Searching for dark matter with SuperCDMS in Creighton Mine
Why is a laboratory kilometres below the Earth’s surface the best place to search for the dark matter that holds entire galaxies together?
After spending most of the last year doing research for a master’s project called “Direct detection of dark matter”, the SuperCDMS experiment seemed like a good place to start this blog. Calculations suggest that roughly 26% of the content of the Universe consists of so-called “dark matter”. Although we are unable to see it, we can observe gravitational effects on other objects, such as galaxies and galaxy clusters, which give evidence for its existence. Currently, the most popular candidate for dark matter is the Weakly Interacting Massive Particle (WIMP). This particle would interact with other particles only through gravity and the weak interaction (one of the fundamental forces that is only effective over very small distances), making it extremely difficult to detect.
One method of searching for WIMPs is direct detection. It is thought that the Milky Way, like other spiral galaxies, is surrounded by a dark matter halo, which causes a wind of dark matter particles on Earth. Trillions of these particles pass through our bodies every second. In fact, this dark matter flux is calculated to be so large that a measurable fraction of WIMPs should scatter off nuclei, even though they interact so weakly. WIMPs can then be detected by observing target nuclei in a detector, following a collision with a dark matter particle.
The SuperCDMS (Cryogenic Dark Matter Search) experiment uses germanium target nuclei in the search for WIMPs, and is run by an international collaboration made up of 111 members from 26 different institutions.¹ In SuperCDMS, germanium crystals with a diameter of 76 mm and a mass of 600 g are arranged in towers and attached to sensors, which give a signal every time an interaction occurs.²
The reason why the location of this experiment is so important is that background noise from other types of particle makes it extremely difficult to identify a dark matter signal. This background radiation leads to electron recoils in the target material, which can be difficult to distinguish from the nucleus recoils due to WIMPs. External sources of radiation can be reduced significantly by siting the detector deep underground to provide shielding from highly penetrating charged particles called muons, which can be generated by cosmic rays. A large number of similar events must be detected before we can be confident that they are not simply due to background radiation. A definitive dark matter signal would allow us to learn more about the nature of WIMPs, such as their mass and direction, and hence gain a much deeper understanding of the content of the Universe.
After a period at the Soudan Mine in Minnesota, construction is underway for the next stage of the SuperCDMS experiment at the SNOLAB facility in the Creighton nickel mine in Ontario. The U.S. Department of Energy, the National Science Foundation and the Canada Foundation of Innovation initially agreed to contribute a total of $32.4 million to cover the construction of the experiment.³ At 2 km deep, SNOLAB is the second-deepest underground laboratory in the world and is so well-shielded by the rock above that only 0.27 muons pass through each square metre every day, compared to around 15 million at sea level.⁴
Some issues that come with operating in this type of location have been taken into consideration. For example, the SNOLAB facility has been built to withstand seismic events due to the mining activity. As well as this, the temperature of the rock at the depth of SNOLAB is 42 ºC, meaning that a significant amount of geothermal heat must be removed using a ventilation system. The location of the SNOLAB facility has also been carefully chosen to avoid rock stress and fracture zones.⁴
This should give an idea of why Creighton Mine is an excellent location for a dark matter direct detection experiment like SuperCDMS, but also how the experiment has been adapted for its environment. Although public tours are not available, a virtual tour of SNOLAB can be found here. I will follow a similar theme in my next blog posts, but will consider a variety of experiments from all over the world, so please let me know if you have any suggestions!
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