Design and Implementation of a Detector for High Flux Mixed Radiation Fields

The main purpose of the LHC Beam Loss Monitoring (BLM) system is the active protection of the LHC accelerators' elements against the quench of superconducting magnets and the damage of equipment caused by the loss of circulating protons. The lost protons initiate a shower of secondary particles, which deposit their energy in the equipment and partly in a radiation detector. If thresholds in the BLM system are exceeded, the circulating LHC beam is directed towards a dump to stop the energy deposition in the fragile equipment. The LHC BLM system will use ionization chambers as standard detectors, and in the areas with very high dose rates Secondary Emission Monitor (SEM) chambers will be employed to increase the dynamic range. The SEM is characterized by a high linearity and accuracy, low sensitivity, fast response and a good radiation tolerance. The emission of electrons from the surface layer of metals by the passage of charged particles is only measurable in a vacuum environment. This requirement leads together with the foreseen operation of 20 years to an ultra high vacuum preparation of the components and even to an additional active pumping realized by a getter pump (NEG). The signal and bias electrodes are made of Ti to make use of its Secondary Emission Yield (SEY) stability and favorable vacuum properties. The sensitivity of the SEM was modeled in GEANT4 via the Photo-Absorption Ionization module together with a custom parameterization for the very low energy secondary electron production using the modified Sternglass formula. The simulations were validated by comparative measurements of several prototypes with proton beams of the CERN PS Booster dump line, the SPS transfer line, the PSI Optis line and by a muon beam in the COMPASS beam line. Tests of the complete acquisition chain were performed in the LHC test collimation area of the SPS and compared to the combined Fluka and GEANT4 simulations. The linearity and long term stability was also tested in the high energy beam dump area of the SPS. A dedicated fixed target experiment was designed in the CERN H4 secondary beam line for testing all the 400 detectors produced in IHEP Protvino. The simulations were also used for the prediction of the signal levels expected in the LHC and for an absolute dose calibration. The comparison of simulations and measurements and of SEM and ionisation chamber measurements resulted in the relative difference range between 8 and 43% for different setups and radiation fields.