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Ruth Sisson, one of the “Calutron Girls” at Oak Ridge, Tenn., ran a machine enriching the uranium used in Little Boy, the Hiroshima bomb.

Since it was not known which uranium isotope separation technique — gaseous diffusion, calutron, or centrifuge — would be the most successful, General Leslie Groves insisted that all techniques be pursued simultaneously.

He spent much of the early war years in Oak Ridge, Tenn., assisting in the development of the Calutron farm there to enrich uranium-235 for the Manhattan Project, which built the first atomic bomb, according to friend and former colleague Jose Alonso.

Dr. Lofgren left his graduate studies to become a full-time employee of the Radiation Lab and led development of the ion sources for the Calutron.

Lofgren left his studies to become a full-time employee of the Radiation Lab and led development of the ion sources for the Calutron.