[Table of Contents] [Previous Section] [Next Section]
[Central Preshower] [Forward Preshower]
The central preshower detector will aid electron identification and triggering and correct electromagnetic energy for effects of the solenoid. The detector functions as a calorimeter by early energy sampling and as a tracker by providing precise position measurements. The cylindrical detector will be placed in the 51 mm gap between the solenoid coil and the central calorimeter cryostat at a radius of 71~cm, and covers the region $-1.3<\eta< 1.3$. The detector will consist of several layers of axial and stereo scintillator strips with a wavelength-shifting~(WLS) fiber readout. An end-view of the detector for one of the designs described below is shown in Fig. 6. A lead absorber before the detector will be tapered in z so the coil plus lead yield two radiation lengths of material for all particle trajectories. The scintillation light from the preshower detector will be transported via clear light-guide fibers to the VLPCs. The readout is again based on the SVX II.
The fast energy and position measurements enable use of preshower information at the trigger level to aid electron identification. The axial layers of the preshower will be used in the Level 1 electron trigger. Monte Carlo studies show that the information from the preshower detector will provide a factor of five reduction in the low-energy trigger rate by applying a pulse height cut and requiring coarse position-matching with tracks. Off-line, the early sampling of the showers and the good position resolution of the detector will provide additional means for identifying electrons.
Two approaches for the scintillator strips are being considered. The first approach involves machining square scintillator strips, shown in Fig. 7 (a), from large scintillator sheets. Six layers of strips will be arranged into z-z, u-u and v-v views with a uv stereo angle of $\pm 20$ degrees. The two layers in the same view will be staggered by a half-strip to reduce detector inefficiency. Each strip will be 5 mm thick and 5 mm wide. There will be 5,760 channels in this design. The WLS fiber will be placed in a `key-hole' groove at the center of the strip. A prototype module of this design was made and tested using the VLPC readout at the fiber tracker cosmic ray test facility at Fermilab. The module is about one meter long and consists of 128 channels. With over 15,000 cosmic rays recorded per strip, the light yields, signal uniformity, attenuation length, edge effects and crosstalk were studied. The effective attenuation length is measured to be around 11 meter and a light yield of 4.5 photoelectrons per millimeter of scintillator traversed has been achieved for cosmic ray muons. The detailed prototype test results for this design are described in Ref. 5. The results show that there is a considerable margin in achieving the desired performance for the preshower detector.
The second option uses extruded triangular strips. Figure 7 (b) shows the end view of this design. Triangular cells eliminate the need for two staggered layers of a square cross section, and therefore, make the detector more compact. In addition, this design improves the position resolution for minimum ionizing particles (MIP) as a result of light-sharing between two neighboring strips. We are considering a two-layer design of triangular strips. Each layer is 5 mm thick with a 5 mm fiber-to-fiber spacing. The inner layer will be arranged along the z direction. The outer layer will be arranged with the strips orthogonal to those of the inner layer. The channel count of this design is about 4,000. A 128 channel module made of triangular strips is being tested at lab 6 for its optical viability.
Calibration of the detector will be done in two steps. Light emitting diodes (LED) will be utilized to provide a quick calibration on-line. Each non-readout end of the WLS fibers will be equipped with an LED. By comparing the one and two photoelectron peaks from the LED light, the relative calibration of the detector elements can be performed. An absolute calibration will be provided by MIP responses of the detector using the data. The Monte Carlo studies show that the MIP peak can easily be identified.
The optimization of the detector design is progressing well. The R&D effort is expected to be completed this summer and a final design will follow.
Two Forward Preshower Detectors (FPS) will cover the pseudorapidity regions $1.4<|\eta|<2.5$. They will consist of wedges joined together to form conical detectors mounted on the faces of the End Calorimeter (EC) cryostats. Each wedge will be composed of an inactive absorber material (two radiation lengths of lead) sandwiched between two finely-segmented active layers. The active layer before the absorber (layer 1) will serve as a track stub detector and the active layer behind the absorber (layer 2) will sample the showers initiated in the absorber.
The FPS will be part of the electron trigger in the forward region. At trigger Level 1, a spatial match between any hit in layer 1 and a hit with energy deposition larger than expected from a minimum ionizing particle in layer 2 can be formed. At trigger Level 2, such a pair of hits can be matched with a tower in the electromagnetic (EM) calorimeter with energy above a threshold. At Level 3, more sophisticated algorithms will match clusters in the FPS and EM calorimeter.
Particle level studies show that a matching resolution of 5 mm or better is required between FPS layers 1 and 2 to achieve a substantial reduction of the rate due to the accidental overlap of charged hadrons and photon showers. More detailed GEANT simulations are in progress to estimate the backgrounds due to early hadronic showers and to determine the optimal segmentation.
In the offline analysis, the FPS will provide additional rejection against backgrounds for electrons by providing a more precise spatial match between CFT or H disk tracks, FPS track-stubs and showers than the calorimeter could provide. This will be especially helpful in identifying electrons in busy environments, e.g. from b decays. The fine segmentation of layer 2 will allow resolution of showers initiated by a single photon and by two closely spaced photons from the decay of high $p_T$ $\pi^0$s.
The detector technology for the active layers has not been selected. We are considering two options: grooved scintillator sheets with optical fiber/VLPC readout or interpolating pad chambers. The scintillator solution would use the same technology as the Central Preshower Detector and will require no additional R&D effort, but would provide some constraints on the possible segmentation. A pad chamber could be segmented in any way but would require mor bulky frames. The choice will be made on the basis of detailed GEANT studies, which will determine what segmentation is required to achieve the goals outlined above.