The uranium scintillator calorimeter (CAL)

The ZEUS collaboration in front of the open ZEUS detector.

The uranium scintillator calorimeter (CAL) covers about 99.8 % of the full solid angle, provides linear and equal response for electrons (e) and hadrons (h, like protons or pions) up to the highest energies, precise energy measurements for hadrons and jets with a relative energy resolution obtained from tests of 35%/SQRT(E) + 1% and 17%/SQRT(E) + 1% for electrons (E in GeV), an angular resolution for jets better than 10 mrad, the ability to discriminate between hadrons and electrons using their different energy depositions and a time re solution of one nanosecond. This time precision should be compared with the time between consecutive bunch crossings of 96 ns. The calorimeter energy response has been calibrated with an accuracy of about 1%.

The calorimeter has a total weight of 700 tons. It is subdivided into 80 modules which form the forward (FCAL), central (BCAL) and backward (RCAL) calorimeters. Each module has a layered structure consisting , of 3.3 mm thick depleted uranium (DU) plates wrapped in stainless steel foils (200 or 400 um thick) as absorber material interleaved with 2.6 mm thick sheets of plastic scintillator as detector. The thicknesses of the absorber and detector plates were selected to achieve equal electron and hadron response (e/h = 1.0, compensation). Up to 185 layers of absorber plus scintillator form the calorimeter modules. Light generated by charged particles traversing the scintillator is collected by plastic wavelength shifters and transported to photomultipliers where the light is transformed into electrical signals.

Behind the third and sixth uranium layers of the calorimeter a detector of 3 cm x 3 cm silicon diodes is partially installed; these will improve the hadron electron separation capability and the spatial resolution of electromagnetic showers.

View of a ZEUS forward calorimeter module (~12t).

A forward calorimeter module positioned on its back beam before wave length shifter installation.

Wavelength shifter installation of a central calorimeter module (~10t).