The Real Neutrino

For 20 years nuclear physicists have used neutrinos (small, uncharged particles) in their calculations. Neutrinos are necessary: without them many nuclear equations would not balance, and the massive branches of nuclear theory might fall to the ground. But no known apparatus has ever detected neutrinos. They were reasoned into existence by Nobel Prizewinners Enrico Fermi and Wolfgang Pauli to fill a theoretical need, and the gnawing suspicion has long persisted that they do not exist. Last week from the Atomic Energy Commission came big news. Neutrinos do exist.

Detecting neutrinos with ordinary instruments is like catching bats with a steam shovel. Since they carry no electric charge and are vanishingly small, they pay little attention to matter. The average neutrino can probably pass through billions or trillions of miles of dense material without being stopped by it. Neutrons do, however, "interact" slightly with protons; so there is a very small chance that if a great many neutrinos pass through a material rich in protons, a few of them will be intercepted in a way that can be detected.

Monstrous Assembly. Several years ago Drs. Frederick Reines and Clyde Cowan Jr., physicists of the Los Alamos Scientific Laboratory, armed themselves with AEC money and went hunting neutrinos. Their first attempts, with a monstrous assembly of special apparatus, were inconclusive (TIME, May 10, 1954). They thought they detected neutrinos streaming out of the AEC's great reactors at Hanford, Wash., but they were not sure. So they returned to Los Alamos and constructed an even more monstrous apparatus.

The theory of neutrinos predicts that when one hits a proton (an infrequent event), a positron (positive electron) is emitted, and the proton turns into a neutron. Both new particles are unstable. The positron hits an electron, and both are "annihilated," turning into gamma rays. The neutron is absorbed by almost any kind of matter, and if a little cadmium is around, the neutrons captured by it give another burst of energy.

330 Eyes. The Reines and Cowan apparatus for detecting neutrinos contains more than 1,000 gallons of liquids (water and hydrocarbons) that are rich in protons. The water contains dissolved cadmium to capture neutrons, and the hydrocarbons give scintillations of light when gamma rays pass through them. The scintillations are recorded by 330 photomultiplier tubes that watch the scintillating liquid like large, unblinking eyes.*

Best hunting ground for neutrinos is near nuclear reactors, from which, by the Fermi-Pauli theory, they stream in vast numbers. So Reines and Cowan took their apparatus to the AEC's Savannah River plant. They set it up in an underground room where it was sheltered from distracting cosmic rays but exposed to a flood of neutrinos from one of the great plutonium-producing reactors.

Two or three times an hour when the reactor was in operation, the detecting instruments registered "an event"--two flashes of light of exactly correct intensity and timing. This meant that a single neutrino (out of many billions per second) had hit a proton (out of billions along its path) and turned it into a positron and a neutron. After watching this happen for a total of 1,371 hours and taking elaborate precautions to eliminate false signals, Reines and Cowan announced that they had really detected neutrinos. AEC Commissioner Willard F. Libby congratulated them on their "magnificent accomplishment." Now nuclear physics can use neutrinos without an uneasy conscience. Further neutrino experiments, Libby hinted, may reveal deep secrets about the structure of matter. They may tell what happened to matter that turned into neutrinos in the hearts of stars billions of years ago and has perhaps been circulating ever since around the universe.

*The photomultiplier tubes are cooled by water flowing between them, and when the apparatus was at Los Alamos, goldfish were thriving in the water. Said Dr. Reines: "There is little interaction between neutrinos and goldfish."

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