Neutrinos: the ghost particles

Scientist Ray Davis went hunting for neutrinos using a detector in the Homestake Mine in South Dakota. Photo: DOE

Imagine: It is 1960 and you (or more likely your dad) meet a young man in a pub. He tells you his name is Ray, and you think he must be mad. He says he wants to go down a gold mine a mile underground to try to see inside the sun in the middle of the night. Or day, it doesn’t matter, because he is not using light but “invisible rays,” or particles, that go right through Earth like ghosts. He is a scientist, Ray Davis Jr., and is not mad. Forty years later he wins the Nobel Prize. The particles are called neutrinos, Italian for “little neutral ones”.

This story starts in Victorian times, with a huge puzzle. Charles Darwin had convinced biologists that all life has been evolving from simple forms for hundreds of millions of years. But, at the rate the sun is shining, without some unknown fuel it would burn out in less than 20 million years.

By the 1920s we had an answer. Einstein had shown that matter can be converted into energy. Nuclear reactions like those in a hydrogen bomb could be the mystery source. But as often happens in science, getting an answer leads to more mysteries.

The energy in nuclear reactions studied in the laboratory didn’t add up. Not enough energy came out of a radioactive nucleus. But scientists know that energy cannot just disappear — it is conserved — so something must be taking it away. In 1930 Wolfgang Pauli suggested they could be tiny particles, like electrons without any electric charge, calling them neutrinos.

In 1956 Pauli got a telegram: Neutrinos had been discovered coming out of a nuclear reactor. Then Ray Davis had that wild idea. Perhaps he could detect neutrinos coming from the nuclear reactions in the sun. Down the mine, he filled a tank with 100,000 gallons of dry-cleaning fluid.

Eventually he extracted a few radioactive argon atoms from chlorine changed by neutrinos from the sun. But something was wrong. Theoretically he should find about two atoms per day, but he found even fewer. Was the giant nuclear reactor in the center of the sun shutting down? If so, we might not know for thousands of years. Then: serious global freezing!

The answer is amazing, and next time I will explain. If you can’t wait, check Fermilab’s site, www.fnal.gov, about experiments studying these “ghost particles.”

This is a version of an article that first appeared in Positively Naperville.