In 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.
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.