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Ken Bloom | USLHC | USA

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Pixel cosmics

While most of my time is spent on computing issues, there is (just a little) more to my research life.  Our group is also involved in building, installing and commissioning CMS’s forward pixel detector.  This is a silicon-based detector which records the position of charged particles.  Wafers of silicon are segmented into little squares (the pixels) that are 150 microns on a side.  When charged particles pass through a pixel, a little bit of charge is liberated in the material.  This charge can be picked up on a tiny amplifier and recorded into the data stream.  By looking at patterns of pixels that have charge in them, we can reconstruct the paths of the particles that passed through.  One particularly striking aspect of these detectors (to me, at least) is the density of readout channels.  The forward pixel detector is a few wheels that are about 10 cm in radius, but the whole detector has millions of readout channels!  (When I was in grad school, I worked on a similar device that had about 26,000 channels.  It seemed like a lot at the time.)

Even though this detector hasn’t seen any beam yet, it is already time to think about replacing it; in a few years, its performance will be impaired by through radiation damage (it sits right next to the beam), and we can apply lessons that have been learned from building the current detector to the new one.

This summer, we set up a little lab down the hall from my office where we can start to study prototype detectors and readout chips for the next round.  We have a few spare pieces from the current detector and a rudimentary readout system for the electronics.  A couple of our students set up a couple of scintillators and phototubes so that we can trigger the readout on cosmic rays that pass through the silicon.  Here is a plot of the pulseheights that they read out of the system during a cosmic-ray run:

The curve that is superimposed on the distribution is a Landau function, which is what you expect to see.  Not bad!  Our guys pretty much got this on the first attempt — good for them.  I’m told that no one has actually seen an output spectrum from real incident particles from these devices for some time.  (We’ll be seeing lots of them in a few months, when the LHC starts up.)  Now we can start to think about looking how the performance varies as we vary the operating parameters, and about testing out prototype detectors when they become available.

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