If you read about the LHC detectors a lot, then you’ve heard about pixel detectors from time to time. Both ATLAS and CMS have one. But what is a pixel detector? Ken explained briefly here—and mentioned, in the process, that when he was my age, their trackers only had 26,000 channels, dinosaurs roamed the earth, they had to walk ten miles to the lab in the snow, and it was uphill both ways—but, anyway, I thought it might be worth expanding on the explanation a bit.
I’ve heard several times that a pixel detector is like a digital camera. Indeed, they both have pixels, but how far does the parallel really go? The biggest, most obvious difference is the shape, which follows from different purposes. Your camera is designed to collect all the light that reaches a particular point, namely the camera lens, and so there’s a flat rectangle of pixels at the spot where the light is focused. A pixel detector at a collider is designed to collect information on particles coming from a particular point, namely the place where the particles collide, so it’s built to surround that point as completely as possible. The schematic of the CMS detector in the upper right illustrates this point nicely; obviously, on a large scale, it looks nothing like the inside of your camera. (A pixel detector has more pixels too, but it’s about the same scale—ATLAS has about 80 million compared with 6 million in my digital camera.)
But the real question is how the individual pixels differ, and the answer turns out to be: not by as much as you might guess. I’m no expert on digital cameras, so I had to look it up, but it turns out that digital camera sensors are actually little arrays of silicon detectors. When a visible photon hits the layer of silicon that makes up a pixel, it knocks an electron out of its place in the silicon. The electron is pulled in one direction by an electric field, while the hole (the empty space where the electron should be) is pulled in the other. The charge thus collected by each pixel is proportional to the number of photons that hit it, and hence the intensity of the visible light; this charge is eventually read out by (for example) a charge-coupled device, and the picture can be assembled.
The basic idea of the silicon detectors we use in particle physics is the same, but we’re looking at fewer particles with much higher energy. Whereas a visible photon has an energy of a few electron volts (eV), the interesting particles passing through our silicon detectors at the LHC will have an energy somewhere from several hundred million eV to several hundred billion eV. Thus, as illustrated at left, when a particle passes through our silicon detector, it knocks loose a bunch (thousands or tens of thousands) of electron-hole pairs and doesn’t stop at all. That’s exactly what we want, actually; this type of detector is for telling us where a particle went, not for absorbing it. (This is called tracking, and it’s a topic for another day.)
That’s step one. In step two, the electron-hole pairs are pulled in opposite directions by an electric field, and pulled into “contacts.” (Actually, specially-doped regions of silicon, if you’re curious.) In step three, the charge built up on those contacts produces a current that flows into our electronics—another topic for another day.
So far I’ve discussed silicon detectors in general; I could just as well be talking about the “silicon strip” detectors that are also used in ATLAS and CMS, for example. The key feature of a pixel detector is that the individual contacts are two-dimensional; for every 0.05 by 0.4 millimeter pixel, there’s a separate circuit and separate electronics. This gives us a very precise measurement of where, exactly, the particle passed through the detector.
Of course, those pixels aren’t so very small—I’m pretty sure they’re actually larger than the ones in your camera. But they have to read out much faster than your camera does, since the LHC produces collisions forty million times a second. They also have to withstand the intense radiation found right next to the collisions, which can damage the silicon structure, for years and still work. It’s challenges like this that make pixel detectors such a complex and expensive job, but they’re vitally important for our physics program—but that, yet again, is a topic for another day.
Update (November 25): “Another day” has arrived, or at least one of them: How Tracking Works
