, and Niels Bohr visited and advised scientists. None of them headed the division to which Feynman was assigned; that role belonged to Bethe.