A new readout system for “Digital Hadron Calorimetry” for the International Linear Collider

A new detector concept called "digital calorimetry" is being developed for the International Linear Collider to achieve unprecedented jet energy resolution. Instead of traditional tower geometry and energy summation from many sampling layers, the new approach detects energy deposition in 1 cm² cells on each sampling layer using discriminators. Jets are reconstructed using hit patterns from each layer, combined with information from inner tracking and the electromagnetic calorimeter. Detector R&D is in progress, and we are building a readout system for a 400,000 channel "prototype detector" to demonstrate this concept. The readout system uses a 64-channel custom integrated circuit called DCAL to record hits from each cell and apply a global timestamp. The chips mount directly on sophisticated front-end boards that are not only an integral part of the charge collection of the detector chambers, but also incorporate digital signal transmission, clock and control, and power and ground. The readout of data is serial, multiplexed into high-speed serial streams and sent to a "back-end" VME system for time-sorting and higher-level triggering. The system can be operated with an external trigger or be self-triggered, and can produce trigger signals from the front-end chips. We have built a "vertical slice" of the readout system, and are using it for tests of Resistive Plate Chambers (RPCs) and Gas Electron Multipliers (GEMs) in a test beam at Fermilab. This represents the first stage in building a 400,000 channel system for reading out the "cubic meter" prototype detector as part of the CALICE collaboration. The components of the system are described, and preliminary system performance is reported.