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The Uranium Calorimeter (UCAL)

NA44 uses nine stacks of a fine-sampling Uranium/Copper/scintillator Calorimeter with wavelength shifter readout, originally designed  [32] for the AFS experiment at ISR. Each stack is sub-divided into six towers. Each tower has two sections, read out separately: a 6 radiation lengths ($6X_0$) long electromagnetic one, and a 3.8 absorption lengths ( $3.8\lambda_I$) long hadronic absorber. Each section is read out by two PMTs collecting light from two light guides that cover left and right flanks of the section. Two characteristic features of this device are

• nearly identical response to charged and neutral pions (not required by NA44)
• self-calibration by measuring fission signal.

NA44 uses UCAL for $e/\pi$ separation. In particular, in this analysis UCAL was used to determine how many pions are lost due to the Cherenkov electron veto in the trigger (see subsection 4.4.3). This is possible due to the longitudinal separation of the tower into electromagnetic and hadronic sections. The electromagnetic showers tend to have a predominant contribution from the electromagnetic section. Due to the low track multiplicity in the spectrometer, the UCAL's granularity is sufficient to associate tracks with towers.