CMS Silicon December 27, 2006Posted by gordonwatts in physics.
Particle Physics detectors, at their heart, use ultra-accurate tracking detectors to look for the particles that fly out of the interaction point. As these particles fly through the detectors they leave little electronic signals in the various detectors. Remember those old black and white movies where a detective would enter the room where a murder occurred, and then sight back a long a string he had drawn from the bullet hole in the window to the body to see what window in the building across the way the shooter had used? Well, these things operate the same way. Except there are many many plates of glass that we can draw the string through.
The most accurate of these detectors in modern physics are made out of silicon. We can frequently determine the position of a particle to better than 1/1000th of an inch. I don’t think it is an understatement to say that when this detector technology was developed it revolutionized the way we do physics and opened up new doors that were closed before.
Ever since we’ve been building the silicon detectors bigger and better (and more expensive). The CMS detector at CERN has one of the largest ones that I know of. 10,000,000 read out channels (imagine the electronics! The cooling to read out that many elements ever 25 nanoseconds!).
While I was at CERN Oliver from CMS gave Joe Rothberg (another UW professor) and I a quick tour of the CMS silicon detector. That thing is truly massive. It is over 2 meters in diameter. DZERO’s, which is only half a meter in diameter, costs over 10 million bucks — not including labor — I’d hate to know what this thing costs.
We got a “we’ve all learned that the hard way” story while we were over there. The single most expensive part of the silicon is the silicon itself. These are large plates of silicon (often 3cm by 5 cm or larger). The silicon is ultra-pure. Regular chips are make of silicon as well, but not nearly as pure; the purity is what drives the expense. As is always the case, we go out for bids when purchasing something like this. Some company always wins, and off we go to the races. In the end, however, we always get burned. Few companies that haven’t done this before, for example, really understand what is involved in making Silicon this pure. Production problems ensue. Delays. In some cases we have to move to back-up bids. Or the company totally fails to deliver. In the end, we go running back to the arms of Hamamatsu. That company will have responded to our first bid, but their price will have been x2 or x3 the cost of the low bid. But they know. They just sit back and wait. Eventually, we come home. 🙂 DZERO did this. Apparently CMS also has done this. It wouldn’t supervise me if ATLAS had (though I don’t know).