Physicist Frederick Reines, who shared the 1995 Nobel Prize for his discovery of the neutrino, died last night after a long illness. He was 80. As a professor at the University of California, Irvine, for 22 years, Reines was instrumental in founding the field of neutrino physics.
With all the attention the neutrino has been getting recently, it is easy to forget that its very existence was once doubted. In 1930, when the only known particles were the electron and the proton, Wolfgang Pauli daringly suggested that the neutrino must exist to conserve energy when certain radioactive particles decayed. It was clear even then that if this ghostly particle did exist, it was practically undetectable--weighing almost nothing and slipping easily through all types of matter without leaving a trace. In the 1950s, Reines and his colleague Clyde Cowan succeeded in pinning down this slippery particle.
The discovery was made with a 400 liters of water and cadmium chloride. Neutrinos emitted from a nuclear reactor collided with a hydrogen nucleus, spawning a positron and a neutron. When the positron was slowed by the liquid and destroyed by an electron, it gave off photons. The neutron bounced around before it hit a cadmium nucleus, and its photons emerged microseconds later. This signal provided proof of the neutrino.
Reines spent the rest of his life trying to understand the neutrino and did many more important experiments to determine its properties. In the late 1960s he was part of a team that detected the first neutrinos produced in the atmosphere by cosmic ray collisions, and in 1987 he co-discovered neutrinos from supernovae. He was also one of the pioneers in developing the huge neutrino detectors that are used to ensnare them.
"He had enormous vision and interest in doing the important and difficult experiments," says Jonas Schultz of UC Irvine. "He didn't really care for the trivial things, and for that I considered him very courageous."