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CMS Silicon December 27, 2006

Posted 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. ๐Ÿ™‚ DZERO did this. Apparently CMS also has done this. It wouldn’t supervise me if ATLAS had (though I don’t know).



1. Kevin - December 27, 2006

The Atlas inner detector actually is larger by volume (2.3 m by 6.8 m for Atlas and 2.6m by 5.4 m) not that this statistic alone tells you that much – i am not sure why every cms talk i hear tells me how much the detector weighs . I guess it depends where all that stuff is. I believe that at least the strip detectors in atlas were manufactured by Hamamatsu – i am not sure about the pixels. D0 tried micron but they kept claiming that the reason that the failure rates were 2 to 3 times over specs was that the mail man must have done it (i tested F wedge detectors).
I agree that almost all of the time trying to go the “el cheapo” route we get burned. I am constantly amazed by how many projects get delayed due to stupid things like cable connectors that fall out of place or shotty soldering of ball grade fpgas. Everyone is always concerned about the high level stuff and it is almost something very basic which gets you. One company put close to 500 boards together incorrectly by putting on one chip backwards – and they did this twice!
A big part of the problem – I think – is that if you are a company the size of Micron or Hamamatsu getting a request for 100 boards is like asking Evian to design a special bottle that holds one drop of water and to make 3 of them total. They are really doing it out of charity rather than making any real money off of it since there are so few components (compared to the huge number of iPods or digital cameras that they churn out) and zero market value for these specialized requests. So its hard to get them to be too worried about it
Happy Holidays

2. Carl Brannen - December 28, 2006

There’s another set of very accurate particle detectors, and that is the emulsion cosmic ray experiments. You leave huge sheets of x-ray film separated by layers of lead on a mountaintop for a year or two, develop the film, and then examine it with a microscope for the particle tracks. A review of exotic stuff in cosmic rays is href=http://arxiv.org/abs/hep-ph/0111163>this, for more, search for things like “centauro” in arXiv. There are some new experiments planned that will combine the emulsion with electronic particle tracking, search for “emulsion” and “hybrid”.

3. Carl Brannen - December 28, 2006


4. gordonwatts - December 28, 2006

Kevin — very true. In the case of Micron, however, I seem to remember reports of our D0 reps who when to the factory floor and it sounded like every order was a special ordre. But I do remember the mailman excuse! ๐Ÿ™‚

Carl — yes — those are amazing accurate. Sadly, we can’t use those for collider detectors (the fluence is too high). But thanks for reminding me of those. I used to be a photog in my spare time and so I’ve always felt a special affinity for those types of detectors.

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