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[Front-End Electronics] [The Inter-Cryostat Detector]
The Calorimeter system upgrade is driven by the need to preserve the excellent performance of the calorimeter for Run II running conditions. For practical reasons, we have chosen not to make any modifications to the uranium liquid-argon calorimeter itself, but to restrict all the changes to the front-end electronics. One component of the calorimeter system that will need modification due to the effect of the solenoid field is the Intercryostat Detector (ICD), which uses phototube readout located in what is presently a field free region.
An upgrade of the calorimeter front-end electronics is required because the minimum bunch crossing time will be reduced and the luminosity will be increased in Run II. The electronics upgrade will preserve as much of the existing infrastructure as possible.
To minimize the effects of pile-up in the calorimeter, we have re-optimized the shaping times. The present peak sampling time of $2.2\mu$s will be reduced to 400ns (matching both the charge drift times in the calorimeter and the expected minimum bunch-spacing at the start of Run II). Since this shorter shaping time increases the sensitivity to noise and reflections on the signal cables, we will replace the present cables from the calorimeter cryostat with cables that are impedence matched ($30\Omega$) to both the cables inside the cryostat and the new preamp input impedance. The calorimeter signals are transported to preamps housed on the calorimeter. The present preamp hybrids will be replaced with new hybrids which will have better noise performance through use of dual low-noise FETs, and increased output drive capability in order to drive the long terminated-cable run from the preamp to shaper circuitry (baseline subtractor or BLS). The new preamps require new preamp motherboards and power supplies, but otherwise use the existing mechanical structure.
The shaper circuitry incorporates significant new elements in the design to provide the necessary pipelining of the calorimeter signals in order to provide time for a trigger decision to be made. The analog pipeline will use a switched capacitor array (SCA) originally developed for SSC work by LBL [6], and modified to match D0 specifications. The SCA will sample the peak of the integrated charge signal from a preamp, shaped to provide a symmetric unipolar signal. The calorimeter signals require 12-bit accuracy with 15-bit dynamic range. This range requirement exceeds that which can be achieved with the SCAs, so a dual-pipeline will be used to accommodate the full dynamic range. To minimize the deadtime at high luminosity, the signals will be toggled between two dual-pipelines. Limiting the trigger to only one in a ``superbunch'' (a superbunch consists of 11 (or 33) bunches at 396 (or 132) ns spacing with about a $2\mu$s gap between superbunches) provides a deadtimeless system in which one of the dual-pipelines is reading out while the other is being filled with data. The gap between superbunches will provide a single baseline sample which will be used to remove long term drifts. The present BLS system (60,000 channels) will be fully replaced (including new power supplies, timing system, and pulser system) except for the mechanical infrastructure (crates, cabling, cooling, shielding).
The performance of the system with regard to pile-up has been simulated, and we find that the capability of the upgrade detector at high luminosity is comparable to that of the present detector at our present lower luminosities. Its performance at 132ns has also been checked and found satisfactory up to luminosities approaching $10^{33}{\rm cm}^{-2}{\rm s}^{-1}$.
The preamp design is finished and we anticipate completing the SCA R&D this year. A modest amount of R&D for the new timing and pulser systems should be completed by the begining of next calendar year.
The Intercryostat Detector provides a single energy sample in the region between the Central and End Calorimeter cryostats. This sample serves to improve the detector performance significantly in the overlap region $1.1<|\eta|<1.4$. The solenoidal field will render the present ICD phototube readout inoperable. Thus the upgrade of this detector system consists of modifying the present scintillator-tile system with waveshifter and fiber readout by moving the phototubes to a lower magnetic field environment. This will preserve the present capabilities of this system.