Calibration of the DELPHI HPC calorimeter during the LEP2 programme

The readout chambers of the DELPHI HPC calorimeter are affected by large ageing which results in a module-dependent loss of gain. The calorimeter response thus needs to be tuned on a module-by-module basis. During the LEP operation phase at energies close to the Z resonance (LEP1), the tuning was based on the high available statistics of Z→e+e− events. In the high energy operation phase (LEP2), no such clear monochromatic physics channel is available with high statistics. A monitoring of the analog response of the readout chambers of each module is provided by the periodic calibration with radioactive 83mKr; using this information to account for module-dependent effects ensures the stability of the energy resolution throughout the LEP2 programme.