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.

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 energy information: the energy deposited in the 11 Full Area silicon Detectors (FAD).
• The position information: the two planes of 1 mm pitch silicon strip placed at 5 and 9 radiation lengths give the x position and the plane at 7 radiation lengths gives the y position (also with 1 mm pitch) of the shower axis.

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

On-line measurements:

• Bg2 is the value of Background 2 (Background 1 supplied by TPC and mainly consist of photons). Bg2 is mainly have deal with electrons (and positrons) but VSAT have a possibility to evaluate the X-ray background by using signal from FAD plane No 1 only. Bg2 number is made by summing single electrons hits per second in all four VSAT modules (for beam energy above 95 GeV the rate is about 2000 Hz). This sum multiplied by the factor to made the Bg2 number which represent the level of radiation risk for the detectors (especially to gas detectors with high voltage). Bg2 number provided for LEP during all the time without connection with DELPHI data-taking. Bg2 equal to 1.0 means that the background conditions is calm, if it equal 5.0 or more that mean that data-taking is impossible due to the high background and high voltage for detectors should be switched off. This value as well as Bg1 can be seen on the LEP Information Page or on LEP TV-sets among CERN.

• L is the on-line luminosity sent to LEP during all the time also without connection with DELPHI data-taking. On-line Luminosity measurements also carried out by two DELPHI detectors: STIC and TPC.

Trigger:

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.

• T1: the energy of the 4 FADs from 2 to 5 (to reject events from the back) is added and then compared with a threshold of about 26 GeV (low threshold). There different types of T1 events - look Electronics Description Page and Physics Explanation for details.
• T2: then the VSAT data-buffer becomes full with T1 events. The limit of events is set able up to 28 events (for 1999 the limit is on 25 events, it was equal 12 during summer of 1998, and 20 during the end of 1998). Mean buffer size depends of how many events is getting by VSAT per time and often other detectors trigger DELPHI DAS by T2.

Off-line measurements:

• Long term Luminosity Measurement for physic analysis together with STIC, DELPHI and LEP.
  Look a short description of the Off-line Data Processing for explanation.
• Long term beam parameters (beam-spot, beam-tilt and angles) behavior.
  Look this page for details about beam parameters and minibunch mismatches recovery process.