Hadronic Endcap Calorimeter
Calorimeters will play a crucial role at LHC. In contrast to other detectors their intrinsic resolution improves with energy, which makes them very suitable detectors at high energy machines. An important aspect of the HEC is its ability to detect muons, and to measure any radiative energy loss.
The HEC is designed to provide coverage for hadronic showers in the range 1.5 < |η| < 3.2 . The HEC detector elements are located in the endcap cryostat at either end of the ATLAS tracking volume. It is a liquid argon (LAr) sampling calorimeter with copper-plate absorbers. This technology was selected as it allows a simple mechanical design to be produced that is radiation resistant and covers the required area in a cost-effective way.
The gaps between the copper plates are instrumented with a read-out structure forming an electrostatic transformer (EST). This structure optimizes the signal-to-noise ratio while reducing the high-voltage requirements and ionization pile-up, and limiting the effect of failure modes such as high-voltage sparks and shorts.
The signals are amplified and summed employing the concept of "active pads": the signals from two consecutive pads are fed into separate amplifiers (based on GaAs electronics) and summed to read-out channels. The boards with the GaAs chips are mounted on the outer radius of the HEC. The use of cryogenic GaAs preamplifiers provides the optimum signal-to-noise ratio for the HEC.
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