The VSAT is a detector in DELPHI, located close to the beam at
7.7m from the interaction point. The physical processes being
monitored are the elastic scattering of electrons and positrons,
Bhabha scattering, used to determine the luminosity , and two photon collisions .
The detector consists of 4 small electromagnetic calorimeter modules located symmetrically
around the LEP beam pipe. The detector modules contain tungsten
absorbers (FAD), each interspaced with silicon planes for the energy
measurement. The electromagnetic shower coordinates, in the
horizontal and vertical directions, are given by silicon strip planes
close to the shower maximum.
The signals are amplified, and transported to electronics where
they are temporarily stored until a decision is taken if it should be
kept or not. The decision is formed by requiring that the energy
deposited in one or more module must exceed a given threshold. The
signals are digitized in case of positive decision.
Due to its
placement near the beam pipe and hence high counting rate, the VSAT
detector provides useful online monitoring of beam parameters and
luminosity. Information is sent to the central DELPHI monitoring
system and to the LEP control room. This information contains
estimates of the beam situation around the DELPHI interaction
point. The picture to the left shows a bhabha scattering as recorded
in VSAT. The signal from each silicon plane is drawn on the box, and
the signals from the three silicon strip planes inside the box.
In order to obtain good results with small errors, are detector
specific parameters, e.g. status of individual channels,continously
monitored Any changes in the parameters are recorded for later usage
in the analysis of the data.
13 october 1997