Visiting the 2km deep SNOLAB
Stephen Hawking visited SNOLAB in Sudbury, Ontario—twice! Once in 1998, and once in 2012. On June 17, 2013, I too, visited SNOLAB with the Royal Astronomical Society of Canada in Toronto (RASCTC) for a tour of the magnificent lab! Originally known as the Sudbury Neutrino Observatory (SNO), where a solar neutrino experiment successfully helped solve the solar neutrino problem, SNOLAB has since expanded to include both neutrino experiments as well as dark matter experiments.
What is the solar neutrino problem? The solar neutrino problem was a large discrepancy between the amount of solar neutrinos predicted by theoretical models and the amount detected in experiments. The reason why there was a discrepancy is because neutrinos oscillate (change "flavors") when they interact with matter. Previous detectors were only able to detect electron neutrinos, while muon and tau neutrinos passed through undetected.
In 2001, experiments at SNOLAB successfully confirmed that neutrinos oscillate and helped solve the solar neutrino problem. Now, experiments prepare to uncover the dark matter particle.
It was not known that neutrinos oscillated and had mass (they were thought to be completely massless) in the Standard Model prior to experiments. An experiment held in Japan called the Super-Kamiokande found that neutrinos did have mass (albeit an amount tiny enough to say they're NEARLY massless) in 1998. In 2001, the SNO experiment produced results confirming that neutrinos oscillate. These findings solved the solar neutrino problem and changed the Standard Model, and the Nobel Prize in Physics was awarded to the leaders of the experiments in 2002.
SNOLAB is located about 2 km below ground level, and nearly 2 km into the Vale Creighton mine. The purpose for placing the lab at this depth was to ensure that only neutrinos get through. Neutrinos interact very weakly with matter, and thus have no problem getting through the Earth. In fact, TRILLIONS of neutrinos pass through your pinky finger EVERY SECOND! Not just you, but EVERYTHING. A neutrino could pass through a LIGHT-YEAR (nearly 9.5 TRILLION km) of lead completely unhindered! These guys are tough to catch! Hunting for dark matter here makes perfect sense for the same reason as studying neutrinos—they are thought to interact very weakly with matter.
At the entrance, we were provided with coveralls, steel-toe boots and hardhats for safety in the mine. The lift cage transported us from ground level into the mine at 2100 ft/min (10.6 m/s)! A pressure change was experienced on the way down (pressure is 25% greater than ambient in the mine). We were given gum to chew while in the lift cage to counteract the discomfort experienced in our ears due to the pressure change (if you've ever had your ears pop on a flight, then you know exactly what I'm talking about). Once we reached the mine, we headed on our 2-km trek to SNOLAB!
Before heading into the actual lab, we first went through a boot wash before going into the shower area. Here, there is a "dirty area", and a "clean area" separated by showers. We had to remove all our gear and leave it in the “dirty area”, then take showers before entering the “clean area”. Once clean, we got disposable suits and hairnets, and sanitized socks and boots. The cleanliness level maintained in SNOLAB is to prevent excess radioactivity from entering the lab and interfering with experiments. Within the reactors, radioactivity is about 100 million times less than that found in normal environments! Even our cameras had to go through a “carwash” before entering the lab! None of the cameras were damaged in the process.
My group was first introduced to the Helium And Lead Observatory (HALO). Here, there is about three million dollars worth of Helium-3 encased in a total of 79 metric tons of lead! When construction of HALO is complete, this observatory will detect supernovae in our galaxy by looking for neutrino bursts. Very exciting!
Next, we were taken to the dark matter experiments! COUPP, one of the experiments, is designed to detect dark matter in the form of Weakly Interacting Massive Particles (WIMPs) by using bubble chambers filled with superheated fluid (CF3I, or trifluoroiodomethane). The fluid is superheated at a level such that gamma rays and beta decay are not detected, thus eliminating their interference with the experiment. When a WIMP passes through the superheated fluid and interacts with it, a single bubble will be formed. Alpha decay will also produce bubbles, but their ultrasonic acoustic signatures can be used to distinguish them from WIMP interactions. Really cool!
Next are the DEAP 3600/MiniCLEAN dark matter detectors! These are two separate detectors in an effort to find the elusive dark matter particle. They both use cryogenic fluids (DEAP 3600 will use liquid Argon, and MiniCLEAN, liquid Neon) for the purpose of detecting WIMPs. They will be hundreds of times more sensitive to WIMP scattering than current dark matter detectors. For MiniCLEAN, photomultipliers will be attached to a giant sphere (in the image posted). It will then be inserted into a vessel which will be filled with the cryogenic fluid. Liquid Argon and liquid Neon are great cryogenic fluids for dark matter detection because they have a high enough density for interaction to occur. When a WIMP interacts with the fluid, scintillation will occur (the area struck by the WIMP will glow). The photomultipliers are extremely sensitive to light, and will multiply the light produced by 100 million, ensuring that any occurring interactions are detected. Awesome!
So, what with SNO? Well, SNO hasn’t been running since 2006. But there are some very cool facts about it I’d like to share! SNO was a Cherenkov radiation detector that used heavy water (deuterium oxide, or D2O) to detect neutrinos. D2O allowed for the different "flavors" to be detected when a neutrino oscillated. The detector held 1000 metric tons of D2O, an amount worth about 300 MILLION dollars! All the heavy water has since been removed from SNOLAB. The detector used in SNO will now be used in a new experiment called SNO+ (we got to see the service area; still amidst construction). Instead of D2O, SNO+ will use liquid scintillator to detect low energy solar neutrinos and geoneutrinos, as well as study neutrinoless double beta decay in Neodymium (Nd).
The tour was an experience of a lifetime! I encourage any of you to visit it if you ever get a chance. Nothing like going to a place where discoveries have been made, and new ones may come! Here's to dark matter being the next discovery at SNOLAB!
A special thank you to Daryn Cressy for making this tour happen, Samantha Kuula for being such a great host and tour guide, and Dr. Clarence Virtue for giving our group such a great tour and being so informative. RASCTC and Charles Darrow, thank you for putting this tour together!
Image Credit: Charles Darrow
Further Reading:
SNOLAB (a multitude of info can be found here): http://www.snolab.ca/
http://www.hep.upenn.edu/SNO/intro.html
Photomultipliers (detailed info): http://learn.hamamatsu.com/articles/photomultipliers.html
Solar Neutrino Problem: http://www.astro.cornell.edu/.../astro201/sun_neutrino.htm
Neutrinos: http://icecube.wisc.edu/info/neutrinos
http://www.astronomynotes.com/starsun/s4.htm
Dark Matter: http://science.nasa.gov/.../focus-areas/what-is-dark-energy/
Deuterium Oxide: http://chemistry.about.com/.../f/What-Is-Heavy-Water.htm
Article by Sophia Nasr.
Sophia Nasr is currently studying astrophysics at York University. She is an active team member at the York University Observatory, a host on York Universe Radio, and the Vice President of the York University Astronomy Club. Actively engaged in astronomical events and scientific outreach, Sophia began writing for Facebook page "All Science, All the Time" to help spread information about astronomy while simultaneously learning more about her passion.