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[Front-End Electronics] [Trigger Detectors]
The advent of the Main Injector requires a major redesign of the D0 muon system triggering and front-end electronics systems. Bunch crossing times of 396 and 132 nsec are smaller than the drift time of the wide angle muon system (WAMUS) and the 132 ns time is less than that of the small angle system (SAMUS). The chambers will be preserved but the existing readout electronics will be replaced. In addition, it is necessary to improve the rejection power of the muon trigger systems considerably in order to handle the factor of ten increase in luminosity.
The WAMUS chambers are extruded aluminum tubes with a rectangular cross section. The wires are 50 $\mu$m gold clad, nickel struck tungsten. The drift distance is 5 cm with field shaping electrodes made of ``glassteel" in a pattern such that the relative charge on two adjacent triangular pads yields a coordinate along the wire direction. The SAMUS chambers are stainless steel tubes with a diameter of 3.2 cm. There is no field shaping, and only drift time is recorded. A three-dimensional readout is achieved by using crossed $x\times y\times u$ tubes. WAMUS covers $|\eta|<2.4$ while SAMUS covers $2.4<|\eta|<3.4$. The detectors are organized in three layers; the A layer for WAMUS is located between the calorimeter and the toroid iron, the B and C layers are located outside the toroids, with the C layer outermost. This notation is built into the acronyms in the discussion that follows.
We will use the existing high voltage, low voltage, gas systems, and coaxial-cable runs to the moving counting house (MCH) and the L2 trigger system. The L1 trigger system will be moved to the detector platform and use new serial data links, greatly simplifying the trigger cable plant. As much of the existing infrastructure will be reused as is possible.
The front-end electronics for the muon system must be replaced to accommodate the shorter bunch-crossing times. Improvements in available electronics make it possible to replace the present discrete-component systems with commercially available integrated cicuits.
The chamber wire signals will be processed by new amplifiers with transformer coupling at the inputs to avoid low impedance ground paths between the chambers and the preamplifier inputs. These commercial-IC based preamplifiers will provide equivalent noise performance to that achieved in the present run. The trigger information will be provided by wire signal latches, with the ambiguities, caused by the ``ganging'' of pairs of tubes, resolved by inserting a lumped delay between the adjacent tubes.
The time digitizers use a KEK-developed four-channel pipelined digital TDC chip called a TMC [7] which matches our needs very closely. The bin width when run at 26 MHz is 1.2 ns and the maximum delay is 4.8 $\mu$s, enough to cover the first level trigger (L1) latency of D0 of 4.1 $\mu$s. The time-of-arrival difference between each end of the tube pair signals is used to break the ambiguities of the repetitive pad pattern. We propose to calculate $\Delta t$ by subtracting time values from adjacent tubes.
The charge will be measured using a commercial 500 ns gated integrator, and digitized by a 10-bit 15-Msps ADC. The ADC signals are pipelined to provide deadtimeless L1 operation. This will allow us to maintain 2 mm pad resolution from 1% measurements of the individual pad pulse heights.
In order to form a trigger using the complete hit map for a particular collision, the signals cannot be used until the maximum drift time has elapsed. Thus the wire signal must be stretched to the maximum drift time, which is greater than the crossing interval. The problem of correlating hits with their corresponding collision can only be accomplished with the previously mentioned auxiliary detectors.
The rate capabilities of the data acquisition system (DAQ) dictate a hierarchal trigger scheme. In the muon system, which uses the TMC chip, the L1 pipeline is integral to the chip. The trigger logic generates a new candidate list of hits at a 53/7 MHz rate. The need to send trigger bits to the L1 processor at this speed defines its bandwidth requirements, which can be handled by serial links with rates as high as 1 Gbit/s developed for telecommunications. These serial links will be used to transmit signals to the L1 trigger and to the L2/DAQ in the counting house.
For the ADCs, we must supply an external pipeline. The present plan is to use a FIFO as a pipeline to provide at least eight-deep buffers. The outputs of the L1 FIFOs are attached to a data bus controlled by a digital signal processor (DSP). This high level processor will perform data formating, including pedestal subtraction, and time to distance conversion, and more extensive processing depending on the high level processing algorithm.
Since the bunch crossing time for Run II will be smaller than the WAMUS (SAMUS) drift time of 700 (200) ns, subsidiary detectors are required to remove the ambiguities and to provide the ``time stamp" for the event. The muon trigger detectors consist of:
The cosmic ray shield was completed and brought into operation during Run Ib. The readout for all these detectors is designed. They will be part of the muon system data stream, reporting to L1 and L2/DAQ.
The experience gained from data taken in Run Ia was invaluable for learning how to operate a muon trigger in the Tevatron environment. In particular, the forward region is very susceptible to albedo from the End Calorimeters and the low-beta aperture stops. Prototype scintillators and pad-pixel chambers have been installed in the present experiment yielding the rates, pulse heights and timing spectra which are required for the upgrade design. Shielding studies have led to reduced rates which are of critical importance to triggers based on a three-element coincidence.
We have found that the effective rate can be reduced in both EFB and EFC by a factor of 50 by using 20 nsec timing gates to reject albedo. Similiar factors are measured for CFA scintillator. We are also measuring the reduction of rates due to soft photons, obtained by using pulse height cuts on the scintillator signals. Clearly, scintillator is the detector of choice, but its use is constrained by the total system cost. We are optimizing the cost/benefit to establish the transition point between scintillator and PWC detectors on the basis of measurements taken in Run Ib. The time to freeze the design is the summer of 1995.