Rich Morin :: tchotchkes

A Little Bit o' Nuthin'

One of the cool things about working at SLAC is being around "Big Science". I had lunch the other day with a physicist and his grad student (or so I surmised) and learned something new and interesting.

I had picked a nice shady picnic table for my lunch. The scientist and his colleague joined me and started chatting about their experiments. I listened in on their conversation and, eventually, got brave enough to ask a few questions. The scientist was happy to indulge me, telling me about an experiment he had done with neutrinos.

Neutrinos are about as close to nothing as it is possible to be (and still be something :-). They have no charge and very little mass. They go right through most matter, with few noticeable effects. With no charge, you can't grab onto them, let alone fling them around. All of this makes them rather, erm, challenging to work with!

Nonetheless, this scientist needed to do so. Specifically, he needed to generate a stream of neutrinos. And, because neutrinos are hard to detect, he needed rather a large stream. Here's how he did it.

Technorati Tags: big science, neutrino, physics

He started with a bunch of protons. Protons are easy to come by; just ionize some Hydrogen and throw away the electrons. They are charged, so you can accelerate them with microwaves, steer them with magnets, etc. Consequently, generating a stream of protons is duck soup (quark!).

As things approach the speed of light, however, any "acceleration" only produces tiny increments in speed. The energy doesn't go away; it just increases the particle's mass. So, these protons are now packing quite a punch.

When a stream of accelerated protons slams into a "target" (e.g., a thin metal plate), a large number of particles and photons comes out the other side. One of these particles, a "muon", has a very short lifetime. When it "decays", it produces a neutrino and some other junk.

Because of the way relativistic interactions work, all of this junk is traveling on (more or less) the same path as the original proton. Thus, he now has a stream of neutrinos, mixed in with a whole lot of other things. Here is where the neutrino's ability to zip through matter comes in handy. To get rid of the junk (but not the neutrinos), he put up a barricade of dense matter (e.g., steel plates).

As the junk particles and photons go through each plate, some of them interact with the steel, producing still more particles and photons. With each interaction, however, some energy gets drained off, so each generation is less energetic. Eventually, none of the junk is energetic enough to get through the plates. This leaves us with a nice, clean stream of neutrinos.

This technique requires quite a bit of steel, to be sure. In this experiment, the scientist used 300 meters (!) of steel plates, stacked up like playing cards. The plates, as it happens, were carved up from decommissioned battleships (swords into plowshares :-). Because they were slightly curved, they didn't stack up entirely neatly, but that didn't keep them from doing their job!

So, having generated a stream of neutrinos, the scientist only needs to detect them. Unfortunately, the neutrino's ability to zip through matter is now a problem. Even with another hundred meters of dense matter (e.g., iron), only one neutrino in a million will interact with anything.

But, since he has as many protons as he needs, that's not a real problem. Generate tens of millions of neutrinos per second, detecting every millionth one (or so). This gives him dozens of "events" per second, which is plenty enough to work with.

Ain't science wonderful?

Technorati Tags: big science, physics, neutrino

A Little Bit o' Nuthin' in Science , Technology - posted at Wed, 07 Jul, 19:39 Pacific | «e» | TrackBack