Rocking the Neutrino Laboratory like its 1999 - an early tour of the first Homestake Neutrino Lab

One of the highlights of 1999 for me was a tour of the Ray Davis Neutrino laboratory at the 4850 level of the Homestake gold mine at Lead, SD.  It was a special tour set up by one of the miners and a physics professor at the South Dakota School of Mines & Technology. My good friend and fellow student, Mark was also there.

At that point, the mine was still operating, although it was late days and everyone knew that it was going to be shut down soon. Mark and I had just graduated from high school and were starting the first year of undergraduate education at Tech.

The mine has several access points. We started at the Ross headframe in Lead. There was a safety briefing at the beginning and everyone was assigned a helmet, eye protection, and steel toed boots. Mark's father, Jim worked at the mine and had previously outfitted us with old timey head protection. Once outfitted and organized, we caught a ride to the 4850 level on the hoist.

John and Mark ready to go

Accessing the lab involves hiking from the hoist access through a large tunnel for something like a half mile. The ventilation system and ore carts meant that the hike was warm or cold and we were constantly hugging the wall to avoid getting squished by miniature ore trains. There was no light, except for the headlamps and lights on the front of the passing ore carts. The tunnels were mostly dry, with occasional wet spots dripping water and making puddles. Ventilation pipes, water pipes, electrical wires and other infrastructure lined the ceiling. All the blowing air and moving equipment fill the tunnels with a roar of sound.

The lab is accessed by a smaller tunnel that shoots off the side of the main tunnel. There are, of course, threatening signs warning off miners without permission and, apparently other trespassers.

The first thing you notice when entering the lab is that it is mercifully quiet and racks of fluorescent lines buzz away, lighting up the facility in ghostly green light. The first room is a laboratory space with chemistry apparatus lining a wall and stacks of lead bricks. Old computers with orange and black screens cast a strange glow.

The strange and wonderful chemistry setup at the 4850 level.

The detector room itself is a large cavernous space almost completely filled with a large steel tank and numerous gray cask boxes about 1 foot by 1 foot by about four feet long. The steel tank is the main neutrino detector and is filled with perchloroethylene, dry cleaning fluid. The casks were scintillator detectors filled with a mineral oil.

Mark, safe from eye injury, but vulnerable to the rocks we were trying to drop on his head the entire trip. The big grey tank under his hand is the main detector.

Scintillators stacked above the main detector in the Davis cavern. 

The way it works is, neutrinos travel through the earth down to the 4850 level and a few of them, maybe one per month, interacts with the chlorine in the perchloroethylene tank. When that interaction occurs the chlorine ion is transformed to an argon isotope and a flash of light is emitted (I think). I think the light is detected by the scintillators. But the most interesting things is how the argon isotope is detected. Periodically, maybe once a month or once a quarter, they filter the entire huge tank of perchloroethylene for argon isotopes. The exact method is unimportant and a mystery to me, but Ray Davis somehow managed to develop a method to distill and collect just a few atoms from a tank that holds one hundred thousand gallons of fluid. The individual atoms of argon isotopes are counted up and used to determine how many interactions occurred in that period.

It is a truly remarkable feat that Davis managed to do this experiment in an active mining environment over many years with a precision good enough to tell that there were not enough neutrinos detected within a factor of three. This result first proved that the sun was the source of neutrinos, the first direct proof of fusion in the sun (as opposed to coal, an actual scientific hypothesis that survived into the 20th century). However, his work, specifically the smaller number of neutrinos they detected turned out to be one of the lines of evidence supporting the idea that there are multiple types of neutrinos and that they oscillate, or change flavor as they travel, and also that they have a small but non-negligible mass. This complete hypothesis for neutrino oscillation came about with a bunch of other work at Japan's Super-Kamiokande and Canada's Sudbury Neutrino labs. Ray Davis deservedly shared the Nobel Prize for this work in 2002.

Left to right, Mark, Prof. Bob Corey from SDSM&T, Jim Hanhardt, and (I think this was Ken Lande from U of Penn., but his current picture looks nothing like this guy. He was really interesting to talk to and the entire experience was inspirational about physics. Too bad I don't remember his name for sure). 

To finish the story, it was announced that the mine would be shut down on September 11, 2000, about a year after this tour. In 2002, Barrick purchased the Homestake mining company and agreed in principal to donate the facility to the state for use as a National Underground Science Laboratory (NUSEL). At this time, the mine went into care and maintenance mode, high grade mining and milling, and environmental cleanup. In June, 2003 Barrick closed the mine by shutting down the pumps and the lower levels were allowed to flood with natural ground water. The Ross and Yates shafts were sealed and access to the mine was effectively shut down. In 2004, the state of South Dakota made funding available to begin the process of converting the mine to a lab. This work required refurbishing the access and pumping out water. In March 2008, pumping began at the 4600 level with a push of a button by the Lab Director, Jose Alonso. The water level dropped down to the 4850 level in May 2009, thus opening the Davis Cavern up to access for eventual construction of the Interim Laboratory.

For more information on the current laboratory efforts, follow this link to the Sanford Underground Laboratory at Homestake, DUSEL.