Our EXO Collaboration meeting began with an excellent tour of the SNOLAB facility. Our guide – Samantha – did a wonderful job of leading our exceedingly large group through the mine and answering all of our questions. While it is not my first time underground – I’ve possibly logged 220 hours underground already at WIPP – this was the deepest I’ve ever been. While WIPP is 2150 ft underground, it is still above sea level. SNOLAB is 2km underground (currently the deepest science facility in the world), which is about 5600 ft below sea level. The depth certainly is noticeable: while both involve pressure changes as you descend in the alarmingly-loud lift, at SNOLAB the pressure change is much greater and the ambient rock temperature is 42C (108F) once you reach the bottom. I was quite concerned that this would make the faciilty a giant sauna – and we were warned to wear little clothing under the mine gear – but it wasn’t too bad. At least, I’ve been as sweaty at WIPP after a long walk and there the temperature is close to room temperature.
I was surprised by certain differences in the mines themselves. The mine SNOLAB is located in (Vale/Inco) is a nickel mine, while WIPP is a salt mine. This makes the mining process very different, especially since SNOLAB is located in a private mine run for profit while WIPP is a government facility for storing nuclear waste. Hence, Vale/Inco is concerned with getting as much nickel ore out as quickly as possible, while WIPP just wants a place to put the nuclear waste (as safely as possible). One big difference is the size – while I’m not sure how big the Vale/Inco mine is in itself, the area we were in today was certainly a small fraction and it likely rivaled the entire size of WIPP. Another is the size and condition of the drifts – at WIPP most of the drifts are very large, flat, and wide, while we walked through some much smaller areas today. There are tracks running through the drifts at the Vale/Inco mine for the carts they use, while at WIPP things tend to be more vehicular. This meant we did more walking today than we usually do at WIPP!
Perhaps the biggest difference (to me) was that SNOLAB has toilets. Proper toilets. WIPP mostly has porta-jons, while there is at least one mini-sewage treatment toilet underground at WIPP (it is a glorified porta-jon). At WIPP, the port-jons are scattered throughout the underground and are pumped out a few times a week. This fills the downwind drifts with an absolutely horrible stench that is overpowering even inside our clean rooms! In the underground at Vale/Inco they have a small sewage treatment facility that can support about 100 people (the SNOLAB facility currently is supporting about 40) and only a small amount of waste needs to be shipped above ground. There seemed to be no smell, even when looking directly into the open treatment tank.
Of course, the most meaningful difference is that everywhere at WIPP that isn’t within our cleanrooms is a salty environment. SNOLAB is a large, class-2000 (or better) clean room facility. Once they finish the expansion, they will have about 5000 m2 of clean room facility. Our clean room facility for EXO-200 is something close to 60 m2 (though it is a “more clean” area than SNOLAB). At WIPP we go in and out of the clean rooms, changing between clean room suits and mine gear. We have to leave the clean room if we want to eat something, use the porta-jon, or scratch our head. At SNOLAB, you leave your mine clothes at the door, take a shower, and put on clean room gear. Then you enter an area where there are physics experiments, computers, eating facilities, and toilets. It was quite civilized!
I’m very impressed with the lab. Not only is there an unimaginably large area – kilometers underground – that has complete infrastructure for scientific work, but it has been implemented at the class-2000 clean room level. Every cable and piece of support structure has been cleaned by hand. There are indescribably huge caverns for future experiments – it feels like walking into the bottom of a professional football arena. And then, crowning it all, is the SNO detector.
The SNO detector has a special place in physics history, because it is (more or less) where measurements were made that proved that neutrinos oscillated. And that meant they had mass. While SNO wasn’t the first experiment to measure solar neutrinos, it did it in a way that was more definitive than previous experiments. This has special meaning for me, as I was in high school at the time and remember when the science magazines covered their first results in 2001. This was the experiment that showed me we didn’t yet know everything about particle physics.
The SNO detector is still there, being refitted to look for a different types of reactions, including neutrinoless double beta decay. I won’t hold it against it that it will be competing with EXO, because it is the most beautiful thing I have ever seen. It is a huge sphere, covered with photomultiplier tubes (PMT’s) looking inwards, with a few looking outside the detector. For most of its operation it was filled with heavy water, where PMT’s would detect rings of light left by relativistic particles. Because it was empty, we were able to look both inside and outside the detector, from above. At 6m in radius, it is a large structure. But more importantly, it is impressive in its perfection. Over a million pieces were brought into the mine to be assembled, including acrylic pieces that form the sphere itself. They were carefully bonded together and inspected, and then carefully sealed. The PMT’s are themselves impressive on their own, but this has a large quantity are precisely installed.
It felt like viewing an ancient cathedral. No picture will be able to capture how small it makes you feel. Numbers alone do not do it justice. Realizing how many people poured their effort into it and how its influence has been felt around the world and how close you are to the location where the most important neutrinos were – it makes history seem so small – how else could someone so insignificant as me get so close to it. It was quite awe inspiring.