DELPHI Very Small Angle Tracker (VSAT)

Technical description

The VSAT detector consists of 4 identical calorimeter modules.

Module composition

Each such module is composed of 11 silicon diodes or so-called FADs (Full Area Detectors). These FADs have size 3 cm x 5 cm with usual thickness 0.3 mm) and separated by tungsten alloy absorbers with size 5.12 cm x 5 cm of two radiation lengths thickness (0.38 cm). FADs have a full depletion voltage around 30 V (operational voltage around -25 V) with capacitance 500 pF.

Three silicon strip planes are 5x5 cm2 in size with 1 mm pitch and placed at 5,7,9 radiation lengths into the modules are used for the (X,Y) shower position measurement. There are two planes for the X measurement with 32 strips (each strip 1 mm wide, 50 mm high) and one for the Y-measurement with 48 strips (each strip 50 mm wide, 1 mm high). Full depletion and operation voltages are equal +25 V for both types of strip planes.

All silicon detectors was made by Micron Semiconductor.

The lead block with 10 radiation lengths thickness placed on back side of each module in order to decrease the number of parasite (coming not from interaction point) events like photon's or off-momentum lepton's hits from the back.

Look on Views of Simulated Events made by GEANT program as a good explanation of one module design and work.

All hardware described above as well as preamplifiers and temperature sensors are closed inside precision metal box with thin ( mm) aluminum window headed toward the interaction point. The cooled nitrogen gas is circulating in these box keeping temperature inside below 60 K (warning level).

Module positions

Modules are as symmetrical with respect to the interaction point as it possible.

The setup is shown with respect to LEP beam-pipe (the origin of coordinate system is the Interaction Point): You should also appreciate the big difference between the horizontal scale (up to 8 meters) and the vertical scale (up to 10 cm)

The modules are labelled B2 (Module 1 or LUM1) and B1 (Module 2 or LUM2) in the backward (with respect to the electrons) direction and F2 (Module 3 or LUM3) and F1 (Module 4 or LUM4) in the forward direction. You can see the bending due to the quadrupole (in the horizontal plane) which give a defocusing effect for X coordinates:

For Y coordinates situation is just opposite - strong focussing effect makes Y distribution less than 1 cm wide.

The beam-pipe which separate two neighboring VSAT modules have an elliptical shape with roughly 11 cm (12 cm before 1998) range between modules in X axis as it shown at the VSAT Photo and here:

By using these four modules VSAT detects electrons, positrons coming from Bhabha scattering and photons between 5 and 7 mrad at polar angle and from -650 to +650 for each side in azimutal angle. Look for more precise information about modules position, size and VSAT geometrical acceptance.

Electronics hardware

The first stage of electronics is a individual charge sensitive preamplifiers placed in the same box as the detectors: two different types for FADs and strips.

Second stage of electronic readout is two so-called tunnel crates located a 3 meters away from pairs of modules (forward or backward) in the LEP tunnel. These crates consist of the following devices: MUX (multiplexing) cards, DPC, APC, energy supply and Tunnel Processor. The main aim of this equipment is to amplify and sharpen the weak signals from VSAT for a long travel to ADC in D2 barrack. One old Olivetti PC placed in D2 barrack is in use as a terminal to control and command tunnel crates.

The last stage of electronic readout is the VSAT crate in DELPHI barrack D2 located a 40 meters away from tunnel crates (forward or backward).
These stations consist of the following devices: FASTBUS and NIM cards with FDDPs (Fast Digital Data Processor - one per diagonal) rules over ADC and LTS (Local Trigger Supervisor) which takes a decision about event types.

There is a Electronics Description Page for detailed review of electronics hardware.

Quantities measured

For each module, data concerning these quantities are produced :

The Typical Histograms Page represent a good example of measured quantities and their distributions.

On-line measurements:

LEP also has it own detector with quite similar design for background and luminosity measurements. It is situated in the collimators 80 cm farther then VSAT (8.5 meters from Interaction Point). Look this note for details.


Dedicated electronics described above is used to produce the local trigger results for the first (T1) and second level (T2). The VSAT (because of its high rate) has an independent data acquisition system that is read out by DELPHI when there is a DELPHI trigger. This system can store up to 28 VSAT event until the buffer will be taken by DELPHI DAS. VSAT can trigger DELPHI only if the buffer is full. If another detector will trigger DELPHI by T2 VSAT event buffer will be taken a cleaned by DELPHI.

Off-line measurements:

  • Data for two-photon physics at low polar angles (single and double tagged events).
    Look Two-Photon Physics Page for details.


    VSAT Notes

    VSAT page in Lund University

    Link to VSAT operation page