Technical description of the FEMC

The Forward ElectroMagnetic Calorimeter consists of two arrays of 5 m diameter; each array is made of 4532 Cherenkov lead glass blocks. The front faces are placed at |z| = 284 cm, covering the polar angles from 8 to 35 degrees and from 145 to 172 degrees. The blocks are truncated pyramids with inner (outer) face dimensions of 5.0 * 5.0 (5.6 * 5.6) cm^2 and depths of 40 cm, corresponding to 20 radiation lengths. Each block is mounted in the detector to point near to the interaction region. A tilt angle of about 1 degree was applied in order to avoid any particle escaping undetected in the insensitive regions between the blocks.

The operation of lead-glass Cherenkov counters is based on the collection of Cherenkov light emitted by the charged tracks of a shower. If n is the refractive index of the medium traversed by a particle and beta its speed in units c=1, part of the light emitted by excited atoms appears in the form of a coherent wavefront at fixed angle with respect to the trajectory (cos(theta) = 1/(beta*n), as beta > 1/n ). The pulse height is related to the number of photoelectrons knocked out of the photocathode by the Cherenkov light.

Lead glass blocks

The Cherenkov signal induced by the charged particles in the shower is read out by a single stage photomultiplier (phototriode) designed to operate inside the DELPHI magnetic field, coupled to a low noise preamplifier.

The phototriode consists of a photocathode, one dynode and an anode as shown in fig.1. The input capacitance for the phototriodes which was used is determined by the distances between the electrodes which vary from one manufacturer to another. Typical measured values range from 5 to 15 pF. The triode gain in a magnetic field of 1.0 Tesla is in the range 7-10.

The average yield of photoelectrons is about 1000 per GeV of deposited energy. The system shows very low noise, corresponding to about 25 MeV of energy deposit (the tipical deposit from a minimum ionizing particle is about 540 MeV).

Distribution of the energies deposited in the FEMC by muons selected by means.

The reconstruction of electromagnetic showers is performed in two stages:

The first stage is an iterative search for energy clusters. In each endcap, the glass with the largest energy deposit not yet associated with others is located and the eight adjacent ones are taken as part of the cluster if their energies exceed a threshold value. A flag is set if a glass had previously been attributed to a contiguous cluster and its energy is then shared between the clusters in proportion to the energies in their central glasses. The cluster energy is then evaluated by summing the energies assigned to it and its coordinates are calculated from their centre of gravity. The energy sharings between contiguous clusters are then refined taking into account the computed energies and positions of the clusters. The process continues until all energies above threshold have been assigned to an energy cluster.
The second stage uses information from the tracking system to distinguish the clusters due to neutral particles from those coming from charged particles. A matching is performed based on a chi^2 comparison between the predicted impact point and the reconstructed shower position. The efficiency of the matching algorithm is however degraded for electrons due to the emission of hard bremsstrahlung photons (more than one radiation length of material is crossed between the interaction point and the FEMC).

After calibration performed in an electron test beam Bhabha events are used to calibrate the detector in the pit. The calibration constants were found to vary on average by less than 1% per year. This procedure calibrates about 90% of the detector, but in the region theta > 32 degrees the electron energy is degraded too much by interactions in the TPC support structures. Counters in this region are therefore calibrated using muons, which deposit 540 MeV in each glass with an energy spread of about 20%. The energy distribution after the detector calibration is compared to that obtained using the calibration constants determined for the previous year . The energy resolution for Bhabha electrons is 4.8 %, degraded as compared to the test beam results by pre-showering of the electrons in the material between the beam intersection point and the detector. The relative precision on the measured energy can be parametrised as :

sigma(E)/E = 0.03+(0.12/sqrt{E})+(0.11/E)

where E is in GeV. For neutral showers of energy larger than 2 GeV, the average precision on the reconstructed hit position in x and y projected to |z| = 284 cm is about 0.5 cm.

Energies of Bhabha electrons as seen by the FEMC, normalized to the beam energy.

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Conception and design : Krzysztof Cieslik

Last update: Philippe Charpentier