What are neutrinos?

Ordinary matter is made of neutrons, protons, electrons and neutrinos. Neutrinos are emitted when neutrons transform into protons in nuclear reactions. They are produced in the nuclear reactions in the sun and in those ignited by the collapse of a dying star. They're so tiny that they can pass through solid matter without colliding with any molecules and they travel at close to the speed of light, making them incredible intergalactic messengers.

What are you hoping to find?

We are looking for neutrinos as a result of a gravitational collapse of a star in our galaxy. Correlating the number of the detected neutrinos with predicted events will help us explain their nature.

Who provides your funding?

The main source of funding for HALO is from the Natural Sciences and Engineering Research Council of Canada (NSERC). Additional funding for HALO installation is provided by SNOLAB. Some HALO collaborators receive additional assistance from the National Science Foundation (NSF), and the United States Department of Energy.

Why is this important?

Under typical conditions, a neutrino is 100 billion billion times less likely than light to interact with matter. This makes supernova neutrinos excellent intergalactic messengers. A signal from a galactic supernova will help refine theroretical supernova models and our understanding of one of the fundamental building blocks of nature.

In addition supernova remnants are extremely important for understanding our Galaxy. They heat up the interstellar medium, distribute heavy elements throughout the Galaxy, and accelerate cosmic rays.

Why is the detector located deep underground?

The Helium and Lead Observatory (HALO) is located at SNOLAB which is 2,000 m underground (6,800 ft). HALO makes astroparticle physics measurements which involves extremely rare events that deposit small amounts of energy in detectors. In fact HALO expects to see less than one event per tonne of detector material in the event of a supernova at a distance of 10 kpc. Therefore HALO 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.. Even at the depth of SNOLAB there are still cosmic rays passing through the experimental area but the rate is about 50 million times less than would occur if the experiment was located on the surface of the Earth.

What is a supernova?

A Supernova (plural supernovae) represents a catastrophic event for a star that effectively ends its active lifetime. These events can briefly outshine entire galaxies and radiate more energy than our sun will in its entire lifetime. They are also the primary source of heavy elements in the universe.

On average, a supernova will occur about once every 25-50 years in a galaxy the size of the Milky Way. Put another way, a star explodes every second or so somewhere in the universe.

Exactly how a star ends its life depends in part on its mass. Our sun, for example, doesn't have enough mass to explode as a supernova. A star can go supernova in one of two ways in which each type is broken up into subtypes.

Type I supernova: star accumulates matter from a nearby neighbor until a runaway nuclear reaction ignites.

Type II supernova: star runs out of nuclear fuel and collapses under its own gravity.

For a star to explode as a Type II supernova, it must be at several times more massive than the sun. Like the sun, massive stars will eventually run out of hydrogen and then helium fuel at its core. However, massive stars above a certain limit will have enough mass and pressure to fuse carbon. Gradually heavier elements build up at the center, and it becomes layered like an onion, with elements becoming lighter towards the outside of the star. Once the star's core surpasses a certain mass (the Chandrasekhar limit), the star begins to implode. Eventually the implosion bounces back off the core, expelling the stellar material into space — the supernova.

Type Ia supernovae are generally thought to originate from white dwarf stars in a close binary system. As the gas of a binary companion star accumulates onto the white dwarf, the white dwarf is progressively compressed, and eventually sets off a runaway nuclear reaction inside that eventually leads to a cataclysmic supernova outburst.

Type 1b and 1c supernovas also undergo core-collapse just as Type II supernovas do, but they have lost most of their outer hydrogen envelopes.

Since neutrinos are thought to originate only from the cores of massive stars as they die, HALO is only sensitive to supernovae resulting from gravitational collapse.

Where can I learn more?

You can explore our website for descriptions of the contruction of HALO, the physics behind the detector and recent results. If you have a specific question and can't find the answer, you can send an email message to cjv#snolab.ca.