A luminosity monitor for LHC

LHC luminosity will reach 10³⁴ cm^-2 s^-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. Special attention was devoted to minimise 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 ~5.10⁶ incident particles while IC is linear over the full studied range. However, because of the radiation environment at LHC, the SEC mode is much preferred at high intensity. A solution actually foreseen is to switch from IC to SEC mode when the intensity is around 5.10⁶ incident particles per second corresponding to an LHC luminosity of 6.10 ³⁰ cm^-2 s^-1