When most mines are shut down, they are sealed up and abandoned. When the second largest gold mine in the world shuts down, it is converted into a cutting edge research facility for studying dark matter.
The Sanford Lab Homestake Visitor Center documents the history of the Homestake Mine and its recent conversion into a lab where research into dark matter is conducted.
The Homestake Mining Company
The Homestake Mine was the premier gold mine in the Black Hills. The original gold deposit was discovered in April 1876 by Fred and Moses Manuel, Alex Engh and Hank Harney. A year after striking gold, they sold their mining rights to George Hearst and two other wealthy investors. Hearst continued to acquire neighboring claims, consolidating the land into one massive mining operation: the Homestake Mining Company. To fund the expansion and equipment of this isolated deposit, Hearst sold shares of the Company. The Homestake Mining Company became one of the longest-listed stocks on the New York Stock Exchange since its listing in 1879. It continued operation until the end of 2001. Low gold prices and increasing mining expenses made mining no longer profitable. By that point 40 million troy ounces of gold had been extracted from the mine over the time of its operation.
For most mines, this would be the end: the water pumps were shut off and the tunnels flooded with ground water. But the Homestake mine’s depth drew the attention of researchers.
Ray Davis, The Homestake Experiment, And The Nobel Prize
96% of the universe, we cannot see. This is dark matter and dark energy and we know so little about it. Attempts to sense dark matter on the earth’s surface have been foiled by cosmic radiation and other noise that obfuscates the signals of dark matter. So, researchers began to conceive of an under ground experiment, somewhere so deep in the earth so as to block other signals. Fortunately, part of the work had already been done for them.
In 1965, Dr. Ray Davis had a 100,000 gallon cylindrical tank installed on the 4950 level of the Homesteak Mine. The tank was so large, it had to be lowered in sections. The tank’s intent was to detect solar neutrinos which had, up to that point, only been a theoretical concept. Davis’ solar neutrino experiment lasted three decades and was the first to successfully detect and count solar neutrinos. His research into neutrinos earned Davis a Nobel Price of Physics in 2002. One of the last remaining parts of the tank is on display at the Sanford Lab Visitors Center.
Dark Matter And Sanford Lab
After the conclusion of Davis’s work and the closing of Homesteak in 2001, the pumps that kept water out of the mine shafts were shut off and the mine began to flood. Yet, on July 10, 2007, the former Homestake Mine was chosen for the site of the Deep Underground Science and Engineering Laboratory (DUSEL), now known as the Sanford Underground Research Facility (SURF).
We take so much for granted: “what goes up, must come down” and the like. But these platitudes come from somewhere. Gravity itself is just a theory and many experiments were conducted before “gravity” even reached that degree of certainty. (Even today, it is imperfect.) Several hypotheses are only now being tested deep in the Homestake mine. At the time of our visit, three experiments were underway: the LUX, MAJORAN, and CASPAR.
The Large Underground Xenon (LUX) Experiment
The human eye takes in light and the brain interprets those light patterns as the world around us. That green and brown blotch is a tree. That blue line is a river. But what of things that don’t emit or reflect light? Dark mater emits no light, so we cannot see it but it is still around us, all the time. The LUX experiment is aimed at detecting Weakly interacting Massive Particles (WIMPs). This describes a particular interaction between dark matter and known particles. At this time, WIMPs are just a theoretical component of dark matter. Since we cannot see them, the experiment uses a tank of xenon to detect WIMPs. A collision with the xenon should emit a photon, which should be detectable.
If they exist as theorized, WIMPs would only be detected in this experiment a few times a year. That’s pretty rare. Research thus far has be inconclusive. Even now, plans to expand the experiment are in progress. LUX-ZEPLIN (LZ) will be 45 times larger than the current LUX experiment, with 10 tons of liquid xenon to increase the likelihood of encountering WIMPs.
Almost a mile below the earth’s surface, within a 54-ton lead-brick shield, and two ultra-pure copper liners, is the MAJORAN experiment. The experiment is monitoring a very particular type of decay called neutrinoless double-beta decay. If this is observed, it would reenforce theories that neutrinos are their own antiparticle. If this research is successful, it would expand our understanding of dark mater and could bring us one step closing to explaining the existence of all matter.
Compact Accelerator System for Performing Astrophysical Research
Compact Accelerator System for Performing Astrophysical Research (CASPAR) is designed to replicate the reactions that scientists believe occurred within stars. These reactions are attributed to half of all the elements in the universe. Scientists are using a nuclear reactor to reproduce some of these complex reactions. After operating for ten years at Notre Dame, the experiment and equipment has shifted to the Sanford Lab.
The Sanford Visitors Center
Research continues in the mine and the Sanford Visitors Center provides an opportunity to learn about Homesteak’s mining history and cutting edge future. The Center houses displays explaining the history and future of the mine. Behind the Center is the open pit mine, only a small fraction of the intricate mining operation that took part deep in the Sanford Mine. There is even a little patch of green on the observation deck, from which visitors can put a golf ball and be guaranteed a whole-in-one. Tours of the hoist house are available from the Visitors Center. Unfortunately, that is as close as the general public can get to the lab itself.