The barrel is constructed of 24 modules with 20 layers of limited streamer mode detectors inserted into 2cm slots between 5cm iron plates in each module. The modularity of end-caps is similar to the barrel with a sampling depth of 19 layers.
The detectors are wire chambers which consist of a plastic cathode forming 8 cells of 0.9x0.9cm with one anode wire (80um made of copper-beryllium) in each. The inner surface of the cathode cells is coated with a poorly conductive graphite varnish.
The hadron calorimeter contains by far the largest volume of gas in DELPHI, and operates with relatively low i-butane content: Ar/CO2/i-butane 10/60/30 percents. With this gas mixture an average charge of 10pC is induced on external readout boards with an HV of 4kV.
The copper clad readout boards are segmented into pads wich pick up the streamer charges. Pads are shaped to form projective towers pointing to the intersection point; in the barrel 5 adjacent pads are combined into a tower, 7 or 4 in the end-caps. Each tower covers an angular region of delta-phi=3.75 and delta-theta=2.96 degrees in the barrel, and delta-theta=2.62 in the end-caps. The dimensions of a typical tower in the barrel are 25x25x35 cm. There are 19008 towers in hadron calorimeter, with 232 and 164 towers in barrel and end-cap modules respectively.
The analogue signals from towers are collected into 96 front-end electronic crates (one per module) for charge integration, multiplexing by 256, 8-bit ADC, zero suppression and a discrimination according to a control signal. The charge integration time is about 2us. A fast discrimination of the signals from group of 4x4 towers (supertowers) is performed and logical combinations of the supertower signals are available for triggering purposes. The zero suppressed data are processed in a local FASTBUS data acquisition system, composed of 12 OCTOPUS receiver cards, an HFB buffer memory and FIP crate processor.