HERMES is an experiment designed to study in a complete and precise way the spin structure of the nucleon at HERA (DESY-Hamburg, Germany). It will measure spin-dependent deep inelastic scattering from the proton and neutron using the longitudinally polarized electron beam of the HERA storage ring at beam energies between 30 and 35 GeV. For these measurements it will use polarized internal gas targets of hydrogen, deuterium and 3He.

Scattered electrons and hadrons produced in the deep inelastic reactions will be detected and identified by a magnetic spectrometer. The angle and the energy of the scattered electron will be determined by track reconstruction from a mini-drift chamber, drift chambers in front of and behind the magnet and proportional chambers in the magnet. Most of the background particles emitted from the target are suppressed at the trigger level by a calorimeter wall which measures the energy of the scattered electrons, rejects pion and provides the trigger for the experiment.

The calorimeter will be built from special radiation resistant lead glass TF 101 to overcome the potential problem of radiation damage in the HERA environment.