Neutrino hunting in a Park City mine
September 2, 2016
If you're trying to understand the secrets of the universe, where do you go? Maybe to the top of a remote mountain, far from city lights?
How about the bottom of a Park City mine?
On this date 50 years ago, a group of scientists from the University of Utah were aiming to do just that. With the help of a grant from the National Science Foundation, they were putting the finishing touches on an experiment designed to prove the existence of elusive particles known as neutrinos.
"These shadowy particles promise to unlock some of the greatest secrets of the universe," astrophysicist Ray Jayawardhana says in his 2013 book, 'Neutrino Hunters.'
"For astronomers … neutrinos offer an exciting new window on the most violent phenomena in nature."
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More on that later. But first, a little background.
If you've been around Park City for a while, chances are you already know something about the Spiro Tunnel, in what is now the Silver Star community: How it's a cool place to stand on a hot summer day. How it was originally built to reach new ore deposits in Thaynes Canyon, and was later reincarnated as an underground ski lift and mining museum. How it's an important source of culinary water for the city.
One hundred years ago this summer, construction began on the tunnel under the direction of Solon Spiro, president and general manager of the Silver King Consolidated Mining Company.
"Preliminary work on the Thaynes Canyon tunnel has been started," The Park Record reported on July 7, 1916. "The total length of the tunnel will be 14,000 feet and at the head of the canyon it will have gained a depth from the surface of 1,800 feet."
Translation: The end of the tunnel will be 1,800 feet below the surface.
As it turned out, the tunnel – which was later named after Spiro – took longer to build, cost a lot more, and uncovered less valuable silver ore than the company had hoped. But it did provide another way to reach the mine's underground workings, and it did help drain water that was the bane of many Park City mines.
Now turn your clock ahead 50 years. By the mid-1960s the mines were in trouble and the major surviving mining company, United Park City Mines, had started a ski resort to help fund its underground operations. At that time the Spiro Tunnel became a part of "the world's first underground ski lift." Skiers traveled three miles deep into the mountain on a converted mine train, then transferred into a "cage" elevator for the ride 1,800 feet up the Thaynes Shaft to the surface. During the summer, visitors could ride the train into the tunnel to an underground mine museum.
At about the same time The Park Record was telling its readers that a chamber was being carved at the end of the Spiro Tunnel to install equipment "for the detection and study of neutrinos – tiny particles with amazing penetrating power.
"Some originate in the far reaches of the universe – others at the top of the earth's atmosphere. Thus, in the heart of a Utah mountain, will be a laboratory to help probe the mysteries of outer space," the newspaper reported in April 1965.
"The type of equipment to be installed, which is now being tested on campus, will distinguish between neutrinos from outer space and those originating in the atmosphere. Because of this, the project will be the only one of its kind anywhere."
Astrophysicist Jayawardhana writes that Austrian physicist Wolfgang Pauli first proposed the existence of neutrinos in 1930 as a way to explain a perceived imbalance in the products of radioactive decay. Neutrinos come from the sun, the stars and other sources of radioactive decay, he says. Like electrons, neutrinos are elementary particles, but they aren't trapped inside atoms. They have no electrical charge, have almost no mass, and hardly ever interact with other particles.
About 100 trillion neutrinos produced by the sun pass through your body every second without leaving a trace, he says. "Neutrinos travel right through the Earth uninhibited, like bullets cutting through fog."
But if these so-called ghost particles are so elusive, how can you detect them?
"Every so often, a neutrino does collide with something, such as a proton inside a water molecule, essentially by accident," Jayawardhana writes. So the trick is to design an apparatus to detect and record these rare collisions.
The next step is to shield your neutrino detector from other sources of background radiation. In the 1940s, a pair of Cambridge University scientists decided to set up their device 100 feet underground in London's Holborn subway station, according to Jayawardhana. Although they found no neutrinos, their strategy was later copied by others – including University of Utah scientists in the 1960s.
"When you're looking for something that's very rare, you want to reduce anything that looks like it could mimic what you're looking for, says Pierre Sokolsky, current professor of physics and astronomy at the University of Utah. "There's a lot of cosmic radiation hitting the Earth that's not neutrinos – that's mostly charged particles. But some of it can fool you. So you want to reduce that as much as possible, and charged-particle cosmic rays get attenuated by the earth. So if you go deep underground, mainly what you see are, hopefully, neutrinos."
The equipment set up inside the Spiro Tunnel included four large concrete water tanks and several hundred cylindrical "spark counters" made up of 33-foot lengths of thin-walled six-inch-diameter steel pipe.
Sokolsky wasn't in Utah when University of Utah scientists launched their Spiro experiment. But he heard about it.
"It was, at the time, a pioneering experiment," he says. "It was very well known for a while because they produced a result which was very exciting in the world of that kind of physics."
In 1969, Utah Professor Jack W. Keuffel and other participants in the Spiro Tunnel experiment published an academic paper concluding that they had observed two "neutrino events" in the Utah detector. Unfortunately, Sokolsky says, their analysis later turned out to be wrong.
"But it turned out to be wrong in a way that was good for Professor Keuffel, because (he) was an extremely honest man, and he was the first person to admit that there was a mistake in the analysis. So, even though it was incorrect, people praised him and his group for being so forthright."
What they had recorded, Sokolsky says, was "just a background thing, a random effect that masqueraded as a real effect."
Another flaw with the Utah experiment, he says, was that the equipment just wasn't big enough to record such a random event.
"People realized that you'd have to build a much larger experiment to actually detect neutrinos," Sokolsky says. "So neutrinos were (finally) detected underground at Kamioka in Japan (in the late 1980s), and there was an experiment also in Ohio in a salt mine that detected underground neutrinos, but those were much larger."
Today, the poster child for neutrino experiments is IceCube, in which scientists have installed sensors to detect the passage of neutrinos through a cubic kilometer of ice deep under the surface at the South Pole. In 2013, the journal Science reported that IceCube had recorded 28 neutrinos traveling from beyond the solar system.
"They're looking at neutrinos which are produced in two different ways," Sokolsky says. "One way (is from) within the centers of galaxies around the supermassive black holes that most galaxies now appear to have. And these things produce massive jets of accelerated particles."
Among these particles, he says, are neutrinos.
"And the nice thing about neutrinos is, because they are zero electric charge and are essentially massless, they travel in a straight line. So, if you detect enough neutrinos, you can actually point back and ask where they came from and pinpoint their origin."
If current theories are correct, scientists should be able to identify which "active galactic nuclei" are producing neutrinos "and learn a lot about the black holes at the center of those galaxies," he says.
"The other way that we can make neutrinos is with very high-energy protons flying through the universe. They interact with what's called the cosmic microwave background radiation left over from the Big Bang. And when they interact with that radiation, they can make neutrinos."
This type of neutrino, which should be coming from all directions, he says, has yet to be detected by scientists.
"If you can detect that radiation, that neutrino flood, then you can actually say something interesting about this microwave radiation, which tells us about the Big Bang and the birth of the universe."
The University of Utah pulled the plug on its neutrino experiment in the late 1970s. Sokolsky says that scientists from several other universities, including Harvard, Purdue and the University of Pennsylvania, later used the same space for a study of spontaneous proton decay. However, by the mid-1980s the scientific experiments in the tunnel had been dismantled.
For more information on the search for neutrinos, look for a copy of "Neutrino Hunters: The Thrilling Chase for a Ghostly Particle to Unlock the Secrets of the Universe," by Ray Jayawardhana.