Leo James Rainwater (December 9, 1917 – May 31, 1986) was an American physicist who shared the Nobel Prize in Physics in 1975 for his part in determining the asymmetrical shapes of certain atomic nuclei.

He also contributed to the scientific understanding of X-rays and participated in the United States Atomic Energy Commission and naval research projects.

His father died in the great influenza epidemic of 1918 and Rainwater and his mother moved to Hanford, California, where she married George Fowler, a widower with two sons, Freeman and John.

[2] Fermi was engaged in neutron moderator studies that would lead to the construction of the first nuclear reactor, while Dunning and Eugene T. Booth had built Columbia's first cyclotron, in the basement of the Pupin Physics Laboratories.

The SAM Laboratories' primary task was the development of gaseous diffusion technology for uranium enrichment, to produce fissile uranium-235 for use in atomic bombs.

Niels Bohr and John Wheeler had developed a theoretical treatment for nuclear fission in 1939 that they based on the liquid drop model of the nucleus.

[13] This was superseded in 1949 by Maria Goeppert Mayer's nuclear shell model, which could explain more about the structure of heavy elements than the older theory but it still had limits.

At a colloquium at Columbia in 1949, Charles H. Townes reported experimental results that indicated quadrupole moments greater than those indicated by the shell model.

He later recalled that: When I made my proposal for use of a spheroidal nuclear model, it seemed to be an obvious answer which would immediately be simultaneously suggested by all theorists in the field.

"[16] With funding from the Office of Naval Research,[2] Rainwater built a synchrocyclotron, which became operational in 1950, at the Nevis Laboratories, on an estate on the Hudson River at Irvington, New York, willed to Columbia University by the DuPont family.

When a reporter rang in 1975 to inform him that he had won the Nobel Prize in Physics, he initially thought that it was for his work on muonic atoms.