The Elusive Neutrino

Physicists last week were watching with interest a complicated apparatus parked near a nuclear reactor at Hanford, Wash. Out of it may come a new branch of physics-or a warning that the structure of physics is threatened with collapse.

The apparatus at Hanford was designed to detect the neutrino, a ghostly particle that the physicists invented to make their nuclear equations come out even. When an atom disintegrates, the mass of its fragments plus the mass equivalent of the energy released should equal the mass of the original atom. Often they do not; a small amount of mass disappears as completely as a snowflake in the ocean. This is serious because the physical sciences are based on the principle that mass can turn into energy and vice versa, but neither can just disappear.

To save the situation, the physicists invented the neutrino, which they think of as a particle with less than one two-thousandth of the mass of an electron. It has no electric charge, and it therefore reacts very slightly with matter, sailing through solid metal or rock almost as if they were empty space. About 5% of the energy of a nuclear reactor (so says the theory) goes off in the form of neutrinos, and most of those that shoot downward pass right through the earth.

The trouble with these bizarre particles is that many physicists fear that they do not exist. They have not been detected and until recently there was little hope of detecting them. But two developments have changed the situation. Nuclear reactors produce floods of neutrinos (if they exist), and such modern detecting devices as the photomultiplier tube and the liquid scintillator are about 1,000,000 times as sensitive as their predecessors.

Flashing Particles. Fortified with this knowledge, two Atomic Energy Commission physicists, Frederick L. Reines and Clyde Cowan Jr., gathered an erudite task force at Los Alamos Scientific Laboratory and went hunting neutrinos. Theory told them that if a neutrino hits a proton, as may happen on very rare occasions, the reaction should yield a neutron and a positron (positive electron). If this happens in a liquid that scintillates in the proper manner, both particles will give flashes of light.

So Reines and Cowan built the world's biggest and most complex scintillation counter. They filled a 28-in. cylinder with toluene, a scintillating liquid that contains lots of protons. In the toluene, they dissolved a small amount of a cadmium compound. Then they surrounded the cylinder with 90 photomultiplier tubes hooked up to respond to pairs of flashes caused by positrons and neutrons.

Taking this ponderous instrument to Hanford, they set it in front of one of the great reactors and surrounded it with a massive lead shield to reduce background radiation from cosmic rays, etc. By commandeering all the lead shielding available at Hanford, they got the background count for pairs of flashes down to 2.15 a minute when the reactor was not operating. When the reactor went to work, releasing floods of neutrinos (if they exist), the count went up to 2.5 a minute.

Neither Reines nor Cowan regards this small difference as conclusive. But they have gained experience, much of it of practical value to the bomb makers at Los Alamos. A second series of experiments will be starting soon. "Then," say Reines and Cowan, "we should be able to say definitely either that the neutrino exists or that it does not exist."

New Revolution? On the answer to this question hang enormous issues. Physicists are already comparing the neutrino hunt with the igth century hunt for the "celestial ether," which was then considered necessary to carry waves of light through the vacuum of space. When the Michelson-Morley experiment (reported in 1887) proved that the ether does not exist, physics was thrown into confusion and had to be rescued painfully by Einstein's relativity.

If the neutrino is abolished, physics may be threatened by a more sweeping revolution. Physicists would balk at admitting that matter or energy just disappears. They would try to explain where matter or energy goes to, and the search might reveal a new world of physics.

If Reines and Cowan succeed in detecting neutrinos, their study may show a "fine structure" in matter that is not suspected now. It may solve mysteries about the whole universe. Most of the neutrinos that were in existence at the birth of the universe (if the universe had a birth date and if neutrinos exist) are probably still cruising round and round, passing with ease through stars and galaxies. They may form a large part of the universe.

This file is automatically generated by a robot program, so reader's discretion is required.