Science at SNOLAB

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Photo by Gerry Kingsley

 

Why is the Universe made of matter instead of anti-matter?

 

What is dark matter?

 

Are there physics outside the Standard Model of Particle Physics?

 

How are heavy elements produced in the universe?

 

These are some of the questions we work to answer at SNOLAB by studying astroparticle physics. Astroparticle physics is a crossover field that studies subatomic particles from astronomical sources. Astronomical sources can produce energetic particles (for example, cosmic rays) in amounts and energies that can't be achieved in particle accelerators, so particle physicists turn to extraterrestrial sources to study some particle properties. Additionally, understanding the properties of these particles gives insight into the creation and evolution of stars, galaxies and ultimately the universe itself, so astrophysicists turn to the tools of particle physics to study the universe. The topics in astroparticle physics experiments at SNOLAB explore are:

  • Low Energy Solar Neutrinos;
  • Neutrinoless Double Beta Decay;
  • Cosmic Dark Matter Searches and
  • Supernova Neutrino Searches.

However, the questions that can be answered at SNOLAB are not limited to particle and astrophysics. SNOLAB’s unique location makes it an idea site for other types of science as well! For instance, a geophysics experiment at SNOLAB helps determine why the earth is hot by measuring geo-neutrinos. Another studies the propagation of earthquakes.

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A common challenge of astroparticle physics experiments at SNOLAB is that they search for extremely rare events. "Rare" means that some experiments expect to see at most one event per tonne of detector material per year. To overcome this, the detectors are very large. They also have to be shielded from background radiation that would interact and hide the physics of interest. 

The majority of background comes from cosmic rays - energetic particles produced throughout the cosmos and which are constantly bombarding the earth. Locating these experiments underground shields them from these cosmic rays. In the case of SNOLAB, the experiments are located 2,000 m underground (6,800 ft). On the earth’s surface, trying to pick out dark matter in the data would be like trying to hear a conversation on your phone at a rock concert – impossible. Going underground to get away from the background radiation is like stepping outside to hear the call. Even at the depth of SNOLAB there are still cosmic rays passing through the experiments (about 1 through every square metre every 3 days) but the rate is about 50 million times less than would occur if the experiment was located on the surface of the earth.

Another source of background radiation is the naturally occurring radioactivity found in the rock the laboratory is built in. This radioactivity is much less energetic than cosmic rays, so a few cm of lead or a few metres of water are enough to shield the experiments from it. As a result, most of the SNOLAB experiments are wrapped in bulky shielding before they begin collecting data. 

However, shielding doesn’t prevent dust with trace amounts of radioactivity getting into the detectors. To protect the experiments, SNOLAB is operated as a large clean room (similar to what is done in the pharmaceutical industry). All equipment entering SNOLAB is carefully cleaned. Personnel coming into the laboratory are required to shower and change into clean room clothing. The level of cleanliness required for the experiments at SNOLAB is extreme. Typically, the interiors of the detectors have 100 million times less radioactivity than what is naturally found in the environment. To achieve these levels of radiopurity, great care is taken in selecting and preparing materials for the experiments.

Astroparticle Physics

Solar Neutrino Physics

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Solar neutrinos are produced by the same fusion reactions in the sun that produce heat and light. Because the sun is so dense, it takes a million years for the light and heat to travel the 700,000 km from the centre of the sun to its surface (and then 8 minutes for the light to travel to earth). Solar neutrinos interact so rarely that they escape almost immediately, traveling at nearly the speed of light. These solar neutrinos allow astrophysicists to study the core of the sun. At the same time, because the sun produces so many neutrinos (about 60 billion solar neutrinos pass through your thumb nail every second!) they can be used to study neutrino properties. 

Solar neutrino experiments like SNO+ will make precise measurements of the solar neutrino spectrum. This will help us understand solar fusion, and ultimately the evolution and fate of the sun. These measurements will also test in detail the mechanisms for neutrino flavour oscillations.

Search for Neutrinoless Double Beta Decay

One common form of radioactivity is beta decay, where a neutron inside a nucleus changes into a proton, an electron and an antineutrino. Examining this process led scientists to consider the neutrino as a particle, to explain the energetics of beta decay. However, some atomic nuclei don’t have enough energy to create a proton, electron, and antineutrino. For a few of these nuclei, it is instead possible to undergo double beta decay, in which the nucleus emits two electrons and two anti-neutrinos at the same time, which requires less energy than beta decay. To date, double beta decay has only been observed in 13 nuclides – this type of decay is very rare. The half-lives of these nuclides (the time it takes for half of the parent material to decay) are typically on the order of 1018 to 1021 years. For comparison, the universe is 1010 years old. Theoretically, there is a process related to double beta decay called neutrinoless double-beta decay, in which the nucleus emits two electrons and no neutrinos. This can only happen if the two neutrinos that would have normally been emitted by the decaying nucleus annihilate each other. This annihilation could occur if the neutrino is a Majorana particle – a particle that is its own antiparticle.  SNO+ is one example of a SNOLAB experiment project that is focused on the search for neutrinoless double-beta decay.

Supernova Watch

A supernova is the spectacular death of a massive star in an explosion that for a few brief weeks outshines the star’s entire galaxy. Neutrinos are intimately connected to supernovae. During the bulk of a star's life, about 99% of the energy produced is in the form of light and about 1% as neutrinos. When a star goes supernova, almost all the energy produced is in the form of neutrinos. Neutrinos are now thought to be an integral part of the explosion mechanism for supernovae and of the production of heavy elements created during the explosion. Neutrinos also escape the exploding star before the light emerges, so detecting a burst of neutrinos from a supernova would allow optical astronomers to study the supernova ‘turning on’ long before it would normally be observed. There is a network of neutrino detectors around the world called the Supernova Early Warning System (SNEWS) and, in the event that a burst of neutrinos is detected, SNEWS will alert the astronomy community. The SNO experiment was part of SNEWS, HALO (a designated supernova detector) is part of the network, and SNO+ will also be involved with SNEWS.

Search for Cosmological Dark Matter

Astronomers and astrophysicists have determined that most of the matter in the universe is ‘dark’ – it doesn’t interact with the electromagnetic spectrum (light). This dark matter makes up about 25% of all the mass in the universe, while the ordinary matter that we are made of only comprises about 5% (the remaining 70% of the universe is so called ‘dark energy’). Dark matter is fundamental to the structure and evolution of galaxies including our own. While it has almost no interactions with ordinary matter through mechanisms that produce light, it does interact with gravity and that gravitational interaction shapes and holds together galaxies. Nobody knows exactly what the dark matter particles are. At one point neutrinos were a candidate, but we now know that while neutrinos do have mass, they are not heavy enough to be dark matter. A current leading candidate for the dark matter particle is a WIMP - a ‘Weakly Interacting Massive Particle’.

At SNOLAB there are a number of dark matter experiments searching for WIMPs. Currently, MiniCLEAN, DEAP-3600, DAMIC, and PICO are taking data, and NEWS and CDMS-SNOLAB will be operating within the next few years.

Geophysics

Geo Neutrinos

Geo-neutrinos are the electron antineutrinos produced by the decay of radioactive materials in the earth - particularly uranium and thorium. Geo-neutrinos were first detected by the KamLAND experiment in Japan. Scientists are interested in geo-neutrinos because they can be used to measure the total amount of heat produced in the earth from radioactivity. Heat from radioactivity is thought to account for between 40% and 100% of the earth's total heat flux. Because SNOLAB is located in the center of the north american continent, it ‘sees’ a different distribution of geo-neutrinos than what is observed in Japan, making the combined data more useful. 

Seismology

Being located 2km underground, SNOLAB is a natural place to study seismology. The now completed PUPS project (Polaris Underground Project at SNOLAB) studied how the propagation of earthquakes changes as they approach the surface of the earth. By placing seismometers at different depths, PUPS tracked earthquakes as they traveled up through the earth.

Genomics and Bioinformatics

Although people have been working underground for centuries, relatively little is known about potential health implications of working under high pressure for extended periods of time. As mines continue to get deeper (Creighton mine is now excavating from the 8200 ft level), the pressure increases and it becomes more important to understand the impacts on workers. The FLAME experiment uses fruit flies to study the metabolic and genomic responses to increased atmospheric pressure. The findings from this research can be used by mining companies to support a healthier workforce.

Another biology experiment is exploring how the reduced background radiation underground impacts cell development and repair. Since life on Earth has evolved in the presence background radiation, scientists hypothesize that removing it may be detrimental to living systems. This research can help determine whether that is the case, and if it is what the negative impacts are.