Astrophysics: Learning from Neutrinos

At Long Island's Brookhaven National Laboratory, Physicist Raymond Davis Jr. is designing one of the most extraordinary instruments known to modern science. When completed, it will be a swimming pool full of cleaning fluid, and will be installed in a deep mine to X-ray the sun.

Scientists have long been fascinated by the sun's center, where all the energy originates that supports life on earth. But the only practical way to observe this arcane spot is to study the neutrinos that are a by-product of its fierce thermonuclear reactions. The ghostly particles pay hardly any attention to matter. All except one in a billion of them pass through the sun's dense material and escape into space.

Rare Reaction. Dr. Davis estimates that about 54 billion solar neutrinos hit each square centimeter (.155 sq. in.) of the earth's surface every second. They have no effect that is normally detectable, but if they happen to collide with atoms of chlorine 37, a small fraction of the collisions results in the manu facture of radioactive argon 37. When it occurs, this rare reaction gives Dr. Davis a chance to count solar neutrinos.

Backed by funds from the Atomic Energy Commission, Dr. Davis plans to set his 100,000-gal. tank in a mine at least 5,000 ft. deep to protect it from cosmic rays. Only neutrinos will reach the tank's supply of perchlorethy-lene, a cleaning fluid containing about one quarter of chlorine 37. Dr. Davis estimates that out of the countless trillions of solar neutrinos that will be passing through the tank, between four and eleven per day will react with chlorine 37 atoms.

Firmer Figure. To detect these few hits, a stream of helium will bubble through the tank, sweeping any argon 37 and carrying it to a charcoal filter. Then a special instrument will count the argon atoms by means of their radioactivity. Their number will be in direct proportion to the total number of neutrinos emitted by the sun.

First thing the neutrinos will measure is the temperature of the core. Astrophysicists now estimate it at 29 million degrees F., but the neutrino observatory will give a firmer figure because the nuclear reaction that produces solar neutrinos is favored by high temperature. If Dr. Davis counts more neutrinos than current formulas predict, astrophysicists will know that the temperature of the core is higher than they have guessed.