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Testing the CMS tracker March 13, 2007

Posted by dorigo in Uncategorized.

At the core of CMS, a giant detector which will be assembled this year inside one of the underground caverns of the Large Hadron Collider at the CERN laboratories in Geneva, lies a marvelous device, a silicon tracking chamber.

Tracking chambers are detectors for charged particles. Their aim is to reconstruct the particle paths using the ionization trails left in their midst. Until a few years ago, trackers were almost invariably designed as large vessels filled with a gas mixture (typical was argon/ethane, but the right recipe could involve some black magic depending on the construction details) and lined with high-voltage wires which collected the ionization and provided position measurements along the track.

Recently, a paradigm shift has occurred, due to the explosion of silicon microstrip detectors and the improved production capabilities of these devices in large quantity. Silicon detectors are 300-micrometer-thick wafers of silicon, usually instrumented with thinly spaced conductive “read-out” microstrips on one side. If a proper voltage is applied to the wafer, the silicon of the wafer becomes void of carriers of electric charge, and the ionization charge left by the crossing particle along its path is collected and read out with ease in the strips.  

Crafting silicon microstrip detectors is a quite complex business, so due to the high precision of the position measurement guaranteed by the microstrips, these detectors have so far been used in collider experiments to instrument small volumes close to the beam line. CMS has instead taken that cutting-edge technology to mass-production mode,  designing its whole tracker out of these wafers. But will these tens of square meters of silicon microstrips work ?

Apparently, yes. In a integration test that is taking place at CERN, last Friday the TIB and the TOB (acronyms for, respectively, Tracker Inner Barrel and Tracker Outer Barrel) have been taking data together for the first time, and they collected the signals of hundreds of thousands of muon tracks from cosmic rays in a day-long exposure.

Here is how a track reconstructed in the full TIB+TOB looks like (thanks Lino Demaria for providing the picture):

In the drawing you see the organization of TIB (inner 8 layers) and TOB (outer 6 layers) silicon detectors, seen in the plane transverse to the beam. Well, the beam is not there yet, but you got the idea. These are thus 14 concentrical cylinders of silicon sensors. The blue points represent position measurements for one cosmic ray event, and the red line shows the fitted path of the track which originated them.

Our job these days is to extract as much information as possible from the data that the integration test is collecting. Indeed, one gathers information not only on operation of the detector itself, but also on the performance of the track reconstruction software.

The CMS Tracker web page has a whole slide show of cosmic ray events, thanks Domenico Giordano and Giuseppe Zito. Enjoy!



1. Eirik - March 13, 2007

Wow, a whole tunnel made of silicon wafers. That must have been increadably expensive. But won’t the silicon deteriorate from all the radiation? The LHC is an amazing machine!

2. Kea - March 14, 2007

Yeah, this is all so amazing. I’d like to have a peek at the detector one day.

3. dorigo - March 14, 2007

Hi Eirik,

LHC is indeed amazing, and so are the experiments instrumenting the interaction regions. The CMS silicon sensors are going to withstand for a long time the enormous flux of charged and neutral particles that will traverse them as LHC starts colliding protons. They are expected to be good for 10 years, during which they may collect an integrated radiation dose in excess of 10 MRad.

In fact, the studies now ongoing of a replacement for the inner layers of the CMS tracker in view of a “Super-LHC” phase, when the machine should be able to deliver a luminosity of 10^35/cm^2s, are focusing on the needs to reconstruct tracks at the higher luminosity, when triggering will be critical. The sensors would be still good otherwise – but the higher occupancy forces the introduction of more pixel layers.

Hi Kea,

you might be surprised, but I have not been in the cavern myself yet! However, I did see the tracker – it is a tiny bit of the whole thing, but the tidy packing of silicon sensors, cooling tubes, electronical components, and power cables is indeed amazing.


4. Andrea Giammanco - March 14, 2007

Thanks to the slideshow I just spotted a clear misalignment problem between two modules: one of the tracks had a zigzag;)

5. dorigo - March 14, 2007

Yep Andrea, I saw it too… These tracks are quite useful for understanding both the tracking algorithm and the alignment.

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