Buried in a Gold Mine, a Particle Accelerator Searches for Stellar SecretsIn August 2015, scientists from the University of Notre Dame went west, the disassembled pieces of a particle accelerator secured in the back of their U-haul. Over 1,000 miles later and nearly a mile down, they started installing the machine in a new home: deep within an old mine in the town of Lead, South Dakota.
Miners first excavated the Homestake gold mine in the 1880s. But in 2006, the mining company donated the industrial site to the South Dakota Science and Technology Authority, and researchers repurposed its protective layers of rock to search for things like dark matter and neutrinos. Today, the Sanford Underground Research Facility looms over the old west town just as it did during the gold rush, cables spooling out of its tallest building and into a shaft that goes down 8,000 feet. And there, the scientists from Notre Dame planned to install their accelerator—called Caspar, the Compact Accelerator System for Performing Astrophysical Research.
It, too, is a repurposed relic. Since 1958, physicists have used Caspar’s core in one form or another: to fuel larger accelerators, to gain insight into radiocarbon dating, to learn about how atoms grow larger. In its most recent incarnation, which will start taking data this fall, Caspar will mimic the fusion that goes on inside stars to learn how they make heavy elements—like the ones that people dug out of the Homestake mine, and that make up solar systems.
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That particle kind of physics is often the purview of Big Science—expensive endeavors like the Large Hadron Collider and the Stanford Linear Accelerator. Hundreds of humans compose one science team; budgets run into billions. But this little old accelerator and its little team, from Notre Dame and the South Dakota School of Mines and Technology, investigates the universe on a different scale. It leads world-class research into the roiling, collision-filled insides of stars while squeezing into a regular room in the belly of a mountain.
The machine that would become Caspar began nearly 60 years ago as a sort of helper accelerator. It sped up a beam of helium atoms toward another accelerator, which would speed them up even more. But in the ’60s, researchers stopped needing that boost. So as Caspar lead Michael Wiescher wrote in a scientific biography of the instrument, the accelerator ”sat unloved and unused near the ion source.” The accelerator moved to the University of Toronto, where it helped people get information they need for radiocarbon dating. But then its caretaker moved on to newer, shinier accelerators, and once again, the machine “became superfluous,” wrote Wiescher.
Until Wiescher himself overhauled it, moved it to the University of Notre Dame in Indiana, and re-animated it with software.
There, he used Caspar to study a kind of reaction that happens inside stars, in which protons slam into alpha particles—two neutrons bound to two protons—and stay there, making more massive objects. And before long, Wiescher saw how to up the team’s astrophysical game: Put their accelerator underground. Thousands of feet down, rock blocks the cosmic radiation that can swamp the small signals from the accelerator.
Barbara Johnston, University of Notre Dame © University of Notre Dame
And so in 2015, piece by piece, vacuum pump by magnet by tube, the scientists loaded the accelerator equipment into the old-school elevator that still shifts up and down the mine shaft, lined with lumber. It took more than the 10 usual minutes to get down, the elevator slowed to a crawl to protect Caspar’s delicate, antique belt and pulley as it descended from the ground floor to the “4850 Level”—4,850 feet underground, where dirt floors are studded with metal tracks and a light breeze blows.
They moved their equipment to the room (with more modern floors) where they would work and, soon enough, start to smash particles into particles. Specifically, the Caspar team wants to learn how stars a little older than the sun synthesize heavy elements. At its front end, Caspar bathes hydrogen or helium gas in radio-frequency energy, which makes the gas produce a beam of protons. That beam shoots into the vacuumed accelerating tube and streams toward the target at the end of the tube. Particles with the right amount of energy slam into the target—often a neon gas that releases the same neutrons that snowball small elements into bigger ones in star cores.
Since then, the team has been reassembling, commissioning, and calibrating. But this fall, Caspar will finally begin its real work. In June, Wiescher and a few colleagues and students from Notre Dame and the South Dakota School of Mines and Technology converged on SURF and talked over beers in the nearby town of Deadwood, famous for hangers-out like Calamity Jane and Wild Bill Hickock. They discussed the accelerator’s cowboy goal: to understand how stars combine atoms and atomic parts to make larger elements, which go on to form planets, people, and gold.
The morning after, in hard hats and protective eyewear, they gathered around computer screens and racks of electronics in the control room. Behind a closed door, Caspar sat bolted to lab benches, metal tubes leading the beam of particles first straight back, then around a 25-degree bend toward the target. When the particles collide with the target, the ensuing reactions mimic those guttural star reactions. The knocked-around particles inside Caspar may only get up to a million volts—compared to the LHC’s 7 trillion—but that’s the point. Some of the universe’s mysteries lie in the crosshairs of a low-energy, low-cost, little accelerator.
But the team isn’t quite there yet. “Right now we are really futzing around, because we have to learn,” said Wiescher. They’ll start, for real, in the fall. He pointed to a wooden board between some controls, as the students kept tapping and staring away. The board is for knocking, commented astrophysicist Dan Robertson, on wood. You know. And although they likely believe more in humor than superstition, a team member nevertheless leaned over and banged his knuckles against it.
First built in 1958, Caspar will mimic the fusion that goes on inside stars to learn how they make heavy elements