Giant Neutrino Detector, 2km Underground

yulek writes: "Yesterday's APOD ran an incredible photograph from the recently completed SNO Detector, a giant geodesic neutrino detector buried 2km (!) underground near Kingston Ontario. Neutrinos are some of the most bizarre subatomic particles, having virtually no mass and able to 'pass through matter like smoke.' The SNO Detecter is definitely one of the coolest and most ambitious experiments i've seen in recent years."

Comments

Neutrino Beam Through Downtown St. Genis

[goingware]

I spent a summer working at CERN doing my senior thesis for a B.A. degree in Physics at UC Santa Cruz (I was working with the Spin Muon Collaboration, mostly working on data analysis software - I made the mistake of walking down the hall at the physics department asking each professor I met if they could use an experienced software engineer who needed a thesis topic! Mmmm... FORTRAN.).

While I was there I noticed that the CERN neutrino beam went right down the main street of the nearby town of St. Genis in France and on into the Jura Mountains. I wonder if the townspeople in St. Genis would feel comfortable knowing they were being irradiated, even if they understood the particles wouldn't interact.

You see, while the detector that's the subject of this story detects neutrinos of cosmic origin, you can also make them artificially, and with controlled energies and other desirable characteristics, by shooting a high energy particle beam into one end of a long pile of dirt.

The particles shower but are then absorbed by the dirt - except for the neutrinos produced by the showers. Enough dirt, and whatever comes out the other end is pretty much pure neutrino beam.

If you put in an intermediate amount of shielding, you get a mix of muons and neutrinos.

The way you detect these artificial particle beams is typically with packs filled with photographic film sealed in a dark chamber. Just beam it for a while and every zillionth particle will leave a little speck on some of the film.

Ever heard of neutrino oscillations? They proposed the theory to explain the lack of expected neutrino flux in one of the earlier underground neutrino detectors. It takes 10,000 years for heat from the center of the Sun to convect to the surface before it can shine directly on the earth, but neutrinos radiate from the core to the earth in 8 minutes because they don't interact.

Only problem is, we weren't getting many neutrinos. The first suspicion was that the Sun had begun to die but the cooling part of the interior hadn't reach the surface yet - that is, we hadn't visibly received the bad news but had found out ahead of time with the neutrino detector.

If neutrinos change identities into types that a given sensor is not sensitive to, though, it would explain this. But for this to be the case, the neutrino would have to have a very small, but non-zero mass. It's been the work of decades to try to measure this mass.

In the particle beam at CERN they would measure the neutrino flux at different points along the beam to see if they got more and less intense as they oscillated between electron, muon and tau neutrinos.

Enjoy!

Mike

Information on Neutrinos

[Covariance]

One of the best sites for information on particle physics (for non-specialists) is The Particle Adventure.

Neutrinos are the least studied elementary particles because of they interact very, very rarely. It's no joke that they can "pass through matter like smoke", as the story said. The typical neutrino can pass through several light-years of lead without interacting once. The only reason they can be detected at all is that a tremendous number of them pass through the Earth every second. I forget the exact number, but it's something like trillions per square meter per second. Even so, a decector the size of SNO will only see a few hundred events per second. On the other hand, this is also why neutrino experiments like SNO or Super-K are so exciting for astrophysicists. The light that we see from the sun has all come from the surface, photons produced in the core can't make it through the sun to get to the earth. Neutrinos produced in the core can easily penetrate the whole of the sun and reach the earth. As a result, a very good neutrino telescope can look directly into the core of the sun. There are a berzerk number of other reasons to be excited about neutrino experiments, see the Particle Adventure for more.

Oh, and if you thought the SNO picture was cool, check out some of the photos on the Super-K, they've pretty much won the best-looking physics experiment ever contest.