The Sudbury Neutrino Observatory

 

People in Sudbury are used to the idea of digging hundreds of metres underground and finding all sorts of valuable things, such as nickel and copper.  But scientists hope to find something even more valuable in the rock beneath Sudbury over the next few months:  namely, answers to some of the most vexing questions about the nature of the universe itself.

Those answers will come from one of the most ambitious scientific projects ever undertaken in this country, the Sudbury Neutrino Observatory. 

Neutrinos are sub-atomic particles, like the protons, neutrons and electrons that make up all matter, but very much unlike them, too, in that it is as close to nothing as it is possible to get.  Neutrinos have no electrical charge and little or no mass.  Their name, coined by physicist Enrico Fermi in 1934, means “little neutral one” in Italian.

Because they have no charge and are so tiny, neutrinos hardly ever interact with ordinary matter in any way.  In fact, in the last second, trillions of them have sleeted through your body, and you didn’t feel a thing.  They also pour right through the Earth itself as it if wasn’t there.  To a neutrino, a mile or a hundred miles of solid rock might as well be empty space.

Despite the fact neutrinos don’t have any appreciable effect on much of anything, scientists would desperately like to no more about them, for two main reasons.  One, they’re produced by the nuclear reaction that powers the sun, and hence can tell us more about how stars work, and two, they may hold the key to the fate of the universe.

That’s where the Sudbury Neutrino Observatory (SNO, for short) comes in.  It’s by far the most sensitive neutrino detecting device every built.

Current theory holds that the sun should be producing a certain number of neutrinos as it goes about its business of shining.  Trouble is, all of the neutrino detectors built to date have detected only one third as many neutrinos coming from the sun as the theory predicts.  Some scientists hold that the apparently missing neutrinos are simply spontaneously changing into a specific type (or flavor, as the scientists call it) of neutrino that current detectors can’t detect.  SNO should be able to shed some light on this topic.  If there really are as few neutrinos coming from the sun as other detectors have indicated, then what scientists think they know about how the sun works is wrong, and a lot of theories are going to have to be rewritten.

The other question SNO is designed to answer once and for all is whether or not neutrinos have mass, and if they do, how much.

The interest in this question stems from the fact that astronomers, when they add up the mass of all the visible matter in the universe–all the galaxies, dust clouds, and so forth–keep coming up with a total so small that it can’t explain how large structures like galaxies formed in the first place, or what keeps them together now.  As a result, astronomers believe as much as 90 percent of the universe is actually made up of “dark matter,” matter which we can’t detect but which nevertheless exerts a gravitational pull because of its mass.  There have been a number of candidates proposed for this dark matter, some of them quite bizarre.  The lowly neutrino, though, is so numerous that if it has mass, even a little, by itself it could go a long way toward solving this “mystery of the missing mass.”  And if it has enough mass, it could even answer another Big Question:  will the universe keep expanding forever until is nothing but an immeasurably thin cold soup of elementary particles, or does it have sufficient mass to eventually stop expanding and collapse back in on itself, ending in a fiery Big Crunch just as it began in a fiery Big Bang, from there possible exploding again, collapsing again, exploding again and so on, essentially going on forever?

These are important questions, indeed.  So what does the Sudbury Neutrino Observatory look like?

At its heart is a 12-metre-diameter sphere made of five-centimetre-thick transparent acrylic.  This sphere is filled with 1000 tonnes of heavy water (borrowed from Atomic Energy Canada Ltd., which uses heavy water in the operation of CANDU nuclear reactors).  Heavy water is simply ordinary water with an extra proton in its nucleus.  It’s called heavy water because it weighs 10 percent more than ordinary water; otherwise it’s indistinguishable.  (A tiny percentage of all water is heavy water.)

This sphere filled with heavy water is suspended inside an outer container filled with ordinary water, which acts as a shield against natural radioactivity from the surrounding rock, which could confuse the sensors, which are incredibly sensitive devices able to detect a single photon of light emitted from anywhere inside the heavy-water sphere.  That’s roughly equivalent to being able to see the light of a candle on the moon from Earth. Ten thousand of these sensors surround the sphere.

The detection of a photon indicates that a neutrino has interacted with the nucleus of one of the atoms of heavy water in the sphere. (That extra neutron in the heavy water nucleus makes such interaction 20 times more likely than it would be with ordinary water, which is why heavy water is used.)

All of this, plus computers, controls, a laboratory area and more, are buried two kilometres underground to keep out the cosmic rays that constantly bombard the Earth’s surface and could also trigger the detectors. 

All around the world, scientists are figuratively holding their breath.  As Stephen Hawking, who doesn’t waste words (since they’re so hard for him to make) succinctly described it, SNO’s search for neutrinos is “Very important science.”

Just think, the fate of the universe may be revealed by information gleaned from a hole in the ground outside Sudbury.

One doubts the miners who originally dug that hold would have believed it.

Permanent link to this article: https://edwardwillett.com/1998/05/the-sudbury-neutrino-observatory/

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