Ben Mottelson was a US-born Danish physicist who shared the 1975 Nobel Prize in Physics with Aage Niels Bohr and James Rainwater for their cooperative research into the structure of atomic nucleus. To explain why atomic nuclei were not spherical, Rainwater had suggested that nuclear particles form an inner nucleus and an outer nucleus, and these two interacting sets produce energy that deforms the symmetrical nucleus. From 1950 to 1953 Mottelson and Bohr, combining the shell and liquid drop nuclear models, provided experimental evidence to support this theory, which made nuclear fusion a practical possibility.