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The HERA beam line provides a horizontal bend of the proton beam by dipoles around 20 m and a vertical deflection by three dipole magnets between 60 and 80 m. The magnetic optical system of the beam line between the intersection point and the Roman Pots are used as a spectrometer.
Due to the fact that the HERA beam line introduces a dispersion in two planes, longitudinal momenta can be measured twice by making use of the horizontal and vertical deflections. The two independent measurements which have to agree within errors, if they belong to a track starting at the interaction point, enable a self calibration of the FPS and provide efficient means to reject backround tracks. The detectors inside the Roman Pot plunger vessels thus have to record two coordinates (x horizontally and y vertically) at two positions behind the interaction region such that a straight line through the measured space points determines a trajectory which can be traced back to the origin by employing the known transfer properties of the magnetic channel. In this sense there are two independently working spectrometers, the stations at 81 and 90 m, approaching the proton beam vertically, and the stations at 63 and 80 m, approaching the proton beam horizontally.
Each station of the two spectrometers is equipped with two identical subdetectors. The two subdetectors seperated by about 6 cm along the beam direction are mounted together with a segmented trigger system of a detector carrier which ensures precise alignment of all components to each other and to the proton beam line. The distance of 6 cm is sufficient for reconstructing vector track elements, it is limited by the diameter of the plunger vessels.
Each subdetector consists of two planes inclined by ± 45° with respect to the vertical pot axis. Here you can find a perspective view of trigger tiles and scintillating fiber hodoscopes of a horizontal detector unit inside a Roman Pot. Each coordinate plane consits of five layers of 48 fibers in the horizontal stations and 20 fibers in the vertical stations, respectively. Each fiber has a diameter of 1 mm and is positioned parallel within the layers with a pitch of 1.05 mm. The layers are staggered by 210 µm, i.e. the pitch divided by the numbers of layers. In addition the equivalent fibers of both subdetectors are staggered by 105 µm. Here you can find a schematic view of the fiberdetector.
In the 81 and 90 m station four of the 1 mm diameter scintillating fibers
belonging to a read out plane are attached to one cell of a 64 pixel position
sensitive photomultiplier (PSPM). The ambiguities are resolved by the scintillator
tiles in front and behind the hodoscope, which also provide a trigger element to
the general H1 trigger mix to signal a candidate particle passing through both
stations.
In the 63 and 80 m station the fibers belonging to a read out plane are
attached to a 124 pixel PSPM multi channel photo multiplier (MCPM). The large channel
number of this device allows to couple each fiber seperately to one PSPM
channel.
Check out the Roman Pots at CERN.
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