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The presented detector is a high-resolution scintillating fiber tracker with a modular design that is currently under development for use in balloon-borne experiments such as the Positron Electron Balloon Spectrometer (PEBS) and possible upgrades for the LHC experiments.
The tracker modules consist of thin scintillating fibers with a diameter of 0.250mm +/- 0.06mm. The fibers are compiled into 64mm wide and 860mm long ribbons of five tightly packed fiber layers fixed by an Epotek 301 adhesive. Within the ribbon, the fibers form an even lattice with a mechanical precision of approximately 20µm for each placed fiber. The fiber ribbons are produced in a workshop at the RWTH Aachen University achieving a high yield with detailed quality control during the full production process.
Two fiber ribbons are glued to light carbon fiber skin / Rohacell foam module carriers with a thickness of 10mm. Optical hybrids carrying eight 1.1mm high and 8mm wide silicon photomultiplier (SiPM) arrays are used for the readout. A linear 32-channel SiPM array that was specifically designed by us for the use in scintillating fiber trackers were produced by Hamamatsu. They have a readout pitch of 250µm with 80 micro-pixels per strip and a photon detection efficiency of 50% and are fixed on both ends of the fiber module. The SiPM arrays on the optical hybrids alternate with small aluminum mirrors in such a fashion that groups of 32 fibers are read out on one end, while the other is covered by a mirror. The next generation of the linear SiPM arrays has been ordered and is expected to be available end of 2009.
For the readout of the SiPM arrays a electrical frontend boards based on the IDEAS VA32-75 preamplifier chip and the new SPIROC preamplifier chip developed by IN2P3/LAL Orsay for SiPM readout were used. The VA32-75 was originally chosen for its low power consumption of 2mW per readout channel. A new version of the SPIROC chip that was designed for the presented fiber tracker will become available by the end of 2009 and feature a comparable power consumption.
Several prototype modules were tested at the PS facilities at CERN in summer 2008 and November 2009. The light yield of the modules, the spatial resolution, the tracking efficiency and the fake rate of the tracker modules have been measured. Among the tested modules the performance for different fibers of type Kuraray SCSF-78MJ with a peak emission wavelength at 437nm and of type Kuraray SCSF-81M are compared.
For the 2008 data a spatial resolution of 0.07 mm was found at a tracking efficiency of 99%. The 2009 tracker modules show a 50% improved light yield and we expect to find an improved spatial resolution. The results will be presented on the Vienna conference.
