2.5 Calorimetry

By converting an incident particle into a shower of secondary particles, calorimeters provide information on the energy of neutral as well as charged particles and can help in particle identification. Electromagnetic calorimeters are designed for the identification of electrons and photons, which induce a shower of electrons, positrons, and photons by elastic Coulomb scattering from nuclei in the converter. On the other hand, hadron calorimeters rely specifically on detecting the hadron showers produced by inelastic hadronic interactions with the nucleons of the converter.

The two principal electromagnetic calorimeters in DELPHI are the HPC in the barrel and EMF in the endcaps. The SAT (before 1994), STIC (from 1994), and VSAT provide additional electromagnetic calorimetry in the very forward region, though are mainly used for the luminosity measurement (see section 2.8). The barrel and endcap Hadron Calorimeters (HAC) are of similar design. The properties of the calorimeters are listed in table 2.3.

Table 2.3: Some characteristics of the DELPHI calorimeters: barrel and forward electromagnetic calorimeters (HPC and EMF respectively), Hadron Calorimeter (HAC), and luminosity monitors (SAT/STIC and VSAT). $ X_0$ is the radiation length and $ \lambda_I$ the nuclear interaction length. The shower energy resolution is given for 45.6 GeV.

\begin{tabular}[t]{\vert l\vert\vert lll\vert r\vert l...
...$--9& 770 & 0.3--0.5 & $24X_0$\ & 12 & & & 5 \\

2.5.1 High-density Projection Chamber (HPC)

The barrel electromagnetic calorimeter, HPC [35], consists of 6 rings of 24 modules each. Each module is a small TPC, segmented with 41 lead partitions, which induce an electromagnetic shower from the passage of electrons or photons, as well as providing the drift field. A pattern of pads at the end of each module measures the radial and azimuthal coordinates, with the $ z$-coordinate coming from the drift time. This technique provides a three-dimensional picture of the evolution through the lead converters of an electromagnetic shower. Scintillators [36] to provide fast triggering on electromagnetic showers are placed close to the position of shower maxima in the HPC.

2.5.2 Forward Electromagnetic Calorimeter (EMF)

The Forward Electromagnetic Calorimeter [37] consists of 4532 lead glass blocks on each side. The Cherenkov signal induced by the charged particles in a shower is read out by the phototriode backing each block.

2.5.3 Hadron Calorimeter (HAC)

The iron yoke for the magnet is instrumented with plastic streamer tubes [38] to detect showers produced by hadrons interacting in the iron. The barrel Hadron Calorimeter consists of 20 layers of limited streamer tubes sandwiched between 5 cm thick iron plates and segmented into 24 modules. A similar arrangement of 12 sectors are used in each endcap. Each tube consists of a plastic cathode surrounding a single anode wire. The streamer charges from the anodes of 4-7 layers are read out together. Since 1995 (1996 in the endcaps), the cathode of each tube has been read out independently, thus increasing the granularity to improve the $ \pi$/$ \mu$ separation [39].

Tim Adye 2002-11-06