The ALEPH Vertex Detector in Firenze

This page is a short review about ALEPH Vertex Detector. The blinking items are related to Florence group activities.

ALEPH is one of the four LEP experiments (the others are DELPHI, OPAL and L3).

The Vertex Detector is the inner device of the ALEPH tracker apparatus; its most powerful feature is the very high spatial resolution which allows to recognize long-living particles (B and D mesons, tau leptons) and measure their mean lifetime.
Therefore the Vertex Detector is very important for the next LEP physics program (LEP200) because long-living particles (b-hadrons) are the signature of Higgs sector physics, the most important research field in the next future.

In the pictures below you can see two beautiful events (in the top-right square you can see the VDET area -not in scale!- with the polygonal shapes representing the two VDET layers; in the top-left one there is a x-y view of the whole Aleph apparatus; in the bottom square there is the event sketch as reconstructed by Aleph near the interaction point).

The Vertex Detector is fabricated by using 6.5 cm x 5.2 cm n-type silicon crystals ~300 um thick.
Each detector has strips on both sides: the junction electrodes are ~2000 strips (p+ implant, 25 um pitch) running parallel to the long edge of the crystal. On the other side there are the electrodes to achieve an ohmic contact with the bulk; these are ~1280 strip (n+ implant, 50 um pitch) running parallel to the short edge of the crystal and thus perpendicular to the p+ strips.
These structures are first fabricated on a circular-shaped crystal (wafer) by a complex fabrication process (similar to that for IC circuits).

Then the physical parameters of wafers and structures (doping concentrations, bulk and implant resistivity, etc...) are measured just to check the process itself.
The tests are performed in a clean room (a low-impurity enviroment laboratory) via special tools.

At last the wafer's external unused parts are cut away to obtain the final rectangular dimensions.

The whole VDET is the result of the electrical and mechanical joining of 144 crystals.

First 3 crystals are joined together by glueing them head to head on the short edge; thus connecting by microbonding the p+ strips we obtain a ~20 cm long strip. The n+ strip are connected by microbonding too, but via a special circuit (kapton) designed and tested in Florence. Finally the electronics for the read out of these 3 crystals is added at one end; in particular we need 16 CMOS integrated circuits with 128 readout channels each one to read out one module. This structure formed by 3 crystals electrically and mechanically joined plus relative readout electronics is called module (below, at the top, you can see a beautiful picture of the p+ side and of the n+ side at the bottom; in this one you can easily see the kapton circuit hiding the cristals). The whole VDET needs 48 modules.

Two modules mechanically joined together form the so called face; a carbon fiber beam is added on a side of the face to achieve a good mechanical rigidity.

Finally with 24 faces we instrument a two-layers cilindrical structure:

  • the inner layer (~9cm radius) is formed by 9 faces;
  • the outer layer (~15cm radius) is formed by 15 faces.

    The VDET is about 40 cm long.

    On the experiment the axis of vdet layers is in coincidence with the beam axis and its middle point is in coincidence with the interaction point. Thus the p+ (n+) strips are used to read out the crossing position phi (zeta) coordinate. The single track spatial resolution is ~10um either on phi and zeta side.

    VDET needs digital and analogic peripheral electronics to function properly; the power supplies are an important part of this stuff. They are designed and tested in Florence and are very sophisticated; each unit, remotely controlled, supplies the bias and the voltages for module detector and electronics; each voltage is switched on/off with the proper ramp to avoid detector damage.

    At CERN there are a lot of informations about VDET; you can read also VDET in numbers.

  • Back to ALEPH Firenze Homepage.

    Giacomo Sguazzoni , 10th June 1996