OWEN CHAMBERLAIN, an American physicist, was best known for his discovery of the antiproton. Protons are elementary (also called fundamental) particles that occur in the nuclei of all atoms. An antiproton is a particle exactly like a proton except that, whereas a proton carries a positive electric charge, it carries a negative charge equal in magnitude to the proton’s positive charge. All the other properties of the antiproton — size, weight, etc — are identical to those of the proton.

The first anti-particle, the positive electron (which has a positive charge equal in magnitude to the electron’s negative charge) was discovered in cosmic radiation in 1931. The antiproton was discovered in 1955 by a team led by Owen Chamberlain and the Italian Emilio Segrè (who died in 1989). The antineutron was discovered by Bruce Cork in 1956.

Chamberlain and Segrè made their discovery using the proton accelerator, called a Bevatron, at Berkeley University in California. The “atom smasher” was able to accelerate protons to such velocities that they had enough energy to form a proton-antiproton pair.

Matter made up of antiprotons, anti-neutrons and positrons is called antimatter. Small amounts of antimatter have been created artificially at, for example, the CERN laboratory near Geneva. When a particle and its corresponding antiparticle collide annihilation occurs. The mass of the two annihilated particles appears in the form of energy — according to Einstein’s equation E=mc2, where m is twice the mass of a proton and c is the speed of light. The annihilation of a minute amount of antimatter would produce a huge amount of energy. If this energy could be harnessed, it could be used for civil and military purposes.

Antimatter has a very short lifetime because it rapidly interacts with ordinary matter and is annihilated (although in a vacuum antiparticles are stable). Cosmologists believe that matter and antimatter were formed during the big bang that created all the matter and energy in the universe from the explosion, in a state of enormous density and temperature. Perhaps a slight excess of matter was created initially so that in time all the antimatter was annihilated by collisions with matter, leaving only the ordinary matter with which we are familiar. It is possible, however, that some remote stars or galaxies may be formed of antimatter. But exactly why the universe soon consisted of matter rather than antimatter is a question yet to be answered.

Owen Chamberlain was born in San Francisco; his father was a radiologist at Stanford University Hospital, California. Chamberlain received a bachelor’s degree at Dartmouth College, Hanover, New Hampshire in 1941 and then studied physics at the University of California.

In 1942 Chamberlain joined the Manhattan Project, set up to produce the atomic bomb. He investigated various aspects of nuclear fission, working with Segrè at the University of California at Berkeley and in the laboratory established at Los Alamos, New Mexico. He participated in the first test of an atomic bomb, in the desert near Alamogordo, New Mexico, on July 16, 1945. The weapon exploded as planned. Chamberlain incorrectly predicted that it wouldn’t work and lost a $5 bet.

In 1946 he began working at the Argonne National Laboratory in Chicago and, while doing his doctorate at the University of Chicago, he investigated the properties of neutrons under the supervision of the eminent Italian physicist Enrico Fermi. The University of Chicago awarded him his PhD in 1949. In 1948 he took up a teaching post at the University of California at Berkeley. His research work, done in Segrè’s group at the Berkeley Radiation Laboratory, included detailed investigations of collisions between protons.

Chamberlain, Segrè and their colleagues fired ordinary protons at very high velocities, into a copper target. The very energetic collisions produced a large number of other particles. A few of them had the same mass as a proton but curved the opposite way in a magnetic field, showing that they carried a negative electric charge. The antiproton was discovered.

For their discovery of the antiproton, Chamberlain and Segrè were awarded the 1959 Nobel Prize in Physics.

Chamberlain and his colleagues spent the next few years studying how antiprotons interact with a number of elements; they used antiprotons to produce antineutrons. In 1958 Chamberlain was appointed Professor of Physics at the University of California, Berkeley. In 1959 he lectured at Harvard University. He retired from the University of California in 1989 but continued to attend weekly seminars up to the week before he died.

Soon after the end of the Second World War, Chamberlain visited the atom-bombed city of Hiroshima and was appalled by the death and destruction wreaked by the bomb he had helped to develop. Like a number of other physicists who had worked on the Manhattan Project, he became politically active in peace issues and in social justice. He publicly opposed the Vietnam War and, in the 1980s, helped to found the nuclear freeze movement. The nuclear freeze was a proposed agreement between the US and the Soviet Union to stop producing new nuclear weapons. Although there was no such agreement, the movement generated much public debate about nuclear arms.

Chamberlain was a Fellow of the American Physical Society and a member of the US National Academy of Sciences. In 1957 he was awarded a Guggenheim Fellowship to study antiparticles at the University of Rome. He was very popular with students because he was approachable, generous with his time and able to explain complex matters simply.

Chamberlain’s third wife, three daughters, a son and two stepdaughters survive him. His second wife died in 1991. His first wife survives him.

Owen Chamberlain, physicist, was born on July 10, 1920. He died on February 28, 2006, aged 85.