10 - H1 - Desy

48 The H1 experiment at HERA
under the small angles and leave the detector through the beam pipe. They are detected
by the coincidence of the signal in two calorimeters, the electron tagger (ETAG) and the
photon detector (PD). The overview of the system is given in the figure 3.9.
synchrotron radiation
2X 0 filter
γ
PD
e
e
ETAG
p
Figure 3.9: The H1 luminosity system. Photons and electrons are detected in the
electron tagger (ETAG) and the photon detector (PD).
The PD is located at z = −103 m and is a tungsten (absorber plates) / quartz (active
material) fibre based sampling calorimeter with the sufficient depth to fully contain electromagnetic
shower of about 25X 0 . The position of the PD makes it strongly exposed
to the synchrotron radiation. The beryllium based filter with depth of 2X 0 is placed
in front of the main calorimeter, in order to protect it from the synchrotron radiation.
Incorporated into the system are two electron taggers, placed in positions z = −5.7 m
(ETAG6) and z = −44 m (ETAG44). The ETAG6 is a SPACAL type calorimeter containing
scintillating fibres. The ETAG44 have been installed already during the HERA
I running period and is based on scintillating crystals. Both of the electron taggers are
used to tag the photoproduction events (Q 2 < 0.01 GeV 2 ).
3.2.4 Time-of-flight system
The Time-of-Flight (TOF) system consists of plastic scintillators installed at various
places within the H1 experiment. The TOF system delivers precise timing information
used to reject non-ep background events which are asynchronous to the strict 96 ns period
of ep collisions at HERA. In particular two scintillators of the veto-wall (VETO) at
z = 8.1 m and z = 6.5 m reject events with a significant time offset to the HERA clock.
3.2.5 Trigger system and data acquisition
The main objective of the trigger system is to provide a fast decision for the acquisition
of interesting ep events while sorting out background events. Background sources are
mainly due to reaction of the protons e.g. reaction of the protons with the gas in the
beam pipe (beam-gas) or with the material of the beam pipe (beam-wall). Beam halo
muons and cosmic muons also yield significant amount of background. The background