By Don Lincoln, Fermilab
Neutrinos come from nuclear reactors and the biggest reactor around is the Sun. So, it stands to reason that we should be able to measure them. And this is what one ambitious researcher set out to do. Ray Davis was an American chemist who worked at Brookhaven National Laboratory.
Ray Davis’s Chemical Experiment
What Davis decided to do was to use a known nuclear process that converted chlorine atoms into argon ones when the chlorine atom was hit by a neutrino. By the way, that particular process, the one where chlorine gets converted into argon in neutrino collisions, was discovered in the early 1960s.
In any event, this experiment sounds all chemical, and the kind of thing you’d expect a chemist to do. However, the reality is truly staggering. Let me give you a sense of why. Davis decided to use as his chlorine source a substance called tetrachloroethylene, but most of us simply call it dry cleaning fluid.
He started a tank of the stuff that held 100,000 gallons. He hauled it to South Dakota and brought it about a mile underground in the Homestake gold mine. He had to bring it so far underground to use the Earth to shield his apparatus from the cosmic rays.
The basic idea was that he would sit there and wait and let the steady rain of neutrinos from the Sun pass through the detector. Some of the neutrinos would interact with the chlorine and make a radioactive form of argon and he’d collect the argon and watch it decay. From the number of decays, he could figure out the number of neutrino interactions.
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Getting Help from John Bahcall’s Prediction
Now, to figure out if he was actually seeing neutrinos from the Sun, he’d need some prediction to compare it to and that prediction was done by an American physicist by the name of John Bahcall.
He took all of the known fusion processes in the Sun, of which there are more than a dozen, and figured how many neutrinos they would make. It turns out that the different solar neutrino processes result in different amounts of energy carried by the various neutrinos.
As it happens, the only process that made neutrinos in the Sun with high enough energy to convert chlorine to argon is when a specific form of boron is made in the Sun. The boron process is just a small fraction of the energy generation in the Sun so Bahcall had his work cut out for him.
Bahcall predicted an incredibly small rate of argon production. How small you might ask? Really small, specifically about five atoms of argon every day. Yeah, you heard that right—five atoms.
So, that’s the task the Davis set out to accomplish. He let the equipment sit for many days and then flushed out all the argon that had been made. The length of the data taking period varied, but if he ran for 10 days, he’d expect to see about 50 atoms.
A Wrong Calculation?
So, what did he see? Well, consistently, experiment after experiment, over a period of years (even decades), Davis consistently measured about a third as many neutrinos as Bahcall predicted. So, what could be going on?
Well, when you’re talking about making and detecting 50 atoms, it’s easy to imagine that you’re just doing it wrong. Maybe Davis was simply not collecting all of the argon. That would be an easy explanation. Except that it isn’t.
That’s because Davis was able to introduce a known amount of argon into his detector using standard radioactive sources and he measured exactly what he expected. The problem wasn’t on Davis’ end.
On the prediction side, Bahcall was trying to calculate the neutrino flux from a tiny fraction of the Sun’s energy budget. Maybe he was just calculating things incorrectly? And, I admit when I was in graduate school in the 1980s and 1990s, I leaned towards the explanation that Bahcall had just somehow made a mistake. It was a comforting, although as we shall see, wrong position to hold.
A Mystery Called Atmospheric Neutrinos
Davis proposed his experiment in 1964 and the first paper came out in 1968. The experiment ran for many years, well into the 1980s, and the measurement was consistently about a third that of the prediction. It was a mystery for a very long time. But there was more data to consider. The solar neutrino problem, as we scientists call it, was not the only neutrino conundrum. There was also a mystery in what we call atmospheric neutrinos.
Atmospheric neutrinos are ones made in the atmosphere via cosmic rays. Protons from space hit the atmosphere, mostly making pions. Those pions decay 100% of the time into muons and a muon neutrino. Muons themselves decay into electrons, a muon neutrino, and an electron neutrino.
At the end of the day, a pion makes two muon type neutrinos to one electron type neutrino. That’s for a negative pion. For a positive pion, all of the charges are reversed, but the outcome is the same—pions eventually create twice as many muon type neutrinos as electron neutrinos. Two to one.
And this prediction is far simpler than the solar neutrino one. It’s simply very hard to imagine that atmospheric neutrinos can exist in any form other than twice as many muon neutrinos as electron ones.
Common Questions about Ray Davis and the Mysterious Atmospheric Neutrinos
Ray Davis was an American chemist who designed an experiment to figure out the number of neutrinos coming from the Sun.
John Bahcall took all of the known fusion processes in the Sun to figure out how many neutrinos they would make.
The atmospheric neutrinos, which were a mystery for a very long time, are made in the Earth’s atmosphere via cosmic rays.