A luminosity monitor for the Large Hadron Collider

The LHC luminosity will reach 1034 cm-2s-1 but special runs at 1028 cm-2 s-1 are foreseen. Thus a luminosity monitor must have a dynamic range of six orders of magnitude. A good tolerance to radiation is also required. A detector using both ionisation and secondary emission techniques has been studied in this context. Its design is based on monitors used previously at the CERN PS and SPS machines. Special attention was devoted to minimize leakage currents. Linearity in both Secondary Emission Counter (SEC) and Ionisation Chamber (IC) modes has been tested from ~104 incident particles to ~108 incident particles. SEC is linear above ~3 x 106 incident particles while IC is linear over the full studied range. However, because of the radiation environment at the LHC, the SEC mode is much preferred at high intensity. A solution actually foreseen is to switch from IC to SEC mode when the incident current on the monitor is ~10-13 A corresponding to an LHC luminosity of ~1031 cm-2 s-1.