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back to: EMC software

The barrel consists of 5760 CsI(Tl) crystals, arranged in 48 polar-angle rows of distinct crystal sizes, each having 120 identical crystals in azimuthal angle (phi). The crystals are grouped in 280 modules, each spanning 7 x 3 crystals in theta and phi, respectively, except for the most-backward module, which has only 6 crystals in theta. The modules are made from 300 mum thick carbon fiber composite (CFC) material and held from the rear by an aluminum strongback. [picture] They are mounted in an aluminum support cylinder, which in turn is fixed to the coil cryostat [picture].

By supporting the crystals from the rear, minimal material is placed in front of them. The EMC front material consists of two 1 mm thick cylinders of aluminum, separated by foam, which provide a gas seal and RF shielding. Additional front material due to the liquid radioactive source calibration system consists of the equivalent of another 3 mm of aluminum. Cooling, cables, and services are located at the back of each module and thus do not add to the inactive-materials budget.

CFC and crystal wrapping material introduce gaps between the crystal active material of about 1.25 mm in each of the theta and phi directions. In order to minimize the loss of photons traversing these gaps, all theta gaps (except the one at 90 degrees) are non-projective by 15-45 mrad for most of the barrel. The gaps in phi are fully projective, causing up to 2.5% of photons traversing inactive material between crystals.

The forward endcap is a conic section, with front and back surfaces tilted at 22.7 degrees with respect to the vertical. The barrel/endcap gap is expected to be about 2 mm. The endcap contains 820 CsI(Tl) crystals, arranged in 8 theta rings, starting at an inner radius of 55.3 cm from the beam line. A 9th ring is currently filled with lead. As in the barrel, the endcap crystals are grouped in modules: there are 20 wedge-shaped modules around in azimuth, each with 41 crystals. The modules are of a CFC construction similar to the one in the barrel, and are also held from behind by aluminum strongbacks, mounted to an aluminum backplate. [picture]

schematic r-z view of the calorimeter
the numbers correspond to the theta index of the crystal rings

schematic view of the calorimeter in the x-y-plane
The z-plane is cut at z=0 and one can see the barrel in front and the projection of the endcap crystals front face onto the x-y-plane in the back. The dotted crystals belong to the last not installed ring.

Properties of Thallium-doped CsI

Radiation Length 1.85 cm
Moliere Radius 3.6 cm
Absorption Length for 5Gev pions 41.7 cm
Density 4.53 g/cm3
dE/dx MIP 5.6 MeV/cm
Light Yield (LY) ~5000 photons/MeV
Light Yield Temperature Coef. 0.001 /K
Peak Emission 565 nm
Refractive Index at Emission Maximum 1.79
Decay Time 940 ns
Hygroscopic slight, less than NaI
LY reduction after 10 kRad ...

Design parameters

performance parameters design actual
geometric acceptance 0.90
material in front of Calorimeter (at 90 degrees) 0.25 X0
energy resolution (1 GeV photon, average of all angles) 1.8%
photon efficiency within acceptance (at 100 MeV) 0.92
theta resolution (1 GeV photon, average of all angles) ...
phi resolution (1 GeV photon, average of all angles) 4 mrad

Ralph Mueller-Pfefferkorn

Created: Oct 12 1998 Last modified: Aug 29 2000