Science at SNOLAB
Photo by Gerry Kingsley
Why does matter dominate over anti-matter in the Universe?
What is the nature of Dark Matter?
What physics, if any, exists beyond the Standard Model of Particle Physics?
What are the mechanisms by which heavy elements are produced in the Universe?
These are questions which we hope to help answer at SNOLAB through the study of astroparticle physics. Astroparticle physics is a cross over field of research that studies subatomic particles associated with astronomical sources. On the one hand, astronomical sources can produce energetic particles (cosmic rays are an example) in quantities and at energies that can't be achieved in Earth based particle accelerators. Thus particle physicists turn to extraterrestrial sources to study some aspects of particle properties. On the other hand, understanding the properties of these particles gives insight into the creation and evolution of stars, galaxies and ultimately the Universe itself. Thus astrophysicists turn to the tools of particle physics to study the Universe on large. The principle topics in astroparticle physics being investigated at SNOLAB 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. There are also other areas of science that SNOLAB can contribute to. For instance, in the field of geophysics an experiment at SNOLAB will help deterimine why the Earth is hot by measuring geo-neutrinos. Another experiment will study the propogation of earthquakes.
A common feature of the astroparticle physics measurements at SNOLAB is that they involve extremely rare events that deposit smallamounts of energy in detectors. "Rare" means that some of these experiments expect to see at most one event per tonne of detector material per year. To overcome this the detectors are made very large and must be shielded from background interactions that can mask the physics of interest. The most pressing form of background comes from cosmic rays - the energetic particles produced throughout the cosmos and which are constantly bombarding the Earth. Similar to the way radiation is kept inside a nuclear reactor by placing many metres of concrete around the reactor core, astroparticle physics experiments shield from cosmic rays by locating the detectors deep underground. This is because the cosmic rays are much energetic than the radiation from nuclear reactors and require thicker shielding. In the case of SNOLAB, the experiments are located 2,000 m underground (6,800 ft). Moving a detector from the surface to underground is analogous to attempting to listen to somebody whispering to you on your cell phone while you are standing in a crowded night club while the band is playing. Locating astroparticle experiments underground is the equivalent to stepping out of the night club to listen to your telephone 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 important source of background that must be overcome is the naturally occurring radioactivity found in the rock out of which the laboratory is carved. This radioactivity is much less energetic than cosmic rays and can be shielded from with materials like lead (10s of cm) or even water (a few metres). Thus experiments located in SNOLAB tend to have bulky shielding placed around them and require large experimental halls ranging in size from a few metres to 10s of meters in size depending on the experiment.
However, shielding will not help if dust from the surrounding rock containing the slight amounts of radioactivity found there get into the detectors. To prevent this, SNOLAB is operated as a large clean room similar to what is done for the pharmaceutical or semiconductor industries. All equipment entering SNOLAB is carefully cleaned. Personnel coming into the laboratory are required to shower and change into clean room clothing. The levels of cleanliness required in the experiments at SNOLAB are 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 the selection and preparation of the materials used in the experiments.
Solar neutrinos are produced by the same fusion reactions in the core of the Sun that produce the heat and light which makes life on Earth possible. The density of the Sun is so great, it takes a million years for the light and heat produced by these reactions to travel the 700,000 km from the centre of the Sun to its surface (and then a mere 8 minutes for the light to travel the intervening 150 million kilometers from the Sun to the Earth). Solar neutrinos on the other hand interact so rarely that they have almost no interactions inside the Sun and escape almost immediately traveling at nearly the speed of light. This property of neutrinos to pass through the Sun as if it was not there offered astrophysicists the opportunity to view into the heart of the Sun and study the solar furnace that powers it. At the same time, because the sun is such an enormous source of neutrinos (about 60 billion solar neutrinos pass through your thumb nail every second) it could be used to study the properties of neutrinos themselves. Experiments such as SNO have made measurements of parts of spectrum of neutrinos from the sun which have revealed new properties of the neutrino (that they undergo flavour oscillations) and confirmed to very good precision that we understand the mechanisms that make the Sun shine.
The next generation of solar neutrino experiments such as SNO+ will be able to make precision measurements of different parts of the solar neutrino spectrum. This will further our understanding of the solar fusion mechanisms 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 of the most common forms of radioactivity is Beta Decay in which a neutron inside a nucleus changes into a proton, an electron and an antineutrino. The result is that the nucleus increases its atomic number (Z) by 1 while the mass (A) remains the same. The resulting nucleus is said to be a "daughter" of the parent nucleus and may itself be unstable and decay. However, for some atomic nuclei this decay is energetically forbidden meaning that it takes more energy to form the electron, antineutrino and daughter nucleus than is present in the parent nucleus. For a few nuclei, it is instead possible to undergo Double Beta Decay in which the nucleus emits 2 electrons and 2 antineutrinos at the same time. The resulting daughter nucleus has changed its atomic number by +2 (Z -> Z+2). To date there are only about 10 nuclides where double beta decay has been observed. These are extremely rare decays. Their half lives (the time it takes for 1/2 of the parent material to decay) is typically on the order of 1018 to 1021 years. For comparison, the universe is 1010 of years old. Theoretically there is a process related to Double Beta Decay called Neutrinoless Double Beta Decay (0νββ). This can only happen if the two neutrinos that would have normally been emitted by the decaying nucleus "cancel" each other out. Such a cancellation can occur if the neutrino has the property of being its own antiparticle. Such a particle is said to be a Majorana particle named after the Italian physicist Ettore Majorana. There is a claim that neutrinoless double beta decay has been observed but it is very controversial. A number of 0νββ experiments are presently underway around the world. If the observation of 0νββ is confirmed, these experiments will make detailed measurements of the neutrino's properties. If the observation of 0νββ is not confirmed, these experiments will push the limits of sensitivity much lower. The discovery of Neutrinoless Double Beta Decay would be very important to particle physics. It would tell scientists important properties of neutrinos. It would allow a measurement of the neutrino mass (the oscillation experiments such as the solar neutrino measurements can only measure the mass differences between neutrinos). It may also help explain through a process called leptogenisis why the universe is dominated by ordinary matter rather than be a more symmetric mix of normal and antimatter. However, Neutrinoless Double Beta Decay is expected to be even more rare than "ordinary" Double Beta Decay. Assuming the claim to have observed 0νββ is incorrect, the current best limits on sensitivity for 0νββ is a half life of 2x1025 years. The next generation of 0νββ detectors will have to be 10 to 100 times bigger than the current efforts.
At SNOLAB, the SNO+ experiment intends to use tellurium as a target material dissolved in liquid scintillator to look for Neutrinoless Double Beta Decay. 0νββ events will be distinguished from regular Double Beta Decay and other backgrounds by their energy deposition in the scintillator. The EXO experiment will use xenon gas in a Time Projection Chamber. EXO intends to identify the daughter nucleus (barium) from the xenon to distinguish both the 2 neutrino and the neutrinoless double beta decays from backgrounds.
A supernova is the spectacular death of a massive star in an explosion that for a few brief weeks out shines the entire galaxy the star is situated in. 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. For a supernova, almost all of the energy produced is in the form of neutrinos. It is now thought that neutrinos are an integral part of the explosion mechanism for supernovae and in the production of heavy elements created during the explosion. Neutrinos also have an important feature in that they escape the exploding star before the light emerges. So detecting a burst of neutrinos from a supernova would allow optical astronomers to study the "turn on" of the supernova long before it would normally be observed. There exists a network of neutrino detectors around the world that belongs to SNEWS (Supernova Early Warning System) and, in the event that a burst of neutrinos is detected, SNEWS will alert the astronomy community (and the the amateur astronomer community as well). During its operation, the now decommissioning SNO experiment was a participant in SNEWS. Future experiments at SNOLAB that can detector supernovae are SNO+ and HALO (which would be a dedicated supernova detector).
Astronomers and astrophysicists have determined with ever increasing certainty that most of the matter in the universe is non luminous or "Dark". Furthermore it is now known that most of the Dark Matter is not even the "ordinary" matter that makes up the chemical elements that compose the Earth and us. This extra-ordinary Dark Matter is called "non baryonic" and 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 appears to have almost no interactions with ordinary matter by mechanisms that produce light, it does interact with gravity and that gravitational interaction shapes and holds together galaxies and even clusters of galaxies. Nobody knows exactly what the Dark Matter particles are. At one point it was thought that they may be neutrinos but we now know that while neutrinos do have mass, they are not heavy enough to explain the Dark Matter. A leading candidate for the Dark Matter particle is that it is a WIMP - a "Weakly Interacting Massive Particle" which is a possible particle predicted by a theory called Super Symmetry.
At SNOLAB there are a number of experiments to search for WIMPs. The PICASSO experiment has been running a number of different detectors at SNOLAB for several years and is searching for spin dependent interactions from WIMPs using fluorine as a target nucleus in the form of freon droplets suspended in a gel matrix. The DEAP/CLEAN program is building detectors based around liquid argon and neon and look for WIMPs by examining the scintillation light deposited by interactions in the cryogenic liquid. The Cryogenic Dark Matter Experiment (CDMS) intends to look for WIMP interactions in large solid state detectors made from germanium. The COUPP experiment is currently operating a 4kg CF3I bubble chamber at SNOLAB, and will install a 60kg CF3I bubble chamber in the fall of 2012.
Geo-neutrinos is the term used for the electron antineutrinos produced by the decay of radioactive materials in the Earth - in particular uranium and thorium. Geo-neutrinos were first detected by the KamLAND experiment in Japan. The interest in geo-neutrinos is that they are a way 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 (present day) heat flux. There is an interest to measure geo-neutrinos at SNOLAB because it is located in the centre of the North American continent and "sees" a different distribution of geo-neutrinos than what is observed in Japan. The SNO+ experiment will do this using a detector containing 1 million litres of liquid scintillator.
Being located 2km underground, SNOLAB has a natural association with the study of seismology. A completed project that took data at SNOLAB was PUPS (Polaris Underground Project at SNOLAB) which studied how the propagation of earthquakes change as they approach the surface of the Earth. By placing seismometers at different depths, PUPS observed earthquakes as they traveled up through the Earth.
It is generally recognized that the first pre-biotic organic molecules on earth and elsewhere in the solar system must have been formed by abiogenic reactions. To date it had been largely assumed that after the evolution of life on earth, biologically mediated reactions overprinted evidence of this pre-biotic abiogenic history. Current research indicates that abiogenic reactions that contributed to the formation of primary organic molecules on the early earth continued to play a significant role in the production of hydrocarbons in the Precambrian rocks of the earth’s crust. In addition, one of the most exciting scientific discoveries of the past decade has been the discovery that microbial life exists in the subsurface at depths hitherto unanticipated - the so-called “deep biosphere”.