Earthwalk Day 550 (26 July 2023), 5358 km: Sanford Underground Research Facility

It’s another of those substandard months. A couple days off due to rain. Then I twisted my foot and had to take more time off. Recovery came just in time for another AQI spike. Now I’m spending a week in Massachusetts with the American Travelling Morrice which leaves me with limited time for walking (and my bike’s back in Syracuse), and no, I don’t wear the pedometer while I’m dancing.

But today was our day off and I took a walk on the Assabet River Rail Trail which allowed me to reach the end of Leg 20. I virtually arrived in Lead, South Dakota , home of what used to be the Homestake Mine , at one time the largest and deepest gold mine in North America. This is the second mine I’ve visited and the reasons are similar.

In the late 1960s physicist Ray Davis designed an experiment to detect electron neutrinos emitted by the sun. To avoid being swamped by cosmic ray backgrounds, the experiment had to be placed deep underground, and the Homestake mine was chosen as the site. Essentially the apparatus was a big tank of dry-cleaning fluid. Neutrinos would convert atoms of chlorine in the fluid into argon, which could be extracted and detected.

The expected rate of argon formation was calculated by theorist John Bacall. Davis’s experiment did indeed detect argon, proving the sun does produce neutrinos — the first neutrinos ever detected from an extraterrestrial source — but at only about one-third the predicted rate. The calculation and the experiment both were scrutinized carefully, but the discrepancy remained. The experiment collected data for an astonishing 24 years, from 1970 to 1994, giving the result very high statistical significance. Later, other neutrino detector experiments showed directly what Davis’s had strongly suggested: that the electron neutrinos emitted by the sun were morphing into muon neutrinos and tau neutrinos, via a mechanism known as neutrino oscillation. By the time they reached Earth, the former electron neutrinos had become an equal mix of electron, muon, and tau neutrinos, accounting for the factor of one-third. Thus the experiment was the first indication that neutrinos are not massless — because neutrino oscillation is impossible unless neutrinos have mass. Davis won a Nobel Prize for his work.

Having produced about 71 cubic meters of gold starting in 1878, Homestake shut down in 2002. In 2007 it was established as the Sanford Underground Research Facility , a laboratory for physics experiments that need to be shielded from cosmic rays including the under-construction DUNE , which will detect neutrinos produced 1300 kilometers away at Fermilab.

So that’s why I’m here. Virtually. Massachusetts really, after some walking and bike riding in the Syracuse area with a couple of friends:

“Butterfly resting on a path”