Deep Down, It’s Still Astronomy

Astronomically Speaking, published in the York University Gazette, December 1999

What do you picture when you imagine an astronomy experiment?

Maybe you imagine a white dome on a mountaintop in Hawaii, or the large radio dish at Arecibo. Maybe you see a lone astronomer out in a field with a small optical telescope that fits on the back seat of an old Toyota hatchback.

It’s a fairly safe bet, however, that you wouldn’t think of a huge tank of water hidden in a mineshaft more than two kilometers underground.

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While most astronomy is geared towards viewing faint objects millions of light years away, an experiment here in Ontario is doing a very different kind of astronomy. Researchers at the Sudbury Neutrino Observatory (SNO) are not peering into the heavens, they’re working deep underground. They’re not looking at distant stars, they’re concerned with the properties of the Sun, our nearest star. And they’re not using sunlight for their investigations, they’re looking for elusive particles called neutrinos.

What all this has to do with old mine shafts and tanks of water, we’ll get to in a minute. First, let’s get aquatinted with neutrinos.

You may have heard the Sun described as something like a giant nuclear reactor. Deep inside the Sun, the heat and pressure are so great that nuclear reactions are occurring at an enormous rate. In the process, huge amounts of energy are released. We feel this energy as warmth on our faces, and see it each morning in a blaze of colour.

What we don’t see, though, is that along with all that light and heat, the Sun is pouring out a staggering quantity of particles known as neutrinos ('little neutral one'). They have no electric charge, and are incredibly light — they might have no mass at all. 

And at this moment, billions of neutrinos are streaming through your body.

You don’t notice this spray of particles because normal matter is essentially transparent to neutrinos. A neutrino could easily pass through the entire Earth. In fact, there is only a fifty-fifty chance that a neutrino would collide with even a single atom as it streamed through a block of lead the size of a whole galaxy!

So even though so many neutrinos are passing through you every second, only a couple will actually scatter off the particles inside your body in your entire lifetime.

These same properties make neutrinos incredibly difficult to detect. Since the chance of a neutrino interacting with an atom is tiny, we have a choice: we can either make a small detector and wait an incredibly long time to see a neutrino bounce off it, or we can make a very, very large detector, and give ourselves many more chances of recording a neutrino collision.

One kind of neutrino detector is little more than a huge swimming pool of water surrounded by very sensitive light meters. Very occasionally, a neutrino from the Sun collides with one of the vast numbers of water molecules, which then emits a flash of light. The patterns made by the flashes tells us something about the number and energy of the solar neutrinos reaching the Earth.

There's a problem, though. A stray cosmic ray from space could easily collide with a water molecule and cause a flash that would be indistinguishable from a neutrino collision. One way to guard against such accidents is to bury the detector underground. At SNO, the water tank is deep, deep underground, just over two kilometers down a mineshaft. At these depths no cosmic rays can reach the detector, but the neutrinos can with ease.

Astronomers know enough about the nuclear reactions in the Sun to calculate how many neutrinos they ought to be seeing. Strangely, past experiments have only found about one half of the expected number of neutrinos — this is called the ‘Solar Neutrino Problem’. It could mean the experiments are all wrong, which isn’t very likely. It could mean we don’t really understand the Sun’s behaviour, but our knowledge of the Sun has been confirmed in so many other, different ways. The only remaining possibility is that we don’t fully understand neutrinos.

Some theories have been suggested that allow the solar neutrinos to spontaneously change into different particles en route from the Sun. This strange quantum-mechanical effect would account for the ‘missing’ solar neutrinos. SNO’s clever design is sensitive to these new particles, and so has a chance of settling this problem once and for all.

SNO has been up and running since May this year (1999). Every once in a while, their light detectors show a flash inside the giant tank of water, betraying a neutrino’s death. It's a wonderful irony that, to understand the Sun, the SNO researchers find themselves down a mineshaft, two kilometers below the surface of the Earth.