The HPC is the barrel eletromagnetic calorimeter and is installed as a cylindical layer outside the Outer Detector. It is mounted on the inside the solenoid.
Each HPC module is a trapezoidal box with a width ranging from 52 to 64 cm and a height of 47 cm. The length is 90 cm, except for modules in the first and last rings which are somewhat shorter. The box is filled with 41 layers of lead separated by gas gaps. An electromagnetic particle showers in the lead and ionizes the gas. The charge drifts to one end of the box, where it is collected by a proportional chamber with pad readout (like in a TPC). Have a look at the display module outside the control room and watch the blinking lights simulate a shower!
Each layer of material is actually a fiberglass-epoxy support (0.1 mm) with lead wires (1.7 mm) glued to both sides. Resistor chains set up a voltage gradient between neighbouring lead wires, giving a constant drift field along z of about 100 V/cm. The 8 mm gaps are filled with 80% argon and 20% methane. The electric and magnetic fields are precisely aligned by adjusting the orientation of each box, making it possible for the ionization electrons to spiral all the way to the readout chamber. The charge transmission is limited by transverse diffusion, giving an attenuation length of 3-4 meters (much greater than the longest drift distance). With a drift speed around 5.5 cm/us, the maximum drift time is 16 us. The 10th sampling gap at about 4.5 radiation lengths (shower maximum) is filled with a scintillator plane instead of gas. It is used for the first level trigger (see below).
Each sampling gap ends in a U-shaped brass cathode (8 by 8 mm) with a 20 um goldplated sense wire. It is the induced cathode pulse that is read out. The cathodes are divided into segments (2-8 cm long), which are connected to their neighbours (above or below) in groups of 3-6. The result is 128 pads in 9 rows, ranging from 2 by 3 cm (first three rows) to 8 by 7 cm (last three rows). It is this pad pattern that defines the granularity in r and phi. The pad signals pass through preamplifiers mounted on the chamber and are sent via 40 m of cable to the counting rooms (B1 and D2).
144 modules with 128 pads each means 18,432 channels. The electronics is grouped into four clusters, two in B1 and two in D2. Each cluster consists of six crates with 32-channel shaper boards and six Fastbus crates with 32-channel digitizer boards. The shaper circuit suppresses chamber-induced tails and amplifies the signal further.
(to be continued)