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Buried in a gold mine, a particle accelerator searches for stellar secrets

In August 2015, Scientists from the University of Notre Dame headed west, the dismantled pieces of a particle accelerator in the back of their U-Haul. More than 1,000 miles later and nearly a mile below, they began installing the machine in a new home: deep in 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 property to the South Dakota Science and Technology Authority, and the researchers used its protective rock layers to look for things like dark matter and neutrinos. Today the Sanford Underground Research Facility towers over the old west town like it did during the gold rush. Cables spooled from the tallest building into a shaft 2,000 feet deep. And that’s where the Notre Dame scientists planned to install their accelerator called Caspar, the Compact Accelerator System for Performing Astrophysical Research.

It’s also a repurposed relic. Physicists have been using Caspar’s core in one form or another since 1958: to power larger accelerators, to gain insight into radiocarbon dating, to learn how atoms get bigger. In his most recent incarnation, which will begin data collection this fall, Caspar will mimick the fusion inside stars to learn how they make heavy elements – like the ones people dug out of the Homestake mine and out of which solar systems exist.

This particle physics is often the job of big science – expensive ventures like the Large Hadron Collider and the Stanford Linear Accelerator. Hundreds of people make up a scientific team; Budgets run into the billions. But this little old accelerator and his little team from Notre Dame and the South Dakota School of Mines and Technology are studying the universe on a different scale. It leads world-class research into the troubled, collision-filled interiors of stars as they squeeze into normal space in the belly of a mountain.

The machine that was to become Caspar began almost 60 years ago as a kind of auxiliary accelerator. It accelerated a beam of helium atoms to another accelerator, which would make it even faster. But in the 1960s, researchers no longer needed that boost. As Caspar director 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 his caretaker switched to newer, more brilliant accelerators, and once again the machine “became superfluous,” wrote Wiescher.

Until Wiescher overtook it himself, relocated it to the University of Notre Dame in Indiana and re-animated it with software.

There he and Caspar investigated a kind of reaction that takes place inside stars, in which protons collide with alpha particles – two neutrons bound to two protons – and stay there, creating more massive objects. And Wiescher soon realized how the team’s astrophysical game could be improved: to get his accelerator underground. Thousands of meters deep, rocks block the cosmic radiation that can flood the accelerator’s small signals.

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