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[Scintillating Fibers] [Photodetectors] [Cosmic Ray Test setup] [R&D and Design]
A scintillating fiber tracker (CFT) surrounds the silicon vertex detector and covers the central pseudorapidity region. The fiber tracker serves two main functions. First, with the silicon vertex detector, the tracker enables track reconstruction and momentum measurement for all charged particles within the range ${\eta} = \pm 1.7$. Second, the fiber tracker provides fast, ``Level 1" track triggering. Combining information from the tracker with the muon and preshower detectors, triggers for both single muons and electrons will be formed at Level 1. These triggers will be critical to take full advantage of the physics opportunities available with the Main Injector.
The scintillating fiber tracker is shown in Fig. 2. A total of about 80,000 scintillating fibers are mounted on four concentric cylinders with average radii of 20,30,40 and 50 cm. Each of the four cylinders supports a ``superlayer" of four doublet fiber layers. Two of the doublet layers contain fibers oriented in the axial direction, parallel to the beam line. These two axial doublets are separated by the 1.5 cm thickness of the support cylinder. The other two fiber layers are oriented at $\pm1.5^\circ-3^\circ$ stereo angles. All fiber doublet layers are constructed so that one layer is offset by one half of the $\sim 900 \mu$m fiber spacing with respect to its partner. This configuration removes all gaps and improves the doublet position resolution. We have studied the tracking and triggering with Monte Carlo simulations and the expected performance is excellent.
The basic detection element is the multiclad scintillating fiber. The inner polystyrene core is surrounded by a thin acrylic cladding, which in turn is covered by a thin flouro-acrylic cladding. These three materials have indices of refraction of 1.59, 1.49 and 1.42, respectively. The addition of the second cladding increases the light trapping by about 70% with respect to single-clad fibers, and improves the mechanical robustness of the fibers. The nominal fiber diameter is $830 \mu$m; the core diameter is $770 \mu$m and each cladding is $15 \mu$m thick. The polystyrene core is doped with 1% p-terphenyl (PTP) and 1500 ppm of 3-hydroxyflavone (3HF). The fiber scintillates in the yellow-green part of the visible spectrum, with a peak emission wavelength of 530 nm.
Eight meter clear multiclad fiber waveguides conduct the light to the photodetectors and are mated to the scintillating fibers by plastic, diamond-polished optical connectors. The photodetectors for the fiber tracker are placed in the platform under the central calorimeter.
The photodetector must be capable of detecting single photons with high efficiency at high rates and with large gain. We will use the Visible Light Photon Counter (VLPC), a variant of the solid-state photomultiplier. Much research and development in collaboration with Rockwell International Corp. has led to a device with the characteristics: greater than 70% quantum efficiency for the wavelength of interest, gain of roughly 20,000, and a rate capability of at least 10 MHz. The VLPC can be operated at full effficiency with a noise rate of 0.1% or less. The VLPC's are manufactured in arrays containing 8 circular pixels each 1 mm in diameter, well-matched to the clear waveguide fibers. The photodetector operates with a bias voltage of about 7.5 volts. The VLPC's operate at a temperature of 6-8 K, requiring the detectors to be maintained in a cryogenic environment. Cryogenic ``cassettes" are being designed which will house 128 arrays for a total of 1024 channels, and maintain them at a stable operating temperature.
The R&D program to develop the fiber system culminated in the operation of a large-scale scintillating fiber cosmic ray test stand at Fermilab. The test stand contained three 128-fiber-wide superlayers (a total of 3072 fibers). Superlayers were mounted at the top and bottom of a carbon-fiber support cylinder, with a third on a flat board on the cylinder axis. Muons with momenta greater than 2.5 GeV/c were selected using a steel filter. The cosmic ray setup was designed to test the major components of the fiber tracker in a configuration as close as possible to the final tracker design. The scintillating fibers were three meters in length, and were optically coupled to eight-meter-long clear waveguides. A cryostat was built to house test cassettes containing 128 VLPC channels each. The test stand was in operation from May through December 1994.
The results [4] of the cosmic ray test were excellent. The VLPC cryostat operated stably and the temperatures of individual cassettes were controlled to better than $\pm 15$ mK, easily good enough for stable VLPC operation. The gain of each of the 3072 channels was monitored by an LED-based calibration system, and overall gains were found to vary less than 1% over the length of the run. The noise rate, which was fixed to 0.1% by setting thresholds on each VLPC channel, also remained constant over the entire run. The light yield and tracking resolution are consistent with expectations. Figure 3 shows the light yield spectrum in photoelectrons for all fibers found on tracks. The most likely value of 8.5 photoelectrons is about a factor of four more than the minimum required for efficient tracking. There was no evidence of any degradation in light yield over the duration of the run. The doublet hit efficiency for cosmic ray tracks is better than 99.9%. The doublet position resolution, plotted in Fig. 4, is found to be 136 $\mu$m.
Currently, a variety of R&D tasks are being completed before construction of the fiber tracker begins. A new set of doublet ribbons with more accurate layer-to-layer registration has been manufactured and installed in the cosmic ray test stand and are expected to improve the position resolution to the theoretical limit of $120 \mu$m. Designs for the fiber ribbon manufacture and for the optical connectors are being finalized. A joint Fermilab-Rockwell study to optimize the VLPC cassette design is near completion. Prototypes for the final calibration system are being tested, and alternative scintillating dyes which may prove superior to 3HF in speed and environmental stability are under investigation.
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