Ben Roy Mottelson Dies at 95; Shed Light on the Shape of Atoms

Ben Roy Mottelson, an American-Danish theoretical physicist who shared a Nobel Prize for revealing how the motion of protons and neutrons could distort the shape of the nuclei of atoms, died on Friday. He was 95.

The Nobel Foundation confirmed his death. No other details were available.

Dr. Mottelson was awarded the Nobel in physics in 1975 along with James Rainwater, a Columbia University physicist, and Aage Bohr, a Danish scientist whose father, Niels Bohr, was awarded the Nobel in physics in 1922.

The three scientists’ discoveries about the nuclei of atoms had an impact on the development of human-made nuclear fusion, which generates energy by combining hydrogen atoms.

Their research concerned a fundamental topic: the forces that shape the nucleus. Before their work, there were two prevailing theories. One said that the nucleus was like a liquid drop — that it was essentially spherical and that the protons and neutrons that make up the nucleus behaved like cohesive molecules in a drop of water.

The second theory was that the nucleus was ordered somewhat like the electrons that orbited it, with protons and neutrons similarly arranged in shells within the nucleus.

Both theories had shortcomings: Neither was able to adequately explain the data arising out of experiments on atomic nuclei. For example, the liquid-drop model could not explain why nuclei sometimes had nonspherical charges.

Dr. Rainwater had worked out some of the theory about the structure and forces in the nucleus in a paper published in 1950. Dr. Mottelson, working in collaboration with Dr. Bohr, developed a theory that the structure could be explained only by simultaneously accounting for the individual and collective rotations of the protons and neutrons — a superimposition of forces.

Their theory, which they published in three papers in 1952 and 1953, perfectly matched experimental data. It showed that the nucleus was not necessarily spherical and could be distorted — that “it can become football-shaped or plate-shaped,” Philip W. Anderson, an American physicist and a 1977 Nobel laureate, wrote in Science magazine in 1972.

Dr. Mottelson and Dr. Bohr later expanded their work, publishing two encyclopedic books, “Nuclear Structure: Single Particle Motion” (1969) and “Nuclear Structure: Nuclear Deformations” (1975). The books remain standard references.

In awarding the Nobel to Drs. Mottelson, Bohr and Rainwater, the Royal Swedish Academy of Sciences cited their “discovery of the connection between collective motion and particle motion in atomic nuclei and the development of the theory of the structure of the atomic nucleus based on this connection.”

Ben Roy Mottelson was born on July 9, 1926, in Chicago, the second of three children of Goodman and Georgia (Blum) Mottelson. His father was an engineer. The family home was a place of lively conversation about scientific, political and moral issues, Dr. Mottelson recalled in an autobiographical sketch for the Nobel Foundation.

He graduated from Lyons Township High School in La Grange, Ill., during World War II and, after joining the Navy, was sent to Purdue University for officers training. After graduating with a bachelor’s degree in 1947, he entered Harvard University for postgraduate studies in nuclear physics. There he studied under Julian Schwinger, the theoretical physicist who was awarded the Nobel in 1965 for his work on quantum electrodynamics. With Dr. Schwinger as his thesis adviser, Dr. Mottelson obtained his Ph.D. in 1950.

Receiving a one-year Sheldon Traveling Fellowship from Harvard, he decided to spend the time at the Institute for Theoretical Physics in Copenhagen (later renamed for Niels Bohr, its founder). Dr. Bohr was still working there, as was his son Aage. It was at this time that the collaboration between Dr. Mottelson and the younger Dr. Bohr began. (In a twist of science, the liquid drop model that was eventually made obsolete by the work of Drs. Mottelson, Rainwater and Aage Bohr had been proposed by Niels Bohr.)

When the Sheldon fellowship ended, Dr. Mottelson received a two-year fellowship from the United States Atomic Energy Commission, which allowed him to stay in Copenhagen. He was then hired by the recently formed European Organization for Nuclear Research, known as CERN, which had been started in Copenhagen before it moved to Geneva.

When the Nordic Institute for Theoretical Atomic Physics was created in 1957, Dr. Mottelson was hired there as a professor. He stayed there the rest of his professional life, with only a brief stint at the University of California, Berkeley, in spring 1959.

He became a naturalized Danish citizen in 1971.

The theory established by Dr. Mottelson and his colleagues on the shape of atomic nuclei came to be universally accepted. But, as often happens in science, it was initially met with some resistance.

In his Nobel lecture, Dr. Mottelson recalled: “I remember vividly the many lively discussions in these years reflecting the feeling of unease, not to say total disbelief, of many of our colleagues concerning the simultaneous use of both collective and single-particle coordinates to describe a system that we all agreed was ultimately built out of the neutrons and protons themselves.”

He married Nancy Jane Reno in 1948 and had three children with her, Malcolm, Daniel and Martha. He married Britta Marger Siegumfeldt in 1983. Information on survivors was not immediately available.