Neutrino Discovery

The shy neutrino is an invisible, almost massless particle that hardly ever interacts with anything. We tell you how scientists first detected this so- called “poltergiest particle” .

This is for Thursday, October 11. On this date in 1995, Frederick Reines received the Nobel Prize in physics for helping to track down the elusive neutrino.

Wolfgang Pauli was the first to suggest that these particles must exist. Neutrinos carry energy away from some nuclear reactions — both human-made reactions — and those that power the sun and other stars. Neutrinos were nicknamed “poltergeist particles” — because they’re so reluctant to interact that they can pass out of the sun and through the Earth without bumping into any other particles. This prompted Pauli to lament, “I have done a terrible thing, I have postulated a particle that cannot be detected.”

Reines and longtime collaborator Clyde Cowan were among those scientists who disagreed — and who actively searched for neutrinos. They buried a 10-cubic-meter tank — containing mineral oil — in the same building as the Savannah River nuclear reactor. When a neutrino collides with elements dissolved in the oil, it releases a characteristic double flash of light. Reines and Cowan sifted through months of data and found that their carefully laid trap had caught an average of three neutrinos per hour. Theirs was the first positive experimental identification of the elusive “poltergeist particle”.

A neutrino is one of the elementary particles. Neutrinos are created in nuclear reactions: for example, in the nuclear fusion that powers our sun and other stars.

The sun is believed to release enormous numbers of neutrinos. They are thought to be electrically neutral — have almost no mass — travel at or near the speed of light NOTE: If the neutrino has ANY mass at all it cannot travel at the speed of light. Only totally massless particles can travel that fast. What’s more, neutrinos can pass through almost anything. Neutrinos created inside the sun soon fly out of the sun’s core and into space. Those that encounter the Earth penetrate our planet’s core and fly out the other side. Thus neutrinos are difficult to detect!

In his nobel prize acceptance speech, Reines recalled how he and his longtime collaborator Clyde Cowan started their search. He said, “In the summer of 1951, we were grounded in Kansas City with engine trouble. At loose ends, we wandered around and started to discuss what was interesting in physics. I said ‘Clyde, let’s work on the neutrino.’ His immediate response was ‘Great idea!’ He knew as little about the neutrino as I did, but he was a great experimentalist with a sense of derring-do. So we shook hands and got off to working on neutrinos.”

Clyde Cowan, an equal participant in the development of the detector and the detection experiment, passed away before the Nobel Prize was awarded. Frederick Reines died in 1998.

Frederick Reines was awarded a 1961 grant from the Research Corporation for his “search for neutrons and gamma rays of extraterrestrial origin.”

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